222 — Knutzen and Martin · PDF file222 — Knutzen and Martin Pediatric Exercise Science, 2002, ... In the growth and motor development area, ... Tutorial. Biomechanics and

222 — Knutzen and Martin

Pediatric Exercise Science, 2002, 14, 222-247© 2002 Human Kinetics Publishers, Inc.

Using Biomechanics to ExploreChildren’s Movement

Kathleen M. Knutzen and LeaAnn Martin

Introduction

Biomechanics is a discipline that studies the motion and effect of forces on bio-logical systems such as the human body. It is an area that has wide application foruse in the exploration of movement across multiple venues including sport, reha-bilitation, and growth and development. The vast majority of applications has beenwith the adult population. However, the application of biomechanical principles tothe study of movement of children is being used with greater frequency, as re-searchers and practitioners expect a more exact determination of movement char-acteristics than previously provided by visual observation only. This paper willexplore the role of biomechanics in the examination of children’s movement andprovide a review of the various approaches that have been applied to the study ofmotion characteristics in children.

Biomechanical applications have proven to be very useful in a variety ofareas where the quantification of movement is important. In orthopedics, for ex-ample, surgical procedures are applied that influence the mechanics of the joint.This can be illustrated by looking at orthopedic management of the child withcerebral palsy where procedures are sometimes complex, involving multiple jointsand planes of dysfunction. An evaluation of gait using combined information thatincludes biomechanical assessment can separate primary deformities from sec-ondary compensations, allowing for a more precise solution and optimization oftreatment (19). The use of biomechanical applications is valuable because it pro-vides objective data that can be used to make diagnostic and rehabilitation deci-sions and in the development of new techniques or new rehabilitation procedures(20).

In the growth and motor development area, biomechanical assessment hasbecome a standard means of exploration. Early researchers in the motor develop-ment area collected a large volume of qualitative information on children’s move-ment, which provided a good base of descriptive data. These data were used todocument the attainment of motor milestones, rather than quantifying specific com-ponents of the motion. Observational rating scales and motor proficiency batter-ies, used to assess the attainment of the milestones, are now being replaced bybiomechanical analysis of movement about several joints. The information can

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The authors are with the Department of Physical Education, Health and Recreation,Western Washington University, Bellingham, WA.

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Biomechanics and Children — 223

lead to more accurate evaluation of not only the outcome of the movement, butalso the manner in which the movement is performed and the determination ofspecific patterns.

Likewise, in the motor control area, the recognition of the influence of bio-mechanical properties on movement control has many researchers integrating bio-mechanical measurements in the study of movement control problems. The pat-terns of limb action may be controlled by motor programming, but the limb move-ments are influenced significantly by mechanical factors such as mass, leverage,and moment of inertia (47). There are both active and passive biomechanical con-straints that influence movement, and these must be assessed to understand mecha-nisms of movement control. With this understanding, movement control problemscan more adequately be analyzed and addressed.

Biomechanics can also play a role in understanding injuries to children (13).The application of biomechanics to injury evaluation can provide information toanalyze movement patterns for injury producing components and insight into dif-ferences between adults and children in terms of response to physical stress. Bio-mechanical assessment can also identify forces that may contribute to injury. Chil-dren undergo unique neuromuscular development and skeletal growth that limittheir power and control capabilities. Special anatomical characteristics of childrens’musculoskeletal structure can be studied biomechanically to determine loads thatcan be accommodated in ligaments, tendons, bone, and specialized growth platetissue.

There is a need to gather additional biomechanical information on move-ment characteristics of children, especially for children with disabilities (4, 35).Comparisons between younger and older children are also needed to determinebiomechanical reasons why younger children are less economical in specific move-ments such as running (50). Mechanical principles that facilitate economy of ef-fort and efficiency are important to define for children attempting to master thefundamentals of a sport skill (24). Thus, the use of biomechanical analysis in theevaluation of children’s movement is necessary and beneficial, providing infor-mation on a variety of aspects of performance and locomotion.

Major issues concerning biomechanical assessment of children’s movementinclude the question of the appropriateness of applying methodologies and tech-niques validated on adults to children, the usefulness of the information providedin a biomechanical analysis, the goal of a biomechanical analysis of children’smovement, and the important biomechanical principles that guide an explorationof movement characteristics in children. What are some of the research questionsthat can be easily addressed using biomechanical techniques? What role will bio-mechanics play in understanding movement characteristics of children? What arethe components of a biomechanical analysis? Finally, what limitations and issuesmust be considered when using biomechanical analysis?

Biomechanical Approachesto the Study of Movement

The discipline of biomechanics addresses motion and the interaction of bodies atrest or in motion. Newtonian laws and principles provide the mechanism for un-derstanding the relationship between force and motion. Biomechanical analysis isthe avenue for determining the cause and effect of motion by studying the forces


that cause motion (kinetics) and by recording the product or effect of motion (ki-nematics).

A biomechanical analysis varies depending on the goal. An analysis can beused to determine mechanisms associated with a disease process or to determinehow a particular condition impacts function. It can be used to clarify where move-ment deficits occur and point to areas where corrections can be made. It can alsobe valuable to determine whether an intervention is effective. These varied appli-cations of biomechanics have been used extensively, especially in the study of gaitas it applies to orthopedic interventions (66).

An examination of the different approaches in a biomechanical analysis ishelpful in determining the usefulness of a biomechanical application. These ap-proaches include kinematic, kinetic, combined kinematic and kinetic, dynamicalanalysis and modeling.

Kinematics

Kinematics is the description of motion, and it is the net effect of forces applied tothe body that create linear or angular motion. This precise description of motionhas been used to determine patterns of motion or to reconstruct motion of points inspace (14). It is important in the conceptual aspect of exploring movement, but itdoes not infer cause of motion, only the effect. Kinematics often times serves as aninitial point of analysis and is the basis for subsequent kinetic analyses.

A kinematic analysis offers a description of movement that has been used byresearchers to study skill acquisition, examine the temporal organization of gait,generate normative data for future use, or compare patterns of movement betweenspecific conditions or individuals. The most common tool for kinematic analysis isthe high-speed video or motion capture system. One or more cameras are used toobtain two- or three-dimensional spatial coordinates of body landmarks. Placingactive or passive markers on the body typically identifies the landmarks. Passivemarkers are light reflecting devices that reflect ambient or projected light and ap-pear on a camera as illuminated spots. Active markers are connected to the record-ing system and are typically light emitting diodes that pulse in a sequential mannerthat can be identified by the data tracking system. In both systems, the markers aresequentially scanned and recorded via computer control. It is becoming more com-mon to see 6 or more cameras in a typical laboratory set up. This minimizes thechance of losing a marker during tracking and makes for an easier transformationto 3-dimensional data. Multiple cameras are calibrated with a set of precalibratedmarkers that allows the creation of a global coordinate system from the cameracoordinates. The linear and angular motion characteristics of segments, segmentalendpoints, and the total body can be calculated once the markers are recorded andtwo or three-dimensional coordinates calculated.

A sample of studies using a kinematic approach is presented in Table 1. Themajority of these studies have employed a kinematic analysis to examine charac-teristics of a particular skill or movement pattern. For example, gait is commonlystudied using a kinematic approach to track the path of the center of gravity or toexamine the angular displacements and the synchronization between angular move-ments. Additionally, this biomechanical approach is used to evaluate the spatialand temporal relationships of gait, including stride lengths, stride widths, and stance


Table 1 Kinematic Approaches to Studying Children’s Movement

Gender/Authors Age (yrs) Selected Findings

Assaiante 26 M 3-8 Oscillations of head are minimized for children from 4et al. (1) 14 F 3-8 years and older; angular oscillations in frontal plane

3 M 20-25 dependent of width of supporting surface; control of3 F 20-25 locomotor equilibrium aims at limiting angular

oscillations of the head.

Assaiante 12 F 5.5-7.5 Head and trunk stabilization about the pitch axis waset al. (2) 6 F young present in children and adults; at landing the head

adults stabilization disappeared in both children and adults.

Bednarczyk 10 M&F 22-52 Children wheel at lower velocities and have more elbowet al. (4) 32 M&F 8-14 extension; adults and children similar in trunk angles,

patterns, and propulsion and recovery phases.

Beuter 8 M 5 Significant variability across the subjects in stance andet al.(8) 7 F 5 deceleration phase of swing; least variability in

acceleration phase of swing.

Boswell 3 M 4-7 Children with cerebral palsy walked at lower velocities,et al. (9) 7 F 4-7 had shorter stride lengths, and had higher cadence

values.

Charlton 8 M 9 Children with Down Syndrome moved more slowly;et al. (15) 6 F 9 wrist trajectories were more variable; more time in

5 M 4 deceleration.2 F 4

Gachound 40 M 6-9 Kinematic features of lifting task did not vary betweenet al. (23) 10 M children and adults; patterns of emg were different.

young adults

Gibson 4 M 12 Older males = more consistent wind-up.et al. (24) 4 M 15 Younger males = overstriding, limited follow through,

lower ball release.

Halverson 22 M 12 Overarm pattern not fully developed at this age; maleset al. (27) 17 F 13 increased ball horizontal velocity more than females.

Hoy et al. 1 M 11 Children with amputations walking with a prosthetic(35) 4 F 11 foot had shorter stride lengths than adults, walked at

increased cadence, greater stride width, asymmetric footangles, lack of propulsion in prosthetic limb.

Hsu et al. 6 M&F 5-7 There were graded differences in decoupling of limb(36) motion, limb excursions and range of motion between 5

year old, the 7 year old awkward, and the 7 year old

(continued)


normal child.

Jackson 4 M Skilled children exhibited greater horizontal and verticalet al. (37) displacement of the center of mass.

Martens 11 M Poor balancers registered higher velocities and moreet al. (49) 8 F displacement of the head.

Parker 3 M 6-7 Children with Down Syndrome exhibited poor heel-toeet al. (51) 3 F 6-7 mechanism in stance, exaggerated hip abduction, out-

toeing, and variable excursions of the upper extremity.

Satern (55) 20 M&F 13 Adults used higher angles of projection, had lower37 M&F 18-21 linear velocities, and had a looser linkage between knee

and elbow motions.

Sutherland 98 M 1-7 Sagittal plane rotations of children 2 years and older iset al. (58) 88 F 1-7 similar to adults. Younger children exhibit more hip

rotation, knee flexion and ankle dorsiflexion. As gaitmatures the cadence decreases with an increase invelocity and step length.

Vander 6 F 5.2-7.9 More range of motion was required to move to a standLinden 4 M 5.2-7.9 using the half-kneeling position; emg was variable.et al. (65)

Van Rossum 10 F 4 & 6 6 year olds were more accurate and had more controlet al. (65) 10 M 4 & 6 over the angle of release of the ball, with elbow angle

fixed during forward movement.

Wood (70) 6 M & F 6 & 9 Motorically delayed children demonstrated smallerhorizontal velocities at takeoff, less trunk inclination,increased trunk rotation, less flexion of arms, minimumheel lift.

Table 1 (continued)


duration. An examination of a subject’s walking pattern would not be completewithout comparing time-distance parameters (9).

An example of the usefulness of the kinematic approach is illustrated inFigure 1, which presents the results of a study by Bednarczyk and Sanderson (4).They were interested in examining the kinematics of wheelchair propulsion ofboth children and adults. By measuring angular displacement of the elbow, trunk,and shoulder, Bednarczyk and Sanderson were able to determine that children dem-onstrated more shoulder extension and abduction than adults, while elbow andtrunk displacements were similar. They summarized by stating that even though


Figure 1 — Angular kinematic wheelchair propulsion patterns for children (top) andadults (bottom) for the elbow (A), shoulder (B), trunk (C), and shoulder abductionangles (D). Bednarczyk, J.H., and D.J. Sanderson. Kinematics of wheelchair propul-sion in adults and children with spinal cord injury. Arch Phys Med Rehabil. 75:1327-24, 1994.


there were differences in the wheeling techniques, the similarities may have beena reflection of the physical containment and restriction imposed by the wheelchair.Other researchers have used kinematic analyses to measure movement controlparameters in children with neurological dysfunction (63).

Another application of the kinematic approach is the use of angle-angle orphase plane plots, which are generated from angle data. In the case of an angle-angle diagram, angular position of one joint is plotted against another to presentsimultaneous kinematic data. This presentation of the data has been used to deter-mine whether there is phase coordination between joints or whether joints areworking independently. An example of the angle-angle diagram taken from Hsuand associates (36) is presented in Figure 2. In this study, shoulder vs. ankle angle-angle diagrams during walking was constructed to compare the patterns of activitybetween older normal, older awkward, and younger subjects. The results clearlydemonstrated differences in patterns where the younger subject exhibited decouplingof joint motion (one joint angle changing while the other is primarily stationary),more limb excursion, and greater range of motion at the ankle joint than the oldernormal subject. The older awkward subject was somewhere in between the twoother subjects in terms of excursions and range of motion.

The phase plane plot is the plot of angular displacement against angularvelocity and provides information on the dynamics of limb trajectories (7, 36).These kinematic data representations are common in the dynamical systems ap-proach that will be discussed later in this paper.

As previously stated, kinematic analysis commonly involves obtaining two-or three-dimensional spatial coordinates of the body, via high-speed cameras. Themeasurement of a point can be done accurately if specific procedures are followed.However, there are accuracy concerns in computations such as acceleration, wherethere is considerable noise in the data. Also, in angular measurements joint centersare sometimes difficult to locate (17) or markers are lost in the field of view be-cause they are behind a segment and not in the field of view of all cameras. Mea-surement of derived values such as total body center of gravity can also lack accu-racy. Errors in kinematic data are typically due to errors approximating masses orjoint centers, marker placement, loss of body parts in the field of view, and pro-cessing of the data through transformation, smoothing, and differentiation tech-niques, and in the accurate determination of events (e.g., takeoff, heelstrike).

Biomechanical applications involving just kinematic assessment providedescriptive data but are usually lacking in clear hypotheses. Examining only thetemporal and spatial components of a motion is limited and does not represent amechanistic approach to solving a movement problem. However, qualitative kine-matic information has been used extensively to assess children’s movement andmay be justifiable in terms of ease of use, both clinically and pedagogically. Thesetypes of biomechanical investigations have contributed to the literature by quanti-fying movement characteristics of children in a variety of activities or locomotorstyles. The disadvantage of just performing a kinematic analysis is that it offers noinformation about causes of motion.

Kinetic Approaches

Kinetics is the study of actions of forces on bodies that allows for a determinationof the cause of motion. Motion is a consequence of the interaction between anobject and surroundings with the actual interaction termed a “force.” Motion occurs


Figure 2 — Angle-angle diagrams of the shoulder versus the ankle during normal(solid) and heel (dotted) walking for a 7-year-old (A), a 7-year-old awkward (B) and a5-year-old (C) child. Hsu, E., S. Bardfield, B.J. Cratty, and A. Garfinkel. Cine-matographical methods to assess associated movements in children. Adapted Phys ActQuart. 6:255-267, 1989.


within a field of external forces and is altered as a result of these forces and otherfactors such as structure of the environment, structure of the system, psychologi-cal state, and the task to be performed (22). Hence, a kinetic approach inbiomechanical analysis provides the reason for movement through determinationof variables such as force, energy, and power.

Forces are not overt and require the use of special equipment to measure.The most common instrumentation used in a kinetic analysis is the force platform.The force platform is typically mounted flush with a surface, allowing for theperformance of a variety of activities on the level surface containing the forceplatform. Force is applied to the surface to accelerate the total body center of massin three different directions relative to the ground: vertical, anteroposterior, andmediolateral. The force platform measures ground reaction force components inthese three directions and records the force effort. With most force platform sys-tems, corresponding torques about these three axes are also recorded. It is com-mon to scale ground reaction forces to body mass or body weight and to definetemporal aspects of the forces with respect to a relative time base. Scaling allowsfor the comparison across different subjects and speeds since ground reaction forcemagnitudes are influenced by body weight and velocity. Ground reaction forcescan be collected easily and with a high degree of accuracy with the force platform.

The kinetic analysis provides direct information about the cause of motions.The location of the resultant ground reaction force (center of pressure) can bemonitored to determine (a) stability during locomotion or other postural tasks, (b)if there are patterns of force production common to specific movement tasks, and(c) the level of force production required for the performance of skills. A sample ofkinetic approaches used to study movement characteristics of children is presentedin Table 2. These studies have primarily employed the use of the force platform todetermine kinetic differences between individuals or conditions, or to explore otherkinetic factors (e.g., power) that contribute directly to success of events such asjumping or cycling.

Figure 3 presents the average force-time curves for three different leg types(able-bodied, prosthetic, non-prosthetic) during running. This work of Engsbergand colleagues (20) demonstrates a typical use of the force platform to differenti-ate between kinetic components of a single footfall. Illustrated are the averagecurves from 200 able-bodied children and 21 children having a trans-tibial ampu-tation (TTA), with examination of the prosthetic and non-prosthetic limbs of thelatter. The analysis of the three ground reaction forces generated during the sup-port phase of running identified numerous differences between the groups. Thefirst vertical peak seen in heel-toe running was present in the non-prosthetic limbof the TTA children, less prominent for the able-bodied, and absent in the pros-thetic limb. In the anteroposterior direction, there were more abrupt changes inmagnitudes for the children with TTA for both limbs. Finally, in the mediolateraldirection, there were variations in both magnitude and pattern across the threeconditions. By examining the kinetic aspects of the support phase, these authorssuggested that the non-prosthetic limb of the child with TTA experienced greaterforces. This information has practical use as normative data and for clinical gaitapplication.

Because the majority of the kinetic analyses use the force platform, its use islimited in its application because it is confined to one area and generally allows forthe collection of one contact, unless multiple platforms are used. Additionally,


Table 2 Kinetic Approaches to Studying Children’s Movement


Berg et al. 14 M 11-12 Knee extension torque at 30 degrees/sec had the highest(6) correlation with sprint times.

Breniere 5 M 15-18 mos. Progression velocity is not reached at the end of the firstet al. (10) 3 F 15-18 mos. step in children; gait initiation depends on initial fall.

Bril et al. 4 M 2-3 Over a two-year period, children decreased step width;(12) 1 F 2-3 increase in velocity due to first step length increase and

later to increased cadence.

Davies et al. 24 M 11 & 14 Peak power in cycling and jumping related; higher(18) 32 F 11 & 14 power output with cycling; power increased with age.

Engsberg 6 M 7-11 There were no differences between right and left limbset al. (20) 5 F 7-11 of able bodied; prosthetic limb trials = non-dominant

role in gait.

Engsberg 36 M 7-8 Non-prosthetic limbs generated greater forces thanet al. (21) 58 F 7-8 prosthetic or the limbs of able-bodied children.

28 M 9-1031 F 9-1026 M 11-1221 F 11-12

Greer et al. 7 M 3-4 Children contacted ground with greater vertical force(26) 11 F 3-4 measures relative to body mass; transition from braking

to propulsion was earlier; boys and girls differed.

Kinoshita 5 M 7 Children had lower shock absorbing capacity in heelet al. (45) 7 M 19-35 pads; adults had higher peak deceleration and larger

9 F 19-35 amounts of forces.

Lebiedowska 5 M 7-14 Developmental factors were not related to swayet al. (46) 14 F 7-14 parameters in the no feedback condition; voluntary

13 M 15-16 feedback resulted in a 20% increase in total sway.9 F 15-167 M 17-189 F 17-18

Ledept et al. 21 M & F 4-8 Duration of gait initiation is independent of gait(47) velocity. Biomechanical constants are determining

factors for initiating gait.

Shirado 15 F 15-18 Patients with scoliosis shifted less weight than normals,et al. (56) 3 M 10-17 time for shift was longer, and there was less weight on

47 F 10-17 the concave side.

Xue-Cheng 200 M&F 7-8 Intoeing children exhibited a lower torque, shortenedet al. (71) forward displacement, and a medial shift of the center of

pressure than matched controls.


Figure 3 — Average vertical (top), anteroposterior (middle) and mediolateral (bot-tom) ground reaction force curves measured during running. The legs of able-bodiedchildren (large dash) were compared with the prosthetic (small dash) and non-pros-thetic (solid line) limbs of children with trans-tibial amputation. Engsberg, J.R., A.G.Lee, K.G. Tedford and J.A. Harder. Normative ground reaction force data for able-bodied and trans-tibial amputee children during running. Prostet Orthot Int. 17:83-89, 1993.


there are issues of targeting and gait speed, since both influence the pattern and theamplitude of the force measures (17). For targeting, it is believed that if a subjector patient is asked to hit a platform that is situated on the ground in front of them,they may make an unnatural adjustment in an attempt to make contact with theplatform, resulting in a ground reaction force recording that is not representativeof a normal movement pattern for that subject. Gait speed is another considerationwhen measuring ground reaction forces, since an increase or decrease in speedwill be reflected in the ground reaction force amplitudes and temporal characteris-tics. A logical adjustment in gait studies using the force platform may be to haveall subjects run or walk at the same speed. This has serious limitations because ofthe inherent nature of locomotion where individuals have preferred velocities.Forcing subjects or patients to walk at slower or faster speeds may result in adjust-ments in gait that are not normal for that subject. All of these factors need to betaken into consideration when using the force platform in a kinetic measurement.For example, if we find that older adults produce lower forces than children, itmay be just that they are walking at a slower pace.

In isolation, the kinetic analysis provides valuable information on forcesacting on the body as a whole. When coupled with kinematic information, a differ-ent level of analysis is allowed where specific movement characteristics can beassociated with force production. Load distribution methods or force productiontechniques can then be attributed to specific joint actions or multijoint coordina-tion strategies. Thus, hypotheses can be developed to assess cause and effect throughkinetic and kinematic applications, respectively.

Combined Kinetic and Kinematic Approaches

To make use of principles of biomechanics, one must examine the cause of themotion in conjunction with a description of the movement characteristics. It isbecoming more common to assess a movement problem using a combination oftechniques to explore both the cause and effect phenomena. Movement patternscan be related to moment and power characteristics of the joints. A complex skilllike jumping can be studied to determine propulsive force at takeoff, programmingof the center of gravity trajectory at takeoff, control of the body in flight, andequilibrium at landing (2). Changes in timing and forces as a skill is practiced canbe quantified, balance strategies to maintain the center of gravity over the base canbe recorded, and impaired ability such a weight transfer during locomotion can bedetermined.

Principles that guide the understanding of human motion explore the rela-tionship between kinetics and kinematics using one of three approaches. Theseinvolve the examination of motion as a result of a force applied (a) at an instant intime, (b) over a period of time, and (c) over a distance (29).

Studies that examine the effect of force or torque at an instant in time use thetraditional application of Newton’s second law of motion, �F=ma (linear) and�T=I� (angular). Using this approach, systems in equilibrium can be examined inthe case where acceleration is zero (statics) or where there are significant linear orangular accelerations (dynamics). In the linear case, if we know the mass of thesystem and can measure the forces acting on the body, we can compute accelera-tion of the total body and examine the cause (forces) and effect (motion) relation-ship. The same is true for the angular analog where we can measure the torques(forces applied a distance away from the axis of rotation), compute the moment of


inertia (I) about an axis, and then compute the angular acceleration (�) to explorethe cause and effect relationships in angular movement.

The effect of a force applied over a period of time can be used to study therelationship between momentum of an object to impulse, the force and time overwhich the force acts (29). This is commonly known as the impulse-momentumrelationship. Momentum values can be used to approximate final velocities andalso show how velocity is developed in the skill. Linear impulse can be calculatedfrom force platform amplitude and temporal values (force * time). Again, if themass of object is known, the velocity can be calculated by dividing the im-pulse values by body mass at any interval. In order to attain accurate angularimpulse calculations, integration (force * time) should occur over short sam-pling times with precise time measurements and a precise center of gravitycalculation (17).

Using angular impulse data, angular momentum can also be calculated orvice versa. Kinematic data is used to calculate angular velocities and angular mo-mentum can be calculated by multiplying the angular velocity times the segmentor object’s moment of inertia about a defined axis. This is equivalent to angularimpulse or the integration of torque over time. To calculate angular impulse (torque* time) first, torque can be measured directly such as on an isokinetic machine, orit can be indirectly calculated by measuring the linear forces and the distance oftheir application from the axis of rotation.

Force applied over a distance is studied through the application of the work-energy theorem. The work done on a segment or by the body as a whole is equal tothe sum of changes in energy level of the segment or the total body center ofgravity. Computing work and energy in a dynamic situation involves multiple steps.First, accurate anthropometric measures of mass, center of mass, and moment ofinertia about a transverse axis through the segment center of mass are calculated.The motion is recorded kinematically with the segmental endpoints identified.Secondly, linear and angular velocities are calculated and an inverse dynamicstechnique is used to calculate joint torques. Once joint torques have been deter-mined, mechanical work, power, and energy are calculated (31). Mechanical en-ergy and work can be calculated using velocity data while torques require the useof acceleration data. Mechanical energy is in four forms: energy due to verticalposition (potential energy), energy of motion in straight lines (kinetic energy),energy of motion in rotation (kinetic energy), and energy of change in shape orconfiguration (strain energy). Mechanical work can be computed by observing thechange in potential or kinetic energies, or calculated after measuring force anddistance. Mechanical work, power, and energy variables calculated for the seg-ments provide good information about control of movement. Power output hasbeen shown to be a good measure of how a skill is acquired and is added to amovement when beginners add more degrees of freedom to a movement (31).Biomechanical investigation into mechanical work generated by muscle, energytransfer across joints, mechanical power output, and the identification of segmentinteractions responsible for generating the required energy for a movement wouldprovide valuable insight into how a child learns a skill.

There are different instrumentation techniques and methodologies used toassess motion when both kinetic and kinematic techniques are utilized. Kineticand kinematic data can be collected concurrently using a force platform and mo-tion capture system that are temporally synchronized. Force and position data arecombined with anthropometric data to compute angular kinematic and kinetic


characteristics. Then, using an inverse dynamics approach, net joint torques can becalculated. In an inverse dynamics calculation, the analysis generally proceeds ona segment-by-segment basis from the more distal segment to the more proximal.The use of the inverse dynamics technique is very important in determining muscleand joint forces which are not practical to measure intrinsically.

Examining cause and effect phenomena of a movement problem using acombination of kinetic and kinematic approaches is occurring with greater fre-quency. Examples of studies that have employed both methods to study movementproblems involving children are presented in Table 3. The work of Horita andcolleagues (34) provides a good illustration of the benefits of using both kinematicand kinetic data to explore a specific movement of children. They studied the stand-ing long jump in both children and adults to compare joint function and bodyconfiguration in this skill. To explore these differences, they collected both videoand force platform data. These were used to calculate net joint moments of bodysegments using the ground reaction forces, the computed linear and angular accel-eration of the segment, and the segment center of gravity. Moment inertia aboutthe segment center of gravity was also computed. The jumper’s performance wasevaluated using a seven rigid link model: head and neck, forearm and hand, upperarm, trunk, thigh, shank, and foot. Joint muscle power was calculated at the neck,elbow, shoulder, hip, knee, and ankle joints using net joint moments, angular ve-locity, and mechanical work. Figure 4 represents the results of their power analysisfor the lower extremity. They reported that the timing of the joint power was thesame for children and adults, and individual joint contribution to the total workdone did not change between adults and children. Adults did demonstrate a morerapid change of hip joint power prior to the lowest point of center of gravity in thecrouch before take off. They concluded that children demonstrated a skilled formin the propulsive phase of the jump but immature joint function in the preliminaryphase of the takeoff and in the flight phase.

The work of Jensen, Ulrich, Thelen, Schneider, and Zernicke (40) is a valu-able example of the use of the inverse dynamics approach. They used a four-cam-era optical imaging system to evaluate the kicking characteristics of infants inthree different postures. The kicking data were analyzed and 3-dimensional coor-dinates were calculated for a three-link rigid body model. Segmental angles andlinear and angular displacement data were calculated for the thigh, shank, andfoot. Using estimates of segmental masses, segmental center of mass locations,and segmental moments of inertia, net joint torque was calculated using the equa-tions of motion. The net joint torque was further separated into gravitational torque(gravity acting through center of mass), motion-dependent (inertial torques andcentripetal torques), and generalized muscle torque (residual torque after gravita-tional and motion-dependent torque are subtracted from net torque). A sample ofthe kinematic and kinetic results of a kick for the hip and knee joints is shown inFigure 5. The combination of kinetic and kinematic data allows for a discussion ofjoint angle changes and corresponding muscle torque production. Jensen et al.(40) reported that joint angle changes did not parallel changes in muscle torqueand specifically found that as the hip joint flexed, the flexor torque decreased asthe hip extended the flexor torque increased. At the knee joint, the muscle torqueand joint angle changes around the joint reversal were in phase and were in con-trast to that seen at the hip joint. These results suggested a spring-like contributionfrom the hip joint that created coordinated limb action out of nonspecific muscleactivations for the infants studied.


Table 3 Combined Kinematic and Kinetic Approaches to StudyingChildren’s Movement


Beuter 14 M 6-9 Ankle joint unstable in high arousal conditions;et al. (7) increased kinetic energy ratios and more translational

and rotational kinetic energy to complete task instressed conditions.

Bril et al. 11 M& Range of speed of walking in children increases(11) F 13-19 mos. substantially in 1st 6 months; duration of step, swing and

double-support decreases with speed increase; similartemporal structure of step compared to adults.

Greer et al. 7 M 3-4 Boys had longer swing phase time; longer overall stride(25) 11 F 3-4 time; higher foot and shank velocities; more vertical

displacement and knee flexion in stance.

Horita 12 M 20 Adults and children have similar gross motor patternset al. (34) 1 M 6 before takeoff; children performed jump using less

7 F 6 negative work at the hip at takeoff; children have lessdeveloped air motion.

Jensen 18 M & F 3-4 Adults and children have similar temporal coordinationet al. (39) 6 M&F patterns in jumping but there were differences in some

young adults spatial measures indicating that the novice jumper lacksthe ability to precisely control the movement.

Jensen 5 M 0.26 mos. Kicks performed in the vertical position demonstratedet al. (40) 4 F 0.26 mos. less hip motion and more synchronous hip and knee

flexion and extension.

Parker 10 F 3-4 In unipedal hopping = no difference across ages exceptet al. (52) 10 F 4-5 that older children generated more impulse per hop per

10 F 6-7 body weight; age affected temporal variable. Both10 F 7 limbs combined = more anticipation of timing and

landing amplitude required.

Slobounov 24 M& F 3-5 COP motion increased as task constraints changed fromet al. (57) bipedal to one-legged stance and from vision to no-

vision.

Tant et al. 24 F 103 mos. Children demonstrated larger peak vertical forces than(59) 6 M 103 mos. that reported for adults; greater forces associated with

early peaks as height increased; longer time in landingas height increased.

Ulrich et al. 8 M&F 24 Adults used muscle torque to initiate and terminate(61) 8 M&F swing; infants displayed multiple patterns of torque.

0.59 yrs.

Wilson 33 M&F 4-7 Velocity of arm and forearm movements and landinget al. (69) position were important discriminators of ability and

development.


There are limitations for each approach, kinetic and kinematic, in biome-chanical analysis. It is important to have accurate information on the physical prop-erties of segments for good kinetic analyses. As previously stated, approximatingmasses and joint centers may be problematic. When combining these approaches,the limitations of the studies can be compounded with the individual consider-ations for kinematic and kinetic measurement, such as accuracy in a kinematicanalysis and environmental constraints with kinetic measurement. However, thebenefits of evaluating the kinematic and the kinetic components of a motion offera more thorough understanding of movement by exploring both the cause andeffect simultaneously.

Figure 4 — Lower extremity joint moments during the standing long jump for adults(left) and children (right). LPC = lowest point of center of gravity before take off.Horita, T., K. Kitamura, and N. Kohno. Body configuration and joint moment analy-sis during standing long jump in 6-yr-old children and adult males. Med. Sci. SportsExerc. 23:1068-1077, 1991.


Dynamical Systems

There are biomechanical applications that are more powerful than a simple kine-matic or kinetic analysis because of the intent of the application. Modeling and theapplication of a dynamical systems approach are two such examples. Both of theseapproaches move the application of biomechanics to a more mature level wherethe complex nature of human movement can be more thoroughly assessed andunderstood.

The use of biomechanics to define and apply dynamical systems theory tomovement problems is becoming more common in the area of motor development

Figure 5 — Hip (top) and knee (bottom) joint and muscle torques in a kicking actionby 3-month-old infants. Jensen, J.L., B.D. Ulrich, E.Thelen, K. Schneider, and R.F.Zernicke. Adaptive dynamics of the leg movement patterns of human infants: I. Theeffects of posture on spontaneous kicking. J Motor Beh. 26:303-312, 1994.


and motor control. The dynamical systems approach is a method for evaluating thechanging states of complex systems (68), such as a child learning how to walk.One or more essential variables are studied over time to identify points of transi-tion where loss of stability occurs. Variables are selected that constrain the degreesof freedom for developmental states, and the transition from state to state is mea-sured biomechanically after scaling the control factors. Variables such as phasingrelationships between limbs in walking, extensor strength, and postural control areexamples of control parameters studied using the dynamical analysis to exploregait (67). The transition stage is studied to determine what parameters might assistwith movement into the next phase. This technique allows one to clarify the emer-gence of a movement pattern as a function of the environment in which the move-ment is performed. Coordination is a state of frequency and phase locking, andthere are different methods that can be applied using a dynamical systems ap-proach. A discrete method evaluates coordination at only one point in the move-ment cycle where a time series of two joint or segment angles can be evaluated orone point can be plotted on another with a specific time lag. A continuous methodcan be applied which evaluates movement coordination over a period of time. Thisallows both a spatial and a temporal analysis while the discrete method evaluatesonly at the temporal level. Angle-angle plots can be generated to show relativemotion, or velocity can be plotted as a function of position to show continuousrelative phase characteristics (28). Different qualitative states of the system dy-namics can be identified by looking at the relative phase between segmental mo-tion such as the arms and legs (64). The concept of variability, especially as it hasbeen related to coordination in the movement patterns of children, is a topic thatshould be evaluated using a dynamical systems approach. Work done with adultshas shown that an increase in variability may not be related to a less stable systembut is rather an indication that the movement is nearing a point where a new pat-tern will emerge such as the transition from a walk to a run and vice versa (48).

The value of the use of dynamical systems analysis for biomechanists is thatit moves the research from a stage of being primarily descriptive to a stage wherethe understanding of the coordination of multiple numbers of degrees of freedomcan be enhanced. Coordinative structures can be defined and studied to identifytransition states and characteristics, and the degree of stability and variability inthe coordination dynamics can be studied (30, 32). It is a multidisciplinary ap-proach to solving movement problems that examines the relationship and inter-face between muscle action, motion, and the inertial reaction force. Through thequantification of timing patterns, a better understanding of control and coordina-tion can be achieved. This is an important function in lieu of the large number ofdegrees of freedom associated with controlled movements (32).

Beuter and Lasko (8) suggests the need for a different approach to examin-ing movement to determine why motor behaviors with only a few degrees of free-dom are successfully created when the potential number of degrees of freedom foruse is numerous. A dynamical systems approach can be applied to discover se-quencing, proportionality of forces, phasing and temporal characteristics, or therepeatability of angle-angle diagrams. It is the study of invariance. Its clinical andpedagogical uses include the quantification of mature and immature movementpatterns, the examination of coordination development, the identification of pa-rameters contributing to stability and variability, and the study of segmental inter-actions associated with specific neuromuscular disorders.


A sample of dynamical systems approaches used to study movement charac-teristics of children is presented in Table 4. Whitall and Getchell (68) examinedthe development of locomotor skills in four children. Film records were collectedon both walking and running gaits. Collective variables associated with transitionfrom walking to running were relative stance, estimated pathway of the center ofmass, and segmental/joint actions. The change between gait forms is illustrated inFigure 6 where the phase portrait of the ankle joint in one infant is shown. Theseresults demonstrate that the ankle joint actions are unstable for both walking andrunning gaits and do not stabilize earlier in the walk than in the run. These phaseportraits offer a dynamical description of the instability of the ankle motion andpoint to a trend that the control of ankle extension may be a key control parameterfor the emergence of running. Parker, Monson, and Larkin (52) have viewed hop-ping as a series of oscillating forces produced by alternating propulsive and forceabsorption phases, with the critical parameters being the forces in the thigh, leg,and foot that overcome the inertia of the system. They also point out that the struc-tural basis of movement is essentially symmetrical; however, human motion is not

Table 4 Dynamical Systems and Modeling Approaches to StudyingChildren’s Movement


Clark 15 M& F Across all four phases of the step cycle, infantset al. (16) 3-10 mos. exhibited an organization that was almost identical to

that of mature walkers.

Holt et al. 9 M 9 The same force driven oscillator formulation used for(33) 6 F 9 adults can be used to predict the cadence of children.

Jeng et al. 10 M 7-12 Preferred walking velocity of children with CP could be(38) 2 F 7-12 predicted with the force driven harmonic oscillator

model when mass and leg length discrepancies wereaccounted for. Different mechanisms were used toachieve optimality goals.

Ledebt 21 M&F 4-8 The duration of gait initiation was independent ofet al. (47) walking speed; values generated using an inverted-

pendulum model were close to the experimental valuescomputed from the force platform data.

Roberton 7 M&F There were common, qualitative changes in theet al. (54) movement; relative timing invariances were present.

Whitall 3 M infants There was a transitional form over the first few monthset al. (68) 1 F infant of running; coordination of knee joint was similar across

3 M 22-35 gaits and ages; ankle joint was less consistent; rate-1 F 22-35 limiting parameters were present in both running and

walking gaits of infants.


necessarily symmetrical. For example, gait is symmetrical (but out of phase be-tween limbs), hopping is symmetrical, and galloping and sliding are asymmetri-cal. The differences in task symmetry present different control problems for thechange in force and timing patterns.

A dynamical systems approach is certainly less common than the traditionalbiomechanical analyses. However, this approach can be important in gaining in-sight into complex movement problems. This technique has been successfully ap-plied to increase understanding of gait and locomotor skill development.

Modeling

Modeling has also been applied to identify the control parameters of a movement.The purpose of modeling is to explore a problem by assessing the accuracy ofpredicting parameters of a movement. Most of these models are behavioral mod-els in that they do not represent the structures themselves but the behavior of thestructures. They are developed and tested with experimental data to determine thesuccess of the prediction. Once verified, the model can be presented with a prob-lem that could not easily be accomplished in an empirical study.

There are many types of muscle models in the literature that model the be-havior of skeletal muscle. These models are generally based on a Hill muscle model.To develop a model, specific parameters are identified as being important for in-clusion in the theoretical prediction equation. The model behavior is comparedwith experimental data to validate the model. For example, muscle forces can bemodeled using information gathered from cadavers which can then be applied andextended to a modeled situation where input factors such as line of action of themuscles, muscle geometries, segment masses, and other components in the model

Figure 6 — Ankle phase plots of average joint displacement plotted against angularvelocity for walking (top) and running (bottom) across five ages. Whitall, J., and N.Getchell. From walking to running: applying a dynamical systems approach to thedevelopment of locomotor skills. Child Develop. 66, 1541-1553, 1995.


can be adjusted. Modeling has been used to estimate internal forces, predict forcesin the joints, generate simulations of complex movements such as gymnastics, andexplore parameters for optimization of a movement. A sample of modeling ap-proaches used to study movement characteristics of children is presented in Table 4.

Other types of models are referred to as mechanical models. For example,Holt and associates (33) used the resonance of a force-driven harmonic oscillatorto predict preferred walking cadence. This technique has been successfully ap-plied to the gait of children (36). Ledept and Breniere (47) have also used an in-verted pendulum model to examine the gait of toddlers, as their center of gravityrotates around the center of pressure. The inverted pendulum rotates around thecenter of foot pressure, and the oscillation period can be calculated taking intoaccount moment of inertia of the body with respect to the center of gravity, bodymass, gravitational force, and the position of the center of mass in relation to theground.

Although their use is less common than the traditional kinetic and kinematicbiomechanical analysis, modeling can be especially valuable in the examinationof developmental changes over time or to improve insight into a number of com-plex movement problems.

Special ConsiderationsLimitations

The application of biomechanics for analysis and correction of human movementhas limitations that directly influence the usefulness of biomechanics as a tool.First, the measurement itself is usually conducted in a laboratory, away from thenatural environment of the participants. Removing individuals from their comfort-able surroundings may cause them to perform differently and thus, not producenatural results. There are also reliability issues with many of the biomechanicalmeasurements, requiring data collection across multiple trials and multiple days.Errors in measurement also continue to influence results in biomechanical studies,especially in the kinematic area where marker placement, camera recordings, anddata processing can dramatically influence results.

Childrens’ Growth and Anthropometric Characteristics

In this review of the use of biomechanics to explore children’s movement, thestudies presented have used the same measurement techniques and instrumenta-tion that are used to measure adult movement parameters. Many of the applica-tions using force platforms and motion analysis systems have resulted in a suc-cessful quantification of children’s movement. However, there is one area wherespecial consideration must be given when evaluating children’s movement biome-chanically. Body dimensions and anthropometric characteristics of children mustnot be ignored in the analysis and must be adjusted accordingly to generate accu-rate data (42). This is especially true during rapid growth and development stageswhere growth can even influence subtle factors such as drag in swimming (60).

There are multiple areas where the body dimensions of children can greatlyaffect measurements in biomechanics. One important area is the computation ofthe center of mass. The center of mass is the point where the entire mass of thebody is considered to be concentrated. Errors occur in the apportionment of mass


to segments and in the estimation of the location of the segmental center of mass.Inaccuracies in the calculation of the center of mass can result in significant errorsin moment calculations (44).

A second area of concern is the identification of joint centers, an importantcomponent of kinematic and kinetic analyses. Bell and associates (5) evaluatedthe accuracy of predicting hip joint center in children and adults. Through radio-graphic analysis, they determined that the location of the hip joint center variednot only between adults and children, but also across genders. This is an importantconsideration when attempting accurate measurement because of its impact oncalculations of hip joint forces and moments.

Angular kinetic analyses also require the knowledge of moment of inertia,defined as the product of incremental mass and distance from the reference pointsquared. When children experience an increase in growth, there are correspondingincreases in the moment of inertia with large increases continuing through pu-berty. Jensen (41) suggests that the alterations in moment of inertia in childrennecessitate the development of greater forces to maintain the same level of accel-eration. As a result, the body must adapt to the increase in activities where rotationis common. The growth in children can alter proportions for segment masses, ra-dius of gyration and moments of inertia—all factors that are necessary in kineticstudies.

The measurement of moment of inertia requires a detailed analysis. A fif-teen-segment model has been used to approximate moment of inertia of each seg-ment. Joint centers are identified from film records and moments of inertia areestimated using major and minor axes of an elliptical zone (41). The common useof adult segmental size and inertial parameters in computations involving childrenmay introduce a large amount of error in the calculation of kinetic data. The massdistribution of the segments is continually changing and may also vary across gen-der and race as well (53). Therefore, every attempt should be made to computemoment of inertia accurately for younger subjects. It is suggested that predictionsbe based on the elliptical zone mathematical model (43).

Moment of inertia in children was examined by Jensen (41) who followedtwelve boys (ages 4–14) for three years. As the children aged, there were corre-sponding mass and mass distribution changes. It was also demonstrated that agewas not an effective predictor of moment of inertia because of a wide distributionwithin each age group. Jensen (43) demonstrated in a later study that as a persongrows, there are changes in the whole body moments of inertia that affect perfor-mance. With mass distribution constantly rearranged during growth, skills such aslayouts and tucks in gymnastics impose different levels of difficulty depending onthe moments of inertia. In males, aged 5–15, there were children who had doublethe moment of inertia of others, suggesting that age alone could not predict themoment of inertia. Moment of inertia about the mass centroid increases with growthand accelerated changes in moment of inertia should be expected. Jensen (41)recorded increases of 12–57% and 8–92% about the transverse and longitudinalaxes, respectively, with extreme changes seen in 12–13 year olds going throughgrowth. To accurately estimate total body changes occurring in the moment ofinertia about the transverse axis, Jensen (41) suggests computing mass*height2.

Biomechanical analyses using children populations must account for bodydimension influences on measurement parameters. Adult parameters cannot beused with children, limiting the kinetic analysis that can be done where known


forces and moments are determined from segmental kinematics. Likewise, seg-mental acceleration, velocity and displacement data cannot be computed from jointforces and moments (41). The influence of changes in body size and dimensionson movement requires further analysis (3).

Summary

The use of biomechanical analysis in studying motion characteristics, solvingmovement problems and examining effectiveness of interventions with children,is beneficial and is occurring with greater frequency. Biomechanics can be studiedin isolation, but it has more value studied in conjunction with other disciplinessuch as neurophysiology, physiology, and motor control (32). The solution of manymovement problems can only be understood by applying a multidisciplinary ap-proach, which examines the motion characteristics from a biochemical, biome-chanical, psychomotor, physiological, and environmental perspective (14). Withcombined efforts and analyses, solutions to movement problems can be identifiedmore accurately and appropriately to the benefit of the clinician, pedagogist,reseacher, and client.

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