Algorithm implementation for the analysis of gait and

posture

André Mendes Marques de Carvalho

Mestrado Integrado em Bioengenharia

Porto, Janeiro 2013

André Mendes Marques de Carvalho

Algorithm implementation for the analysis of gait and

posture

Monografia do Mestrado Integrado em Bioengenharia

Faculdade de engenharia da Universidade do Porto

Porto, 2013

iii

Abstract

Some of the most common motor action are the postural control and gait

control. This is of critical importance as gait is the main way of human locomotion.

Both gait and the ability to remain in a standing position are highly affected by factors

such as somatosensorial inputs, ground inclination and health problems. The goal of

this study is to implement an algorithm capable of analyzing the EMG and force signal

recorded during standing and gait and associate the specific muscles activations during

body movements and standing maintenance. To accomplish that Matlab will be used

to operate previously collected data from both gait and standing movements. The

EMG data will be treated in the time domain and patterns of muscle activation are

expected to be found during cyclic movements. Also the force data will be examined as

to the magnitude, peak interval and event duration.

iv

Index

Abstract…………………………………………………………………………………iii

Introduction……………………………………………………………………………...1

Motor control…………………………………………………………………………….2

Musculoskeletal system………………………………………………………….3

Postural control…………………………………………………………………..6

Gait control………………………………………………………………………8

Instrumentation…………………………………………………………………………10

Force Plate………………………………...……………………………………11

Electromyography………………………………………………………………13

Work plan………………………………………………………………………………15

References………..…………………………………………………………………….16

1

Introduction

The complexity of the motor system is reflected on the complexity of the

actions humans can achieve (Enoka 2008). Nowadays gait and postural analysis are

mainly done for clinical research. This means that the purpose is not to study individual

problems but to study the specific condition of a group of patients. (Baker, A., 2006)

The objective of such analysis depends on the investigation. Some are interested in

gait patterns, speed, the work made by witch muscles during specific activities,

metabolic cost, etc. (Andreia, 2009). The goal is clear, to understand the mechanisms

associated with gait and posture control.

In order to study this mechanisms force plate, EMG, visual tracking, kinematics

as well as many other techniques can be applied. The main problem is to understand

and interpret correctly the signal of such measurements. To do that time or frequency

domain analysis of the collected signals are made. These signals can determine the

body center of pressure and the force exerted on the ground at all times and the rate

of muscular activation for a given task.

From this data a better understanding of the human gait and posture processes

can be achieved and proper medical evaluations can be made. It is known that gait and

postural pattern can be influenced by health state, age, shoes, body alignment and

many other factors. The main goal of this study is to implement an algorithm capable

of analyzing the EMG and force signal recorded during standing and gait and associate

the specific muscles activations during body movements and standing maintenance.

2

Motor Control

The complexity of the motor system is reflected on the complexity of the

actions humans can achieve (Enoka 2008). These actions are a result of the

interactions between the musculoskeletal and nervous systems, the first acting as a

support and the second as the coordinator (Ghez and Krakauer 2000). Being so, all

movement is achieved by the coordinated response of the muscles to neural

commands (Bawa 2002).

Some of the most common motor action are the postural control and gait

control. The postural control is a rather complex skill based on the interactions of

dynamic sensorimotor processes and can be separated into two main functional goals:

the postural orientation and postural equilibrium (Horak 2006). As for the Gait control

this is a complex action in which the body center of mass (COM) is continuously

unbalanced so the body weight is transferred from one leg to another with the

minimum energy consumption (Alonso et al. 2002).

For a motor action to take place it is needed a proper alignment of all body

sections so it can maintain its stability despite all the unbalancing forces (Mackey and

Robinovitch 2006). This way motor control can be defined as all the processes from

which the nervous system, based on visual, vestibular and somatosensory inputs,

controls the musculoskeletal system to create coordinated movements and skilled

actions while maintaining body posture (Junior and Barela 1996).

3

Musculoskeletal system

The musculoskeletal system is composed by over 200 bones and 600 muscles

that allows around 224 degrees of freedom and is the effector component of all

biomechanical movement (Duarte 2001). The bones act as the support and protective

system for various organs and tissues while the muscles can contract to realize

movement.

The muscle is a hierarchical structure that contains fascicles, muscle fibers,

myofibril and actin and myosin filaments as shown in figure 1.

Figure 1- Schematic of muscle components [from Medicalook 2013]

For a contraction to take place several structures of the nervous system must

be involved. At the lowest level of organization there’s the spinal cord which sends all

motoneurons and their associated muscle fibers form the neuromuscular system. This

system has been considered as the interface between muscles and nervous system

(kernel 2006).

The basic control unit is called motor unit. This term was introduced in 1925 by

Sherrington and Liddell, and it is from this unit that the nervous system controls the

movement and force of the muscle (Ghez and Krakauer 2000). The motor unit is

composed by one motor neuron and all the muscle fibers to which it is connected

(figure 2).

4

Figure 2 - Motor unit schematics [Baileybio 2012]

This is the smallest amount of muscle fibers the nervous system can activate to

perform a given task as once the motor neuron is activated all its associated muscle

fibers contract. Depending on the movement and the force necessary to perform it the

muscle fiber can be activated from only a few to all of them (Kernel 2003).

Once a motor neuron is activated it generates a potential difference in its

innervated muscle fiber resulting in an action potential (figure 3). This happens every

time an unbalanced distribution of ions inside and outside the cells is achieved (Loeb

and Ghez 2000). During cell depolarization there’s an influx of sodium (Na+) and an

efflux of potassium (K+).

5

Figure 3 - Membrane potential of a muscle cell [Wikipedia 2013]

During rest the membrane potential is usually around -70 mV. Once the

threshold potential is reached, approximately -55 mV, the cell depolarizes opening its

sodium channels. The influx of sodium channels raises the membrane potential up to

+40 mV. After this the sodium channels close and the potassium channels expel

potassium from inside the cell, lowering its membrane potential (Seeley 2003).

6

Postural control

Posture is referred to as the ability to maintain the body in a given position and

translates the result of a permanent muscular activity that opposes the instability

created by the gravitational force. As shown in figure 4 the postural control is the

result of an efficient coordination between the visual, vestibular, somatosensory and

musculoskeletal systems (Fernandes 1998)

Figure 4 - Postural control mechanisms [jneuroengrehab 2013]

The two main functional goals of postural control are postural orientation and

postural equilibrium. Postural orientation integrates sensory information to actively

align the head and body with respect to gravity visual surrounding, support surfaces

and internal references. This is achieved once all external and internal forces are

balanced. Postural equilibrium refers to the movements necessary to stabilize the

body center of mass during stability disturbances. It depends on individuals

experience, goals and postural displacement (Horak 2006).

The postural control is not an easy task, the surface area is quite small and its

body center of mass is located quite high making this a very instable position. To

maintain Postural control the nervous system is continuously making adjustments.

These adjustments are a result of the sensorial input and the appropriated muscular

responses (Pais 2005). The standing position is characterized by small and spontaneous

postural changes. There are two main contributions to the stability of the standing

position, the corporal alignment capable of reducing the gravitational forces effect as a

good alignment reduces the energy expenditure (figure 5) and the muscular tonus

(Shumway-Cook and Woollacott 2001).

7

Figure 5 - Body center of mass alignment and gravitational force [adapted from newbalancecalgary 2013]

As the standing position is unstable muscles from all over the body are

continuously contracting to maintain body balance. These muscular contractions give

rise to small movements which in turn act to stabilize the body position (Horak 2006).

8

Gait Control

The primary goal of walking is the locomotion of the body from one place to

another. To achieve this humans alternate their support from one leg to another,

always keeping one foot on the ground, in an action where the body center of mass is

displaced from its regular position allowing for the body to move forwards. The

movements that take place during walking are cyclic and optimized to decrease energy

expenditure (Silva 2009).

To perform a regular walking an individual needs to be able to maintain

movement patterns, be able to maintain a dynamic equilibrium between the body

center of mass and the moving support and to change his movement patter in

response to any external disturbance. The Walking cycle is every movement that

occurs between the initial contact of one foot in the ground and the instant that foot

touches the ground again (Monteiro 2004).

Each walking cycle has two steps, one for each leg, and it is divided by two main

actions, the step and the swing (figure 6). The step refers to the contact between the

foot and the ground and it correspond to 60% of the cycle time. The swing corresponds

to the other 40% and is the time in which a leg is progressing thru space (Gamble et al

1998).

Figure 6 - walking cycle [adapted from Perry 2005]

The two prevailing theories of human walking are the inverted pendulum and

the six determinants of gait as shown in figure 7. The inverted pendulum theory states

that it is beneficial for the stance leg to behave like a pendulum, where the kinetic

energy is converted into potential energy and vice versa, saving this way more than

9

50% of the required mechanical energy (Cavagna, 1996). The six determinant of gait

are kinematic features of gait proposed to minimize the energetic cost of locomotion

by reducing the vertical displacement (Kuo, 2007). Among those kinematic features we

have the pelvic rotation and the flexion of the knee as a way of reducing the body

center of mass vertical variation.

Figure 7 - Two major walking theories [kuo 2007]

Independently of the theory the muscular activity during the walking cycle can

be divided into the ground strike step in which the foot touches the ground with much

positive work from the quadriceps, the pre-load step in which the elastic energy is

stored into the Achilles tendon and the quadriceps continue doing positive work,

propulsion step with major positive work by the anterior tibialis and the swing step

where no force is being exerted to the ground. Despite the absence of ground contact

some muscular activity can still be found in anterior tibialis, hallucius and digitorum

longus extensor (Norkin, 1992).

10

Instrumentation

In order to study the gait and postural control mechanisms force plate, EMG,

visual tracking, kinematics as well as many other techniques can be applied. These

techniques work on the premise that the human body is not statically stable so a

control system is necessary to stabilize the body. Some of the techniques allow

monitoring the body center of pressure while others measure the small electric

potentials required to activate a muscle and other the position of the body segments.

It is known that ageing, lesion, footwear and many others factors contribute to

an abnormal postural and gate pattern. As the feet reaches the ground in different

angles or as the center of mass displaces from its regular position the stability

movements required to hold the body change.

Biomechanical instrumentations give us the ability to study gait and postural

patterns as well as all the variables and diseases that can change a normal pattern

increasing energy expenditure and increasing the probability of a fall. This is specially

important for elderly people as this is one of the major causes of dead and health

related problems (Stevens et al, 2006). It also allows quantifying motor impairments

and to keep tracking of diseases progressions. This kind of instrumentation is also used

in rehabilitation to monitor improvements of stroke patients.

11

Force plate

Force plates are used by many researchers to evaluate the postural stability by

analyzing the time-varying coordinates of the center of pressure. These measurements

have been shown to be highly correlated with the center of mass, this way the

horizontal or vertical ground reaction forces are proportional to its respective center of

mass acceleration (Hasan et al 1996).

There are several types of force platforms depending on its transducer being

the most common the strain gauge and the piezoelectric. These sensors are between

two rigid plates to minimize energy absorption in order to acquire the most precise

data possible. The strain gauge varies its resistance with an according deformation and

the piezoelectric generates a potential difference to an according deformation. The

most common force plates have four sensors located on the four corners of the

platform and each one measure the three force components and the three moments

of those forces as shown in figure 8 (Pais 2005).

Figure 8 - Force platform and measured force components [Pais 2005]

While using force plates it needs to taken into account the plate installation. A

proper installation can increase data quality. Force plates need to be screwed to a solid

and leveled plane. This way the plate is isolated from any vibrations produced in the

vicinity. Also the upper plate must be leveled with the ground so it doesn’t induce gait

alterations (AMTI 2013).

Force plates require auxiliary hardware to work as shown in figure 9. As the

plate output is an analogic signal it need to be converted to a digital signal so a

computer can read the data. This is done with an A/D converter. All the hardware

equipment is connected with shielded cables to avoid noise contamination of the

12

collected signal. There is also a piece of software that manages the data acquisition

and storage.

Figure 9 - Force plate components [AMTI 2013]

13

Electomiography

The activation of the muscle fibers generates an electrical field that can be

measured with electrodes, generating the electromyographic signal (Basmajian and

DeLuca, 1985). This way a direct measurement of the muscular activity can be made.

Also the EMG signal can provide information related to the degree of muscular

activation, the rate of force production, the number of motor units recruited, etc.

Theoretically the EMG signal would correspond to the sum of all motor units action

potentials (Basmajian and DeLuca, 1985).

The electric signals generated by muscular activity can be detected with two

different types of electrodes, intramuscular and surface electrodes (Farina et al, 2004).

The intramuscular electrodes are used to study individual motor units. They are

introduced with a needle and they get in direct contact with the muscles fibers. The

main problems regarding intramuscular electrodes are the limited amount of muscle

fibers that it can detect and the inconvenience related to invasive methods (Basmajian

and DeLuca, 1985). As for the surface electrodes they are painless and have a larger

area of activity can be pickup. The classic systems for surface detection are composed

of two large electrodes. This system delivers the sum of all the electric activity on the

detection area. It allows the study of the global activity of a muscle. Its main

disadvantages are its susceptibility to crosstalk; this means that a muscle activity can

be felt on the vicinity locations inducing to misleading readings (Rau et al. 1997).

Usually the frequency of an EMG signal can go from 25 to several kHz as for the

amplitude it usually has very small values ranging from 0.1 to 1 mV for surface

electrodes (DeLuca, 1993). Due to the small amplitude and the sensitivity of the

electrodes, the signal must be filtered, to avoid low frequencies noise contamination,

and amplified before it can be read by a computer. Figure 10 shows a typical EMG

signal. The main types of signal noise sources are the power supply, which introduces a

50-60Hz noise that can be eliminated with a high pass filter (Raez, 2006) and crosstalk.

Figure 10 - Typical EMG signal [adapted from Biopac Systems]

The processing of the EMG signal can be made either in the time or frequency

domain. The time domain signal analysis objective is to characterize the intensity of

14

the signal during muscle activation. As part of this analysis we have the rectification,

integration, and mean root square. The rectification is the elimination or

transformation of the negative values of the signal into positive ones, figure 11. The

mean root square is the technique that correlates in a more precise way the behavior

of the motor unit during activation. For this reason this is the most used procedure

(Basmajian and DeLuca, 1985). This value depends on the number of motor units

recruited, event duration, electric configuration and the characteristics of the

instruments. Despite the fact that the mean root square and the integral are both a

way of measure the area bellow the signal (figure 11), only the mean root square has a

physical meaning. Also as the mean root square is the mean square sum of the signal

values there’s no need for prior rectification (Oliveira, 2007).

Figure 11 - EMG signal processing example [Biopac Systems]

The Frequency domain signal characterization has been frequently used to

study muscle fatigue (Komi and Tesh, 1979). Stulen and DeLuca (1981) have shown a

linear relation between mean and median frequency and the conduction speed of

action potentials in muscle fibers.

15

Work plan

As the data for this analysis have already been collected from another

researcher there is no need to concern about an experimental procedure for EMG and

force plate. The data is going to be analyzed using Matlab. The first step is to acquire

all the necessary information about the data gathering procedures so frequency of

acquisition, layout and all other relevant experimental variables are checked.

After that a small algorithm for automatic reading and separation of the data is

going to be made. This allows storing each muscle EMG signal and force component on

separate arrays. The necessary normalization, filtering (if necessary), rectification, off-

set removal will then be made. Regarding the force data amplitude, peak intervals and

inflection points will be determined. As for the EMG signal this data will be studied as

for its frequency during activation, its mean root square, amplitude, duration and peak

interval. Both data must then be synchronized to study the influence of specific

muscular groups on the different gait cycle steps. Some modifications to the work plan

may occur if necessary. By the end of this work it is expected that a tool for

automatically analyses of EMG and force data from gait and posture control is created.

16

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