Wrist Flexors are Steadier than Extensors

754 Training & Testing

Salonikidis K et al. Wrist Flexors are Steadier … Int J Sports Med 2011; 32: 754–760

accepted after revision May 14, 2011

BibliographyDOI http://dx.doi.org/10.1055/s-0031-1280777Published online: September 12, 2011Int J Sports Med 2011; 32: 754–760 © Georg Thieme Verlag KG Stuttgart · New YorkISSN 0172-4622

Correspondence Prof. Ioannis G. Amiridis Department of Physical Education and Sports Sciences at Serres Aristotle University of Thessaloniki Agios Ioannis, Serres 62110 Greece Tel.: + 30/23/1099 1058 Fax: + 30/23/2106 4806 [email protected]

Key words ● ▶ force variability ● ▶ isometric ● ▶ electromyography ● ▶ coactivation

Wrist Flexors are Steadier than Extensors

skilled participants [ 25 ] . This greater steadiness was not accompanied by diff erences in either the agonist or antagonist EMG activation. The hypotheses to justify why experts are more accu-rate than sedentary individuals with the same level of force imply diff erences in the agonists activation strategy, muscle typology and mechanics [ 25 ] . However, it is well known that force fl uctuations could also depend on the pres-ence of visual feedback, task specifi city, type and intensity of the muscle contraction and physical activity status of the individual [ 7 ] . It is of interest to study why the neural mecha-nisms that underlie the fl uctuation around an average (given) level of isometric force produc-tion, involve a non uniform activation of several diff erent muscles around a joint [ 7 ] . The interest is growing when one also considers that partici-pants who displayed high Coeffi cient of Variabil-ity (CV) of force values for the agonists (plantarfl exors) did not necessarily exhibit high values for the antagonists (dorsifl exors), and vice versa [ 30 ] . The lack of steadiness correlation between muscles is consistent with the notion

Introduction ▼ Variability around the mean in the force output is interpreted as “problematic” random variation. The amplitude of fl uctuations around an average value can depend on the test muscle, activation level of the motor unit pool and age [ 31, 32 ] . In contrast to the predictions of Fitts [ 8 ] , Hamilton et al. [ 12 ] proposed that motor-output variability or noise is lower in strong and bigger muscles compared to weak and smaller muscles. The functional signifi cance of this prediction is that accuracy can be maximized by preferentially activating the larger muscles in a synergistic group. Fluctuations of muscle force have been exten-sively studied in the fi rst dorsal interosseus [ 7 , 19, 21 ] , elbow fl exors [ 11 ] , knee extensors [ 27, 32 ] , plantarfl exors [ 30 , 34 ] and wrist fl exors muscles [ 25 ] . In our recent study, it was demon-strated that highly skilled individuals have a greater ability to perform steady submaximal isometric wrist fl exions at matched levels of Maximal Voluntary Contraction (MVC) than less

Authors K. Salonikidis , I. G. Amiridis , N. Oxyzoglou , P. Giagazoglou , G. Akrivopoulou

Affi liation Laboratory of Neuromechanics , Department of Physical Education and Sport Sciences at Serres , Aristotle University of Thessaloniki , Greece

Abstract ▼ To examine torque variability in 2 antagonistic muscles 20 individuals performed maximal and submaximal (5, 10, 20, 50 and 75 % of Maximal Voluntary Contraction, MVC) isometric wrist fl exions and extensions (5 s) at 5 diff erent angles (230, 210, 180, 150 and 130°). The EMG activ-ity of Flexor Carpi Ulnaris (FCU) and Extensor Digitorum (ED) was recorded and quantifi ed as the integral of EMG. Participants showed higher maximal isometric torque (32.43 ± 11.17 vs. 17.41 ± 3.84 Nm) and lower coeffi cient of vari-ability during wrist fl exion compared to exten-sion. The normalized agonist EMG increased across higher levels of torque for both wrist

muscles. Interestingly, the coactivation of ED during wrist fl exion was greater compared to the coactivation of FCU during wrist extension at 50 and 75 % of MVC, regardless of wrist angle (f.e.: at 180° and 75 % of MVC the normalised EMG of ED as antagonist was 14.84 ± 5.18 % vs. 9.33 ± 6.94 of the FCU). It is concluded that a stronger isomet-ric wrist fl exion is more steadily produced, with greater coactivation compared to a weaker wrist extension, independently from muscle length and torque level. Even if the relative contribution of antagonists to the resultant torque is to be considered, altered activation patterns responsi-ble for diff erences in force fl uctuations could be suggested.

755


Training & Testing

that muscles with widely varying neuromuscular properties produce uncorrelated fl uctuations, while similar characteristics between muscles (e. g., muscle size and strength, motor unit number, recurrent inhibition, corticospinal input) tend to result in correlated force variability within the same individuals [ 12 , 32 ] . However, force fl uctuations could also be dependant on the level of force produced as well as on the muscular length. A sigmoidal relationship between variability of force and level of force production in continuous isometric contractions has been proposed [ 5 ] . No wrist angle eff ect on torque variability meas-urements is also reported, suggesting that the length of the fl exor muscles does not infl uence the steady application of force [ 25 ] . Moreover, daily use-induced technique improvements seem to be associated with an “independency” in the motor unit fi ring rate, lower motor unit synchronization and substantially lower common drive [ 26 ] . Therefore, it is of interest to study the physiological mechanism responsible for the motor output in a well used muscle and its antagonist that is less used as a prime mover in daily activities. In line with this, the hypothesis of this study was that wrist fl exors present a greater steadiness than extensors during isometric torque production. To our knowl-edge, no information is available on potential diff erences in torque variability between 2 antagonistic muscular groups of the upper limbs, such as the wrist fl exors and extensors. The aim of this study was to examine whether the ability to sus-tain a constant application of submaximal strength is aff ected by the muscular group. The isometric torque steadiness was studied over a wide range of angles and submaximal percentage of the MVC.

Material and Methods ▼ Participants 20 young males (age: 21.6 ± 1.3 years, height: 181 ± 4.5 cm, mass: 79.9 ± 6.7 kg) participated in this study. None of the participants had previous experience with this specifi c isometric task in order to avoid any learning eff ect. All participants were right-handed and free from any neurological or musculoskeletal impairment or disease. The experimental procedure was explained and prior to their inclusion in the study, the partici-pants signed an informed consent form. Approval for the experi-ment was obtained from the institutional ethics committee on human research in accordance with the declaration of Helsinki and the ethical standards of the IJSM [ 14 ] .

Strength measurements After a standardized warm-up (2 sets of 10 submaximal wrist curls for fl exors, allowing a barbell of 5 kg to roll out of palms down to fi ngers and extensors with the palms facing down; 5 min stretching), maximal and submaximal isometric data dur-ing wrist fl exion and wrist extension were collected at 5 diff er-ent angular positions (230: maximal extension, 210, 180: anatomical zero, 150 and 130°: maximal fl exion) using a Kin Com ® (Chattannooga) isokinetic dynamometer. Participants were familiarized with the apparatus over a period of 1 week (3 sessions of 40 min) and were advised to refrain from any kind of physical activity for 48 h prior to testing. All tests were con-ducted at the same time of day (15:30–18:00) to avoid any chronobiological eff ect. The rotation axis of the lever arm and the wrist joint were adjusted and positioned according to the Kin Com System Manual. The right upper arm was restrained by

2 velcro straps and the forearm was strapped on a special manipulandum, which allowed to secure the elbow joint at 120°. The wrist joint remained free, the hand was positioned in a ver-tical position and the fi ngers were clasped to the lever arm. Par-ticipants performed 3 isometric MVC (duration 5 s), in each angular position, randomly presented. The maximum torque produced for a minimum period of 1 s was used for the calcula-tion of torque levels. Following MVC assessment, the partici-pants performed submaximal actions at 5, 10, 20, 50 and 75 % of MVC. Particularly, the participants were asked to develop and maintain, as stable as possible, the respective torque level for 5 s. 2 eff orts were performed at each level of MVC. Visual feedback of the exerted torque during the isometric task was provided on-line both arithmetically and as a large bar histogram on the com-puter screen which was placed in front of the subject at eye level. First, the participants were informed about the type of visual feedback they were about to receive. They were then asked to watch the computer screen during contraction. The predeter-mined (target) level of torque (for example 20 % MVC) was marked on the screen by the experimenter and was visible for both the examiner and the participant. Consequently, upon ini-tiation of the contraction, the participants could watch their recorded torque which was displayed as a continuous rising bar histogram on the computer screen. When the target torque level was reached, the participant was instructed to maintain his eff ort at this level for a period until the 5 s duration was com-pleted. A 2 min rest was allowed between trials and the best eff ort was analyzed. To avoid any fatigue eff ect the tests of wrist extension and fl exion were performed on separate days.

Electromyography (EMG) A TEL100D (Biopac Systems, Inc., Goleta, CA) system consisting of shielded electrode lead assemblies (bipolar silver/silver chlo-ride electrodes, centre-to-centre: 2 cm) interfaced to a portable amplifi er/transmitter (TEL100M, CMRR > 110 db at 50/60 Hz, bandwidth = 10–500 Hz) was used to record the EMG activity of Flexor Carpi Ulnaris (FCU) and Extensor Digitorum (ED) muscles. Electrodes were attached over the palpable bellies of the FCU (at ¼ of the line between the medial epicondyle of the humerus and the pisiform bone) and ED muscles (at ¼ of the line between the lateral epicondyle of the humerus and the midpoint of the line connecting the styloid process of the ulna and the radius) [ 16 ] . Both Kin Com (100 Hz) and EMG systems were interfaced to a Biopac MP100 Data Acquisition unit sampling at 1024 Hz. Fol-lowing data collection, the EMG signals were full-wave rectifi ed. Low impedance (Z < 500 Ω) at the skin-electrode interface was obtained by shaving, abrading the skin and cleaning it with alco-hol. Special caution was given to the standardization of the elec-trode position. Signals were band-pass fi ltered (cut off : 10–330 Hz) and digitally sampled at a frequency of 1000 Hz (full-wave rectifi cation, fourth-order, zero-lag Butterworth, cut off : 100 Hz). Integrated EMG activity over the 3 s of each isometric submaximal wrist fl exion was expressed as a normalized value of the EMG during MVC.

Data analysis The fi rst and the last second were excluded from the analysis to avoid the transient phases in torque development. For the remaining time interval (3 s), torque fl uctuations were quanti-fi ed by calculating the SD of torque and the Coeffi cient of Varia-bility (CV = (SD/mean) * 100). All EMG data collected were analyzed off -line using the Acq Knowledge ® (3.7.3) software. The

756


Training & Testing

integral of EMG activity over the 3 s of submaximal isometric action was expressed as a normalized value of the EMG activity observed during MVC. The coactivation of the antagonist was quantifi ed as a percentage at each specifi c angle using the nor-malized EMG of the same muscle when acting maximally as an agonist.

Statistical analysis All data are presented as means ± SD. A 2 (joint action) × 5 (level of torque) × 5 (wrist angle) repeated measures Analysis of Vari-ance (ANOVA) was performed to examine maximal torque diff er-ences between fl exion and extension at diff erent wrist angles. In addition, a 2 (joint action) × 5 (level of torque) × 5 (wrist angle) ANOVA was performed to examine diff erences in CV and nor-malized EMG of agonists and antagonists on joint action, target torque and wrist angle. All signifi cant main eff ects and interac-tions were further analyzed using post hoc Tukey test compari-sons among the levels of each factor. The level of signifi cance was set at P < 0.05.

Results ▼ Maximal isometric torque – angular position relationship ● ▶ Table 1 presents the maximal isometric torque – angular posi-tion relationship for both actions. The maximal torque ranged from 25.52 ± 6.08 Nm (180°) to 32.43 ± 11.17 Nm (230°) during wrist fl exion (mean CV: 2.21 ± 0.55, 2.40 ± 0.56, 2.40 ± 0.73, 2.19 ± 0.36, 2.44 ± 0.37 for 230, 210, 180, 150 and 130°, respec-tively) and from 9.00 ± 3.08 Nm (230°) to 17.41 ± 3.84 Nm (130°) during wrist extension (mean CV: 3.82 ± 0.63, 4.13 ± 0.89, 4.38 ± 1.2, 4.01 ± 1.37, 5.80 ± 1.46 for 230, 210, 180, 150 and 130°, respectively). ANOVA revealed a signifi cant main eff ect for joint action (F 1,19 = 163.35, P < 0.001) suggesting that maximal torque production was greater during fl exion compared to extension. Maximal torque decreased with wrist angle (F 4,76 = 12.23, P < 0.001). A signifi cant joint action × angle interaction (F 4,76 = 5.48, P < 0.01) showed a diff erent pattern of decrease in each joint action. Specifi cally, post hoc analysis for wrist fl exion revealed that the maximal torque decreased from 230 to 210° ( P < 0.05) but did not signifi cantly change any further at 180, 150 and 130°. By contrast, for wrist extension, the maximal torque signifi cantly ( P < 0.001) decreased across all successively lower angles until 130°.

Force variability ● ▶ Fig. 1 presents the torque raw data from a participant per-forming the submaximal isometric task at 5, 10, 20, 50 and 75 % of MVC during wrist fl exion and extension task (180°). ● ▶ Fig. 2 shows the CV of torque during fl exion and extension at each

angle (230, 210, 180, 150, and 130°) across the 5 target torque levels (5, 10, 20, 50, and 75 % of MVC). The CV of torque was lower during fl exion compared to extension (F 1,19 = 104.22, P < 0.001) ( ● ▶ Fig. 2 ). In addition, the CV signifi cantly decreased across higher torque levels (F 4,76 = 64.41, P < 0.001). A signifi cant joint action × target torque level interaction (F 4,76 = 19.41, P < 0.001) revealed that this decrease was greater in wrist exten-sion than fl exion ( ● ▶ Fig. 2 ). Specifi cally, during wrist extension, for the anatomical zero (angle 180°), the CV of torque decreased signifi cantly between 5 and 10 % ( P < 0.001), 10 and 20 % ( P < 0.001), 20 and 50 % ( P < 0.001) and 50 and 75 % of MVC ( P < 0.001). By con-trast, during wrist fl exion, for the anatomical zero (angle 180°), the CV of torque decreased signifi cantly only between 5 and 10 % ( P < 0.05) and 50 and 75 % of MVC ( P < 0.01). The CV of torque signifi cantly increased with decreasing wrist angle (F 4,76 = 18.22, P < 0.001), showing that the greater the wrist angle, the more constant was the torque production. Again, this was true only for wrist extension and not for fl exion as confi rmed by a signifi cant joint action × wrist angle interaction (F 4,76 = 17.57, P < 0.001). Post hoc analysis revealed that the CV of torque during wrist fl exion was aff ected by the wrist angle only at 10 % of MVC ( P < 0.001). By contrast, during wrist extension, the CV increased with decreasing wrist angle across all levels of torque ( P < 0.001).

EMG activity Agonist muscle A signifi cant 3 way interaction (joint action × wrist angle × torque level) (F 16,304 = 2.86, P < 0.001) as well as a signifi cant joint action × wrist angle (F 4,76 = 4.51, P < 0.05) and joint action × torque level (F 4,76 = 18.79, P < 0.001) interaction indicates that the ago-nist EMG activity was dependent on the joint action, the wrist angle and the torque level. Specifi cally, post hoc comparisons performed separately at each torque level revealed that the EMG activity of both muscles increased between 180 and 150° and between 150 and 130° ( P < 0.05) only at 5 and 10 % of the MVC. The level of EMG activity increased signifi cantly across higher levels of torque production ( ● ▶ Table 2 ). Moreover, a signifi cant joint action × torque level interaction indicates a diff erent slope of increase for each muscle across the torque levels. Regardless of wrist angle, at low levels of torque (5, 10 and 20 %), the EMG activ-ity of FCU was lower than that of ED, however at 75 % of maximal torque the EMG of FCU was higher than that of ED muscle.

Antagonist muscle A signifi cant 3 way interaction (joint action × wrist angle × torque level) (F 16,304 = 7.09, P < 0.001) as well as a signifi cant joint action × wrist angle (F 4,76 = 6.92, P < 0.001), joint action × torque level (F 4,76 = 47.30, P < 0.001) and wrist angle × torque level (F 16,304 = 1.82, P < 0.05) interaction indicated that the antagonist EMG activity was dependent on the joint action, the wrist angle and the torque level. The antagonist EMG increased signifi cantly across higher levels of torque for both muscles ( ● ▶ Table 2 ). Post hoc comparisons performed separately at each wrist angle indi-cated that the antagonistic activity of ED was greater than that of FCU at 50 and 75 % of MVC ( P < 0.01).

Discussion ▼ The main fi nding of this study was that, during an isometric action, a greater in size and well used muscle (Flexor Carpi

Table 1 Maximal isometric torque (mean values ± SD) at 5 angular positions during wrist fl exion and extension (n = 20).

Wrist angular positions (°) Flexion (Nm) Extension (Nm)

230 32.43 ± 11.17* 9.00 ± 3.08 210 28.32 ± 9.64* 11.37 ± 2.69 180 25.52 ± 6.08* 12.89 ± 3.26 150 26.86 ± 4.12* 14.95 ± 4.16 130 28.12 ± 4.67* 17.41 ± 3.84 *signifi cantly greater from extension (P < 0.05)

757


Training & Testing

Ulnaris, FCU) shows more steadiness than its smaller and less used antagonist (Extensor Digitorum, ED). At high percentages of the MVC (50 and 75 %), the antagonistic activity of ED during fl exion was greater than the coactivation of the FCU during extension.

Greater muscle size and daily use A given force can be more accurately generated by a bigger than a weaker muscle. This is in agreement with Hamilton et al. [ 12 ] who postulated that a muscle with more motor units has a lower coeffi cient of variation of force than a weaker muscle with fewer motor units. Even if we did not examine motor unit fi ring pat-terns, it is tempting to consider that a more stochastically inde-pendent discharge of motor units in bigger muscles might have contributed to the lower force fl uctuations compared to the smaller antagonists [ 26 ] . Diff erences in central control signals, range of motor unit recruitment, motor unit synchronization, spike train noise, and number of motor units recruited are also some of the possible sources of motor noise [ 12 , 17 ] . The stronger fl exors (FCU: PCSA = 363.6 ± 34.3 mm 2 , predicted maximal tetanic tension = 89 ± 8.4 N) may have more active units to pro-duce a given force level compared to their weaker antagonists (ED: PCSA = 130 ± 11.1 mm 2 , predicted maximal tetanic ten-sion = 31.9 ± 2.7 N) [ 23 ] . It could be hypothesized that a greater motor unit number of the FCU compared to ED muscle is active during the submaximal isometric actions. These motor units have a lower threshold and smaller twitch force and, according to the size principle [ 15 ] , they are recruited before those with a high threshold and large twitch force, placing a ‘lower bound’ on the level of motor noise in the larger muscle [ 13 ] .

Another plausible explanation could be the daily diff erence of muscle usage patterns for wrist fl exors and extensors. The wrist fl exors are more frequently used (2–3 times more used than their antagonists and their contraction is 40–60 % stronger), prone to injury (tendinitis, tunnel syndromes, stress fractures, etc.) and not similarly activated across the whole range of wrist motion (unpublished observations). Thus, it can be hypothe-sized that the greater daily use of the wrist fl exors results in a greater stability compared to their antagonists. Our results extend the results of Tracy [ 30 ] showing that the weaker dorsi-fl exors present a greater variability compared to the stronger plantarfl exors. In this study, the values for CV of force were con-sistently larger for dorsifl exors compared to plantarfl exors across a large range of forces, suggesting a diff erent transforma-tion of the descending input to the motor unit pools into motor output, despite similar task goals, visual feedback, and gross muscle activation between the muscles [ 32 , 33 ] . This is in agree-ment with the statement that smaller muscles with fewer motor units and more corticospinal input exhibit greater normalized fl uctuations when their gross level of activation is similar [ 12 ] . In line with previous studies [ 5 , 30 ] , our results showed clearly that that the CV decreased as the torque target increased, regard-less of the joint action.

Coactivation In our study, regardless of angle, a greater coactivation was observed at 50 and 75 % of MVC during the most steady isometric wrist fl ex-ion, compared to the extension. It is well known that forcing antag-onist coactivation increases stability, aids the ligaments in maintaining joint stability, equalizing articular surface pressure

% ofMVC

wrist flexion

wrist extension

5

S

10 20 50 75

FCU

ED

FCU

ED

0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

S0 21 3 4 5 6 7

2.00.0 V–2.0

2.00.0 V–2.0

100 wrist flexion

80

60

40% o

f MVC

20

00 2 4 6

Time (s)8

wrist extension100

80

60

40% o

f MVC

20

00 2 4 6

Time (s)8

Fig. 1 Representative variability during submaximal isometric constant-torque (above) and EMG activation (below) of the FCU and ED muscles acting as agonist and antagonist at 5, 10, 20, 50 and 75 % of MVC during wrist fl exion and extension (180°).

758


Training & Testing

[ 1, 10 ] . However, many other studies showed that steadiness does not depend on either the degree of the antagonist coactivation [ 4 ] or alternating pattern of agonist-antagonist coactivation during isometric and slow anisometric contractions [ 7 , 21 ] . As in the current study, the coactivation level, based on EMG activity, is commonly used in the literature. However, the EMG recordings have to be taken into consideration with caution because of 4 serious limitations: a. the cancellation phenome-non could induce an underestimation of the total motor output and therefore underestimate the EMG activity recorded during this study. The surface EMG cannot reveal (consider) any cancel-lation of overlapping positive and negative phases of motor unit potentials [ 6 ] or synchronization of motor unit enhancing the amplitude without any change in the number of action poten-tials [ 33 ] ; b. the contribution of the antagonists depends on their electromechanical effi ciency. The alterations of the antago-nist EMG value due to angular position, mainly observed in our study for the FCU muscle, are not suffi cient to estimate the con-tribution to the resultant torque and the mechanical impact of the coactivation phenomenon [ 2 , 3, 28 ] ; c. the activation of sev-eral muscles during wrist fl exion or extension is not similar and the redundancy of the activation patterns seems equivocal. For example, extensor carpi radialis and extensor carpi brevis are 2 of the muscles acting to extend the wrist. The desired level of net extension torque could be achieved by activating either one of

these muscles or both in combination [ 12 ] . However, in our study, the electrodes placement was identical between partici-pants, inducing a similar bias to the recruitment; d. the cross-talk in forearm muscles and the signal contamination is very large due to the proximity of relatively small muscles [ 20 ] . Thus, the potential contribution of the antagonists to the force fl uctu-ations remains an unresolved issue.

Muscular length Only for wrist extensors, the CV of force signifi cantly increased with decreasing wrist angle, across all levels of torque, showing that the greater the wrist angle, the more constant was the torque production. This length specifi city to the steadiness of torque application could be in accordance with a previous study showing that the fl uctuations in net torque are increased with modulations in activation strategy of agonist muscles that are induced with altered muscle length [ 29 ] . However, this seems in disagreement with another recent study demonstrating that changes in fascicle length in the tibialis anterior are not associ-ated with variability during anisometric actions [ 18 ] . A possible explanation for the observed diff erences in torque variability could be related to diff erences in the architectural characteris-tics between the 2 antagonistic muscles. To our knowledge, no evidence exists in the relevant literature explaining the relation-ship between pennation angle and force variability. However, it

12 FlexionExtension

10

8 *

230°

6

4

2

05 10 20

Target moment (% MVC)

Coef

ficie

nt o

f var

iatio

n (%

)

50 75

12

10

8*

*

210°

6

4

2

05 10 20


Coef

ficie

nt o

f var

iatio

n (%

)

50 75

12

10

8 **

180°

6

4

2

05 10 20


Coef

ficie

nt o

f var

iatio

n (%

)

50 75

12

10

8

**

*

* *

130°

6

4

2

05 10 20


Coef

ficie

nt o

f var

iatio

n (%

)

50 75

12

10

8

*

*

150°

6

4

2

05 10 20


Coef

ficie

nt o

f var

iatio

n (%

)

50 75

FlexionExtension

FlexionExtension

FlexionExtension

FlexionExtension

Fig. 2 Coeffi cients of variation at 5 angles (230, 210, 180, 150 and 130°) during wrist fl exion and extension, across all levels of torque. *: signifi cantly greater from fl exion at the same percentage ( P < 0.05).

759


Training & Testing

4 Burnett RA , Laidlaw DH , Enoka RM . Coactivation of the antagonist muscle does not covary with steadiness in old adults . J Appl Physiol 2001 ; 89 : 61 – 71

5 Christou EA , Grossman M , Carlton LG . Modeling variability of force during isometric contractions of the quadriceps femoris . J Motor Behav 2002 ; 34 : 67 – 81

6 Day SJ , Hulliger M . Experimental simulation of cat electromyogram: evidence for algebraic summation of motor-unit actionpotential trains . J Neurophysiol 2001 ; 86 : 2144 – 2158

7 Enoka RM , Christou EA , Hunter SK , Kornatz KW , Semmler JG , Taylor AM , Tracy BL . Mechanisms that contribute to diff erences in motor perform-ance between young and old adults . J Electromyogr Kinesiol 2003 ; 13 : 1 – 12

8 Fitts PM . The information capacity of the human motor system is controlling the amplitude of movement . J Exp Psychol 1954 ; 47 : 381 – 391

9 Friden J , Lieber RL . Evidence for muscle attachment at relatively long lengths in tendon transfer surgery . J Hand Surg 1998 ; 23 : 105 – 110

10 Gardner-Morse MG , Stokes IA . The eff ects of abdominal muscle coac-tivation on lumbar spine stability . Spine 1998 ; 23 : 86 – 92

11 Graves AE , Kornatz KW , Enoka RM . Older adults use a unique strategy to lift inertial loads with the elbow fl exor muscles . J Neurophysiol 2000 ; 83 : 2030 – 2039

12 Hamilton AF , Jones KE , Wolpert DM . The scaling of motor noise with muscle strength and motor unit number in humans . Exp Brain Res 2004 ; 157 : 417 – 430

13 Harris CM , Wolpert DM . Signal-dependent noise determines motor planning . Nature 1998 ; 20 : 780 – 784

14 Harriss DJ , Atkinson G . International Journal of Sports Medicine – Ethical Standards in Sport and Exercise Science Research . Int J Sports Med 2009 ; 30 : 701 – 702

15 Henneman E , Somjen G , Carpenter DO . Excitability and inhibitability of motoneurons of diff erent sizes . J Neurophysiol 1965 ; 28 : 599 – 620

16 Hermens HJ , Freriks B , Disselhorst-Klug C , Rau G . Development of rec-ommendations for SEMG sensors and sensor placement procedures . J Electromyogr Kinesiol 2000 ; 10 : 361 – 374

17 Jones KE , Hamilton AF , Wolpert DM . Sources of signal-dependent noise during isometric force production . J Neurophysiol 2002 ; 88 : 1533 – 1544

is stated that the actions produced by muscular contractions are profoundly infl uenced by the architecture of the muscles that produce the motor output [ 9 , 22 ] . The architectural features (muscle length, mass, fi bre pennation angle, fi bre length, sar-comere length, physiologic cross-sectional area and fi bre length/muscle length ratio), determined with the use of laser diff rac-tion techniques, showed that wrist muscles could have diff erent architectural properties [ 23 ] . Finally, the maximum force was attained in the maximally extended wrist position for fl exors and maximally fl exed wrist position for extensors ( ● ▶ Table 1 ), confi rming another study combining morphological data from cadavers with a mathematical model [ 24 ] . In conclusion, we observed that 2 antagonistic muscular groups distinguished by muscle size and daily use present signifi cant diff erences in torque variability. This observation was accompa-nied by diff erences in both agonist and antagonist muscle activa-tion patterns. Because the quality of performance is assessed from the deviation between an established criterion and a per-formance outcome further examination of other factors, such as motor unit behaviour is required to examine whether force vari-ability is related to skill level.

References1 Baratta R , Solomonow M , Zhou BH , Letson D , Chuinard R , D’Ambrosia

R . Muscular coactivation. The role of the antagonist musculature in maintaining knee stability . Am J Sports Med 1988 ; 16 : 113 – 122

2 Billot M , Simoneau E , Van Hoecke J , Martin A . Coactivation at the ankle joint is not suffi cient to estimate agonist and antagonist mechanical ontribution . Muscle Nerve 2010 ; 41 : 511 – 518

3 Billot M , Simoneau EM , Ballay Y , Van Hoecke J , Martin A . How the ankle joint angle alters the antagonist and agonist torques during maximal eff orts in dorsi- and plantar fl exion . Scand J Med Sci Sports 2011 doi: 10.1111/j.1600-0838.2010.01278.x

Table 2 Mean values ± SD for normalized aEMG (%) for agonist and antagonist muscles of wrist joint during fl exion and extension movement at 5 diff erent angular positions.

Flexion Extension

% of MVC Agonist (FCU) Antagonist (ED) Agonist (ED) Antagonist (FCU)

230°

5 3.48 ± 2.14 2.92 ± 1.02 9.72 ± 3.48* 1.71 ± 1.17 10 7.21 ± 2.82 4.01 ± 1.42 13.99 ± 5.10* 2.17 ± 1.26 20 17.50 ± 7.67 4.41 ± 1.49 24.93 ± 10.44* 3.51 ± 3.17 50 51.50 ± 16.97 10.94 ± 4.37† 53.17 ± 21.19 6.12 ± 3.46 75 79.47 ± 13.70** 17.29 ± 4.48† 67.14 ± 18.37 9.57 ± 4.67

5 3.11 ± 1.11 2.53 ± 1.13 9.86 ± 3.63* 1.38 ± 0.93 10 7.30 ± 2.41 1.98 ± 1.00 13.64 ± 5.76* 2.01 ± 1.82

210° 20 17.65 ± 5.30 4.13 ± 1.74 22.89 ± 9.44* 3.38 ± 3.26 50 54.68 ± 15.78 9.65 ± 2.67† 45.85 ± 16.62 5.46 ± 3.64 75 75.92 ± 9.27** 15.61 ± 4.95† 63.16 ± 19.68 10.49 ± 7.91

5 3.58 ± 0.73 2.41 ± 1.05 10.76 ± 3.77* 1.42 ± 1.33 10 6.96 ± 1.94 2.15 ± 0.84 13.79 ± 4.32* 1.98 ± 2.43

180° 20 17.98 ± 6.98 4.35 ± 1.31 19.79 ± 6.39* 2.78 ± 3.16 50 50.87 ± 13.41 8.73 ± 1.64† 46.30 ± 12.24 5.46 ± 3.32 75 77.25 ± 13.94** 14.84 ± 5.18† 64.43 ± 18.04 9.33 ± 6.94

5 4.74 ± 1.71 2.78 ± 1.08 11.89 ± 2.62* 1.65 ± 1.73 10 8.39 ± 2.63 2.75 ± 1.21 16.36 ± 4.77* 1.72 ± 1.43

150° 20 17.37 ± 5.72 4.13 ± 1.36 25.40 ± 6.13* 3.18 ± 2.70 50 49.44 ± 10.97 9.79 ± 3.27† 56.36 ± 16.85 6.02 ± 4.14 75 69.93 ± 14.36 18.02 ± 6.04† 70.49 ± 15.33 7.53 ± 5.75

5 6.53 ± 1.91 2.88 ± 0.73 12.40 ± 4.98* 2.85 ± 2.56 10 10.72 ± 3.07 5.08 ± 2.20 18.94 ± 4.87* 2.53 ± 2.54

130° 20 20.67 ± 7.53 4.64 ± 1.44 33.81 ± 10.61* 3.03 ± 2.20 50 52.05 ± 13.50 9.70 ± 2.83† 57.33 ± 16.99 5.43 ± 4.25 75 75.44 ± 10.45** 22.25 ± 4.54† 66.21 ± 18.78 7.35 ± 6.45

*: signifi cantly higher than agonist (FCU), **: signifi cantly higher than agonist (ED), †: signifi cantly higher than antagonist (FCU) ( P < 0.05)

760


Training & Testing

27 Shinohara M , Yoshitake Y , Kouzaki M . Alterations in synergistic muscle activation impact fl uctuations in net force . Med Sci Sports Exerc 2009 ; 191 – 197

28 Simomeau EM , Billot M , Martin A , Van Hoecke J . Antagonist mechanical contribution to resultant maximal torque at the ankle joint in young and older men . J Electromyogr Kinesiol 2009 ; 19 : e123 – e131

29 Taylor AM , Christou EA , Enoka RM . Multiple features of motor-unit activity infl uence force fl uctuations during isometric contractions . J Neurophysiol 2003 ; 90 : 1350 – 1361

30 Tracy BL . Force control is impaired in the ankle plantarfl exors of eld-erly adults . Eur J Appl Physiol 2007 ; 101 : 629 – 636

31 Tracy BL , Mehoudar PD , Ortega JD . The amplitude of force variability is correlated in the knee extensor and elbow fl exor muscles . Exp Brain Res 2007 ; 176 : 448 – 464

32 Tracy BL , Enoka RM . Older adults are less steady during submaximal isometric contractions with the knee extensor muscles . J Appl Physiol 2002 ; 92 : 1004 – 1012

33 Yao W , Fuglevand AJ , Enoka RM . Motor-unit synchronization increases EMG amplitude and decreases steadiness of simulated contractions . J Neurophysiol 2000 ; 83 : 441 – 452

34 Yoshitake Y , Kouzaki M , Fukuoka H , Fukunaga T , Shinohara M . Modula-tion of muscle activity and force fl uctuations in the plantarfl exors after bedrest depends on knee position . Muscle Nerve 2007 ; 35 : 745 – 755

18 Jesunathadas M , Rudroff T , Enoka RM . Changes in muscle fascicles of tibialis anterior during anisometric contractions are not associated with motor-output variability of the ankle dorsifl exors in young and old adults . Eur J Appl Physiol 2010 ; 110 : 1175 – 1186

19 Keen DA , Yue GH , Enoka RM . Training-related enhancement in the con-trol of motor output in elderly humans . J Appl Physiol 1994 ; 77 : 2648 – 2658

20 Kong YK , Hallbeck MS , Jung MC . Crosstalk eff ect on surface electro-myogram of the forearm fl exors during a static grip task . J Electro-myogr Kinesiol 2010 ; 20 : 1223 – 1229

21 Laidlaw DH , Hunter SK , Enoka RM . Nonuniform activation of the ago-nist muscle does not covary with index fi nger acceleration in old adults . J Appl Physiol 2002 ; 93 : 1400 – 1410

22 Lieber RL , Friden J . Functional and clinical signifi cance of skeletal mus-cle architecture . Muscle Nerve 2000 ; 23 : 1647 – 1666

23 Lieber RL , Jacobson MD , Fazeli BM , Abrams RA , Botte MJ . Architecture of selected muscles of the arm and forearm: anatomy and implications for tendon transfer . J Hand Surg Am 1992 ; 17 : 799 – 804

24 Loren GJ , Lieber RL . Tendon biomechanical properties enhance human wrist muscle specialization . J Biomech 1995 ; 28 : 791 – 799

25 Salonikidis K , Amiridis IG , Oxyzoglou N , De Villareal E , Zafeiridis A , Kel-lis E . Force variability during isometric wrist fl exion in highly skilled and sedentary individuals . Eur J Appl Physiol 2009 ; 107 : 715 – 722

26 Semmler JG , Nordstrom MA . Motor unit discharge and force tremor in skill- and strength-trained individuals . Exp Brain Res 1998 ; 119 : 27 – 38

Documents

Wrist Flexors are Steadier than Extensors