Clinical Biomechanics 1988; 3: 153-I 58

Exercise moderation of foot function during walking with a re-usable semirigid ankle orthosis

J Hamill PhD

G Morin MS,ATC

P M Clarkson PhD

R 0 Andres PhD

Biomechanics and Exercise Biochemistry Laboratories, University of Massachusetts, Amherst, Massachusetts, USA.

Summary

A study was conducted to investigate the effects of a re-usable semirigid ankle orthosis on the support phase of the walking stride in both pre- and post-exercise conditions. Ten young, adult males were required to complete ten trials in each of four orthosis/no orthosis and pre-/ post-exercise conditions. Data were collected via a force platform and a high speed camera. The analysis consisted of the evaluation of selected ground reaction force parameters and kinematic parameters describing rearfoot motion. The exercise regimen consisted of 70 maximal eccentric actions of the ankle everters, with 15 s between each action. The results revealed significant differences between the orthosis/no orthosis conditions for the variables describing the mediolateral action of the ankle during walking. Only the rearfoot touchdown angle was affected by the exercise regimen. The data indicated that the semirigid orthosis moderated ankle joint mechanics, although the measured values were within normal bounds.

Relevance Although ankle injuries account for the majority of injuries in athletics, efforts to produce a prophylactic orthosis for the ankle have generally not been successful. The semirigid orthosis used in this study appeared to accomplish the goals of a successful prophylactic: moderate ankle joint motion within the normal bounds of the joint action.

Key words: Ankle joint, Orthosis, Kinematics, Kinetics, Rearfoot motion

Introduction

The ankle is one of the most vulnerable areas of the body during athletic participation and, as a result, is highly susceptible to trauma. Ankle injuries account for the greatest number of days lost in athletics’. O’Donoghue’ and Distefano’ reported that 85% of all ankle injuries were sprains resulting from forced inversion, plantar flexion or a combination of both these actions. As a result, great emphasis is presently being placed on ankle injury prevention and rehabilitationA*.

The usual means of preventing and protecting ankle injuries is prophylactic taping, wrapping or strapping.

Retunwd for revisiorl: 4 November 1087 Amq~ted: 4 January 19X8 C‘orrerpo,lderrcearltf wpr,rint reyuwr to: J Hamill PhD, Department of Exercise Science. University of Massachusetts. Amherst. MA 01003. USA 0 1988 Butterworth & Co (Publishers) Ltd 026H~033/88/030 15346 $03 .oo

However, prophylactic devices such as those previously mentioned have generally not been successful in modify- ing or restricting foot function in possible traumatic situ- ations. Studies by Ferguson5, Liberia’ and Mayhew” suggested that ankle prophylaxes might restrict neces- sary ankle mechanisms. Hamill et al.” and McIntyre et al.‘O reported no changes in foot function with the appli- cation of prophylactic taping or a Cramer ankle strap in either pre- or post-exercise conditions, lending support to the belief that these ankle devices do not affect mediolateral function of the foot. However, the exercise conditions used in these studies were those that generate an immediate and temporary localized fatigue in the muscle and were not representative of a traumatic injury.

One possible alternative ankle support to taping, wrapping or strapping is a re-usable semirigid support system”,“. This system, which is worn inside the shoe, consists of two pieces of a thermoplastic material

154 Clin. Biomech. 1988; 3: No 3

Figure 1. Semirigid orthosis after application

moulded to fit the foot and ankle in the form of a stirrup (Figure 1). Air sacks on the inside of the moulded stir- rups are inflated to provide additional support. The sup- port covers the malleoli and continues up the medial and lateral portions of the shank to approximately 15 cm above the malleoli.

The purpose of this study was to investigate the mod- ification of foot function by a re-usable semirigid ankle orthosis in both pre- and post-exercise conditions using both kinetic and kinematic data. The semi-rigid ankle orthosis utilized in this study was the Air Stirrup (Air Cast, Inc., Summit, NJ 07901). The exercise regimen, administered by the experimenter, included eccentric muscle actions which have been shown to produce mus- cle damage and profound weakness. Recovery from such exercise can take 2-S days’“-‘h. This exercise regi- men should provide a good model to simulate an ankle injury.

Methods

Subjects

Ten young, healthy males served as subjects in this experiment after signing informed consent forms in

accordance with University policy. The subjects ranged in height from 1.70 to 1.85 m (mean + s.d. = 1.77 -t 0.05 m) and in weight from 61 to 81 kg (mean + s.d. = 72.6 kg + 6.84 kg). All subjects were free from lower extremity injury at the time of testing and had no prior history of ankle injury for at least 5 years.

Experimental set-up

The experimental set-up consisted of an Advanced Mechanical Technology Incorporated (AMTI. Newton. MA 02158. USA) force platform interfaced to an IBM 9000 microcomputer via an analoguc to digital conver- ter. The force platform was centred in a large testing area. Six raw voltage signals were sampled at 500 Hz. amplified and electronically processed by a signal con- ditioner to provide the three orthogonal ground reaction force and moment components. In addition. a photo- electric switching system was used to monitor the walk- ing speed of the subjects over the criterion 4 m distance. The criterion speed was 1.4 m/s (+ 5% ).

A 16 mm high speed camera (LoCam. Visual Instrumentation Corp., Burbank. CA 91502. USA), equipped with a IS-150 mm zoom lens, was positioned 4 m directly posterior to the force platform. with the optical axis of the camera parallel to the direction of the locomotor path. Data sampling was accomplished at 100 Hz. A 100 Hz pulsed signal applied to dn internal LED timing light enabled the framing rate of 100 fps to be accurately verified.

Protocol

Subjects were required to complete ten force platform trials in each of four conditions. The conditions were:

1) pre-exercise (Cl); 2) pre-exercise with ankle brace (C2); 3) post-exercise (C3) and 4) post-exercise with ankle brace (C4).

Subjects wore the same brand and model of shoe in each condition to minimize subject-shoe interactions. Subsequent to either Cl or C2 (randomized for each subject). the subjects performed an exercise regimen administered by the experimenter of 70 maximal eccen- tric muscle actions of the ankle everters with 15 s between each action. Prior to and immediately following the exercise, isometric strength of the ankle everters was assessed using a fixed strain gauge and pen recorder. At the conclusion of the exercise bout, the subjects filled out a perceived pain questionnaire and completed con- ditions 3 and 4 (randomized for each subject). The ques- tionnaire required subjects to rate their perceived pain on a scale of 1 (normal) to 10 (very painful). The decreases in strength and muscle pain were used as indicators of exercise-induced muscle weakness and damage.

At the beginning of the test session, each subject was prepared for filming by placing reference markers along the lower extremity and on the rear of the shoe accord- ing to the protocol of Nigg l7 Two markers were placed . on lower leg, one on the Achilles tendon just above the

heel cap of the shoe and the second 1.5 cm above that marker in the centre of the leg. Two other markers were placed on the shoe, one on the upper part of the heel cap and one just above the midsole of the shoe. Subjects were given ample time to warm-up, to become accus- tomed to the experimental area and to adjust to the criterion walking speed. Each subject was then required to complete ten successful trials with the left limb in each of the four test conditions. Of the ten force platform trials, three randomly selected trials were filmed. A suc- cessful trial was one in which the subject contacted the force platform in a normal stride pattern at the desig- nated pace. Trials were deemed unsuccessful if the sub- ject appeared to reach for the force platform or if the positive and negative force profiles of the anteropos- terior force component were not symmetrical.

Data analysis

The first step in ground reaction force data evaluation was to normalize the force-time data by dividing by body mass to aid in making between-subject compari- sons. Individual trial data were evaluated to obtain a set of 13 force, relative time and impulse variables (8 verti- cal. 2 anteroposterior and 3 mediolateral) describing the ground reaction force components’x,‘“.

The kinematic data were obtained from the film using a motion analyser in conjunction with a graph pen digitizer (Science Accessories Corp., Southport, Con- necticut 06490) interfaced to a Zenith 148 microcompu- ter. Coordinates of the background references and four body segment markers, located for the calculation of rearfoot angle, were identified and digitized. Digitizing began four frames prior to foot contact and continued until four frames subsequent to toe-off. The film data were smoothed using a recursive low-pass Butterworth digital filter. The frequency cut-off for each X and Y coordinate was determined by the procedure of Jackson’” and in most cases ranged between 4.5 and 9 Hz. Calculation of the rearfoot angles was accomplished according to the protocol described in Clarke et al.“. The rearfoot angle-time data were then differentiated with respect to time using a first central finite difference method to predict rearfoot angular vel- ocity. Values describing touchdown angle, maximum calcaneal eversion, time to maximum calcaneal ever- sion, total rearfoot motion, maximum calcaneal ever- sion velocity and time to the maximum velocity were generated for each trial”.

Statistical analysis

Ten trial mean values for each of the ground reaction force variables and three trial means for each of the kinematic variables for each condition for each subject were statistically evaluated using a two-way analysis of variance technique with a repeated measures design. This analysis provided the main effects of orthosis (with versus without), exercise (pre versus post), and the interaction term.

Hamill et a/.: Exercise moderation of foot function 155

Results

The mean (+ s.d.) isometric strength of the ankle ever- ters was 16.8 (+ 2.4) kg prior to the exercise and 9.5 (& 6.8) kg immediately following the exercise. This amounted to a significant loss in strength of 44% (p<O.Ol). The mean (+ s.d.) of perceived pain was 4.7 (+ 2.1)) indicating considerable discomfort.

The mean values and standard deviations for all ground reaction force and kinematic variables in each condition are presented in Tables 14. The statistical analyses on the vertical force (Fz) variables produced no significant F-ratios for any variable as a result of the exercise regimen. Only one variable, the relative time to the second maximum force, resulted in a significant orthosis main effect and a significant orthosis by exercise interaction term (p<O.O5). The significant interaction resulted from the decrease in the relative time to this force as a function of the brace, while the exercise caused no significant alteration in the timing of this event. In addition, no statistically significant results were derived from the analyses on the anteroposterior variables.

The statistical analyses on the mediolateral ground reaction force component variables revealed no statisti- cally significant interactions nor any significant exercise main effects for any of the variables. Two variables, force excursions (O-30% of total support) and force excursions (O-100% of total support) had significant F- ratio values for the main effect orthosis (p<O.O5). The two significant excursion variables indicated differences between the orthosis/no orthosis conditions in both the pre- (Cl versus C2) and post-exercise conditions (C3 and C4). In each case, the means were less in the orthosis condition than in the no orthosis condition.

The statistical analyses conducted on the rearfoot kinematic variables resulted in no statistically significant interactions. Two variables, touchdown angle and total rearfoot motion, revealed significant differences as a result of the exercise regimen (p<O.O5). In the former case, the exercise regimen caused the foot to be in a more inverted position at touchdown and in the latter case, the total rearfoot motion was reduced. The effect of the ankle brace resulted in significant differences in three variables, reducing maximum calcaneal eversion angle, total rearfoot motion and time to maximum cal- caneal eversion velocity values (p<O-05). Since touch- down angle and maximum calcaneal eversion angle are not independent of total rearfoot motion, any significant change in either of those variables should cause a sig- nificant change in total rearfoot motion.

Discussion

The purpose of this study was to describe the effects of a re-usable, semirigid, air stirrup ankle orthosis on foot function in both pre- and post-exercise conditions using both ground reaction force and kinematic rearfoot data. In terms of the vertical ground reaction force compo- nent, which describes the forces applied by and absorbed through the leg during the support phase, no force or impulse values were significantly different

156 Clin. Biomech. 1988; 3: No 3

whether the ankle orthosis was worn or not in either the pre- or post-exercise conditions. It has been reported in previous studies that the midsole of the shoe primarily determines changes in shock absorption”.‘“. Since all subjects in this study wore the same type of shoe, no ver- tical force component differences were expected. Only the relative time to the second maximum force was affected by the orthosis and only in the pre-exercise con- dition. The second maximum force is generally consi- dered to be the vertical propulsive force or the force that propels the body’s centre of gravity into the air in prep- aration for swinging the trail leg through for the next support. The delay in applying this force caused by the orthosis did not appear to be detrimental to the gait cycle because the delay was just over 1%. It is possible that this delay was caused by the novelty of wearing the ankle orthosis. since the delay did not appear in the post-exer- cise condition.

The lack of significant differences in the anteropos- terior component of the ground reaction force data indi- cated that the subjects were walking at a constant rate throughout the conditions and were under no apparent stress as a result of the ankle orthosis. This was an interesting finding in light of that reported by McIntyre et al.“‘, who suggested that taping procedures reduced the plantar flexion ability of the ankle during locomotion and this in turn may affect the transition from braking to propulsion in the sagittal plane. It appeared that the orthosis did not affect this ability. This was probably due to the lack of forefoot intervention and the stirrup design of the orthosis.

Ankle support systems, acting as prophylactic devices, are primarily designed and worn to moderate mediolateral function of the foot during the support phase of the locomotor cycle. The studies that have investigated prophylactic devices in a dynamic situation have shown that these devices did not moderate mediolateral function”‘“. The mediolateral component of the ground reaction force is the shear force exerted parallel to the locomotor surface and perpendicular to the direction of motion. This component has been associated with the mediolateral function of the foot18.‘9. Significant differences were seen in the two force excur- sion variables in this study. Force excursions describe the sum of the absolute deviations of the force compo- nent in the mediolateral direction. In this study, force excursion values when wearing the orthosis were always smaller than in the no-orthosis condition, suggesting that the orthosis indeed moderated mediolateral func- tion.

This finding was supported by the kinematic variable, maximum calcaneal eversion angle, which revealed decreases from 12.5 to 8.2” in the pre-exercise condition and 11.3 to 6.6” in the post-exercise condition. It is interesting that the orthosis decreased the force excur- sion values during the first 30% of the footfall, that is, during the period of rapid calcaneal eversion. Of note is the fact that the force excursion and calcaneal eversion angle values in both the orthosis and no-orthosis condi- tions were within the normal range of values”.“.

Further evidence of the rearfoot control of the orthosis is seen in the increase in time to maximum cal-

Table 1. Means (s.d.) and statistical analysis of vertical ground reaction force variables

Relative time to 1st max.force (%)

1 st max. force (N/kg)

Relative time to lstmin.force(%)

1st min.force (N/kg)

Impulse to 1st min.force (N.s)

Relative time to 2nd max. force (%)

2nd max. force (N/kg)

Averageforce (N/kg)

Pre-exercise Post-exercise Cl c2 c3 c4

21.01 20.96 21.24 21.27 (3.29) (3.01) (2.65) (1.78) II.06 Il.02 II.10 IO.97 (0.82) (0.78) (0.66) (0.67)

43.47 42.95 42.40 43.21

(3.48) (3.13) (3.39) (3.24) 7.21 7.33 7.37 7.41

(0.46) (0.42) (0.50) (0.43)

0.495 0.496 0.500 0.506 (0.062) (0.070) (0.061) (0.054)

75.17 76.31 75.58 75.70 (1.89) (1.51) (1.55) (1.40) IO.69 IO.65 IO.70 IO.72 (0.57) (0.46) (0.42) (0.49) 7.78 7.82 7.80 7.80

(0.19) (0.18) (0.11) (0.13)

F-ratios Orthosis Exercise

<I <I

1.48 <I

1.22 <I

2.00 4.20

<I <I

14.68* <I

<l <I

1.54 <I

* P<O.O5 Relative time: per cent Force: N/kg of body mass Impulse: N.s normalized to body mass

Hamill et al.: Exercise moderation of foot function 157

caneal eversion velocity in the orthosis conditions. The time to maximum calcaneal eversion velocity increased by 55.4% pre-exercise and 57.1% post-exercise. It would appear that the brace is affecting the rate of change of calcaneal eversion velocity.

The average mediolateral force over the footfall was not significantly different between either orthosis or exercise conditions. The indication was that the orthosis was preventing the ankle from extreme fluctuations but was allowing the ankle to function within normal

Table 2. Means (sd.) and statistical analysis of anteroposterior variables

Pre-exercise Cl c2

Post-exercise c3 c4

F-ratios Orthosis Exercise

Braking force (N/kg) 0.985 0.989 0.975 0.971 <l <I (0.101) (0.085) (0.086) (0.064)

Propelling force (N/kg) 0.962 0.957 0.966 0.969 <I <I (0.113) (0.105) (0.122) (0.108)

Force: N/kg of body mass

Table 3. Means (s.d.) and statistical analysis of mediolateral variables

Pre-exercise Cl c2

Post-exercise c3 c4

F-ratios Orthosis Exercise

Excursions O-30% of support (N/kg)

Excursions O-l 00% of support (N/kg)

Average force (N/kg)

2.71 2.33 2.66 2.36 21.08* <I (0.55) (0.36) (0.48) (0.45)

4.02 3.61 3.96 3.61 28.31* <I (0.56) (0.38) (0.49) (0.48) 0.276 0.272 0.277 0.258 2.69 <I

(0.079) (0.086) (0.077) (0.064)

??P-CO.05 Excursions: N/kg of body mass Force: N/kg of body mass

Table 4. Means (s.d.) and statistical analysis of kinematic variables

Pre-exercise Cl c2

Post-exercise c3 c4

F-ratios Orthosis Exercise

Touchdown angle (“I

Max. calcaneal eversion angle (“)

Time to max. calcaneal eversion angle (ms)

Total RF motion (“I

Max. calcaneal eversion velocity (‘.s-‘1

Time to max. calcaneal eversion velocity (ms)

-2.5 -2.7 (3.19) (5.21)

12.5 (4.63)

8.2 (3.39)

127 (Ill)

15.0 (5.14)

-184.7 (166.7)

347 (128)

10.9 (5.32)

-0.1 (3.71)

11.3 (4.02)

265 (87) 1 I.4 (5.97)

-137.7 (87.8)

1.3 <I 6.20* (3.47)

(Zl ) 21.03, 2.69

357 3.82 <I (130)

7.9 10.88* 5.64* (4.13)

-111.7 4.56 <I (83.01)

6.02, <l

??Pc0.05 Angle: degrees Time: milliseconds Velocity: degrees. s-’

158 Clin. Biomech. 1988; 3: No 3

bounds, that is, to apply a certain force over the footfall. The exercise produced muscle weakness, as evi-

denced by a 44% loss in strength. Strength losses from 30 to 50% have been shown after eccentric exercise”.“. The exercise also resulted in subjective reporting of mus- cle pain. which has been used as an indicator of muscle damage after eccentric exercise’“.‘3.“‘. The damage induced by eccentric exercise is considered to occur in the myofibrils as well as connective tissue’5.‘h. Despite the evidence of weakness, there was no significant dif- ference in mean vertical, anteroposterior or mediolat- era1 ground reaction force variables between pre- and post-exercise conditions. However, the exercise regi- men did affect the angle at which the foot was placed on the ground. Generally, during locomotion, the foot may contact the ground in a slightly supinated or neutral pos- ition. During touchdown. the foot must act as a shock absorber and be a flexible adaptor to the walking sur- face. In this study. the pre-exercise touchdown angle values revealed a slightly everted position (-2.5 and -2.7”). Post-exercise. however, the angles were in a more inverted position. The muscle weakness induced in the peroneals may have caused a disruption of the nor- mal synergies between the peroneals and the anterior tibialis, resulting in a more inverted foot at touchdown. It would appear that the exercise regimen. while not pro- ducing gross kinetic or kinematic changes, may have been sufficient to cause a change in the orientation of the foot at touchdown to a position which, in an unprotected ankle, may lead to a propensity for inversion type sprains. The addition of the ankle support would appear to assist in the prevention of sudden inversion type injuries.

It is possible that the ground reaction force and rear- foot kinematic data after eccentric exercise may be more affected during forms of locomotion that are more stressful, such as running or responding to uneven sur- faces. However. even the modest changes seen in this study revealed that the re-usable air stirrup orthosis was effective in moderating ankle function.

Conclusions

The following conclusions appear warranted, based on the results of this study:

The re-usable air stirrup did not affect the vertical or anteroposterior force components of the ground reaction forces to any degree, although a period of adjustment to the brace appeared necessary. The re-usable air stirrup reduced the force excursion of the mediolateral component and several kinematic variables describing rearfoot motion, although the values were still within the normal range for walking. The orthosis acted as a prophylactic ankle device dur- ing walking, since ankle function was moderated but the degree of moderation was within normal limits for the subject population.

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