ON THE MUSCULAR PROTECTION OF ANKLES

Participants: M. J. Milia, J. A. Ashton-Miller, M. Siskosky, J. P. Boylan, S. Veerapanen, E. M. Wojtys

Keywords: ankle, muscle, stiffness

Introduction

The evertor muscles are responsible for preventing an involuntary inversion rotation of the ankle or, worse, an inversion sprain. We are not aware of any studies that have examined the dynamic ability of the evertor muscles to prevent forced inversion when actually landing from a step or jump. When one lands on a hard foreign object from a step or jump, the object will exert a vertical ground reaction force (GRF) under the foot that tends to forcibly invert the foot. The more medial the location of that object under the foot, the greater the muscular evertor moment required to prevent unwanted inversion (Ottaviani et al. 1995). When standing on one foot, healthy young males have sufficient evertor strength to resist a (GRF) of magnitude 1-bodyweight (1-BW) a distance of 34 mm medial to the midline of the subtalar joint (Ashton-Miller et al. 1996). We tested the hypothesis that if the foreign object was positioned such that the GRF acted only 12 mm medial to the subtalar joint, then the evertor muscles should possess sufficient strength to prevent forced inversion when either stepping onto one heel with a ~1-BW GRF, or landing on the forefoot with a ~2.5-BW GRF.

Materials and Methods

Ten healthy young males (mean (SD) age: 24.6 yrs; mean (SD) body weight: 777 (54) N; right dominant foot; foot size 10-11) were recruited for this experiment. Subjects were equipped with a special right shoe with one of two detachable sub-soles: (a) an experimental sole with a segmented hard plastic fulcrum, measuring 14 mm high x 6 mm thick, running parallel to the length of the foot 12 mm medial to the foot midline (Sole A); (b) a flat cork sole (Sole B). The subject initially stood bipedally on a special platform which rendered it impossible for them to feel or see which sole was fixed under their right shoe. Six Optotrak kinematic markers were affixed to the midline of rear foot (2 markers), midline of the lower leg (2 markers), over the L5 spinous process and T1 process and tracked at 1 kHz. Subjects were trained to move forward from the platform and step onto a 6-channel AMTI force plate a 1-BW GRF under the heel, or jump onto it landing on the forefoot with a 2.5 BW GRF, then standing unipedally for 3 seconds. In trial Block 1 (5 trials) each unknowingly wore Sole A and were instructed to step in a "comfortable manner" per the above instructions. In trial Block 2 (5 trials), Block 1 was repeated with instruction "to try to prevent touchdown of the outside of your foot" caused by an unwanted 16° inversion rotation of their foot. All trials were then repeated with the standardized jump. Touchdown was signaled by a microswitch. In trial Block 3 (10 trials) subjects were presented with 7 dummy trials (Sole B) and 3 stimulus trials (Sole A) in pre-randomized order. The primary outcome was the group success rate in preventing touchdown from a step or jump under the three test conditions.

Results

Step Results: Analysis of the first 300 trials showed that when landing on their heel the mean GRF peaked at 0.92-BW in 300-400 msec after foot contact. Subjects failed in 100% of Block 1 trials, 57% of Block 2 trials, and 80% of Block 3 stimulus trials (p<0.001). Jump Results: Analysis of the first 300 trials showed when landing on their forefoot the mean jump GRF peaked at 2.47-BW in 30-40 msec, too short a latency for any stretch or long-loop reflexes from helping to preventing forced inversion. When landing on their forefoot from the jump, subjects failed in 90% of Block 1 trials, 80% of Block 2 trials, and 93% of Block 3 stimulus trials (p<0.001).

Conclusions

When subjects were able to anticipate full ankle inversion, they could volitionally increase their ankle stiffness to reduce the probability of full inversion by nearly 50% when landing from a step, and 10% when landing from a jump. Therefore, training athletes to anticipate these situations may be helpful in preventing ankle injuries.

References: 1. Ottaviani RO et al.: AJSM 23(4): 418-423, 1995.

2. Ashton-Miller JA et al.: AJSM 24(6): 800-809, 1996.