Compensatory muscle activation caused by tendon lengthening post-Achilles tendon rupture

Abstract

Purpose: The purpose of this study was to establish a relationship between the lengthening of the Achilles tendon post-rupture and surgical repair to muscle activation patterns during walking in order to serve as a reference for post-surgical assessment.

Method: The Achilles tendon lengths were collected from 4 patients with an Achilles tendon rupture 6 and 12 months post-surgery along with 5 healthy controls via ultrasound. EMG was collected from the triceps surae muscles and tibialis anterior during overground walking.

Results: Achilles lengths at 6 and 12 months post-surgery were significantly longer (p < 0.05) on the involved side compared to the uninvolved side, but there were no side-to-side differences in the healthy controls. The integrated EMG (iEMG) of the involved side was significantly higher than the uninvolved side in the lateral gastrocnemius at 6 months and for the medial gastrocnemius at 12 months in the patients with Achilles tendon rupture; no side-to-side difference was found in the healthy controls. The triceps surae muscles' activations were fair to moderately correlated to the Achilles lengths (0.38 < r < 0.52).

Conclusions: The increased Achilles tendon length and iEMG from the triceps surae muscles indicate that loss of function is primarily caused by anatomical changes in the tendon and the appearance of muscle weakness is due to a lack of force transmission capability. This study indicates that when aiming for full return of function and strength, an important treatment goal appears to be to minimize tendon elongation.

Fig 1

The mean iEMG during one gait cycle for all injured subjects. The asterix (*) indicates a significant increase in iEMG. Note the significant increases existed in the triceps surae cases, but not in the TA

Fig 2

The difference in iEMG during one gait cycle compared to the difference in length of the Achilles tendon for healthy and injured subjects. The right leg is positive for the healthy subjects and the involved side is graphed as positive for the injured subjects. The (▲) denote the healthy subjects and the (■) denote the injured subjects. Note the (a) LG, (b) MG, and (c) SL each have a moderate to fair correlation while the (d) TA has no correlation

Fig 3

If Achilles tendon length increases, the muscle length must decrease an equal amount. The figure at left shows that, given a muscle force-length curve at a constant activation level, if the length of the muscle decreases, its corresponding force will decrease. The figure at right shows that to produce the same amount of force at that new length, the activation level must be increased (as marked by shifting to a new force-length curve). Hence, higher EMGs are needed for the same task if the muscle length shortens