Differential mechanical behaviour of superficial and deep aponeurosis and tendon during in situ cyclic contractions of rabbit medial gastrocnemius muscle.

Abstract

Aponeuroses are sheet-like extensions of tendon that transmit forces from muscle fascicles but experience more complex and heterogeneous loading than tendons. Despite this, most muscle models treat tendon and aponeurosis as a single, uniform elastic element in series with muscle. In this study, we examined strains of the superficial and deep aponeurosis and distal tendon of the rabbit medial gastrocnemius in situ under dynamic, physiologically relevant cyclic contractions. We found that, unlike tendon, aponeurosis did not exhibit strain patterns proportional to muscle force. Instead, both aponeuroses showed peak strain early in the activation phase and remained largely constant length during subsequent muscle shortening or lengthening, despite changes in force and belly length. These patterns were absent during passive cycles, indicating context-dependent behaviour of the aponeuroses. The superficial aponeurosis showed considerable variability in strain patterns across animals, particularly during active shortening, consistent with previous in vivo measures. Our findings challenge the assumption that aponeurosis and tendon function as a single series elastic element. Instead, aponeurosis exhibits distinct mechanical behaviour due to its complex structure and loading from the underlying muscle tissue. These results emphasize the need for muscle models that represent aponeurosis and tendon as separate structures, reflecting their unique mechanical behaviours and roles during contraction.