Impaired gastrocnemius medialis muscle-tendon decoupling reduces gait efficiency in spastic paresis syndrome

Muscle-tendon unit interactions are crucial for efficient walking but remain unstudied in adult-onset spastic paresis. This cross-sectional study evaluated the relative contributions of the Achilles tendon, muscle fascicles, and muscle belly to the dynamics of the gastrocnemius medialis during gait in individuals with stroke and incomplete spinal cord injury, focusing on Equinus and Non-equinus patterns.

Sixty participants (20 with stroke, 18 with incomplete spinal cord injury, 22 healthy controls) underwent 3-dimensional gait analysis synchronized with ultrasound. Participants were stratified into "Equinus" (N = 19) and "Non-equinus" (N = 19) groups based on terminal stance ankle dorsiflexion. The force-length relationship (maximal torque, T_max; optimal fascicle length, L_opt) of the gastrocnemius medialis was determined on an isokinetic dynamometer using supramaximal tibial nerve stimulation. Muscle-tendon unit, belly, and tendon kinematics were calculated throughout the gait cycle.

T_max was reduced in both pathological groups, but L_opt was significantly shorter only in the Equinus group (P < 0.001). During walking, both pathological patterns showed a disruption of the spring-like mechanism (loss of fascicle-tendon decoupling). The Non-equinus pattern was characterized by significantly longer tendons and reduced muscle-tendon unit excursion. Critically, fascicles in both pathological groups operated beyond the optimal length of the force-length curve during stance.

Adaptive gait patterns in adult spastic paresis are primarily discriminated by tendon adaptations rather than muscle belly dynamics. The observed functional paradox (structurally short muscles operating in an over-stretched state during stance) highlights a biomechanical trade-off for surgical tendon lengthening. Such intervention may be detrimental to muscle force-generating capacity by shifting fascicles further onto the descending limb of the force-length relationship. These findings suggest that personalized assessments could help optimize clinical decisions, including the development of therapeutic strategies designed to restore tendon stiffness and improve force-transmission efficiency.

NCT06474117.