ABSTRACT
BACKGROUND: Previous literature has shown in vivo changes in muscle-tendon interaction during exhaustive stretch-shortening cycle (SSC) exercise. It is unclear whether these changes in muscle-tendon length during exhaustive SSC exercise are associated with changes in mechanical efficiency (ME). The purpose of the study was to investigate whether changes in platarflexor contractile component (CC) length, tendon length, and changes in plantarflexor muscle activity could explain reduction in ME during exhaustive SSC exercise. METHODS: Eight males participated in an exhaustive hopping task to fatigue. Mechanical work and energy expenditure were calculated at different time-points during the hopping task. Furthermore, hopping kinetics and kinematics, medial gastrocnemius (MG) muscle activity, and in vivo ultrasound of the MG were also collected at different time-points throughout the hopping task. RESULTS: ME did not change during the hopping protocol despite shorter tendon and longer CC lengths as subjects approached exhaustion. Percent decreases in pennation angle and muscle thickness were most strongly correlated to time to exhaustion (r=0.94, p⩽0.05; r=0.87, p⩽0.05; respectively). Percent changes in CC length change and pennation angle were strongly correlated to percent decrease in maximal voluntary isometric plantarflexion (MVIP) force (r=-0.71, p⩽0.04; r=0.70, p⩽0.05; respectively). Braking/push-off EMG ratio increased from initial pre-fatigue values to all other time points showing neuromuscular adaptations to altered muscle lengths. CONCLUSION: Findings from the current study suggest that changes in CC and tendon lengths occur during repetitive hopping to exhaustion, with the amount change strongly related to time to exhaustion. ME of hopping remained unchanged in the presence of altered CC and tendon lengths.
