Running-induced fatigue influences lower extremity muscle synergy and related biomechanics

Background

Fatigue changes muscle activation patterns as an adaptation strategy in multi-joint movements. However, due to groups of muscles involved in running, the across-muscle neurological regulation before and after fatigue is unclear. The central nervous system may employ low-dimensional modular structures, known as muscle synergies, to describe the muscle coordination in a multi-joint movement such as running.

Research question

This study aimed to explore the effect of fatigue on lower extremity muscle synergy and biomechanical characteristics in running.

Method

Twelve healthy male participants were recruited. The kinematic, kinetic, and surface electromyography data of nine muscles (i.e., gluteus maximus, biceps femoris, rectus femoris, vastus medialis, vastus lateralis, tibialis anterior, medial gastrocnemius, lateral gastrocnemius, and soleus) were synchronously recorded on a treadmill at 12 km/h before and after a load-increasing fatigue intervention. Muscle synergy was calculated by the non-negative matrix factorization algorithm. Muscle weight and activation curves were used to identify the co-activation before and after fatigue.

Result

Maximum ankle plantarflexion velocity, maximum knee flexion angle, and hip range of motion significantly increased after fatigue. Vertical stiffness and work in the lower extremity significantly decreased after fatigue. Six muscle synergies (SYN1–6) were clustered before and after fatigue, corresponding to four major functions in the running gait cycle. The number of muscle synergy did not change significantly after fatigue, while the muscle weights and activation curve changed.

Significance

Muscle synergies were stabilized and were less affected by fatigue, corresponding to specific biomechanical functions within the running gait cycle. Meanwhile, changes in muscle weights and activation curves reflected neuromuscular adaptations to fatigue and may be associated with alterations in lower extremity kinematics during continuous movement after fatigue.