Soleus muscle stiffness is regulated by scaled activation to manage unpredictable and predictable walking perturbations

During unexpected drop-like gait perturbations, the body centre of mass (CoM) energy must be absorbed reactively by the lower leg muscles, challenging muscle-tendon unit (MTU) function and body stability. Anticipation and prior experience may enable adjustments of muscle activation in advance to improve the perturbation response. Kinematics, electromyographic activity (EMG), soleus fascicle length, and total CoM energy were measured during unperturbed walking, unpredictable and adapted (experience-based) drop-like perturbations as well as during hole negotiation gait to investigate the interplay of muscle activation, MTU decoupling mechanisms and contractile conditions for the CoM energy management. The soleus force-length and force-velocity relationships were also determined to assess the force-length-velocity potential during the tasks. CoM energy decreased substantially after touchdown in the hole during both perturbations and hole negotiation, indicating energy absorption by the musculoskeletal system. In the unpredictable perturbation, a rapidly increased EMG activity after drop initiation and an almost isometric fascicle behavior close to optimal length throughout the CoM energy absorption phase was found, despite MTU lengthening. In the adapted perturbation, an initial isometric contraction accompanied by high EMG activity was observed, followed by active fascicle lengthening at decreasing EMG activity. Clear fascicle lengthening in combination with low EMG activity was found during hole negotiation. The findings suggest a regulation of muscle stiffness by scaled activation that tunes the contribution of muscle and tendon to the MTU length changes (i.e. tendon decoupling), to facilitate high fascicle force-length-velocity potentials and tendon energy buffering mechanisms in response to drop-like perturbations and hole negotiation gait.