Ankle Muscle Stiffness and Joint Torque Responses to Neuromuscular Electrical Stimulation

Background: Neuromuscular electrical stimulation (NMES) is widely used in rehabilitation, yet its effects on muscle stiffness and the relationship between stiffness and joint torque remain unclear. We quantified NMES-induced changes in soleus (SOL) and tibialis anterior (TA) stiffness and ankle torque during voluntary, electrically evoked, and combined activation. Methods: Thirteen healthy adults performed isometric plantarflexion and dorsiflexion with ankle fixed at 90°. Shear wave elastography measured muscle stiffness during voluntary contractions (10–100% maximum voluntary contraction, MVC), NMES (motor threshold to 100% maximum tolerable stimulation, MTS), and combined voluntary activation (10–30% MVC) plus NMES (80% MTS). Results: NMES increased SOL and TA stiffness and ankle torque with stimulation intensity (p < 0.05). At 100% MTS, SOL stiffness was comparable to MVC, whereas TA stiffness remained lower (p < 0.05). Electrically evoked torque reached approximately 35% of MVC. Combined activation increased stiffness and torque beyond voluntary activation alone, but responses were consistently lower than predicted by linear summation. Muscle stiffness remained strongly associated with joint torque across activation modes. Conclusions: NMES produces graded, muscle-specific mechanical responses while preserving the overall stiffness–torque relationship. Combined voluntary and electrically evoked responses are non-additive, highlighting distinct muscle- and joint-level mechanical effects of NMES.