Study of the electrically evoked EMG and torque output during the muscle fatigue process in FES-induced static and dynamic contractions

This study compared the different fatigue processes of electrically elicited static and dy namic muscle contractions of quadriceps :in five paraplegic subjects. An electrodynamometer was utilized for recording the generated torque output and controlling the isokinetic dynamic movements. Stimulus-evoked electromyography (EMG), after artifact suppression, was used to observe the fatigue process. Besides torque output, the measured EMG features included peak to peak amplitude, rise time to peak and peak to peak duration. The measurements were modeled by a hyperbolic tangent function, which allowed observance of their time constants, inflection times and relative residual levels during the fatigue process. The rime constants as well as the inflection time of torque output and EMG features were compared between static and dynamic contractions. The inflection times of torque output and all EMG features measured during static contraction were found larger than those of dynamic contraction, In EMG temporal features, the inflection times of rise time to peak and peak to peak duration in static contractions were also found significantly larger than those of dynamic contractions. These conditions indicated that, from either contractile (torque curves) or excitation (EMG features) aspects, the electrically elicited muscle contraction in dynamic movement is prone to fatigue earlier acid faster.