The functional electrical stimulation system is being developed for restoring function of spinal cord injured patients. But limitations of current system are electrode failure, muscle fatigue, cumbersome electrical hardware, and an inefficient usermachine interface.

Muscle fatigue in tetanic isometric contractions is defined as the decrease in force due to sustained stimulation.

This study comprises a comparison of fatigue between normal control (N=10) and spinal cord injured (SCI) patients (N=4) induced by electrical stimulation (ES) at different frequency (20, Hz, 80 Hz) and stimulation pattern (continuous, intermittent), and a comparison between maximal voluntary contraction (MVC) and ES in normal controls, respectively.

Fatigue was expressed in two ways: the time for the force to fall to 50 percent of the initial force (T⁵⁰), and the percentage of the initial contraction force (ICF) remaining at T50.

The results were as follows:

The T⁵⁰ of MVC was significantly shorter than that of 20 Hz continuous ES in normal control (p<0.01). The T⁵⁰ of normal controls was significantly longer than that of SCI patients with 20 Hz, 80 Hz continuous ES, respectively (p<0.01). The T⁵⁰ of 20 Hz contunous ES was significantly longer than that of 80 Hz continuous ES in normal control and SCI patients, respectively (p<0.01). And the percentage of the ICF remaining of intermittent ES at the point of T⁵⁰ in continuous ES is greater than that of continuous ES in 20 Hz, 80 Hz of normal control and SCI patients, respectively (p<0.05).

In conclusion, muscle fatigue phenomenon was noted less with low frequency and intermittent ES than with high frequency and continuous ES.