Tolerance and conditioning to neuro-muscular electrical stimulation within and between sessions and gender

1) If healthy subjects can be conditioned to tolerate clinically useful electrically induced muscle contraction; and 2) If there is a gender difference in response to such conditioning. Healthy volunteers (10 males, 11 females, mean age of 27.6 ± 5.8 yrs) were tested during each of 6 testing sessions. Maximal voluntary isometric contractions (MVIC) of the right quadriceps femoris (RQF) recorded by a computerized dynamometer. Electrical stimulation delivered through two surface electrodes and stimulation amplitude increased until the subject indicated to stop. After a 1 min rest the amplitude increased again to the same phase charge level, and the electrically induced contraction (EIC) was recorded by the dynamometer. Measurements of stimulation amplitude were repeated in each of 10 stimulation bouts per session. Measurements of EIC were repeated in session six. Statistical analyses included Multivariate ANOVAs, and Newman-Kuel's post-hoc tests (p < 0.01). Mean values of phase charge increased from session 1 to 6 for all subjects. Males tolerated significantly higher phase charge. The mean %MVIC torque generated by female subjects was initially only 11.2 ± 21.6% but reached 42.9 ± 25.4% at the end of the 6th session. Males' %MVIC torque values were significantly higher reaching 49.0 ± 41.6% and 73.5 ± 18.7% in the first and last trials respectively. Using the criterion that electrically induced contractions must be at least 25% of MVIC to be considered clinically useful, 36% of females were below this threshold at the end of the last session. In contrast, all males exceeded the 25% MVIC threshold at the end of the study. Most healthy subjects can be conditioned to electrical stimulation of the quadriceps, but depending on the criteria of therapeutic value and gender, some males and even more females may not reach the desired stimulation goal in 6 sessions. Females may require more conditioning sessions to reach contraction levels of therapeutic benefits. The reason(s) for the confounding factor of gender remains unknown. Key PointsNeuromuscular electrical stimulation (NMES) can strengthen skeletal musclesTolerance to NMES improves within 6 sessionsConditioning is a key to eliciting stronger contraction and to increasing the number of subjects that can benefit from NMESHealthy males can tolerate higher stimulusintensity and higher electrically induced quadriceps femoris contraction.