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study was conducted to determine: 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
WORDS: Neuromuscular electrical stimulation, tolerance, conditioning,
gender.
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