Peripheral nerve excitation and plantar flexion force elicited by electrical stimulation in males and females

Study Design: Two-group (gender) posttest only using a sample of convenience. Objectives: Our study examined the effect of gender on selected stimulus properties, perceived pain thresholds, and maximally tolerated level of contraction of the plantar flexor muscle group. Background: There is a dearth of literature regarding gender as a factor that may influence the outcome following the application of neuromuscular electrical stimulation. Data from other disciplines suggest that males and females may not have similar tolerance to electrical stimulation. Methods and Measures: Eleven women (mean age, 28.3 years +/- 5.6 years) and 9 men (mean age, 33.2 years +/- 6 years) participated in a single session of electrical stimulation. A pair of 4.5 x 4.5 cm, self-adhesive, synthetic, polymer electrodes was placed over the medial and lateral heads of the gastrocsoleus muscle group. Electrical stimulation was provided by a battery-powered pulsatile stimulator generating a symmetric biphasic waveform, 200-mu second phase duration, and pulse rate of 50 pulses per second. During testing, the subjects were seated on a specially constructed chair that incorporated a foot pedal attached to a piezoelectric force transducer to measure plantar flexion force. Stimulation amplitude was increased slowly until 4 thresholds (sensory, motor, pain, and maximal pain) were sequentially achieved. At each threshold, stimulus peak voltage, peak current, and phase charge used to elicit that threshold were recorded. Plantar flexion force was also recorded when stimulation was sufficient to achieve pain and maximal pain thresholds. Results: The peak voltage, peak current, and phase charge needed to elicit the 4 thresholds did not differ between women and men at any threshold examined (2-factor ANOVA; Newman-Keuls post hoc tests). However, significantly higher stimulus peak voltage, peak current, and phase charge were recorded for each of the 4 thresholds (sensory, motor, pain, and maximal pain tolerance) for both groups. The plantar flexion force elicited by electrical stimulation was significantly lower in female subjects than in male subjects at both pain threshold and maximally tolerated stimulation levels. Conclusions: Gender can influence the magnitude of electrically induced plantar flexion contraction force. We hypothesize that females may require longer conditioning periods to achieve therapeutic levels of muscle contraction.