Novel perspective on contractile properties and intensity‐dependent verification of force–frequency relationship during neuromuscular electrical stimulation

Purpose The aims of the present study were: (a) to examine the effect of the stimulus intensity on force‐frequency and torque fluctuation–frequency relationships during Neuromuscular electrical stimulation; and (b) to identify a novel parameter that can be used to evaluate muscle contractile properties. Methods: Electrically elicited joint torque involving the quadriceps femoris muscle was recorded during neuromuscular electrical stimulation at two different stimulus intensities in 19 healthy men. Stimulation frequencies were set at 5–40 Hz with a duration of 10 s. Evoked joint torque was compared among all stimulation frequencies between the two stimulus intensities (68 and 113 V). The torque fluctuation at each stimulation frequency as the change in the contraction pattern was also compared between the intensities. Torque and torque fluctuation were normalized at each frequency by the largest torque or torque fluctuation, respectively. We extracted a novel parameter: the arrival point of tetanic contraction based on force‐frequency and torque fluctuation‐frequency curves. Results: There were significant differences in normalized torque at 5–25 and 40 Hz and in normalized torque fluctuation at 15–30 and 40 Hz between the two stimulus intensities. Extracted parameters showed no significant difference between the intensities. Conclusion: The results suggest that force–frequency relationships during neuromuscular electrical stimulation are influenced by the intensity of stimulation applied to the quadriceps femoris muscle. However, we consider that it is possible to simultaneously evaluate contractile properties using the novel parameter.