Electrically-evoked force loss of the knee extensors is equivalent for young and old females and males.

Data are scant on sex-related differences for electrically-evoked contractions, which assess intrinsic contractile properties while limiting spinal and supraspinal adaptations to mitigate fatigue. Furthermore, the few studies that exist use stimulus frequencies considerably higher than the natural motor unit discharge rate for the target force. The purpose of this study was to compare force loss to electrically-evoked contractions at a physiological stimulus frequency among young females (n=12), young males (n=12), old females (n=11) and old males (n=11). The quadriceps of the dominant leg were fatigued by 3 min of intermittent transcutaneous muscle belly stimulation (15 Hz stimulus train to initially evoke 25% of maximal voluntary force). Impairment of tetanic contractile impulse (area under the curve) did not differ between sexes for young or old adults or between age groups, with a pooled value of 55.2±12.4% control at the end of fatigue. These data contrast with previous findings at 30 Hz, when the quadriceps of females had greater fatigue resistance than males for young and old adults. The present results highlight the impact stimulus frequency has on intrinsic fatigability of muscle; the findings have implications for future fatigue paradigms and treatment approaches when utilizing electrical stimulation for rehabilitation. Novelty bullets: • Fatigue was not different between sexes with a stimulation frequency comparable to discharge rates during voluntary contractions • These results highlight that stimulus frequency not only influences fatigue development but also between-group differences.