Author(s): CASSOLI, I., PLACE, N., MILLET, G.Y., Institution: UNIVERSITY OF LAUSANNE, Country: SWITZERLAND, Abstract-ID: 2453

INTRODUCTION:
Neuromuscular fatigability can be defined as an acute reduction in maximal voluntary contraction (MVC) force, englobing neural and intramuscular factors. When assessed in a laboratory environment, the origin and extent of fatigability are known to be sex-dependent. Recently, a portable device (Myocene(R)) has been developed for on-field evaluation, which allows quantifying potential sex difference in fatigability outside the laboratory. This study aimed at comparing a field device to a laboratory standard (femoral nerve supramaximal stimulation) in characterizing knee extensor fatigability after repeated contractions in men vs. women.
METHODS:
Twenty young, healthy volunteers (10F) performed an intermittent isometric fatigue protocol (5s/5s, 50% maximal voluntary contraction (MVC) force) until failure or for a maximum of 120 repetitions, followed by a 1-min sustained MVC with the knee extensors. Knee extensor neuromuscular function including MVC force, maximal voluntary activation level (VAL) derived from the twitch interpolation technique, force response to 100 and 10 Hz paired stimuli (PS100 and PS10) on relaxed muscles and estimates of prolonged low-frequency force depression (PLFFD) (PS10/PS100 ratio vs. the Powerdex index(R), a submaximal-intensity low-to-high frequency evoked force ratio) were quantified before, immediately after and 10 min after exercise. Measurements were compared between the portable device equipped with a built-in stimulator and the laboratory isometric chair equipped with a strain gauge and coupled with supramaximal femoral nerve stimulation. Linear mixed models (sex x time x Device) were used to analyze MVC force, VAL, PS100 and PLFFD.
RESULTS:
The protocol induced a significant MVC drop in men (-24.7 +- 13.2%, p < 0.001), while the reduction was not significant in women (-13.6 +- 14.3%, p = 0.064). While voluntary activation decreased similarly between sexes, PS100 dropped only in men (-25.9 +- 11.2%, p<0.001) but not in women (-19.4 +- 8.1%, p = 0.725). When expressed as relative changes from baseline, PLFFD dynamics were similar between devices (Time x Device interaction, p = 0.90), although the PS10/PS100 estimated a significantly greater magnitude of fatiguability (-24.6 +- 14.6%) compared to the Powerdex(R) (-18.7 +- 11.2 %, Device effect, p = 0.004), with no sex difference.
CONCLUSION:
Our findings show that the lower fatigability in women vs. men can be similarly tracked by the portable device and the laboratory standard. Thus, despite absolute offsets in raw values, the field device provides a valid estimation of fatigability and is sensitive enough to detect sex difference in fatigability.