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1 Department of Exercise Science, Musculoskeletal Research Laboratory, Syracuse University, Syracuse, NY, USA
2 Department of Exercise Science, Musculoskeletal Research Laboratory, Syracuse University, Syracuse, NY, USA; Department of Physical Medicine and Rehabilitation, SUNY Upstate Medical University, Syracuse, NY, USA
* To whom correspondence should be addressed. E-mail: bcclar01{at}syr.edu.
The purposes of this study were 1) to evaluate sex differences in back extensor endurance capacity during isometric and isotonic muscular contractions; 2) to determine the relationship between absolute load and endurance time and 3) to compare males (n = 10, 22.4 ± 0.69 [SE] years) and females (n = 10, 21.7 ± 1.07 years) on neuromuscular activation patterns and median frequency shifts in the electromyogram power spectrum of the lumbar and hip extensor muscles during fatiguing submaximal isometric trunk extension exercise. Subjects performed isotonic and isometric trunk extension exercise to muscular failure at an intensity of 50% maximum voluntary contraction force. Females exhibited a longer endurance time during the isometric task when compared to males (146.0 ± 10.9 vs 105.4 ± 7.9 seconds), but there was no difference in endurance performance during the isotonic exercise (24.3 ± 3.4 vs 24.0 ± 2.8 repetitions). Absolute load was significantly related to isometric endurance time in the pooled sample (R2 = 0.34), but not when males and females were analyzed separately (R2 = 0.05 and R2 = 0.04, respectively). Electromyographic data showed no differences in neuromuscular activation patterns; however, sex differences in median frequency (MF) shifts were observed. Females demonstrated a similar fatigability in the biceps femoris and lumbar extensors, whereas in males, the fatigability of the lumbar musculature was more pronounced than the biceps femoris. Additionally, the MF of the lumbar extensors demonstrated a greater association with endurance time in males than females (R2 =0.45 vs R2 = 0.19). These findings suggest sex differences in muscle fatigue are influenced by muscle contraction type, and frequency shifts in the electromyogram signal, but not by alterations in the synergistic activation patterns.
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