Russ, D. W., Bartholomew, D. M., Towse, T. F., & Kent-Braun, J. A. (2003). Influence of duty cycle on sex differences in fatigue during maximum-effort exercise. Medicine and Science in Sports and Exercise, 35(5), Supplement abstract 802.

This study determined if sex differences in fatigue are due to the protocol of the investigation. Men and women were exposed to four fatigue protocols:

• sustained (100% duty cycle) maximum voluntary contractions for 3.5 minutes;
• maximum voluntary contractions with 5s/5s contract/relax cycle at 50% duty cycle performed for four minutes;
• maximum voluntary contractions with 5s/5s contract/relax cycle, performed during ischemia (a condition where the oxygen-rich blood flow to a part of the body is restricted);
• maximum voluntary contractions with 7s/3s contract/relax cycle at 70% duty cycle for five minutes.

The fatigue index of MVCpost/MVCpre and changes in central activation ratio (CAR) were calculated for all protocols.

Ischemia and increasing duty cycle intensity increased fatigue and central activation failure in both genders. A sex difference in favor of women was found in protocols A and B but not in C or D. Increasing the duty cycle intensity from 50% to 70% eliminated the gender difference as did producing an ischemic condition.

"Interestingly, a sex difference was present with A (100% duty cycle), consistent with the possibility that force fell below a threshold for occlusion in women, but not men, due to 33% lower MVCs in women than men. This would allow greater perfusion, attenuating fatigue in women. This force threshold may be a minor factor when sufficient rest time between contractions is present." (p. S145)

Implication. Perfusion/reperfusion during exercise may play an important role in sex differences in fatigue.

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