Minyoung Kwak, Pasquale J. Succi, Brian Benitez, Jody L. Clasey, FACSM, Haley C. Bergstrom. University of Kentucky, Lexington, KY.

BACKGROUND: Critical force (CF) reflects the highest force that can be maintained for a very long time without fatigue. Physical working capacity at the rating of perceived exertion (PWCRPE) is the highest force output that can be maintained for a very long time without an increase in RPE. This study compared the force, time to exhaustion (Tlim), and muscle oxygen saturation responses (SmO2) of men and women during continuous, isometric, handgrip holds to failure (HTF) at two fatigue thresholds, CF and PWCRPE. METHODS: The CF and PWCRPE were estimated for 10 men (Mean ± SD: Age 24.8 ± 4.1 yrs) and 10 women (Age 26.5 ± 3.5 yrs) from handgrip HTF at 4 different submaximal intensities (% maximum voluntary isometric contraction [MVIC]) force. CF (kg) was the slope coefficient of the total work (Wlim) for the 4 holds plotted as a function of Tlim. RPE was recorded every 10s during the 4 holds. PWCRPE (kg) was the y-intercept of the slope coefficients for the RPE vs. time relationships plotted as a function of force. During CF and PWCRPE HTF, Tlim was recorded and SmO2 responses were measured from the flexor digitorum superficialis with near-infrared spectroscopy. Analyses included 2-way mixed model ANOVAs and polynomial regression (p<0.05). RESULTS: For absolute force, there was a main effect (p= 0.027) for sex (collapsed across threshold: Men 7.3 ± 2.8kg, Women 5.5 ± 1.8kg). There were no sex x threshold interactions (p=0.531-0.747) or main effects for relative force (collapsed across sex and threshold, %MVIC= 18.9 ± 6.4%) or Tlim (collapsed across sex and threshold, Tlim= 685.6 ± 386.0s). There were negative, quadratic relationships between SmO2 and time for CF (R2=0.852, p=0.013, SmO2Δ= -6.7 ± 17.1%) and PWCRPE (R2=0.910, p<0.001, SmO2Δ= -5.8 ± 16.6%) for the men, and a negative, cubic response for PWCRPE for the women (R2=0.885, p=0.016, SmO2Δ= -0.7 ± 10.9%). However, there was no relationship between SmO2 and time for CF for women (r2=0.076, p>0.05, SmO2Δ= -0.5 ± 8.6). CONCLUSIONS: The lack of differences in relative force or in the sustainability (Tlim) of CF and PWCRPE indicated that these thresholds reflect a similar intensity for the men and women. Interestingly, the negative, quadratic patterns of responses for SmO2 at CF and PWCRPE for the men, but no change at CF and a small (<1%) decrease in SmO2 at the PWCRPE for the women, suggested that men may desaturate more during a fatiguing handgrip task.

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