Methodological issues influence determination of critical force during intermittent exercise: Time to task failure vs. contraction time

Ansdell et al. (2019) have conducted an interesting study aiming to assess whether a sex difference in exercise tolerance and fatigue exists when exercise is prescribed relative to the intensity–duration relationship. In total, 20 participants (10 male and 10 female) were included, who underwent a large number of tests and measurements: four intermittent isometric knee extension trials to task failure to determine critical force (CF) and the curvature constant (W′); two fatiguing tasks at +10 and −10% relative to CF; neuromuscular function (maximal voluntary contraction, MVC; voluntary activation, VAMNS; quadriceps potentiated twitch force, Qtw.pot); near-infrared spectroscopy (muscle oxygenation); and non-invasive haemodynamic monitoring (mean arterial blood pressure, heart rate and cardiac output). The main results were that females demonstrated a greater relative CF (24.7 ± 2.5 vs. 20.8 ± 2.3% MVC), greater exercise tolerance during intermittent exercise performed at 110% CF (3,742 ± 1,035 vs. 1,826 ± 765 s) and a lower degree of contractile impairment compared to males. Although we fully acknowledge that both rationale and hypotheses were well described and grounded, there is an important methodological issue related to the determination of CF during intermittent exercise that we would like to discuss. Ansdell et al. (2019) used a linear regression between the force impulse at task failure from the four submaximal trials against the time to task failure to estimate CF (the slope of the regression) and W′ (the y-intercept) (Ansdell et al. 2019; see Fig. 1A). This methodological approach was based on two previous studies conducted by Burnley et al. (2009, 2012). However, Burnley et al. (2009, 2012) estimated CF and W′ by plotting the torque impulse against contraction time (Burnley 2009; see Fig. 3A, and Burnley et al. 2012; see Fig. 1B). We did not have the original data but we reconstructed the intensity–duration relationship based on the mean data of males shown in Table 1 of the study of Ansdell et al. (2019). These data were reconstructed using two different approaches: (i) force impulse against time to task failure (Ansdell et al. 2019) and (ii) force impulse against contraction time (Burnley et al. 2009, 2012) (Figure 1). Although W′ was the same (23 415 N s), CF expressed as absolute (161 N) and relative values (22.7% MVC) was substantially lower via the method utilized by Ansdell et al. (2019) compared to the method utilized by Burnley et al. (2012) (CF = 268 N; 37.8% MVC). Using these ‘new’ parameters (i.e. CF and W′), we estimated the impulse accumulated above CF during the four submaximal trials (i.e. 326, 361, 396 and 432 N). The method utilized by Ansdell et al. (2019) produced a different value of impulse accumulated above CF for each submaximal trial, with all values being substantially greater than W′. However, the impulse accumulated above CF using Burnley’s method was similar among the different submaximal trials and close to W′ (Table 1). Similar results were found for females (data are not shown). Below, we present evidence indicating that the method used by Ansdell et al. (2019) underestimated the CF, precluding an appropriate comparison between males and females during intermittent isometric exercise at intensities above and below CF.