Metabolic instability vs fibre recruitment contribution to the  slow component in different exercise intensity domains

This study focused on the steady-state phase of exercise to evaluate the relative contribution of metabolic instability (measured with NIRS and haematochemical markers) and muscle activation (measured with EMG) to the oxygen consumption ($$) slow component ($$) in different intensity domains. We hypothesized that (i) after the transient phase, $$, metabolic instability and muscle activation tend to increase differently over time depending on the relative exercise intensity and (ii) the increase in $$ is explained by a combination of metabolic instability and muscle activation. Eight active men performed a constant work rate trial of 9 min in the moderate, heavy and severe intensity domains. $$, root mean square by EMG (RMS), deoxyhaemoglobin by NIRS ([HHb]) and haematic markers of metabolic stability (i.e. [La^-], pH, HCO₃^-) were measured. The physiological responses in different intensity domains were compared by two-way RM-ANOVA. The relationships between the increases of [HHb] and RMS with $$ after the third min were compared by simple and multiple linear regressions. We found domain-dependent dynamics over time of $$, [HHb], RMS and the haematic markers of metabolic instability. After the transient phase, the rises in [HHb] and RMS showed medium-high correlations with the rise in $$ ([HHb] r = 0.68, p < 0.001; RMS r = 0.59, p = 0.002). Moreover, the multiple linear regression showed that both metabolic instability and muscle activation concurred to the $$ (r = 0.75, [HHb] p = 0.005, RMS p = 0.042) with metabolic instability possibly having about threefold the relative weight compared to recruitment. Seventy-five percent of the dynamics of the $$ was explained by [HHb] and RMS.