Oxygen supply/uptake mismatch during incremental stimulation of the human tibialis anterior

Background Local oxygen supply/uptake mismatch (O 2 M) of active muscles may be influenced by the effectiveness of microvascular adaptations to metabolic demand. The aim of this work was to provide a novel, objective method to measure O 2 M through incremental stimulation. Methods The tibialis anterior of 20 subjects was electrically stimulated at the main motor point. The stimulation protocol involved five single twitches at maximal stimulus amplitude ( V max = full motor unit recruitment) and five 90-s steps with increasing amplitude and frequency of the intermittent stimulation. Changes in force, oxygenated Hb (O 2 Hb) and deoxygenated Hb (HHb) were recorded. Contraction time (CT) of single twitches, fatigue index (FI) and O 2 M were calculated. Results F-statistics identified two groups of muscles, one with FI < 0.69 (group F, more fatiguable) and one with FI > 0.69 (group R, less fatiguable). Group R showed significantly lower CT, higher basal O 2 Hb, lower ΔHHb and lower O 2 M with respect to group F. Conclusion On the basis of the relationship between O 2 M and FI, the proposed method distinguished two different functional capacities of human TA muscles. This tool could be applied to estimate the kinetics of aerobic adaptation to increasing metabolic demand in muscles affected by myopathies or subjected to rehabilitation or training programmes.