Muscle contraction force conforms to muscle oxygenation during constant‐activation voluntary forearm exercise

Abstract

In electrically stimulated skeletal muscle, force production is downregulated when oxygen delivery is compromised and rapidly restored upon restoration of oxygen delivery in the absence of cellular disturbance. Whether this 'oxygen-conforming' response of force occurs and is exercise intensity dependent during stable voluntary muscle activation in humans is unknown. In 12 participants (six female), handgrip force, forearm muscle activation (EMG), muscle oxygenation and forearm blood flow (FBF)were measured during rhythmic handgrip exercise at forearm EMG achieving 50, 75 or 90% critical impulse (CI). Four minutes of brachial artery compression to reduce FBF by ∼60% (Hypoperfusion) or sham compression (adjacent to artery; Control) was performed during exercise. Sham compression had no effect. Hypoperfusion rapidly reduced muscle oxygenation at all exercise intensities, resulting in contraction force per muscle activation (force/EMG) progressively declining over 4 min by ∼16% at both 75 and 90% CI. No force/EMG decline occurred at 50% CI. Rapid restoration of muscle oxygenation after compression was closely followed by force/EMG such that it was not different from Control within 30 s for 90% CI and after 90 s for 75% CI. Our findings reveal that an oxygen-conforming response does occur in voluntary exercising muscle in humans. Within the exercise modality and magnitude of fluctuation of oxygenation in this study, the oxygen-conforming response appears to be exercise intensity dependent. Mechanisms responsible for this oxygen-conforming response have implications for exercise tolerance and warrant investigation.

Priming exercise accelerates oxygen uptake kinetics during high-intensity cycle exercise in middle-aged individuals with type 2 diabetes

“…Consequently, the reliance on [PCr] degradation and glycogenolysis would be reduced, attenuating the rate of fatigue development and thus, delaying motor unit recruitment ( DiMenna et al, 2010 ). In addition, this altered muscle activation response to priming exercise concomitant with the elevated TOI is consistent with the ‘oxygen-conforming’ effect, which has been demonstrated under involuntary and voluntary small muscle activation ( Fitzpatrick et al, 1996 ; Drouin et al, 2022 ), although the mechanisms governing the oxygen-conforming response remain to be elucidated.…”

The ‘normal’ adjustment of oxygen delivery to small muscle mass exercise is not optimized for muscle contractile function

“…To create conditions where cellular disturbance is attenuated, participants completed rhythmic handgrip exercise targeting muscle activity to produce contractions at 90% critical power. To identify this target activation, as per Drouin et al (2022), ten 2 s contractions at 90% critical power were completed with 30 s of rest between contractions. The average muscle activity of the 10 handgrip contractions required to achieve 90% critical power was used as the muscle activation maintenance target.…”

“…Flexor digitorum superficialis muscle activation was recorded via four mini wireless bipolar electrodes (Delsys, Natick, MA, USA) placed according to guidelines reported elsewhere (Drouin et al, 2019) -using guidance from Bickerton et al (1997). Muscle activity was wirelessly recorded via a Delsys Trigno Base (Delsys) and then transmitted through a Trigno EMG 1−16 adapter (ADInstruments, Colorado Springs, CO, USA) and PowerLab interface cable to be recorded in Labchart 7 software (ADInstruments) at 2000 Hz per channel.…”

“…This current perspective, however, does not consider the implication of reduced or increased oxygen delivery on muscle contractile function. First, to maintain force of contraction with reduced oxygen delivery, there must be a compensatory increase in muscle activation J Physiol 601.4 (Drouin et al, 2019;Fulco et al, 1996) -this results in increased metabolic disturbance (Fulco et al, 1996;Hogan et al, 1999). If muscle activation does not increase, force of contraction conforms to oxygen delivery (Drouin et al, 2022;Fitzpatrick et al, 1996;Luu & Fitzpatrick, 2013;Wright et al, 2000) -but cellular disturbance is prevented (Hogan et al, 1996).…”

“…To test the hypothesis that the 'normal' adjustment of oxygen delivery is optimal for muscle performance we used remote skeletal muscle metaboreflex activation to increase arterial blood pressure and thereby increase exercising forearm muscle perfusion (Tschakovsky & Hughson, 1999). As a measure of exercising muscle excitation-contraction coupling performance (force for a given muscle activation) we quantified the ratio of forearm muscle EMG relative to contraction force during constant rhythmic forearm handgrip exercise (Drouin et al, 2019(Drouin et al, , 2022) at 90% of critical power. We speculated that if the normal adjustment in exercising forearm muscle oxygen delivery was not optimal, we would observe an improvement in muscle force relative to EMG during metaboreflex-mediated hyperperfusion.…”