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

What is the central question of this study? In electrically stimulated skeletal muscle, force production is downregulated when oxygen delivery is compromised and rapidly restored upon oxygen delivery restoration. Whether 'oxygen conforming' of force production occurs during voluntary muscle activation in humans and whether it is exercise intensity dependent remains unknown. What is the main finding and its importance? Here, we show in humans that force at a given voluntary muscle activation does conform to a decrease in oxygen delivery and recovers rapidly and completely with restoration of oxygen delivery. This oxygen-conforming response of contraction force appears to happen only at higher intensities.

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.