Fatigue-independent alterations in muscle activation and effort perception during forearm exercise: role of local oxygen delivery

Abstract

The oxygen-conforming response (OCR) of skeletal muscle refers to a downregulation of muscle force for a given muscle activation when oxygen delivery (O2D) is reduced, which is rapidly reversed when O2D is restored. We tested the hypothesis that the OCR exists in voluntary human exercise and results in compensatory changes in muscle activation to maintain force output, thereby altering perception of effort. In eight men and eight women, electromyography (EMG), oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb), forearm blood flow (FBF), and task effort awareness (TEA) were measured. Participants completed two nonfatiguing rhythmic handgrip tests consisting of 5-min steady state (SS) followed by two bouts of 2-min brachial artery compression to reduce FBF by ~50% of SS (C1 and C2), separated by 2 min of no compression (NC1) and ending with 2 min of no compression (NC2). When FBF was compromised during C1, EMG/Force (1.58 ± 0.39) increased compared with SS (1.31 ± 0.33, P = 0.001). However, EMG/Force was not restored upon FBF restoration at NC1 (1.48 ± 0.38, P = 0.479), consistent with C1 evoking skeletal muscle fatigue. When FBF was compromised during C2, EMG/Force increased (1.73 ± 0.50) compared with NC1 (1.48 ± 0.38, P = 0.013). EMG/Force returned to NC1 levels during NC2 (1.50 ± 0.39, P = 0.016), consistent with an OCR in C2. TEA (SS 2.2 ± 2.3, C1 3.9 ± 2.5, NC1 3.4 ± 2.7, C2 4.6 ± 2.7, NC2 3.9 ± 2.8) mirrored changes in EMG. It is noteworthy that during the second compromise and then restoration of muscle oxygenation EMG and TEA were rapidly restored to precompromise levels. We interpreted these findings to support the existence of an OCR and its ability to rapidly modify perception of effort during voluntary exercise. NEW & NOTEWORTHY In healthy individuals, when force output is maintained during rhythmic handgrip exercise, muscle activation and perception of effort rapidly increase with compromised muscle oxygen delivery (O2D) and then return to precompromised levels when muscle O2D is restored. These findings suggest that an oxygen-conforming response (OCR) exists and is able to modify perception of effort during voluntary exercise. Therefore, similar to fatigue, an OCR may have implications for exercise tolerance.

Elucidating the role of KIR channels in rapid vasodilatation during transition in exercise intensity and different levels of muscle fibre recruitment

“…However, the difference in EMG amplitudes was maintained at similar levels to control trials with significantly higher muscle activation during high versus low J Physiol 598.20 fibre recruitment exercise. Partial occlusion of blood flow is previously shown to increase muscle activity (Drouin et al 2019) which can explain the observed increase in EMG amplitudes during BaCl 2 administration. Furthermore, venous [K + ] and forearm K + efflux were higher in both exercise patterns compared to resting values, as expected.…”

“…Partial occlusion of blood flow is previously shown to increase muscle activity (Drouin et al . 2019) which can explain the observed increase in EMG amplitudes during BaCl 2 administration. Furthermore, venous [K + ] and forearm K + efflux were higher in both exercise patterns compared to resting values, as expected.…”

Near-infrared diffuse correlation spectroscopy tracks changes in oxygen delivery and utilization during exercise with and without isolated arterial compression

“…The goal was to apply enough compression pressure to reduce radial artery blood velocity by 40 -50% and maintain that pressure for 2 min. The use of this arterial compression model to reduce blood velocity and shear rate has been utilized by others (8,17,37,38,44). Although others have used a pneumatic piston or linear actuator device to apply constant pressure to the brachial artery during the compression at rest (37,38), this approach is less feasible during exercise because of subject movement (38).…”

“…This model has previously been used at rest and during rhythmic handgrip exercise to reduce shear rate (8,37,38,44) and downstream oxygen delivery . The major advantage of this approach over the more conventional cuff occlusion technique is that arterial oxygen delivery is reduced without disrupting venous return, which can alter local skeletal muscle hemodynamics as well as systemic hemodynamics via the metaboreflex (1,10,17,34). We hypothesized that, during compression, NIR-DCS would effectively track reductions in skeletal muscle microvascular perfusion (measured by DCS) and compensatory reductions in tissue saturation (i.e., increased oxygen extraction) (measured by NIRS) to maintain skeletal muscle oxygen consumption throughout exercise.…”

KIR channel activation links local vasodilatation with muscle fibre recruitment during exercise in humans

“…Although the reasons for this are unclear, we have previously shown that intra-arterial administration of BaCl 2 does not affect maximal voluntary contraction force (Crecelius et al 2013a). Interestingly, Drouin et al (2019) recently reported rapid increases in muscle activation when blood flow to the exercising tissue was partially occluded. Thus, it is possible that compromised blood flow and oxygen delivery during BaCl 2 administration resulted in greater muscle activation for a given workload.…”