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.
In a single bout maximal effort isometric forearm handgrip exercise test (MXT), contraction impulse exhibits exponential decay to an asymptote equivalent to critical impulse (CI). It is unknown whether oxygen delivery (O2del) and consumption (V ̇O2) achieved at CI are maximal. Healthy men participated in a randomized crossover trial at Queen’s University (Kingston, ON) between October 2017-May 2018. Participants completed a MXT and forearm incremental exercise test to limit of tolerance (IET-LOT) (7 completed MXT followed by IET-LOT vs. 4 completed IET-LOT followed by MXT) within a 2 week period. Data is presented as mean±standard deviation. Maximal forearm blood flow (FBF) and O2del were not different in the 11 men (21±2.5years) between MXT and IET-LOT (FBF: 473.8±132.2 mL/min vs. 502.3±152.3 mL/min;P=0.482,ηp2=0.015; O2del: 85.2±23.5mL/min vs. 92.2±37.0mL/min;P=0.456,ηp2=0.012). However, MXT resulted in greater maximal V ̇O2 than IET-LOT (44.5±15.2mL/min>36.8±11.4mL/min;P=0.007,ηp2=0.09), due to greater oxygen extraction (54.0±10.0%>44.4±8.6%;P=0.021,ηp2=0.185). As CI was 88.6%±8.2% of IET-LOT contraction impulse, maximal O2 cost of contractions in MXT was greater than IET-LOT (0.45±0.14mL/min/Ns>0.33±0.09mL/min/Ns;P<0.001,ηp2=0.166). In healthy men, MXT identifying CI results in similar peak oxygen delivery but greater peak V ̇O2 via increased extraction compared to an IET-LOT, indicating increased oxygen cost. MXT-CI may better estimate maximal V ̇O2 than traditional IET-LOT for this exercise modality.
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