Force At A Set Voluntary Forearm Muscle Activation Conforms To Muscle Oxygenation At High Intensities

Abstract

In electrically stimulated muscle, the oxygen conforming response of skeletal muscle is characterized by a down regulation of muscle force for a given electromyography (EMG) when oxygen delivery is reduced, which is rapidly (within 60-sec) reversed when oxygen delivery is restored. Whether this oxygen conforming response exists in voluntary human exercise is unknown. PURPOSE: To test the hypotheses that oxygen conforming 1) occurs during voluntary handgrip exercise 2) is exercise intensity dependent 3) is independent of sex. METHODS: In 6-men and 6-women, handgrip force, forearm EMG, oxyhemoglobin (O2Hb, %) and mean blood velocity (MBV, cm/s) were measured while participants completed 15-min of rhythmic handgrip exercise targeting a forearm EMG equal to that required to achieve 50, 75 and 90% of critical power handgrip force output. At minute 7, 4-min of brachial artery compression was either completed to reduce MBV by ~50% (experimental, EX) or compression was completed next to the brachial artery to simulate compression without affecting MBV (sham, SH). Exercise continued for 4-min after ending compression. RESULTS:Compression: At 4-min, forearm MBV was reduced in all intensities (50%, EX: 11.62 ± 1.44, SH: 24.57 ± 3.07; 75%, EX: 13.64 ± 1.65, SH: 34.35 ± 4.82; 90%, EX:16.71 ± 2.18, SH: 38.63 ± 3.58, all P < 0.05), O2Hb was reduced in 50% (EX: 62.68 ± 5.10, SH: 70.97 ± 4.63, P < 0.05) and 90% (EX: 49.53 ± 3.98, SH: 70.80 ± 2.93, P < 0.05) intensities but not the 75% intensity (EX: 54.95 ± 4.79, SH: 64.37 ± 3.59, P = 0.08). The amount of force produced per EMG (Force/EMG) was reduced in the 75% (EX: 0.90 ± 0.03, SH: 1.01 ± 0.02, P < 0.05) and 90% (EX: 0.86 ± 0.27, SH: 1.01 ± 0.02, P < 0.05) intensities but not the 50% (EX: 0.97 ± 0.03, SH: 1.00 ± 0.02, P > 0.05) intensity. End compression: Forearm MBV and O2Hb was restored within 30-sec for all intensities (90%, EX: 55.98 ± 11.15; SH: 70.72 ± 9.93, P = 0.1289) and force/EMG was restored within 60-sec for 90% (EX: 0.98 ± 0.02; SH: 1.02 ± 0.02, P = 0.279) and 75% (EX: 0.98 ± 0.02; SH: 0.98 ± 0.02, P = 0.929). No sex differences were observed. CONCLUSION: 1) The rapid restoration of force/EMG (within 60-sec) upon restoration of O2Hb is consistent with the existence of an oxygen conforming response in voluntary human exercise that 2) exists only at higher intensities (75% and 90%). 3) This response is independent of sex.