Forearm Blood Flow and Motor Unit Recruitment during Fast and Slow Ramp Exercise

Abstract

During a progressive exercise test, the relationship between oxygen uptake and work rate is dependent on the slope of the ramp forcing function. It is unclear however, the extent that muscle blood flow may be affected by either fast or slow ramp exercise. PURPOSE: To examine the forearm blood flow (FBF) response to dynamic handgrip exercise during fast (FR) and slow (SR) ramp exercise to fatigue. METHODS: Six subjects (5 male and 1 female; age 28 ± 3 yrs (±sem)) performed dynamic handgrip exercise in the supine position on two separate occasions. The forcing function consisted of fast ramp (FR, 0.50 kg/min) or slow ramp (SR, 0.25 kg/min) exercise until the contraction rate (30/min) could no longer be maintained. Maximal FBF was determined by post occlusive reactive hyperemia (PORH). During exercise, FBF was measured continuously by Doppler ultrasound and expressed relative to forearm volume. Forearm flexor muscle activity was measured using surface electromyography (EMG) and analyzed in the time (iEMG, % of maximum voluntary contraction, MVC) and frequency domains (MdPF; % change from unloaded contractions). RESULTS: Fatigue was associated with a higher load and shorter exercise duration during FR (6.5 ± 0.5 kg; 686 ± 70s) than SR (5.4 ±0.4 kg; 1408 ± 143 s, p<0.05). HR and MAP increased (p<0.05) from rest to end exercise during FR and SR; no difference in HR or MAP was observed between ramp protocols. FBF was similar between FR and SR protocols at end exercise (FR, 14.1 ±1.6 ml/min/100ml; SR, 14.5 ±1.7 ml/min/100ml). FBF at end exercise, expressed as a percent of PORH, was similar between protocols (FR, 30.9 ± 4.1%; SR, 31.8 ± 4.5%) suggesting that blood flow was not limited during either FR or SR exercise. In addition, there was no difference in either iEMG (FR, 15.9 ± 3.2% MVC; SR, 12.8 ± 2.4% MVC) or MdPF (FR, −15.6 ± 5.6%; SR, −11.3 ± 4.4%) at end exercise between ramp tests. However, motor unit recruitment was closely coupled to exercise load since the ratio of iEMG to end exercise WR was similar between FR (5.3 ± 1.3) and SR(5.4± 1.3). CONCLUSION: In spite of the differences in the load and work rate achieved at fatigue during FR and SR handgrip exercise, no difference was observed between FBF, HR or MAP suggesting that the slope of the ramp forcing function does not alter cardiovascular adaptations to dynamic handgrip exercise.