Ischemic exercise and the muscle metaboreflex

In exercising muscle, interstitial metabolites accumulate and stimulate muscle afferents. This evokes the muscle metaboreflex and raises arterial blood pressure (BP). In this report, we examined the effects of tension generation on muscle metabolites and BP during ischemic forearm exercise in humans. Heart rate (HR), BP, P(i), H₂PO₄/^-, and pH (³¹P-NMR spectroscopy) data were collected in 10 normal healthy men (age 23 ± 1 yr) during rhythmic handgrip exercise. After baseline measurements, the subjects performed rhythmic handgrip for 2 min. At 2 min, a 250-mmHg occlusion cuff was inflated, and ischemic handgrip exercise was continued until near fatigue (Borg 19). Measurements were continued for an additional 30 s of ischemia. This protocol was performed at 15, 30, 45, and 60% of the subjects' maximum voluntary contraction (MVC) in random order. As tension increased, the time to fatigue decreased. In addition, mean arterial pressure and HR were higher at 60% MVC than at any of the other lower tensions. The NMR data showed significantly greater increases in H₂PO₄/^-, P(i), and H⁺ at 60% than at 15 and 30% MVC. Therefore, despite the subjects working to the same perceived effort level, a greater reflex response (represented by BP and HR data) was elicited at 60% MVC than at any of the other ischemic tensions. These data are consistent with the hypothesis that, as tension increases, factors aside from insufficient blood flow contribute to the work effect on muscle metabolites and the magnitude of the reflex response.