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1 Division of Cardiology, Department of Medicine, and 3 Department of Radiology, Center for Nuclear Magnetic Resonance Research, Pennsylvania State University College of Medicine, The Milton S. Hershey Medical Center, Hershey 17033; and 2 Lebanon Veterans Affairs Medical Center, Lebanon, Pennsylvania 17042
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, Pi,
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
, Pi, 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.
autonomic nervous system; exercise pressor reflex; blood pressure; nuclear magnetic resonance; handgrip
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