Muscular reflex stimuli to the cardiovascular system during isometric contractions of muscle groups of different mass.

Abstract

SUMMARY1. The cardiovascular responses to voluntary isometric contractions performed by human subjects are determined by the proportion of maximal tension achieved by the muscles contracting, and not by the mass of the contracting muscles, nor by the absolute tension achieved (Lind & McNicol, 1967; confirmed here). When two or more muscle groups contract simultaneously at different relative tensions, the increments in heart rate and blood pressure are the same as when the muscle group at the higher relative tension contracts alone at that tension (Lind & McNicol, 1967). It is known that there are both central and reflex stimuli to the cardiovascular system in exercise, and the present study examines whether the muscular reflex stimuli are related to the proportion of maximal tension achieved or to the mass of contracting muscle. 3. When human subjects perform isometric contractions, a pressor response can be maintained beyond the conclusion of the exercise by occluding muscle blood flow. This response is generally attributed to a reflex set up in the muscle by the action of chemical factors on afferent nerves. When comparable pressor responses were evoked by comparable proportional efforts with either the whole hand or the little finger, it was found here that the pressor responses remaining during the period of post-exercise occlusion were greater when the greater mass of muscle had been exercised.4. It is concluded that the muscular reflex drive in isometric exercise is related to the bulk of contracting muscle.

The human pressor response during and following voluntary and evoked isometric contraction with occluded local blood supply.

“…This conclusion has been drawn by others following experiments of this nature (Lind et al 1966; McCloskey McCloskey & Streatfeild, 1975;Mitchell et al 1981) and the response is believed to be mediated via stimulation of unmyelinated group III and IV afferents (Coote et al 1971).…”

“…In the experiments of Lind, McNicol & Donald (1966), McCloskey & Streatfeild (1975) and Mitchell et al (1981) isometric contractions were performed under conditions of circulatory arrest and blood pressure progressively increased for the duration of the contraction. After cessation of muscular activity but with the circulation still arrested, blood pressure showed a small rapid fall to a lower though still elevated level.…”

Cardiorespiratory Response to Dynamic and Static Leg Press Exercise in Humans

“…If we exclude the effect of the one-or two-legged issue which determines the blood pressure and HR responses to static contraction, the size of active muscle mass or the percentage of MVC should be considered here. Many studies have discussed this issue comparing the responses in different parts of the body or only in small muscle masses McCloskey and Streatfield, 1975;Mitchel et al, 1980;Seal et al, 1983). However, based on this research, it should be recognized that the magnitude of VO 2 and HR depend on the percentage of MVC when the active muscle mass is the same and vice versa.…”

The role of muscle mass in the cardiovascular response to static contractions

“…One of the findings is that the magnitude of the increase in blood pressure and heart rate during sustained contractions is related to the percentage of the maximal voluntary (MVC), and the cardiovascular response is unrelated to the mass of the J. H. MITCHELL AND OTHERS muscles engaged in the contraction (Lind & McNicol, 1967;McCloskey & Streatfield, 1975).…”