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Muscular reflex stimuli to the cardiovascular system during isometric contractions of muscle groups of different mass
Insights
Cardiovascular responses to isometric exercise depend on the relative tension achieved, not muscle mass. However, post-exercise reflex responses are greater with larger muscle groups, indicating muscle bulk influences reflex drive.
Area of Science:
- Exercise Physiology
- Cardiovascular Regulation
- Neuroscience
Background:
- Cardiovascular responses during isometric exercise involve central and reflex stimuli.
- Previous research suggests responses are proportional to relative tension, not absolute tension or muscle mass.
Purpose of the Study:
- To investigate whether muscular reflex stimuli in isometric exercise relate to the proportion of maximal tension or the mass of contracting muscle.
- To differentiate between central and reflex influences on cardiovascular responses during isometric contractions.
Main Methods:
- Induced isometric hind-limb contractions in anesthetized dogs and cats via spinal ventral root stimulation.
- Measured cardiovascular responses (heart rate, blood pressure) in animals during single and combined limb contractions.
- Utilized post-exercise muscle blood flow occlusion in human subjects performing isometric contractions with varying muscle masses (whole hand vs. little finger).
Main Results:
- In animals, combined hind-limb contractions produced greater pressor responses than single-limb contractions.
- No significant differences in heart rate responses were observed between single and combined limb contractions in animals.
- In humans, post-exercise pressor responses during blood flow occlusion were significantly greater when larger muscle groups (whole hand) were exercised compared to smaller muscle groups (little finger) at comparable proportional efforts.
Conclusions:
- The cardiovascular response to voluntary isometric exercise is primarily determined by the proportion of maximal tension achieved.
- Muscular reflex drive during isometric exercise is influenced by the bulk (mass) of the contracting muscle.
- Findings suggest a dual control mechanism where relative tension dictates immediate cardiovascular response, while muscle mass modulates sustained reflex activity.
Abstract:
1. 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. 2. Isometric hind-limb contractions were induced in anaesthetized dogs and cats by stimulation of spinal ventral roots. Pressor responses to contraction of both hind limbs were greater than responses to contractions of either hind limb alone. No differences were observed between heart rate responses to single or combined hind-limb contractions. 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.