Heart rate at the onset of static exercise in man with partial neuromuscular blockade.

In human subjects the length of the first complete cardiac cycle after the onset of static finger or hand muscle contractions was recorded. Graded contractions as well as maximal contractions in which muscle fatigue was provoked by ischaemia were studied. Maximal finger muscle contractions were also weakened by partial neuromuscular blockade with either decamethonium or tubocurarine. The change in R-R interval at the onset of exercise was variable and in part dependent on the length of the last R-R interval before the onset of exercise. The largest decreases were seen when the preceding R-R interval was long. During graded submaximal and maximal contractions a correlation was demonstrated between the change of the R-R interval at the onset of exercise and the contraction intensity. At the onset of the weakest contractions a 3% prolongation of the R-R interval was seen while the strongest contractions were associated with a shortening of 8%. At the onset of all maximal contractions, whether fatigued due to ischaemia or weakened by decamethonium or tubocurarine, a decrease in R-R interval was seen independent of the force developed. When compared at the same resting R-R interval, contractions performed with hand muscles elicited a greater shortening of R-R interval than contractions with finger muscles. The results suggest that the initial change in heart rate at the onset of static muscle contractions is related to the voluntary effort rather than to the force developed, the state of the working muscles or to the type of muscle fibres involved. It may thus be elicited by 'cortical irradiation' rather than by a 'muscle-heart reflex'. Its size depends on the length of the preceding R-R interval, the intended contraction intensity and the muscle mass involved.