Cardiac responses to maximal anisotonic isometric contractions during handgrip and leg extension

A group of 14-healthy men performed anisotonic isometric contractions (AIC), for 60 s, at an intensity of 100% maximal voluntary contraction force (MVC) during handgrip (HG) and leg extension (LE). Heart rate (fc), stroke volume index (SVI) and cardiac output index (QcI) were measured during the last 10 s of both AIC by an impedance reography method. Force (F) exerted by the subjects was recorded continuously and reported as a relative force (Fr) (% MVC). The F generated during MVC was greater for LE than for HG (502.1 N compared to 374.6 N, P < 0.001). The rate of decrease in Fr was significantly slower for LE than HG for the first 25 s of the exercise (phase 1 of AIC). The Fr developed by the subjects at the end of AIC was 40% MVC for both LE and HG. The increase in fc was greater for LE (63 beats.min-1) than for HG (52 beats.min-1), P < 0.01. The SVI decreased significantly from the resting level by 17.0 ml.m-2 and by 18.2 ml.m-2 for LE and HG, respectively. The QcI increased insignificantly for HG by 0.09 l.min-1.m-2 and significantly for LE by 0.56 l.min-1.m-2 (P < 0.001). It was concluded that although both AIC caused a significant decrease in SVI, greater increases in fc and Qc were observed for LE than for HG. The greater fc and Qc reported during LE was probably related to the greater relative force exerted by LE during phase 1 of AIC. It seems, therefore that central command might have dominated for phase 1 of AIC but that the muscle reflex also contributed significantly to the control of the cardiac response to the high intensity AIC.