Effects of varying central command and muscle mass on the cardiovascular responses to isometric exercise.

To determine if the central command signal associated with isometric exercise is mass-dependent, 20 subjects (nine male, 11 female; 23 +/- 1 years) performed four 5-min bouts of supine isometric exercise with a large (quadriceps; LEG) and small (forearm; ARM) muscle mass. For each extremity, one bout entailed maintaining a constant force (CF; 20% maximal voluntary contraction) and the other constant electromyographic activity (CE; approximately 20% MVC initially). Central command was assumed to increase with CF and remain unchanged with CE. Heart rate increased more with LEG than ARM (P<0.001) and, in LEG, was higher in CF than CE at min 5 (P<0.001). Mean arterial pressure was higher in LEG (P<0.001) by min 2 and 10 +/- 3 mmHg higher in LEG CF than LEG CE by min 5 (P<0.001). Ratings of perceived exertion were highest in LEG CF (P<0.001); LEG CE did not differ from ARM CE (P<0.001) by min 4. The ARM responses did not differ between CF and CE in any variable. These data suggest that muscle mass influences the central command signal during isometric exercise and central command modulates this response in larger muscle masses.