Blood pressure and heart rate response to static exercise in relation to electromyographic activity and force development

Abstract

Five healthy men performed static knee extension (90.degree. knee angle) with 1 leg for 5 min. In 1 series of experiments the force was held constant at 20% of the isometric maximal voluntary contraction. In the other the initial force development was also 20%, but the smoothed, rectified electromyographic activity, (SREMG) recorded after 5 s of contraction was kept constant. Heart rate (HR), arterial mean blood pressure (MBP) (measured 20 cm proximally to the elbow in a brachialis), EMG (surface electrodes) from the lateral portion of m. quadriceps femoris, and force were continuously recorded. HR and MBP increased .apprx. 40% in the force-constant experiments and .apprx. 20% in the SREMG-constant experiments. The greatest increase occurred during the first 2 min of contraction. In the force-constant experiments SREMG doubled, with the most marked increases occurring during the very early and late parts of the contraction. SREMG correlated with both HR (r = 0.85) and MBP (r = 0.98). In the SREMG-constant experiments force declined quickly during the 1st minute and remained at .apprx. 12% maximal voluntary contraction thereafter. The observed cardiovascular responses to static contraction can best be explained as the result of the combined action of central and peripheral drives, the central drive being related to the central activity for the recruitment of motor units, and the peripheral drive being mediated through chemoreceptors in the exercising muscles.