Different Behavior of Forearm Blood Flow during Intermittent Isometric Handgrip in a Thermo-neutral and a Hot Environment.

The purpose of the present study was to evaluate the different behavior of forearm blood flow (FBF) during intermittent isometric handgrip (IIHG, 6 seconds contraction +6 seconds relaxation) in a thermoneutral versus a hot environment. Six healthy men performed three times of 5-minute period IIHG at three different work loads (10%, 20% and 30% MVC). The IIHG was performed with the right hand. The experiments of IIHG were performed at two different air temperatures (25 degrees C and 40 degrees C, RH: 50%), and an experiment without work was also carried out at 40 degrees C (RH: 50%). The physiological responses measured involved heart rate (HR), cardiac output (CO), mean arterial blood pressure (MBP), FBF, and skin blood flow (SBF). HR, CO and SBF showed higher values during IIHG at 40 degrees C than at 25 degrees C. MBP had a tendency to increase with the intensity of work load at both 25 degrees C and 40 degrees C. Since intramuscular pressure might increase during contraction periods at 20% MVC and 30% MVC at 25 degrees C, FBF was significantly higher during relaxation periods than during contraction periods. FBF showed similar values between contraction and relaxation periods at 10% MVC at both temperatures. The present study suggested that FBF was sufficient for active muscles during IIHG at 10% MVC. FBF of contraction periods was close to that of relaxation periods at 40 degrees C due to the modulation of the sympathetic outflow to the muscles and/or the decreased efficiency of the muscle pump. It was suggested that FBF showed different behaviors during HHG at 25 degrees C versus at 40 degrees C.