The purpose of the present study was to determine the influence of water immersion levels on both cardiorespiratory responses and muscle activities in lower extremity during treadmill walking. Four males and one female (mean age ± SD, 25.0 ± 2.0 yr) were employed as subjects. They performed treadmill walking both in water and on land. Five levels of water immersion were set : diaphragm, navel, groin, midthigh and knee. Pulmonary ventilation (VE), oxygen uptake (Vo₂), heart rate (HR), dyspnea sensation (DS), and leg fatigue sensation (LFS) were measured during exercise.
　The electromyogram (EMG) of six muscles, Tibialis anterior (TA), Medial gastrocnemius (MG), Soleus (SOL), Vastus medialis (VM), Rectus femoris (RF), and Biceps femoris (BF) were recorded by means of bipolar surface electrodes in the left lower extremity. At the diaphragm, navel, groin, and midthigh levels, VE, Vo₂ and HR were enhanced by decreasing the water immersion level, but the influence on the knee was approximately the same as on the midthigh. At the diaphragm level, the smallest integrated EMG (IEMG) of MG and SOL in one walking cycle at 90 m/min seemed to be caused by force of buoyancy. IEMG of VM and RF was enhanced by decreasing the water immersion level. In contrast, IEMG of RF was suppressed concomitantly with the decrease of the water immersion level.
　A significant correlation (r= 0.974 P< 0.01) was observed between the IEMG total for all six muscles in the lower extremity at each water immersion level (for one minute duration) and Vo₂. This result indicated that cardiorespiratory responses during treamill walking both in water and on land were caused by muscle activity in the lower extremities.

DESCENTE SPORTS SCIENCE Vol.16/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE