SUPINE EXERCISE DURING LOWER-BODY NEGATIVE-PRESSURE EFFECTIVELY SIMULATES UPRIGHT EXERCISE IN NORMAL GRAVITY

Exercise within a lower body negative pressure (LBNP) chamber in supine posture was compared with similar exercise against Earth's gravity (without LBNP) in upright posture in nine healthy male volunteers. We measured footward force with a force plate, pressure in soleus and tibialis anterior muscles of the leg with transducer-tipped catheters, calf volume by strain gauge plethysmography, heart rate, and systolic and diastolic blood pressures during two conditions: 1) exercise in supine posture within an LBNP chamber during 100-mmHg LBNP (exercise-LBNP) and 2) exercise in upright posture against Earth's gravity without LBNP (exercise-1 G). Subjects exercised their ankle joints (dorsi- and plantarflexions) for 5 min during exercise-LBNP and for 5 min during exercise-1 G. Mean footward force produced during exercise-LBNP (743 +/- 37 N) was similar to that produced during exercise-1 G (701 +/- 24 N). Peak contraction pressure in the antigravity soleus muscle during exercise-LBNP (115 +/- 10 mmHg) was also similar to that during exercise-1 G (103 +/- 13 mmHg). Calf volume increased significantly by 3.3 +/- 0.5% during exercise-LBNP compared with baseline values. Calf volume did not increase significantly during exercise-1 G. Heart rate was significantly higher during exercise-LBNP (99 +/- 5 beats/min) than during exercise-1 G (81 +/- 3 beats/min). These results indicate that exercise in supine posture within an LBNP chamber can produce similar musculoskeletal stress in the legs and greater systemic cardiovascular stress than exercise in the upright posture against Earth's gravity.