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

Exercise within a lower body negative pressure (LBNP) chamber in supin e posture was compared with similar exercise against Earth's gravity ( without LBNP) in upright posture in nine healthy male volunteers. We m easured footward force with a force plate, pressure in soleus and tibi alis anterior muscles of the leg with transducer-tipped catheters, cal f volume by strain gauge plethysmography, heart rate, and systolic and diastolic blood pressures during two conditions: 1) exercise in supin e posture within an LBNP chamber during 100-mmHg LBNP (exercise-LBNP) and 2) exercise in upright posture against Earth's gravity without LBN P (exercise-1 G). Subjects exercised their ankle joints (dorsi- and pl antarflexions) for 5 min during exercise-LBNP and for 5 min during exe rcise-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 exe rcise-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 d id not increase significantly during exercise-1 G. Heart rate was sign ificantly higher during exercise-LBNP (99 +/- 5 beats/min) than during exercise-1 G (81 +/- 3 beats/min). These results indicate that exerci se in supine posture within an LBNP chamber can produce similar muscul oskeletal stress in the legs and greater systemic cardiovascular stres s than exercise in the upright posture against Earth's gravity.