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This report is the first systematic evaluation of the effects of prolonged weightlessness on the bipedal postural control processes during self-generated perturbations produced by voluntary upper limb movements. Spaceflight impacts humans in a variety of ways, one of which is compromised postflight postural control. We examined the neuromuscular activation characteristics and center of pressure (COP) motion associated with arm movement of eight subjects who experienced long-duration spaceflight (3--6 mo) aboard the Mir space station. Surface electromyography, arm acceleration, and COP motion were collected while astronauts performed rapid unilateral shoulder flexions before and after spaceflight. Subjects generally displayed compromised postural control after flight, as evidenced by modified COP peak-to-peak anterior-posterior and mediolateral excursion, and pathlength relative to preflight values. These changes were associated with disrupted neuromuscular activation characteristics, particularly after the completion of arm acceleration (i.e., when subjects were attempting to maintain upright posture in response to self-generated perturbations). These findings suggest that, although the subjects were able to assemble coordination modes that enabled them to generate rapid arm movements, the subtle control necessary to maintain bipedal equilibrium evident in their preflight performance is compromised after long-duration spaceflight.

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Continuous intramuscular electromyograms (EMGs) were recorded from the soleus (Sol), medial gastrocnemius (MG), tibialis anterior (TA), and vastus lateralis (VL) muscles of Rhesus during normal cage activity throughout 24-h periods and also during treadmill locomotion. Daily levels of MG tendon force and EMG activity were obtained from five monkeys with partial datasets from three other animals. Activity levels correlated with the light-dark cycle with peak activities in most muscles occurring between 08:00 and 10:00. The lowest levels of activity generally occurred between 22:00 and 02:00. Daily EMG integrals ranged from 19 mV/s in one TA muscle to 3339 mV/s in one Sol muscle: average values were 1245 (Sol), 90 (MG), 65 (TA), and 209 (VL) mV/s. The average Sol EMG amplitude per 24-h period was 14 microV, compared with 246 microV for a short burst of locomotion. Mean EMG amplitudes for the Sol, MG, TA, and VL during active periods were 102, 18, 20, and 33 microV, respectively. EMG amplitudes that approximated recruitment of all fibers within a muscle occurred for 5-40 s/day in all muscles. The duration of daily activation was greatest in the Sol [151 +/- 45 (SE) min] and shortest in the TA (61 +/- 19 min). The results show that even a "postural" muscle such as the Sol was active for only approximately 9% of the day, whereas less active muscles were active for approximately 4% of the day. MG tendon forces were generally very low, consistent with the MG EMG data but occasionally reached levels close to estimates of the maximum force generating potential of the muscle. The Sol and TA activities were mutually exclusive, except at very low levels, suggesting very little coactivation of these antagonistic muscles. In contrast, the MG activity usually accompanied Sol activity suggesting that the MG was rarely used in the absence of Sol activation. The results clearly demonstrate a wide range of activation levels among muscles of the same animal as well as among different animals during normal cage activity.

Microgravity was used to study accelerometrically recorded microvibration (MV) and postural tremor (PT) at reduced muscle tone on one cosmonaut before, during, and after an 8-day space flight on the Russian Mir station. MV of the relaxed forearm in the 1 g environment showed the typical 7- to 13-Hz resonance oscillations triggered by the heart beat. In 0 g, these pulsations shifted to below 5 Hz and the waveform became similar to an ultralow frequency acceleration ballistocardiogram. PT of the arm stretched forward showed an irregular waveform in 1 g. In 0 g, the higher-frequency components were reduced and again an ultralow frequency ballistocardiogram emerged. As a control, hand force tremor was recorded as well; it was not affected by the gravity condition. A second-order analog with muscle stiffness (C) as parameter was used to evaluate the measurements. For MV it could be shown that cardiac impacts produce damped resonance oscillations when C is high enough (1 g). At low C (0 g), this resonance phenomenon is essentially filtered out. For PT both neuromuscular and cardiovascular forces produce an irregular output; when C is lowered (0 g) the higher-frequency content is strongly reduced. It is concluded that both MV and PT waveforms are sensitive to musculoskeletal stiffness, such that at the lowest stiffness achieved the cardiac impact dominates. In 1 g, the cosmonaut's data were not significantly different from the results in a control group (n = 6).