During space flight, astronauts experience alterations in muscle performance that may cause significant impairments in the performance of operational tasks immediately following landing on a planetary surface. The effects of reduced strength to body weight ratio on the performance of functional tests that are representative of critical mission tasks for lunar and Mars operations were examined. The investigators tested human subjects on an integrated suite of functional tests while wearing a special suit that allows external loads to be applied in order to manipulate subjects’ strength to body weight ratio. Ths study assisted investigators in 1) documenting the degree to which a reduction in strength to body weight ratio affects functional task performance, and 2) identifying critical strength to body weight thresholds that need to be protected in order to maintain task performance capability.
Subjects completed each of the exploration tasks without external load and with an external load equal to 20, 40, 60, 80, 100, 120% of body weight applied via a weighted suit. Task performance metrics were plotted against the subjects’ strength-to-body-weight (including the external load), power-to-body-weight, and total-work-to-body-weight ratios (upper and lower body muscle performance measures were analyzed). Best fit fractional polynomials were generated in order to understand the relationships between muscle-performance-to-body-weight measures and task performance capability. Relationships were task specific.
Spline regression was used to identify muscle function thresholds for each task. Upright seat egress and walk was the most difficult task according to the spline regression analysis thresholds. Thresholds normalized to body weight were 17.8 N/kg for leg press isometric force, 17.6 W/kg for leg press power, 78.8 J/kg for leg press work, 5.9 N/kg isometric knee extension and 1.9 Nm/kg isokinetic knee extension torque. Leg press maximal isometric force/body weight was the most reliable measure for modeling performance of ambulatory tasks. Laboratory-based manipulation of relative strength has promise as an analog for spaceflight-induced loss of muscle function. Muscle performance values normalized to body weight can be used to predict occupational task performance and to establish relevant strength thresholds.
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