Initial State: Skeletal muscle protein turnover is governed by the differences in rates of muscle protein synthesis versus muscle protein breakdown. Muscle atrophy occurs when there is a negative net rate of turnover of myofibrillar  proteins. Understanding the role of muscle protein synthesis, breakdown and overall protein turnover during the time course of long duration space flight mission would be beneficial in making informed decisions regarding the best countermeasures approaches to implement. If countermeasures are not fully effective, strategies to address the remaining perturbing factor (reduced synthesis versus increased breakdown) would likely yield the most fruitful benefits. Current techniques for analysis of protein synthesis and breakdown require muscle biopsy samples for analysis which are prohibitive at this time because measurements or sample collections would need to be conducted at various time points with crew members aboard the International Space Station.

Interim Stages:
Identification and validation of novel technologies for measuring protein turnover
Determination of in-flight time course of changes in skeletal muscle protein synthesis, breakdown and net turnover
Establish relevance of turnover measurements to exercise prescription modifications
Determination of in-flight time course of changes in skeletal muscle mass or volume
 

Approach: Solicitations for novel approaches will be sought for measuring human skeletal muscle turnover using techniques that are compatible during space flight aboard the ISS. Studies using validated techniques would then be conducted during long duration missions.