What Are Good Muscle Endpoints for Translational Studies?

What Are Good Muscle Endpoints for Translational Studies? Jörn Rittweger()1,2 1Institute of Aerospace Medicine, German Aerospace Center, Cologne, Germany 2Department of Pediatrics and Adolescent Medicine, University of Cologne, Cologne, Germany © The Author   Abstract Muscles matter to our health because of their size, their involvement in energy metabolism and their relevance for locomotion. Adequate selection of good endpoints is crucial for successfully designing translational studies. At least eight different muscle functions matter to health, namely the mechanical functions of exerting force, velocity, power, elastic storage and braking power, the two metabolic functions of substrate uptake (e.g. carbohydrates, lipids and amino acids) and substrate provision (e.g. lactate and amino acids) and secretory functions. However, specific endpoint tests have been validated for muscle force and power only. Walking speed and grip strength demonstrate good predictive value for hard clinical endpoints, such as disability, loss of autonomy and death. Vertical jump power also has good ecological validity and construct validity, and it depicts excellent test-retest reliability, which is an important advantage with regard to the study of power. Assessment of muscle mass, e.g. by magnetic resonance imaging, dual energy X-ray absorptiometry or bioelectrical impedance, should be considered as an important secondary endpoint to enhance construct validity. Further secondary endpoints should be included wherever they are likely to enhance the plausibility of the study outcome and assessment of test-retest reliability at baseline is always recommended. Well-established methods exist for three relevant muscular endpoints, namely power, strength and muscle mass, and these endpoints lend themselves to utilization in clinical studies. However, such validated methods lack a number of additional muscle functions that are scientifically only emerging. This applies foremost to the metabolic function of muscles but also to its role in storage and dissipation of mechanical energy.