In vivo human gracilis muscle active force-length relationship is explained by the sliding filament theory

The sliding filament theory explains skeletal muscle fibre force change as a function of length based on the overlap of actin and myosin filaments. Although this length-tension (LT) relationship has been well investigated in animal models, it is not known whether this microscopic sarcomere LT property can be scaled up five orders of magnitude to explain the LT behaviour of a long human muscle such as the gracilis. The goal of this study is to validate the sarcomere LT curve in humans based on human filament length combined with in vivo experimental data. Intraoperative measurements of maximal tetanic force and muscle-tendon unit length at four different joint configurations (JC) were obtained from 19 patients undergoing free functioning muscle transfer surgery. With physiologically measured fibre length and estimated sarcomere shortening resulting from tendon compliance, we show that 79.7% variance in isometric force data is explained by a simple human sarcomere LT model. This study demonstrates that the human whole muscle LT relationship can be modelled by the sliding filament theory given patient-specific fibre length, filament length, tendon compliance and sarcomere shortening. KEY POINTS: Whole human gracilis muscle isometric length-tension relationships were measured in the operating room. Grouped whole muscle raw length-tension curves showed no obvious form. The width of each experimental length-tension curve was highly variable across subjects and used to predict fibre length (serial sarcomere number). After whole muscle length-tension curves were normalized to each patient's serial sarcomere number, the whole human muscle length-tension curve was well predicted by the sliding filament theory.