Direct intraoperative measurement of isometric contractile properties in living human muscle.

Skeletal muscle's isometric contractile properties are one of the classic structure-function relationships in all of biology allowing for extrapolation of single fibre mechanical properties to whole muscle properties based on the muscle's optimal fibre length and physiological cross-sectional area (PCSA). However, this relationship has only been validated in small animals and then extrapolated to human muscles which are much larger in terms of length and PCSA. The purpose of this study was to measure directly the in situ properties and function of the human gracilis muscle to validate this relationship. We leveraged a unique surgical technique in which a human gracilis muscle is transferred from the thigh to the arm, restoring elbow flexion after brachial plexus injury. During this surgery we directly measured subject specific gracilis muscle force-length relationship in situ and properties ex vivo. Each subject's optimal fiber length was calculated from their muscle's length-tension properties. Each subject's PCSA was calculated from their muscle volume and optimal fiber length. From these experimental data we established a human muscle fibre-specific tension of 171 kPa. We also determined that average gracilis optimal fiber length is 12.9 cm. Using this subject-specific fibre length we observed an excellent fit between experimental and theorical active length-tension curves. However, these fibre lengths were about half of the previously reported optimal fascicle lengths of 23 cm. Thus, the long gracilis muscle appears to be composed of relatively short fibres acting in parallel that may not have been appreciated based on traditional anatomical methods. KEY POINTS: Skeletal muscle's isometric contractile properties represent one of the classic structure-function relationships in all of biology and allow scaling single fibre mechanical properties to whole muscle properties based on the muscle's architecture. This physiologic relationship has only been validated in small animals but is often extrapolated to human muscles which are orders of magnitude larger. We leverage a unique surgical technique in which a human gracilis muscle is transplanted from the thigh to the arm to restore elbow flexion after brachial plexus injury, to directly measure muscles properties in situ and test directly, the architectural scaling predictions. Using these direct measurements, we establish human muscle fibre-specific tension of ∼170 kPa. Further, we show that the gracilis muscle actually functions as a muscle with relatively short fibres acting in parallel, verses long fibers as previously thought based on traditional anatomical models. Abstract figure legend Schematic of a unique surgical procedure in which the gracilis muscle is removed from the medial thigh and transplanted into the bed of the biceps brachii muscle to restore elbow flexion after brachial plexus injury as a free functioning muscle transfer (top panel). During this surgery and prior to removal of the gracilis from the lower limb we have the unique opportunity to measure the subject specific gracilis muscle force-length relationship directly. The muscle's nerve was stimulated to produce an isometric contraction while gracilis force was measured at the distal insertion tendon using a buckle force transducer (bottom left). By moving the lower limb into four positions we can recreate the normalized muscle force-length relationship (bottom right) and, using subject-specific fibre lengths (solid grey line) we can accurately predict the muscle properties. In contrast, literature anatomical fascicle length values (dashed grey line) do not accurately predict the muscle's properties. The long gracilis muscle appears to be composed of relatively short fibres that may not have been appreciated based on traditional anatomical methods. This article is protected by copyright. All rights reserved.