Direct measurement of human skeletal muscle specific tension and fiber length
Authors: Lieber Richard , Binder-Markey Benjamin , Persad Lomas , Shin Alexander , Kaufman Kenton 
Published: May 2023

Skeletal muscle’s isometric contractile properties are one of the classic structure-function relationships in all of physiology allowing for extrapolation of single fiber mechanical properties to whole muscle properties based on the muscle’s optimal fiber 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. We leveraged a unique surgical technique in which an intact, living, 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. We hypothesized that the human gracilis would have a functional fiber length of about 23 cm, based on the anatomical literature, and a specific tension of 225 kPa, based on the physiological literature. Data are provided as mean±standard deviation of the mean. Twelve subjects (7 males, average age 54±12 years). In situ passive and active tension were directly measured using a buckle force transducer (BFT) placed on the distal gracilis tendon. Passive tension was measured in each of four joint configurations (JC) prior to muscle stimulation, and, to produce active tension, the anterior branch of the obturator nerve was stimulated. The muscle was progressively lengthened by extending the knee and abducting the hip. Since we could not measure optimal fiber length directly in our subjects during isometric testing we used a functional surrogate for optimal fiber length, the full-width at half-maximum (FWHM) defined from their force vs. length relationship. The FWHM is the width of the active force vs. muscle length curve at half the maximum force. A linear relationship between FWHM and optimal fiber length has been established across several mammalian species and muscles. Thus, to calculate FWHM, we fit each patient’s calculated fully active force vs. muscle length data to a quadratic curve and then calculated width at half the calculated maximum force from the equation. Optimal fiber length was then calculated from the previously established linear relationship between FWHM and optimal fiber length. Each subject’s PCSA was calculated from their muscle volume and optimal fiber length.From these experimental data we established a human muscle fiber-specific tension of 171±84 kPa (n=12). We also determined the average gracilis optimal fiber length was 12.9±3.3 cm (n=12). Surprisingly, these fiber 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 fibers acting in parallel that may not have been appreciated based on traditional anatomical methods. We thus suggest that the value of 171 kPa be considered the best current estimate of human muscle specific tension. This work was supported the Department of Veterans Affairs grant 1 I01 RX002462 and, in part, by Research Career Scientist Award Number IK6 RX003351 from the United States (U.S.) Department of Veterans Affairs Rehabilitation R&D (Rehab RD) Service. Pilot projects leading to this protocol were supported by the Shirley Ryan AbilityLab Catalyst Grant Program, National Science Foundation Graduate Research Fellowship under Grant No. 1255833 and the Mayo Clinic Graduate School. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.