Musculoskeletal modelling forms an important asset in research on understanding human motions and neuromuscular performance. The reliability of musculoskeletal model outcomes depend on the accuracy of the parameters, particularly Optimal Fiber Length (OFL). However, the OFL may vary considerably between populations. To date it is unknown how such variations might affect musculoskeletal modelling. Therefore, in vivo determination of OFL is essential for subject-specific models, and understanding population-specific differences for improvement of model diversity. This study developed an in vivo methodology for measuring the OFL that is easily accessible for large scale implementations. It was applied it to the m. rectus femoris for validation.

OFL was derived from the force-length relationship by measuring muscle force, calculated from knee moments, muscle-tendon moment arm, and fascicle length using ultrasound. Muscle activation was standardized via electrical stimulation. The protocol was separately evaluated for validity, reliability, and usability.

Results indicated that knee moment and muscle-tendon moment arm measurements deviated from literature values due to experimental setup limitation, and active fascicle lengths could not be reliably estimated due to the complex muscle architecture. Consequently, the current approach did not yield valid OFL estimates. This study provides insight into the challenges of developing reliable in vivo measurement techniques.

Future studies should employ improved experimental approaches such as increasing electrical muscle stimulation, use of dynamometry and more advanced ultrasound techniques, and applications to other muscles and joints, ultimately, providing a foundation for in vivo estimation of OFL, facilitating investigation of population-specific differences and improving diversity in musculoskeletal modelling.