Advancing Skeletal Muscle Force Assessment Using Animal and Human Model

Through conducting groundbreaking studies, collecting first-of-its-kind in vivo measures of human muscle including sarcomere length and force production, this dissertation describes the advances in skeletal muscle force production assessment using intramuscular pressure (IMP). To improve the understanding of IMP studies were carried out to 1) investigating fluid movement in skeletal muscle, 2) advance sensor design, 3) translate studies of IMP from animals to humans, and 4) implement IMP clinically to measure muscle properties during surgery. With this novel in vivo data, 5) patient-specific computational modeling was advanced to improve surgical simulations.;1) Fluid movement in skeletal muscle: In order to investigate variations of pressure in skeletal muscle, the novel technique of injecting fluorescent microspheres into the rat tibialis anterior (TA) and evaluating fluid movement was developed. The fluorescent microspheres dispersed further for passively lengthened muscle compared to static, suggesting the increase of passive force induces fluid movement.;2) Advances in IMP sensor design: With the development of fiber optics and sampling technology, a new sensor was used to investigate IMP. The FISO Inc. product was initially designed for detecting changes in fluid environments (i.e. blood vessel pressure), thus several modifications were required in order to translate the sensor for use in a solid organ. Working closely with colleagues at the University of San Diego, a protective housing was developed and tested. The final design was a nitinol sheath with barbs on the end to enable anchoring in the muscle during testing. The sensor performed well when tested in rabbit TA, displaying high correlation to force.;3) Translational studies: With a new housing design for the sensor, the best insertion technique was evaluated in rabbit TA to prepare the sensors for translation to human studies. Experiments were performed to evaluate the optimal method of insertion in relation to the direction of muscle fibers. Parallel insertion had the highest correlation to force production when compared to perpendicular insertion during passive muscle lengthening.;4) Implement IMP clinically: Free functional muscle transfers (FFMT) are complex surgical procedures designed to return function to the upper limb following traumatic brachial plexus injury. Three surgical teams work in parallel to transfer the gracilis muscle and associated neurovascular bundle from the leg to the debilitated upper limb. Unfortunately, without precise intraoperative measures, the team relocating the gracilis to the upper limb is unable to accurately apply the appropriate tension and attach the donor muscle for optimal function. A complex protocol was developed to collect several novel in vivo gracilis muscle properties: muscle cross-sectional area; muscle and sarcomere length at varying joint angles; passive and active IMP with tendon attached; passive and active IMP and force with tendon transected; and overall muscle volume with the muscle excised. Sarcomere length was greater than previously observed in humans and force measurements suggest the gracilis muscle operates on the descending limb of the force length curve.;5) Computational modeling: Through computational modeling and simulating movement of the musculoskeletal system, interactions of individual components, like a single muscle, can be analyzed to improve our understanding of their contributions to the overall system. This facilitates investigations to improve human locomotion, surgical intervention, and joint loading. Muscle properties collected during in vivo studies were used to develop and investigate a patient-specific model. In vivo sarcomere length values agreed with the model. However, in order for in vivo force measurements to align with the model, non-physiologic fiber lengths were required as model parameter inputs. Future work on the model will allow surgical parameters, such as muscle tension and attachment sites, to be altered in a virtual environment to evaluate the effects on overall force production. These parameters can then be implemented during FFMT surgery to improve the patient's arm function.;In conclusion, this research resulted in valuable findings for IMP and novel muscle properties. Studies translated IMP technology into the clinic and future applications hold promise for improving patient-specific computational models to guide surgical procedures and improve force production in the transferred muscle.