PROJECT SUMMARY/ABSTRACT Volumetric muscle loss (VML) is a debilitating condition that leads to chronic functional impairment due to the irrecoverable loss of muscle tissue. There are no physical rehabilitation or surgical standards of care for VML. As such, there is a critical need to develop therapeutics for muscle regeneration. Animal model studies have shown that increased mechanical loading of muscle via electrical stimulation or wheel running is a promising strategy for partially restoring function. Specifically, electrically stimulated eccentric contraction (ESEC) training where the muscle-tendon unit is elongated while the muscle is contracted has been most promising. Regardless of the therapeutic approach to treating VML, and even more broadly to muscle function in general, there are no techniques for noninvasively and directly measuring the in vivo force generating capacity of muscle in animals. Shear wave tensiometry has recently been introduced as a noninvasive technique for assessing in vivo muscle-tendon loading in humans. Loading is quantified by tracking the vibrational behavior of propagating shear waves. While this technology has proven useful in assessing muscle-tendon loading across a range of human health conditions, it has not been scaled or evaluated in animal models, although it holds substantial potential in this context. Accordingly, our overall objectives are to (1) develop a small animal tensiometer capable of measuring reductions in in vivo muscle forces due to injury, and (2) determine the effectiveness of ESEC on improving muscle force generation following injury. In Aim 1, we will develop a shear wave tensiometer that integrates with our established rodent ankle dynamometer. Following VML to the lateral gastrocnemius in a rodent model, wave speed via tensiometry and ankle torque via dynamometry in injured animals will be compared to controls throughout recovery. This will mark a pivotal first step in validating the effectiveness, accuracy, and reliability of tensiometry against established methodologies in the field. In Aim 2, using the same VML model, ESEC will be applied after injury to assess its effectiveness on improving functional and histological outcomes. Successful completion of these aims holds dual significance. First, we will have validated the use of tensiometry as a noninvasive, direct measure of in vivo muscle function in small animal models. Long-term, we envision this technology as a wearable sensor to characterize muscle loading in a wide range of animal models of disease, injury, development, and aging. Second, we will show that ESEC training has the potential to make significant improvements in muscle function following VML. This will set the stage for investigating synergist treatment strategies for VML when combined with complementary therapeutics such as biomaterials or drugs.