SUMMARY Achilles tendon ruptures have increased 10-fold in the last 3 decades but rehabilitation loading protocols remain generalized with unpredictable and generally poor outcomes. While clinical consensus exists supporting early tendon loading to promote healing, the loading magnitudes and frequencies that maximize functional outcomes remain unknown. This high-risk high-reward study will leverage an established small animal model to systematically determine whether precision rehabilitation loading mitigates deleterious muscle- tendon structural changes, thus preserving ankle function. To test the effects of daily active muscle loading, nerve stimulating cuffs will be surgically implanted on the tibial nerve. By stimulating the tibial nerve, precise muscle-tendon loads will be delivered daily to promote tendon healing and prevent muscle remodeling. In aim 1, animals will undergo simulated tendon rupture, surgical tendon repair, and immobilization followed by cage activity. Throughout immobilization, animals will receive daily muscle excitation profiles to stimulate active muscle contractions. These muscle contractions will be delivered below the tendon strain threshold to ensure that the active muscle contractions do not further damage the healing tendon. This timeline has direct translation to the recovery timeline that patients are prescribed following an Achilles tendon rupture. In aim 2, the effects of sex as a biologic variable will be tested by including equal numbers of male and female rats in each experimental group. The primary outcome of this study is active plantar flexor torque generate over the entire range of ankle motion. By calculating the total work done by the ankle, these experiments will provide the most rigorous evidence to date in support of precision rehabilitation. The secondary outcomes of this study are tendon elongation and muscle remodeling, which patient and computational experiments confirm are the mechanical mechanisms that govern ankle function. This study will be the first to identify precision rehabilitation loading profiles that preserve muscle-tendon structure and restore ankle function following Achilles tendon rupture and repair. Importantly, patients can generate prescribed ankle loading profiles when provided with visual feedback during calf strengthening activities. This direct translation to clinical care will shift the rehabilitation paradigm away from generalized loading to precision loading.