To characterize the type and magnitude of lower extremity physical performance deficits in military service members who have undergone rehabilitation after limb salvage or transtibial amputation. Cross-sectional prospective. Level I trauma military medical and rehabilitation center. Service members with lower extremity trauma resulting in limb salvage (n = 20) or unilateral transtibial amputation (n = 14) compared with uninjured actively training service members (n = 123). Control participants and individuals with amputation were tested during a single session. Participants with limb salvage were tested with and without the use of a custom carbon fiber orthosis. Physical performance as measured using four-square step, sit-to-stand 5 times, and timed stair ascent tests. Secondary outcomes included the associations between these lower extremity activity measures to determine the interrelationship of activity limitations. The ability of service members to rapidly ascend stairs, a demanding lower limb mobility task, is limited after amputation and limb salvage. However, performance on an agility test similar to the four-square step test approximated normative levels. Differences between individuals with amputation or limb salvage were less than 1 second for all tests and were not statistically significant. Correlations were observed among the physical performance measures in the tested patient populations, particularly between the sit-to-stand and timed stair ascent tests. Severe limb trauma significantly affects performance, particularly during tasks requiring lower extremity strength and power. Individuals with amputation or limb salvage who were provided a custom carbon fiber orthosis and intensive rehabilitation had similar performance. Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.