Background: Unilateral transtibial amputees (TTAs) show subtle gait variations while using different prosthetic feet. These variations have not been detected consistently with previous experimental measures. In this research project a novel measure for quantifying kinetic gait differences among prosthetic feet, called Symmetry in External Work (SEW), was introduced. The reliability of the SEW measure was calculated and its correlation with selected clinical measures was determined. The SEW measure was then applied to determine gait differences among four categories of prosthetic feet during various functional activities. Methodology: The study was conducted in three phases. Phase I was a single subject Pilot study to determine the sensitivity of the SEW measure to different prosthetic feet and functional activities. In Phase II, test-retest reliability of the SEW measure was determined for 5 TTAs during the functional activities of level walking, incline walking, decline walking, ascending stairs, descending stairs, sit-to-stand and stand-to-sit. The agreement between SEW values obtained from F-scan and force plates was also calculated. In Phase III, 11 subjects underwent 6 testing sessions over a period of 10-12 weeks. In session 1 subjects were tested wearing their existing prostheses (Baseline session), were given specialized prosthetic training and were tested again after 2 weeks (Training session). For sessions 3 through session 6, subjects were tested with a study socket and one of four randomized test feet. The test feet were SACH, SAFE, Talux and Proprio foot, classified as K1_foot, K2_foot, K3_foot and MP_foot, based on Medicare Functional Classification Level. The Step Watch Activity Monitor (SAM) recorded their daily activities in the 10-14 day accommodation period between two sessions. At each testing session, subjects completed the Prosthesis Evaluation Questionnaire (PEQ-13) and Usability questionnaire, and were evaluated using the Amputee Mobility Predictor (AMP) and the 6-minute walk test (6MWT). Ground reaction forces were collected using F-scan in-sole sensors as subjects performed the above mentioned functional activities. SEW values between the intact limb and the prosthetic limb were computed by integrating vertical ground reaction forces. Intra-class Correlation Coefficients (ICCs) were calculated to determine test-retest reliability and a repeated measure ANOVA was used to establish differences between the prosthetic feet. Results: The ICC values for Test-Retest reliability ranged from 0.84-0.94 for the various functional activities. There was moderate agreement between the SEW values calculated with F-scan and force plates. The SEW values were significantly different between the K3_foot and other feet during level walking and decline walking. During stair ascent, the MP_foot had a significantly higher SEW value than the other feet. There were no differences between feet during incline walking and stair descent. Training resulted in a significant improvement in symmetry from the Baseline session during sit-to-stand, while for stand-to-sit there were no differences between sessions. The PEQ-13 score was not different between sessions, while the Usability scores were significantly different between the K3_foot/MP_foot and K1_foot/K2_foot. The AMP score had a significant increase following Training while the 6MWT score showed a significant increase in the distance walked with the K1_foot, K3_foot and MP_foot over Baseline. The output of SAM did not show any difference in the number of steps or activity level of subjects. There was good to excellent correlation between the SEW values for level walking and other clinical outcome measures. Discussion and Conclusion: The SEW measure had excellent test-retest reliability and the agreement between the F-scan and force plates values could not be established because of a small sample size. The variations in SEW values were the result of distinctive designs of prosthetic feet. The high SEW value of the K3_foot can be attributed to its heel-toe foot plate and "J" shaped ankle spring design, which allowed for greater late stance stability. The active dorsiflexion feature of the MP_foot caused a change in strategy, typically used during stair ascent, resulting in a greater inter-limb symmetry. The SEW measure is a viable method to detect kinetic gait differences among prosthetic feet and represents a resource-effective alternative to traditional gait laboratories. It has several advantages and with further development of in-sole sensor technology, the potential for clinical use due to its relatively low cost instrumentation and minimal subject intervention.