Category: Midfoot/Forefoot Introduction/Purpose: Single-limb heel rise (HR) is used to evaluate muscle performance and clinical outcomes in people with foot and ankle pathology. While the midfoot contributes 13% of total foot/ankle work to forward propulsion during gait, healthy HR performance remains primarily characterized by ankle function. Similar to its function during gait, the midfoot may also generate power during HR to provide the stability needed for ankle push-off power. Further characterization of HR performance by evaluating midfoot kinetics may advance understanding of foot function in the clinical application of the HR task. The purpose of this study was to examine ankle and midfoot power and work during single-limb HR in healthy adults. Methods: Twelve healthy adults [Mean (SD): Age 31.3 (4.9) years; BMI 25.2 (3.3) Kg/m2; 50% male] performed twenty barefoot single-limb heel rises. Multi-segment kinematic and ground reaction force data were recorded with an electromagnetic motion capture system and force plate. Subject-specific, three segment foot models (tibia, rearfoot, forefoot) were derived. Inverse dynamic calculations were performed to obtain ankle and midfoot peak positive and negative powers (joint torque x segmental velocity), as well as the timing of peak powers (% of the heel rise). Midfoot work (integral of power with respect to time), reflecting energy production, was examined as a percentage of the entire work performed at the ankle and midfoot during HR. Group descriptive data were examined as mean (standard deviation) [95% confidence interval]. Results: Ankle peak positive power was 2.8 (0.8) [2.2 - 3.3] W/Kg and occurred during the ascent phase at 7% HR (Figure 1). Midfoot peak positive power was 0.5 (0.2) [0.3 - 0.6] W/Kg occurring at 6%. Ankle peak negative power was 2.1 (0.6) [1.7 - 2.4] W/Kg and occurred during the descent phase at 84% HR. Midfoot peak negative power was 0.4 (0.2) [0.2 - 0.5] W/Kg occurring at 83%. The proportion of midfoot work to total work performed was 13.5 (5.7) [9.8 - 17.1] %. Conclusion: Study findings advance understanding of foot function by demonstrating the contribution of midfoot power and work during HR in healthy adults. Midfoot peak powers were approximately 18% of ankle powers and occurred at similar times during HR. Further, midfoot energy production appears to be an important aspect of healthy HR performance because approximately 14% of the total foot/ankle work occurs at the midfoot. Since active internal muscle and ligament mechanisms are attributed to midfoot power generation, practitioners should consider midfoot tissue loading and muscle performance when implementing the HR task in clinical practice.