The compressive mechanical properties of four prosthetic liner materials (urethane, two silicones, and a thermoplastic elastomer) were compared to human muscle as a function of geometric flow constraint and loading rate. Loading rate effects on calcaneal heel pad were also measured. The mechanical properties evaluated were from force-displacement data obtained during material tests and included stiffness, the percentage of energy absorbed, and residual displacement (or thinning) 8 seconds after unloading. The geometric flow constraint involved a 25.4-mm diameter piston pushed into an 11.5-mm deep cylindrical cavity with radial piston/cylinder clearances of 0.8 mm, 1.55 mm, and 3.2 mm. The haversine wave-form load-controlled tests increased the load from 50 N to 550 N in 10.0, 1.0, and 0.2 seconds; loading rates of 0.1 Hz, 1.0Hz, and 5.0 Hz. The variation of test parameters had a large and mixed effect on the different materials. The average stiffnesses for each material over all geometries and loading rates were as follows: urethane, 537 N/mm; silicone A, 442 N/mm; human muscle, 394 N/mm; thermoplastic elastomer, 220 N/mm; heel pad, 134 N/mm; and silicone B, 99 N/mm. The average percentages of energy absorbed were as follows: human muscle, 90%; silicone A, 74%; urethane, 71%; heel pad, 59%; thermoplastic elastomer, 56%; and silicone B, 41%. The average residual displacements were as follows: urethane, 0.23 mm; silicone A one layer, 0.43 mm; silicone B, 0.47 mm; thermoplastic elastomer, 0.66 mm; silicone A two layers, 0.74 mm; human muscle, 1.09 mm; and heel pad, 1.6 mm. Impact tests were also performed that involved dropping a 4.45 N mass 151 mm onto the liner materials placed on a piezoelectric sensor. The 820 N impact force without liner materials was decreased to 186 N with muscle, 258 ˙N with urethane or two layers of silicone A, 268 N with a thin layer of heel pad, and 309 N for the thermoplastic elastomer. A single layer of silicone A and the silicone B material both ripped repeatedly during impact testing. Using standard material selection methods, urethane appears to be the optimal prosthetic liner material by offering the unique combination of best response (via highest stiffness), best impact protection (via lowest impact forces), and least thinning (via least residual displacement 8 seconds after unloading). The preliminary data reported here suggests that the differences in comfort reported by amputees who have used multiple liner material types are justified.