Influence of excess weight on lower-extremity vertical stiffness and metabolic cost of walking

The purpose was to test whether lower-extremity vertical stiffness and gait mechanics explain differences in energy cost of walking (Cw) between individuals with normal weight (NW) and obesity (OB). Ten OB (33.1 +/- 2.0 kg m(-2)) and 10 NW (24.2 +/- 1.3 kg m(-2)) walked for six minutes on an instrumented treadmill at 1.25 m s(-1) while Cw, lower-extremity kinematics, and vertical stiffness (K-vert) were measured. NW completed another trial with a loaded vest (NWL) to simulate the BMI of the obese group. Cw was 24% greater in OB (277.5 +/- 45.3 J m(-1)) and 23% greater in NWL (272.7 +/- 35.7 J m(-1)) than NW (211.0 +/- 27.0 J m(-1), P < 0.005). Mass-specific Cw (Cw(kg)) wasn't different between conditions (P = 0.085). Lower-extremity K-vert was 40% higher in OB (32.7 +/- 5.2 kN m(-1)) than NW (23.3 +/- 4.7 kN m(-1), P < 0.001), but neither was different from NWL (27.5 +/- 3.4 kN m(-1), P > 0.05). Mass-specific K-vert (P = 0.081) was similar across conditions. K-vert was related to Cw (r = 0.55, P = 0.001). Cw(kg) wasn't different between NW or OB, but there was a negative correlation between BMI and Cw(kg) driven by lower Cw(kg) in NWL. Cw and K-vert covaried in proportion to body mass, but mass-specific K-vert was unrelated to Cw(kg). Mass-specific K-vert was lower in NWL than OB due to NWL's greater angle of attack, center of mass displacement, and joint range of motion.