Effect Of Excess Weight on Lower-Extremity Vertical Stiffness, Muscle Activation, And Metabolic Cost Of Walking

Abstract

PURPOSE: To study the relationship between energy cost of walking (Cw), lower-extremity vertical stiffness (Kvert), and muscle activation across various BMIs. METHODS: This study compared Cw, Kvert, and muscle activation between obese (OB), normal weight (NW), and NW individuals carrying a load (NWL). A sample of 10 NW (24.2 ± 1.3 kg m-2) and 10 OB (33.1 ± 2.0 kg m-2) individuals (29.5 ± 11.7 yr, 14 females, 6 males) walked for six minutes on an instrumented treadmill at 1.25 m s-1 while oxygen uptake, muscle activation, and lower-extremity kinetics and kinematics were measured using an indirect calorimeter, electromyography, and a 3D optical motion tracking system, respectively. RESULTS: Cw was 24% greater in OB (277.5 ± 45.3 J m-1) compared to NW (211.0 ± 27.0 J m-1, P = 0.001), 23% greater in NWL (272.7 ± 35.7 J m-1) compared to NW (P = 0.002), but similar between OB and NWL (P = 0.955). Mass-specific Cw (Cwkg) was not statistically different between NW (2.99 ± 0.24 J m-1 kg-1) and OB (2.85 ± 0.18 J m-1 kg-1, P = 0.382), NW and NWL (2.74 ± 0.24 J m-1 kg-1, P = 0.071), or OB and NWL (P = 0.602), however a negative correlation (r = -0.44, P = 0.008) existed between BMI and Cwkg. Kvert was higher in OB (32.7 ± 5.2 kN m-1) than NW (23.3 ± 4.7 kN m-1, P < 0.001), but NWL (27.5 ± 3.4 kN m-1) was not different from either. Kvert per kilogram (P = 0.081) and muscle activation (P > 0.05) were similar across all conditions. A positive correlation existed between Kvert and Cw (r = 0.55, P = 0.001), but not when expressed per kilogram (r = 0.22, P = 0.120). Center of mass (COM) vertical displacement was similar between OB (3.3 ± 0.4 cm) and NW (3.5 ± 0.7 cm, P = 0.875), OB and NWL (3.9 ± 0.7 cm, P = 0.076), and NW and NWL (P = 0.194). Angle of attack between the leg and ground was similar between OB (104 ± 6 deg) and NW (106 ± 6 deg, P = 0.731) and between NW and NWL (110 ± 3 deg, P = 0.198), but 5% lower in OB than NWL (P = 0.045). CONCLUSION: Cwkg decreased from the NW to NWL condition. Kvert was greater at higher BMIs, but per kilogram was lower in NWL than OB likely due to NWL having a greater angle of attack and COM displacement. The linear relationship between Kvert and Cw is likely due to the greater muscle force needed to support larger body masses. Contrary to our hypothesis, Cwkg was not different between NW and OB and neither Kvert nor muscle activation explained the variability in Cwkg.