Locomotion Pattern Alters Apparent Joint Stiffness During Unloaded And Loaded Bipedal Ambulatory Tasks In Women

Abstract

Soldiers often perform bipedal ambulatory tasks with load carriage. During marching they are encouraged to use a walk pattern instead of running (RN) despite translating at a velocity above their gait transition (GTV), known as forced marching (FM). Apparent joint stiffness provides an opportunity to quantify the modulation of lower extremity elasticity during dynamic activity in regard to joint loading. Moreover, joint stiffness potentially has implications on mechanical efficiency. However, little is known how load magnitude and locomotion pattern affects joint stiffness at relative velocities. PURPOSE: To determine interactive effects of load magnitude and locomotion on lower extremity joint stiffness in women. METHODS: Twelve healthy females (24.75 ± 2.17 years, 60.98 ± 9.74kg) completed 2 testing sessions collecting kinematic (100 Hz) and kinetic (1000 Hz) data. Subjects wore combat boots and a dual-sided weighted vest. Trials were conducted at body weight (BW) and loaded; +25%, +45%. At each load, 2 locomotion types (RN and FM) were performed at 10% above their GTV. Joint angles (θ) were relative and moments (M) normalized to system weight. Joint stiffness (K) [sagittal plane] calculated as ΔMjoint/Δθjoint during the braking portion of stance phase. Multifactorial RMANOVA, load by locomotion (3x2), were conducted on each K for each limb (dominant [DOM] & nondominant [NON]) separately. Bonferroni-corrected pairwise comparisons were conducted when necessary (α=p<.05). RESULTS: Kankle had a main effect of locomotion (DOM: p=.02, NON: p=.002); RN (.81 ± .22 Nm/kg/°) ~82.5% greater than FM (.14 ± .04Nm/kg/°) for both limbs. Kknee had a main effect of locomotion (DOM: p<.001, NON: p<.001); RN (.09 ± .01 Nm/kg/°) ~80.5% greater than FM (.03 ± .004 Nm/kg/°) for both limbs. For KHip had a main effect of load (NON: p=.01); +45% ~10% less than +25%. CONCLUSIONS: Ankle and knee joint stiffness were not significantly altered by changes in load magnitude, demonstrating the importance of locomotion pattern on modulating apparent joint stiffness. Unlike the ankle and knee, the hip may only mediate lower extremity stiffness with the addition of load. Greater observed joint stiffness during RN may exhibit the greater elastic energy potential that can be leveraged for energy absorption and horizontal propulsion compared to FM.