title: Tradeoffs between impact loading rate, vertical impulse and effective mass for walkers and heel strike runners wearing footwear of varying stiffness.
abstract: Humans experience repetitive impact forces beneath the heel during walking and heel strike running that cause impact peaks characterized by high rates and magnitudes of loading. Impact peaks are caused by the exchange of momentum between the ground and a portion of the body that comes to a full stop (the effective mass) during the period of the impact peak. A number of factors can influence this exchange of momentum, including footwear stiffness. This study presents and tests an impulse-momentum model of impact mechanics which predicts that effective mass and vertical impulse is greater in walkers and heel strike runners wearing less stiff footwear. The model also predicts a tradeoff between impact loading rate and effective mass, and between impact loading rate and vertical impulse among individuals wearing footwear of varying stiffness. We tested this model using 19 human subjects walking and running in minimal footwear and in two experimental footpads. Subjects walked and ran on an instrumented treadmill and 3D kinematic data were collected. As predicted, both vertical impulse (walking: F(2,54)=52.0, p=2.6E-13; running: F(2,54)=25.2, p=1.8E-8) and effective mass (walking: F(2,54)=12.1, p=4.6E-5; running: F(2,54)=15.5, p=4.7E-6) increase in less stiff footwear. In addition, there is a significant inverse relationship between impact loading rate and vertical impulse (walking: r=-0.88, p<0.0001; running: r=-0.78, p<0.0001) and between impact loading rate and effective mass (walking: r=-0.88, p<0.0001; running: r=-0.82, p<0.0001). The tradeoff relationships documented here raise questions about how and in what ways the stiffness of footwear heels influence injury risk during human walking and running.
authors: Harvard University, Department of Human Evolutionary Biology, 11 Divinity Ave., Peabody Museum, Cambridge, MA 02138, United States. Electronic address: baddison@fas.harvard.edu.
authors: Brian J
authors: BJ
authors: Addison
authors: Harvard University, Department of Human Evolutionary Biology, 11 Divinity Ave., Peabody Museum, Cambridge, MA 02138, United States.
authors: Daniel E
authors: DE
authors: Lieberman
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D005260:Female; D005684:Gait; D006365:Heel; D006801:Humans; D008297:Male; D012420:Running / Q000293:injuries* / Q000502:physiology*; D012775:Shoes; D013314:Stress, Mechanical; D016138:Walking / Q000502:physiology*; D055815:Young Adult
keywords: Heel strike running; Impact; Walking