Running with a load increases leg stiffness
Introduction
The mechanics of human running have often been characterized using a spring-mass model (e.g. Arampatzis et al., 1999, Blum et al., 2009, Donelan and Kram, 2000, Farley and Gonzalez, 1996, Lipfert et al., 2012, McMahon and Cheng, 1990). In a spring-mass model, the leg is treated as a massless linear spring, and the leg spring stiffness is related to the peak vertical ground reaction force and the change in stance phase leg length. Analyses of spring-mass models have suggested that leg stiffness increases in proportion to body mass among a wide range of animals (Farley et al., 1993). Running while carrying load is common in humans, but it is unknown how carrying load influences running mechanics and leg stiffness.
During the early stance phase of running, the distance between the center-of-mass and the foot decreases, as a result of flexion of the hip, knee, and ankle, and reaches a minimum near the middle of the stance phase (Cavagna et al., 1976, McMahon and Cheng, 1990). Leg stiffness is therefore related to lower extremity joint angles (Gunther and Blickhan, 2002, Kuitunen et al., 2002). Early studies of vertical hopping showed that leg stiffness decreased when subjects hopped with greater knee flexion angles (Greene and McMahon, 1979). Walking with progressively larger loads increases peak stance phase hip flexion (Silder et al., 2013), knee flexion (Birrell and Haslam, 2009, Silder et al., 2013), and ankle dorsiflexion angles (Silder et al., 2013), but it is unknown if subjects run with greater joint flexion when carrying a load.
Stance phase joint flexion angles and ground contact time can affect the peak vertical ground reaction force. McMahon et al. (1987) showed that when subjects ran with more lower extremity joint flexion (i.e. “Groucho running”) ground contact time increased and the peak vertical ground reaction force decreased. During both walking and running, the peak vertical ground reaction force increases less than the added load (Silder et al., 2013, Teunissen et al., 2007). For example, when subjects were asked to walk with load equal to 30% of their body weight, the peak vertical ground reaction force increased by an average of only 15% (Silder et al., 2013), and when asked to run with 30% of body weight, the peak vertical ground reaction force increased only 12%, compared to no load (Teunissen et al., 2007). During walking, subjects mitigate the increase in ground reaction force by increasing ground contact time and increasing flexion of the lower extremity joints (Silder et al., 2013), but the effects of load carriage on ground contact time and lower extremity joint angles during running are unknown.
We were interested to see whether subjects show similar adaptations when running with load as they do when walking with load. We expected that the peak vertical ground reaction force would increase and therefore hypothesized that leg stiffness would also increase when running with load. We further hypothesized that subjects would increase ground contact time and flex their hip, knee, and ankle joints more when running with load. We sought to test these hypotheses to understand how subjects adapt to running with load.
Section snippets
Methods
Twenty-seven recreational runners (16M, 11F; 33±8 years; 70±9 kg; 1.75±0.09 m) provided informed consent to participate in this study according to a protocol approved by the Stanford University Institutional Review Board. Subjects were excluded if they could not run comfortably for a minimum of one hour at a speed of 3 m/s or faster.
All running trials were conducted on a split-belt force-instrumented treadmill (Bertec Corporation; Columbus, OH, USA) at each subject׳s self-reported 10 km training
Results
Dimensionless leg stiffness increased when running with load (p=0.001, Table 1). Leg stiffness increased because the peak vertical ground reaction force increased (p<0.001), and the change in stance phase leg length decreased (p=0.025). Post-hoc analyses revealed that leg stiffness increased between running with no load and running with 20% (p=0.002) and 30% (p=0.006) of body weight. The only other significant pair-wise increase in leg stiffness was between the 10% and 30% load carriage
Discussion
This study tested the hypothesis that running at a constant speed while wearing weight vests with an additional 10%, 20%, and 30% of body weight would increase leg stiffness. In support of this hypothesis, dimensionless leg stiffness increased when running with load because of a simultaneous increase in the peak vertical ground reaction force and a decrease in the change in stance phase leg length. We also tested the hypothesis that running with load would increase ground contact time and peak
Conflict of interest
We, the authors, have no conflict of interest in the subject matter related to this work.
Acknowledgments
We thank Darryl Thelen, Rebecca Shultz, Phil Cutti, Chris Frankel, Stanford Human Performance Lab, and HyperWear®. Funding for this project was provided by the NIH Grants U54 GM072970 and R24 HD065690, and a Stanford Dean׳s Postdoctoral Fellowship.
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