Tradeoffs between impact loading rate, vertical impulse and effective mass for walkers and heel strike runners wearing footwear of varying stiffness
Introduction
The human foot is subjected to repeated impact forces during walking and heel strike running, evident as visible impact peaks in vertical ground reaction forces. Impact peaks are caused by the inertial change in some portion of the body over a brief period of time, usually during the first 10–50 ms of stance. The generation and attenuation of impact forces have been the focus of much research because their potential role in the etiology of various repetitive stress injuries is unclear and intensely debated (Folman and Wosk, 1986, Collins and Whittle, 1989, Nigg, 2001, Nigg, 2010, Gill and O’Connor, 2003, Milner and Ferber, 2006, Wen, 2007, Pohl and Hamill, 2009, Daoud and Geissler, 2012). In addition, how footwear affects the generation of impact forces has been heavily investigated because of the perceived role of footwear in mitigating discomfort and preventing injuries that may result from impact peaks (Hume and Hopkins, 2008, Nigg, 2010).
During the impact phase of stance, defined as the time period from the onset to the zenith of the impact peak, the impulse of the net external force changes the momentum of some portion of the body.where and are the beginning and end times of the impact phase, is the vertical ground reaction force, is the effective mass, is acceleration due to gravity, and and are the vertical velocities of at and , respectively. We define as the portion of the body׳s mass that decelerates to zero during the period of the impact peak; therefore may contain mass from the foot, shank, thigh or other body segments (Dempster and Gaughran, 1967, Bobbert and Schamhardt, 1991, Chi and Schmitt, 2005, Lieberman and Venkadesan, 2010, Shorten and Mientjes, 2011). We define the impact peak as the first peak in vertical ground reaction force, and it thus contains the summation of both high and low frequency ground reaction forces. (Shorten and Mientjes, 2011).
While the frequency components of the vertical ground reaction force are important for understanding how the body generates impact peaks, the purpose of this study is to understand how impact peak magnitude (), impact loading rate (), and vertical impulse, variables that have been implicated in the etiology of various musculo-skeletal injuries, are influenced by footwear heel stiffness. Extensive experimental and modeling studies of the effects of footwear heel stiffness on and have shown that softer footwear heels decrease largely due to increases in the time duration of impact () rather than changes in (Light and McLellan, 1980, Lafortune and Hennig, 1996, Wakeling and Liphardt, 2003). Experimental results concerning are largely inconclusive, with studies finding that less stiff footwear heels can increase, decrease or have no influence on (Clarke and Frederick, 1983, Nigg and Bahlsen, 1987, Lafortune and Hennig, 1992, Hennig and Milani, 1993, Wakeling and Liphardt, 2003). Modeling studies predict that footwear heel stiffness should decrease and that muscle activity in the lower limb can modulate (Nigg and Liu, 1999, Zadpoor and Nikooyan, 2007, Zadpoor and Nikooyan, 2010). Vertical impulse and have been studied in the context of kinematic variation (Chi and Schmitt, 2005, Lieberman and Venkadesan, 2010), but have yet to be studied in the context of variations in footwear heel stiffness.
We can use the impulse–momentum model (Eq. (1)) to investigate how footwear heel stiffness influences , , and vertical impulse by solving Eq. (1) for
Previous experiments using this impulse–momentum model on barefoot individuals have found that varies with gait pattern, kinematics and joint stiffness, and that averages 6.3% of body mass during walking heel strikes and ranges between 2 and 10% of body mass during heel strike running (Chi and Schmitt, 2005, Lieberman and Venkadesan, 2010). is also expected to change with footwear heel stiffness because a less stiff interface between the foot and ground slows the exchange of momentum between the body and the ground. Decreasing the stiffness of footwear heels while holding all other variables constant increases (Fig. 1) (Light and McLellan, 1980, Nigg and Bahlsen, 1987, Lafortune and Hennig, 1996, Whittle, 1999, Shorten and Mientjes, 2011), which will result in a greater portion of the body coming to a stop during the period of the impact peak. Thus, less stiff footwear heels will result in larger within a given gait pattern. In turn, a larger vertical impulse will result from the increase in in less stiff footwear (Fig. 1).
The impulse–momentum model makes additional predictions. If less stiff footwear heels decrease as reported elsewhere and increase as predicted by the model, then there should be a tradeoff between and in walkers and heel strike runners wearing footwear of varying stiffness. Similarly, if less stiff footwear decreases and increases vertical impulse as predicted by the model, then there should be also be a tradeoff between these variables in walkers and runners wearing footwear of varying stiffness. The objective of this study is to test these predictions in human walkers and runners wearing footwear of varying stiffness.
Section snippets
Subjects
Twenty-two healthy adult subjects (13 female – average (SD) body mass (kg): 59.2 (6.63), height (cm): 165 (7.99); 9 male – body mass (kg) 78.9 (7.64), height (cm) 181 (6.93)) between the ages of 19 and 37 participated in this study, which was approved by the Institutional Review Board of Harvard University. Subjects gave their informed consent to participate and the experiments were conducted at the Skeletal Biology and Biomechanics Lab of the Department of Human Evolutionary Biology at Harvard
Effect of footpad stiffness on measured and calculated variables
In both walking and running, was significantly different between conditions (Fig. 3A and D; Table 1; walking: F(2,54)=18.12, p=9.5E−7; running F(2,54)=15.33, p=5.3E−6). was 19% and 29% greater in the control condition than on the hard pad for walking and running, respectively (walking: p=2.7E−7; running: 3.4E−6). was 20% and 24% greater on the hard pad than on the soft pad for walking and running, respectively (walking: p=0.0001; running: p=0.0002).
Vertical impulse was 28% and 35%
Discussion
In this study, we investigated how variations in footwear heel stiffness influenced several aspects of walking and heel strike running impact peaks (including , and vertical impulse) that have been implicated in the etiology of various repetitive stress injuries. Our study used impulse–momentum mechanics, which models impact events as the exchange of momentum that occurs between the ground and some portion of the body () over a brief period of time (). It is important to note that
Conflicts of interest
The authors have no known conflicts of interest.
Acknowledgments
We would like to thank David Pilbeam, Andrew Biewener, Lorna Gibson, Anna Warrener, Eric Castillo, Erik Otarola-Castillo, Will Fletcher, Christine Wu, Victoria Wobber, Kevin Chen and Patrick Dixon for help with study design, statistics, and manuscript preparation.
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