Elsevier

Journal of Biomechanics

Volume 48, Issue 7, 1 May 2015, Pages 1318-1324
Journal of Biomechanics

Tradeoffs between impact loading rate, vertical impulse and effective mass for walkers and heel strike runners wearing footwear of varying stiffness

https://doi.org/10.1016/j.jbiomech.2015.01.029Get rights and content

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.

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.titf(Fzmeffg)dt=meff(vfvi)where ti and tf are the beginning and end times of the impact phase, Fz is the vertical ground reaction force, meff is the effective mass, g is acceleration due to gravity, and vi and vf are the vertical velocities of meff at ti and tf, respectively. We define meff as the portion of the body׳s mass that decelerates to zero during the period of the impact peak; meff 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 (Fmax), impact loading rate (F), 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 Fmax and F have shown that softer footwear heels decrease F largely due to increases in the time duration of impact (Δt) rather than changes in Fmax (Light and McLellan, 1980, Lafortune and Hennig, 1996, Wakeling and Liphardt, 2003). Experimental results concerning Fmax are largely inconclusive, with studies finding that less stiff footwear heels can increase, decrease or have no influence on Fmax (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 Fmax and that muscle activity in the lower limb can modulate Fmax (Nigg and Liu, 1999, Zadpoor and Nikooyan, 2007, Zadpoor and Nikooyan, 2010). Vertical impulse and meff 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 Fmax, F, meff and vertical impulse by solving Eq. (1) for meffmeff=titfFzdtΔv+gΔt

Previous experiments using this impulse–momentum model on barefoot individuals have found that meff varies with gait pattern, kinematics and joint stiffness, and that meff 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). meff 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 Δt (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 meff within a given gait pattern. In turn, a larger vertical impulse will result from the increase in meff in less stiff footwear (Fig. 1).

The impulse–momentum model makes additional predictions. If less stiff footwear heels decrease F as reported elsewhere and increase meff as predicted by the model, then there should be a tradeoff between F and meff in walkers and heel strike runners wearing footwear of varying stiffness. Similarly, if less stiff footwear decreases F 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, F 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). F 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). F 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 Fmax, F 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 (meff) over a brief period of time (Δt). 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.

References (37)

  • J.M. Wakeling et al.

    Muscle activity reduces soft-tissue resonance at heel-strike during walking

    J. Biomech.

    (2003)
  • J.M. Wakeling et al.

    Soft-tissue vibrations in the quadriceps measured with skin mounted transducers

    J. Biomech.

    (2001)
  • M.W. Whittle

    Generation and attenuation of transient impulsive forces beneath the foot: a review

    Gait Posture

    (1999)
  • A.A. Zadpoor et al.

    Modeling muscle activity to study the effects of footwear on the impact forces and vibrations of the human body during running

    J. Biomech.

    (2010)
  • A.A. Zadpoor et al.

    A model-based parametric study of impact force during running

    J. Biomech.

    (2007)
  • R. Alexander

    Principles of Animal Locomotion

    (2003)
  • P.R. Cavanagh

    Biomechanics of distance running

    (1990)
  • T.E. Clarke et al.

    Effects of shoe cushioning upon ground reaction forces in running

    Int. J. Sports Med.

    (1983)
  • Cited by (51)

    • Optimal shear cushion stiffness at different gait speeds

      2019, Journal of Biomechanics
      Citation Excerpt :

      This repetitive vertical impact loading typically occurs at the initial contact (first 10% of the stance phase) of the gait cycle and the loading varies in magnitude from approximately 1.5 to 5 body weights (Blackmore et al., 2016; Hreljac, 2004). To reduce the vertical impact loading, footwear scientists have been focusing on innovating the cushion materials of the midsoles and outsole (Addison and Lieberman, 2015; Chang et al., 2014a; Lin et al., 2017; Trama et al., 2019; Wang et al., 2012; Wei et al., 2018). Although the vertical loading has been the focus, the posterior shear force (PSF) generated by the movement between the feet and ground may contribute to lower extremity injury among different sports events (Boyer and Nigg, 2006; Fietzer et al., 2012; Lorimer and Hume, 2014, 2016; Napier et al., 2018; Villwock et al., 2009; Yavuz et al., 2008).

    • Biomechanical Study on Footwear Comfort and Safety for the Elderly

      2023, Yiyong Shengwu Lixue/Journal of Medical Biomechanics
    View all citing articles on Scopus
    View full text