Elsevier

Journal of Biomechanics

Volume 49, Issue 9, 14 June 2016, Pages 1765-1771
Journal of Biomechanics

Foot strike pattern differently affects the axial and transverse components of shock acceleration and attenuation in downhill trail running

https://doi.org/10.1016/j.jbiomech.2016.04.001Get rights and content

Abstract

Trail runners are exposed to a high number of shocks, including high-intensity shocks on downhill sections leading to greater risk of osseous overuse injury. The type of foot strike pattern (FSP) is known to influence impact severity and lower-limb kinematics. Our purpose was to investigate the influence of FSP on axial and transverse components of shock acceleration and attenuation during an intense downhill trail run (DTR). Twenty-three trail runners performed a 6.5-km DTR (1264 m of negative elevation change) as fast as possible. Four tri-axial accelerometers were attached to the heel, metatarsals, tibia and sacrum. Accelerations were continuously recorded at 1344 Hz and analyzed over six sections (~400 steps per subject). Heel and metatarsal accelerations were used to identify the FSP. Axial, transverse and resultant peak accelerations, median frequencies and shock attenuation within the impact-related frequency range (12–20 Hz) were assessed between tibia and sacrum. Multiple linear regressions showed that anterior (i.e. forefoot) FSPs were associated with higher peak axial acceleration and median frequency at the tibia, lower transverse median frequencies at the tibia and sacrum, and lower transverse peak acceleration at the sacrum. For resultant acceleration, higher tibial median frequency but lower sacral peak acceleration were reported with forefoot striking. FSP therefore differently affects the components of impact shock acceleration. Although a forefoot strike reduces impact severity and impact frequency content along the transverse axis, a rearfoot strike decreases them in the axial direction. Globally, the attenuation of axial and resultant impact-related vibrations was improved using anterior FSPs.

Introduction

Trail running is an outdoor activity which consists of running distances from approximately 20 km to >300 km with positive and negative elevation changes ranging from 500 to >20,000 m. Epidemiological studies reported that ultratrail runners are exposed to musculoskeletal injuries including knee issues and stress fractures of the femur, hip, tibia, fibula and foot (Hoffman and Krishnan, 2014, Lopes et al., 2012). Hoffman and Krishnan (2014) reported an annual stress fracture incidence of 5.5% and observed that a greater running distance during the year was one of the risk factors for stress fractures. The large incidence of osteo-articular injuries may be directly related to the high number of foot-ground contacts experienced. During an ultratrail of ~160 km, a typical runner would experiences approximately 120,000 foot-ground contacts, likely subjecting his joints, bones, cartilage and other structures to a much higher stress than those faced by a short-distance runner, even if running speed is much lower in ultratrails. About 50–75% of all running injuries are overuse injuries due to the constant repetition of the same movement (van Mechelen, 1992). Hence, repetitive shocks are thought to be a significant factor in the development of spinal injuries and degenerative changes in joint and cartilage (Lafortune et al., 1996) such as tibial stress fractures (Dickinson et al., 1985) and osteoarthritis by causing microfractures of osseous tissues that may impair the ability of osteo-articular structures to attenuate mechanical stress (Malekipour et al., 2013, Radin et al., 1973, Valiant, 1989).

Downhill running is the most strenuous exercise for skeletal and muscle structures (Malm et al., 2004). Compared to level running, downhill running induces higher peak accelerations at the tibia (Chu and Caldwell, 2004, Giandolini et al., 2015, Hamill et al., 1984, Hardin and Hamill, 2002), sacrum (Mizrahi et al., 2000) and head (Chu and Caldwell, 2004), higher transverse peak tibial acceleration, i.e. along the tibial anteromedial aspect (Giandolini et al., 2015), and increased loading rate and braking force (Gottschall and Kram, 2005). As a consequence, runners tend to alter their running mechanics in order to better cushion impacts in downhill running through increased knee flexion (Mizrahi et al., 2000, Mizrahi et al., 2001), hip flexion and plantarflexion (Chu and Caldwell, 2004) at foot strike. Also, the negative/eccentric work percentage, which represents the phase of shock absorption during stance, has been shown to increase for ankles and knees during downhill running (Buczek and Cavanagh, 1990, Eston et al., 1995).

The foot strike pattern (FSP) influences impact magnitude and the stress applied to the locomotor system (e.g. Boyer et al., 2014; Divert et al., 2005; Giandolini et al., 2013; Kulmala et al., 2013; Lieberman et al., 2010; Shih et al., 2013). Three main FSPs have been observed: rearfoot strike (RFS), midfoot strike (MFS), and forefoot strike (FFS). Typically, adopting MFS–FFS patterns induces a more plantarflexed ankle and flexed knee at initial contact, inducing a more vertical alignment of the tibia relative to the running surface (Ahn et al., 2014, Shih et al., 2013, Yong et al., 2014), lower ankle stiffness and greater knee stiffness (Hamill et al., 2014, Laughton et al., 2003) compared to a RFS.

Vertical and resultant impact, as quantified by the loading rate, has been shown to decrease with a FFS (Boyer et al., 2014, Divert et al., 2005, Giandolini et al., 2013, Kulmala et al., 2013, Lieberman et al., 2010, Shih et al., 2013). Also, lower peak accelerations and impact-related frequency content along the axial tibial axis in habitual forefoot strikers were recently observed (Gruber et al., 2014). This result contradicts the findings of Laughton et al. (2003) showing an increase in axial tibial shock when forefoot striking in habitual rearfoot strikers. During a trail running race, we observed in a single runner exhibiting a FFS profile that axial peak tibial accelerations decreased as he used a more pronounced heel strike (Giandolini et al. 2015).

Previous studies mainly focused on the vertical force or axial shock acceleration during level running and only a few considered the braking force (Boyer et al., 2014, Cavanagh and Lafortune, 1980, Laughton et al., 2003) or transverse shock acceleration (Giandolini et al., 2015, Lafortune, 1991). However, impact severity in the braking direction and transverse plane should not be neglected since it was previously demonstrated that bone is weaker under shear strains than compression strains (Turner et al., 2001). The importance of the transverse component in the evaluation of shock intensity was previously highlighted by Lafortune (1991) during level running. Hardin and Hamill (2002) also suggested that transverse shocks were greater during downhill than level running because of the shift in orientation of the normal vector. When using RFS, the anteroposterior loading rate decreases, while the vertical loading rate increases (Boyer et al., 2014, Cavanagh and Lafortune, 1980, Laughton et al., 2003). Finally, we observed in a case study that adopting a more pronounced RFS was associated with greater transverse peak tibial acceleration (Giandolini et al., 2015).

Considering that trail runners are exposed to great mechanical stress especially over downhill sections, and are at greater risk of osseous injuries, studying whether the FSP used in downhill running influences impact severity is of great interest. Due to the FSP-associated changes in lower-leg geometry at initial contact, it is also relevant to focus on axial, transverse and resultant shock accelerations. Our purpose was to assess the influence of FSP on the intensity and attenuation of axial, transverse and resultant shock accelerations during an intense downhill trail run. Unlike previous studies performed exclusively in level running, we hypothesized that adopting anterior FSPs in downhill running would increase axial shock accelerations but decrease transverse acceleration, as observed in a single runner during a trail running race.

Section snippets

Participants

Twenty-three experienced male trail runners (age: 39±11 years, height: 176±6 cm, mass: 71.5±9.6 kg, weekly running duration: 4.8±2.4 h per week) were recruited and gave their written informed consent to participate in this study, which was approved by the local ethics committee and conducted in agreement with the Declaration of Helsinki. Inclusion criteria were that subjects completed one official trail running race of at least 45 km and were injury-free for the six months preceding the experiment.

Results

The total running time was 34±6 min. A total of 439±34 steps per subject were analyzed over the six sections (73±21 on average per section). The average running speed over the analyzed sections was 3.79±0.77 m s−1. Mean±SD across the six sections were presented for impact-related and FSP-related parameters in Table 1. Along the run, five subjects presented more than 66% RFS steps, 7 subjects presented from 33% to 66% RFS steps, and 11 subjects presented RFS steps less than 33%. There was no

Discussion

Our purpose was to examine whether FSP influences the impact shock intensity and attenuation in downhill trail running. The main findings were that (i) the impact intensity along the transverse axis is as severe as along the axial axis, and (ii) FSP differently influences the axial and transverse components of shock accelerations.

Impact measurements indicate that the transverse component of tibial acceleration should not be neglected in the measurement of shock severity. Indeed, axial and

Conclusion

During trail races, downhill sections expose runners to severe mechanical stress. This study provides further insights into the mechanical stress on runners during downhill trail running. Two important results were that (i) transverse shock acceleration should not be neglected in the assessment of impact severity and (ii) foot strike pattern differently influences axial and transverse shock intensity and frequency content. Adopting a more anterior foot strike pattern leads to a higher shock

Conflict of interest statement

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

We confirm that

Acknowledgments

This study was supported by Amer Sports Footwear as part of the doctoral work of Marlene Giandolini. We declare that we have no conflict of interest. We warmly thank our subjects for their participation and Sébastien Pavailler (Salomon SAS and University of Savoie Mont Blanc) for his help during the experiment.

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