Incorporating subject-specific geometry to compare metatarsal stress during running with different foot strike patterns

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Takeda, 2003, Milgrom et al., 1985) with 10% of all fractures occurring in the 10 metatarsals and 80% -90% of these located in the second and third metatarsals 11 (Chuckpaiwong et al., 2007).  (Burr et al., 1998) and can increase its susceptibility to stress 18 fracture (Burr, 2011). Metatarsal stress fractures are thought to develop due to cyclic 19 overloading with an intermediate remodelling process, which initially weakens the 20 bone prior to increasing strength (Martin et al., 2015). Without sufficient recovery, 21 further loading can lead to excessive microdamage accumulation (Milgrom et al.,22 2002, Schaffler and Jepsen, 2000) and increased risk of stress fracture.

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Runners can be categorised by their foot strike pattern . Most 25 commonly runners land on their heel (rearfoot strike) (de Almeida et al., 2015). This is 4 26 associated with an early impact peak in the vertical ground reaction force time history 27 and high vertical force loading rates (Lieberman et al., 2010). Conversely, those who 28 do not rearfoot strike display a less distinct impact peak and the vertical force loading 29 rates are typically lower than for a rearfoot striker (Ahn et al., 2014, Lieberman et al., 30 2010). There has been much discussion about the merits and shortcomings of each 31 foot strike pattern, which is confounded when the influence of footwear is considered.

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Running in a minimalist shoe tends to result in a more anterior initial foot contact, 33 resulting in increased loading at the metatarsal phalangeal joint (Firminger and 34 Edwards, 2016) -a potential mechanism for increased stress fracture risk. Metatarsal  have been used to estimate internal forces acting on the metatarsals during both 49 running and walking. Previously beam theory has provided valuable insight into 50 potential mechanisms for second metatarsal stress fracture (Gross and Bunch, 1989). 51 This previous research modelled the metatarsal as a hollow elliptical beam and 52 concluded that more accurate, subject-specific bone geometry is required to improve 53 understanding of stress fracture risk.

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The aim of this study was to quantify the forces and stresses acting on the second 56 metatarsal during barefoot running, using a subject-specific mathematical model, 57 including accurate metatarsal geometry. Further, this model was used to compare 58 metatarsal loading in rearfoot (RF) and non-rearfoot (NRF) strikers during running. It 59 was hypothesised that NRF strikers would experience greater peak internal second 60 metatarsal loading than RF strikers. own running shoes during running over a pressure plate. Participants were not 77 informed of their foot strike pattern. This was later compared to their barefoot trials: 78 two participants displayed a foot strike that was consistently more anterior when 79 barefoot than shod, and these participants were included in the NRF group; two further  The forces acting on the second metatarsal were estimated using a model similar to 125 Stokes et al. (1979) and Gross and Bunch (1989), with stress calculated at the upper 126 and lower surfaces of the bone at the midpoint of the shaft, using a similar approach 127 to that of Meardon and Derrick (2014). Lastly, stress can be calculated using these derived forces and bending moments, and 172 the cross-sectional geometry of the metatarsal at midshaft. independent T-Test. Effect sizes were calculated using Cohen's d (Cohen, 1988).

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Time of peak stress was reported for descriptive purposes.

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There were no differences in weight or age between groups, however the NRF group foot contact in NRF than RF strikers, but it is important to note that the peak stresses 290 occurred at the same point during stance in the two foot strike categories. Therefore 291 the significant differences in stress between these groups only occurred in early stance 292 when the stress magnitudes were lower. The magnitude of bone stress is understood 293 to be important when considering risk of stress fracture, but it is not well-established 294 what magnitudes of peak stress may be detrimental. The results from the present 295 study suggest that whilst externally measured forces differ between runners with 296 different foot strike types, this does not equate to a difference in the magnitude of peak 297 metatarsal stress during running, and therefore may not influence the risk of stress 298 fracture via this mechanism. This does not mean that changing from a rearfoot strike 299 to a more anterior foot strike can be recommended as the results from the present 300 study were obtained from runners using their habitual foot strike. Changing foot strike 301 would introduce unaccustomed activity which was not considered in this analysis.

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The external forces acting under the metatarsal head were significantly greater in the 304 NRF runners than RF runners and this was also the case for all calculated variables, 305 other than shear forces, that did not include subject-specific bone geometry. However  Habitual non-rearfoot strikers experience greater second metatarsal stresses during 334 early stance than habitual rearfoot strikers during barefoot running, but similar peak 335 stresses. This is despite non-rearfoot strikers experiencing greater peak external 336 loading under the metatarsal head, resulting in greater peak bending moments about 337 the midshaft than rearfoot strikers. These findings emphasise the importance of 338 including subject-specific geometry when estimating bone stress and further supports 339 the suggestion that external forces should not be assumed to be representative of 340 internal loading.