1. Introduction
Running is a highly accessible exercise modality associated with a plethora of physiological [
1] and psychological [
2] benefits. However, running is also associated with a very high incidence of chronic pathologies [
3], with as many as 20–80% of runners experiencing such pathologies annually [
4]. Bone stress fractures represent one of the most commonly occurring chronic injuries in runners, accounting for as many as 30% of all running-related musculoskeletal injuries [
5]. The tibia has long been regarded as the most vulnerable site of stress fractures [
6,
7], with as many as 74% of all such injuries being observed at this location [
8]. Tibial stress fractures typically occur at the anterior diaphyseal region of this bone [
9]. Stress fractures are particularly problematic pathologies owing to their lengthy recovery period and high probability of re-injury [
10].
As a cyclical activity, running imposes continuous loads onto the skeletal system, which has the capacity to initiate bone fatigue [
11]. Strain is considered to be the closest analogue for actual structural damage to the bone itself [
12]. As in vivo strains during running have been shown to be considerably lower than the ultimate strength of bone, stress fracture pathologies are considered to be representative of a mechanical fatigue phenomenon [
13], often expressed as an inverse power law association [
14]. Stress fractures transpire due to the accrual of microscopic damage within the bony matrix [
15]. Permitting sufficient rest between each running exposure allows time for bone remodelling, which may enhance bone integrity [
16]. However, if the rate of damage accrual is greater than that of bone remodelling and adaptation, small cracks may materialize in the bony matrix, which proliferates into stress fractures [
17]. Importantly, when the tibia experiences low strain magnitudes, damage accumulation is reduced, and the tissue is afforded a greater duration to repair microcracks; yet with high strains, the degree of damage accretion exceeds the repair and adaptation process [
18]. Therefore, ascertaining tibial loading patterns that attenuate strain magnitudes during running may aid in the prevention of stress fracture pathologies.
Running shoes serve as the principal interface connecting the foot and ground and have thus been posited as a pivotal mechanism that may influence the biomechanical factors associated with the aetiology of chronic injuries [
19]. Modern footwear (henceforth termed conventional footwear) typically feature a high degree of midsole cushioning, particularly in the rear portion of the shoe, stability and motion control technology designed to reduce rearfoot eversion and arch support systems. In addition to conventional footwear models, footwear manufacturers have introduced running shoes with varying levels of midsole cushioning, presenting both minimal and maximal running shoe options [
19]. Minimal running shoes are characterized by a low or zero heel-toe drop, enhanced midsole flexibility and reduced mass [
20]. On the other hand, maximal running shoes, despite featuring a low heel-toe drop, incorporate a significantly greater amount of midsole cushioning, spanning the entire length of the shoe [
19].
Tibial accelerations and the loading rate of the vertical ground reaction force are frequently utilized as proxy indicators for tibial loading and have long been proposed as potential contributors to the development of tibial stress fractures [
21]. Substantial research interest has been directed towards examining the impact of minimal and maximal running shoes on tibial accelerations and vertical loading rates during running. It has been observed that minimal running shoes are linked to heightened vertical loading rates and tibial accelerations in comparison to both traditional [
19,
22] and maximal running shoes [
19,
22,
23]. However, no significant disparities in vertical loading rates and tibial accelerations have been identified between maximal and conventional running shoes [
19,
22,
23,
24].
Recent evidence has shown that surrogate measures, such as tibial acceleration and loading rates of the vertical ground reaction force, are not representative of tibial bone loading in running [
21]. Finite element modelling has been shown to provide more realistic estimates of in vivo tibial bone strains [
25], directly linked to the aetiology of stress fractures [
12]. Indicating that this technique can be utilized to make informed predictions of the damage potential. Significant advances in finite element analyses made in recent years now allow computational probabilistic modelling of the tibia to be undertaken [
25,
26] in order to quantify the probability of tibial stress fractures in runners utilizing different footwear modalities. However, neither of the aforementioned approaches has been utilized to examine differences between minimal, maximal and conventional running shoes during running.
Therefore, the aim of this study is to investigate the effects of minimal, maximal and conventional running footwear on tibial strains and stress fracture probability via a collective finite element analysis and computational probabilistic modelling-based approach. The findings from this investigation will yield new information regarding the effects of minimal, maximal and conventional footwear on tibial strains during running, but also on longitudinal stress fracture probability. This investigation hypothesizes that minimal footwear will increase tibial strains and tibial stress fracture probability in relation to both conventional and maximal footwear.
4. Discussion
The aim of the current investigation was to examine the effects of minimal, maximal and conventional running footwear on tibial strains and stress fracture probability using a cumulative finite element analysis and computational modelling-based approach. This is the first examination of the effects of minimal, maximum and conventional footwear using a concurrent approach of the aforementioned techniques and may therefore yield new information regarding the effects of minimal, maximal and conventional footwear on tibial strains during running, but also on longitudinal stress fracture probability. This investigation tested the hypothesis that minimal footwear will increase tibial strains and tibial stress fracture probability compared to both conventional and maximal footwear.
The current study showed, using musculoskeletal simulation, that ankle joint contact forces in the posterior, axial and medial directions were significantly greater in minimal footwear. To the authors’ knowledge, this investigation is the first to examine three-dimensional ankle joint contact loading in these footwear; however, this observation concurs with previous analyses adopting external indices as pseudo-measures of tibial loading [
19,
22,
23,
24]. The strike index denoted that the minimal footwear was associated with a significantly more anterior and, on average, midfoot strike location [
33]. A more anterior strike location has been shown to increase the ankle plantarflexion moment and plantar flexor muscle forces [
21]. Therefore, the greater forces that were observed in the minimal footwear condition in the muscles crossing the ankle joint, i.e., medial and lateral gastrocnemius, were to be expected, and it is proposed that the increased ankle joint contact forces were mediated as a function of these enhanced muscle kinetics [
46].
Stress fractures are representative of a mechanical fatigue phenomenon, whereby high-magnitude strains without sufficient rest between loading exposures are responsible for the initiation and progression of microscopic damage in the bony matrix, which ultimately results in injury [
17]. It is notable therefore in partial agreement with our hypotheses, that both strain magnitude and strained volume were significantly greater in the minimal and conventional footwear compared to maximal. It is proposed that this observation is related primarily to the aforementioned increases in three-dimensional ankle joint contact forces in the minimal footwear and medially directed contact forces in the conventional footwear. Furthermore, as the plantar flexor muscle forces, which facilitate posterior tibial bending, were significantly greater in the minimal and conventional footwear compared to maximal, it would also be expected that tibial strains due to bending would also be increased. Owing to the association between tibial strains and tibial bone damage, this investigation shows that minimal and conventional footwear appear to place runners at increased risk from the mechanical parameters linked to the aetiology of tibial stress fractures [
12].
Importantly, this investigation showed that tibial stress fracture probability was significantly greater in the minimal compared to maximal footwear. Taking into account the parameters included in the probabilistic model, such increases were mediated firstly as a function of the significantly greater tibial strains allied with the increased number of daily loading cycles required to complete the required modelled daily distance. This investigation therefore indicates that minimal footwear places runners at a significantly increased risk from tibial stress fractures in comparison with maximal running shoes. Taking into account the high incidence of tibial stress fractures in runners [
6,
7], their debilitating and painful presentation, as well their high rate of re-occurrence [
9], the findings from this study indicate that compared to minimal footwear, maximal running shoes appear to be effective in attenuating runners’ likelihood of developing a tibial stress fracture.
Scrutinization of the ankle joint contact forces against previous analyses showed that they were similar to other analyses using musculoskeletal simulation techniques at similar running velocities [
25,
26]. Similarly, the strains experienced by the tibia were also comparable to those observed by previous analyses at the same or similar running velocities [
26]. Finally, in relation to the tibial stress fracture probabilities, our values are consistent with other probabilistically derived failure rates at similar running speeds [
25,
26]; and importantly, the acceleration of risk over the first 40 days, as well as the overall incidence, is also consistent with the epidemiological literature for runners experiencing tibial stress fractures [
52]. However, like all research, this investigation is not without limitations. Firstly, whilst our finite element model was scaled to individual participant dimensions, person-specific bone geomorphologies and, indeed, material properties were not considered. As both tibial bone geometry and density influence tibial strains [
42,
53], the model adopted within this study may not have quantified tibial strains with complete accuracy. Importantly, sex is considered to be an independent risk factor for tibial stress fractures, and epidemiological literature has shown that females are at four times greater risk compared to males [
52]. It is not known whether our findings are generalizable to female runners, and it is therefore recommended that the effects of minimal, maximal and conventional footwear also be examined using probabilistic tibial stress fracture modelling in female runners. Finally, the lack of mechanical testing that may have yielded important information regarding key footwear biomechanical indices such as longitudinal bending stiffness, flexibility, friction and midsole hardness may also serve as a drawback to this study. Future analyses may wish to examine these parameters to elucidate further mechanistic information regarding susceptibility to chronic pathologies when running in minimal and maximal footwear.