Running footwear is potentially an extrinsic risk factor that may modify the training load an athlete can tolerate before incurring a running-related injury. However, there is currently only limited evidence on the relationship between shoe features and running-related injury risk. Amongst others, it is currently unclear to what extent shoe cushioning influences injury risk.
A recent randomized trial including 800+ recreational runners demonstrated a protective effect of greater shoe cushioning on running injury but did not explain the underlying mechanism. In this trial, the participants randomly received one of two running shoe models that differed by ∼35% in their cushioning properties. A 6-month follow-up revealed that those using the Soft shoe version had a lower injury risk compared to those having received the Hard version (HR= 1.52; 95% CI = 1.07–2.16; Soft shoe group is the reference). Furthermore, a stratified analysis showed that, in contrast with popular belief, only lighter runners benefitedfrom this protective effect of greater shoe cushioning (HR= 1.80; 95% CI = 1.09–2.98), while no effect was observed in heavier runners (HR= 1.23; 95% CI = 0.75–2.03).
The use of cushioning systems in running shoes is based on the assumptions that external impact forces relate to injury risk, that footwear cushioning material can reduce these impact forces, and that cushioning itself has no other detrimental effect on injury risk. Some literature data suggests impact force-related variables, such as vertical impact peak force (VIPF) and vertical instantaneous loading rate (VILR), to be related to running injury risk, but the evidence is inconsistent
The main objective of this study was to seek a functional explanation to the protective effect of the Soft shoeversion observed previously.
This was a participant- and assessor-blinded randomized trial with a biomechanical running analysis atbaseline and a 6-month follow-up on running exposure and injury risk, comparing two running shoe prototypes which only differed regarding their cushioning properties.
Leisure-time runners (n = 848) were randomly allocated to the study groups. Volunteers were included if they were in good health, aged 18–65 years, and capable of performing 15 min of consecutive treadmill running.
Two versions (Hard and Soft cushioning) of an anonymized conventional running shoe model were specifically designed and provided by a sport equipment manufacturer. Apart from the cushioning properties, the two versions were identical, with a mass of 337 ± 17 grams. Global stiffness differed by 35.4% (94.9 ± 5.9 and 61.3 ± 2.7 N/mm in the Hard and Soft versions, respectively).
The baseline biomechanical running analysis was performed on a split-belt treadmill instrumented with force plates using the allocated shoe type. The protocol consisted of a 3-minute warm-up, followed by an 8-minute habituation phase at the self-declared preferred running speed, followed by a 2-min data collection phase at the same running speed. The 3-dimensional ground reaction forces were recorded at 2 kHz.
An analysis of variance (ANOVA) was used to compare the means between the two study groups, with speed as co-variable given that the participants were tested at their self-declared preferred running speed.
Given that a lower injury risk was previously observed in the Soft shoe group of the same cohort, and considering prior literature findings on impact forces and injury risk, the authors hypothesized that VILR and VIPF would be lower in the Soft shoe group.
Contrary to these expectations, a significantly higher VIPF was observed in the Soft shoe group compared to the Hard shoe group (1.53 ± 0.21 vs. 1.44 ± 0.23, respectively; p < 0.001, 6.9%), and the effect size was medium.
However, the proportion of steps with detectable VIPF was lower in the Soft shoe group (84 vs. 97%, Soft shoe and Hard shoe groups, respectively; p < 0.001). A longer time to VIPF was observed in the Soft shoe group (46.9 ± 8.5 vs. 43.4 ± 7.4 milliseconds, Soft shoe and Hard shoe groups, respectively, p < 0.001; 7.8%;medium effect).
No significant differences were observed for VILR (60.1 ± 13.8 vs. 58.9 ± 15.6 BW/s for Soft and Hard shoe group, respectively; p = 0.070) or any other kinetic variable.
When stratifying the participants according to body mass, the differences between the two shoe groups regarding VIPF, the proportion of steps with detectable VIPF and the time to VIPF remained statistically significant in both lighter and heavier runners. No other difference between the shoe groups was observed in either of the body mass categories, except for a slightly longer time to peak force observed in the Hard shoe group among the heavier runners.
The findings demonstrate that the beneficial effect of greater cushioning cannot be explained by a decrease in vertical impact peak force and loading rate. Taken alone, these GRF metrics are not appropriate markers to illustrate the relationship between shoe cushioning and injury risk, while delayed VIPF and the proportion of steps displaying a VIPF may be of relevance here.
Malisoux L et al. (2021) Effect of shoe cushioning on landing impact forces and spatiotemporal parameters during running: results from a randomized trial including 800+ recreational runners, European Journal of Sport Science, 21:7, 985-993.
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