The aim of this study was to compare impact loading during running with regards to different footwear in preschool children. We hypothesized that running barefoot and in minimalistic shoes would increase impact loading compared to standard running shoes in 3- and 4-year-old children because in these footwear conditions they displayed a rearfoot-midfoot footstrike with a significantly less plantar flexed ankle compared to 5- and 6-year-olds [13]. The results of this study showed contrary findings than we hypothesized. However, the minimalist shoes displayed the highest impact loading in 3-year-old children.
In 3-year-old children, barefoot running reduced impact loading compared to both shod conditions.
This study showed that vertical impact loading (VILR) was significantly lower in the barefoot condition compared to minimalist or standard running shoes in 3-year-old children. This evidence is in the agreement with results of statistical parametric mapping of the VGRF whereas impact peaks (3-11% stance phase) reach significantly higher values in the minimalist and standard running shoes than in the barefoot condition. Barefoot running may be a painful physical activity and it has been shown that only 35% adult runners maintain rearfoot strike on hard surface compared to 80% on a soft surface [39]. We also observed this in our study in the 3-year-old preschoolers (31% RFS, 47% MFS, 22% FFS), even though a previous study suggested insufficiency to forefoot strike landing in such a young children based on less plantar flexed ankle at footstrike [13].
In terms of footstrike and mechanical loading, a recent study by Zeiniger et al. [40] reported that 3-year-old children who adapted a heel strike pattern during barefoot walking had lower impact forces and calcaneal loading compared with flatfoot initial contact early walkers (high loading on anterior part of calcaneus). It seems to contradict the findings of the current study. However, we should take into account the fact that running is a different mode of locomotion than walking imposing higher external forces on body tissues. Also a flatfoot/midfoot strike in adult runners exposes the body to lower loading rates compared to rearfoot strikers [41]. Our previous study showed that in a cohort of 48 preschoolers (aged 3-6 years) [13], there was a significant main effect of footwear on strike index. However, this effect disappeared when only the 3-year-old group was considered for the analysis in this study (One-way repeated measures ANOVA). Indeed, 3-years-old children showed mostly a midfoot strike pattern, particularly in barefoot condition. A midfoot strike and higher nociceptive stimulation of lower limb flexion reflex could consequently reduce an abrupt impact loading in short period of time after initial contact [42–44]. On the other hand, from the perspective of cutaneous nociceptive stimulation, it seems that 3-year-olds in minimalist shoes with a rearfoot-midfoot strike immediately after foot contact are not able to react due to lower sensitivity of the location of stimulus (heel has higher pain threshold than rest of the foot) [44]. In addition, the VGRF curve in the current study increased immediately after foot contact (0-4% of stance phase) in minimalistic shoes and reached higher values than in standard running shoe. An explanation could be that the peak of VILR usually occurred earlier in minimalist shoes and its magnitude is relatively very high in the youngest children. An absence of a cushioning sole probably could lead to a higher mechanical loading immediately after foot contact. In previous studies, there have been reported several occult fractures of calcaneus in toddlers without any history of significant acute injury [45, 46]. Some researchers suggested that it could indicate some possible overuse of calcaneal bone from locomotion [40]. From this perspective, one should be careful and reconsider using of minimalist shoes and excessive exposure to running activities on hard surfaces in 3-yr-old children because this could be possible contributing risk factor of calcaneal injuries in such a young children.
In 4-6 year-old children footwear did not affect impact loading as did in 3-year-olds
There is only one previous study, published by Hollander et al. [10] that investigated the effect of footwear on running biomechanics in children (aged 6-9 years). This aforementioned study reported significantly lower magnitude of impact peaks in barefoot running and in minimalistic shoes compared to standard running shoes during treadmill running (fixed velocity at the level of 10 km/h which is close to running pace in the current study based on dimensionless speed). In the current study, we showed that from age 4 to 6, children demonstrated more forefoot strike patterns only during barefoot running compared to standard running shoes. Surprisingly, a forefoot strike pattern in 5-year-old children led to lower impact forces when comparing barefoot and standard running shoes. It seems that, from impact loading perspective, 4 to 6-year-old children are not as sensitive to changing footwear or could possibly adjust their movement patterns via a lower joint limb re-alignment (e.g. more plantar flexed ankle and more eccentric capacity) [13, 47] when they are not fatigued. However, children in the present study (4 to 6-year-olds) in minimalist shoes had the highest loading rate despite a midfoot-forefoot strike. Previously in adult runners, different results regarding to loading rate with respect to footwear and footstrike have been observed. For instance, Rice et al. [24] found significantly lower loading when runners used minimalist shoes and a forefoot pattern compared to rearfoot and forefoot strikers running in SRS. Moreover, Hollander et al. [17] demonstrated that adult runners had significantly lower impact loading during an acute response to barefoot running compared to running in SRS. This was not seen in the 4-year-old and 6-year-old children in the current study. Nevertheless, the differences in VGRF impact peak between barefoot and SRS would be confirmed in all preschool years (3, 4, 5 and 6-year-olds) if the least significance difference technique of the post hoc analysis was used as previously recommended by several researchers [48, 49].
Due to a lack of studies investigating impact loading in children during running, it is difficult to compare loading rates values with previous research. However, Wei et al. [35] compared shod running biomechanics (including VILR) between preschool children (3-6 years, mean age = 4.2 years, SD = 1.6) and young adults (mean age = 35.1, SD = 9.5). Their preschoolers displayed VILR about 63.1 BW/s (SD = 11.8) and adults 59.4 BW/s (SD = 12.9). These authors did not find any differences in vertical instantaneous loading rate (VILR) during submaximal speed running between preschoolers and adults in their usual shod condition. However, they used a different method of evaluation of impact loading originally developed for adults.[50] Values of VILR in SRS of the current study were about 190 BW/s which is approximately three-fold greater compared to the preschoolers of Wei et al. [35]. Some authors [51] suggested that 70-75% children between 4.0-4.9 years of age reached the mastery level in running skill (qualitative assessment); however, our data showed that, from the impact loading perspective, they did not. Although, this study did not analyze age related changes of impact loading, there is an obvious trend of decreasing loading particularly in shod conditions. Therefore, future studies should focus to age related changes of impact loading in different footwear conditions.
Strengths and limitations of the study.
Because VGRF could have a different course and shape in preschool children than in adults as seen in the prior studies that revealed a higher VGRF impact peak (1st peak) than the active peak (2nd peak) and could occurred sooner than in adults [21, 25, 50]. We felt it was more appropriate to analyze the VGRF component as a continuous variable and combine it with discrete variables such as VILR [24]. Therefore, we suggest that this approach could allow a more comprehensive understanding of footwear/footstrike effect on impact loading phenomenon in young children.
In terms of limitations, even though all participants were considered as habitually shod, we did not collect information about their specific footwear type that they usually wore (e.g., experience with standard running shoes). Uniform footwear used during testing in this study could increase internal validity, but concurrently decrease external validity. In addition, all statistical tests used a conservative approach with alpha level correction for multiple testing, even though some researchers suggested no corrections for biological data [48, 49]. Moreover, we analyzed only an acute response to changing footwear conditions and we did not analyze age related changes of impact loading or morphological changes. Therefore, it is not feasible to draw any conclusions with recommendations for parents or teachers. On the other hand, the findings of this study could mark a starting line for further investigations. Future research should be addressed to investigate longitudinal changes of impact loading and morphological changes of the preschoolers and older children´s bodies regarding to exposure to the different footwear conditions.