The aim of the present study was to investigate the effects of a 4-week MS walking intervention on foot parameters (i.e., foot posture and MLA height and stiffness), local and composite ROM of the DC, balance during single-leg stance, and muscle strength of the DC. Walking in MS for four weeks, resulted in a significantly improved FPI-6 score at M2, which could be maintained also after a 4-week intervention pause. Although the FPI-6 slightly increased from M1 to M3 also for the control group, there was no improvement between M1 and M2. All other foot parameters showed no significant results. For the ROM parameters, there were also no significant differences between groups. The MS group showed improved balance during single-leg stance at M2 and M3 (yet, CoP path at M2 not significant). Although the composite strength measurements of the DC (i.e., Bunkie Test and IPC) showed no significant interaction between groups over time, the descriptive data indicates an improvement for the MS group from M1 to M2, which was maintained or even slightly increased at M3. Nevertheless, also to control group showed an improvement at M3 compared to M1. For the isolated isokinetic hamstrings strength there were no significant differences between groups.
Foot parameters
Previous research has shown that wearing MS (for eight weeks or longer) is associated with improved IFM and EFM strength compared to baseline or non-MS shoed populations (Holowka et al., 2018; Johnson et al., 2016; Miller et al., 2014; Xu et al., 2017). Although prior studies agreed that IFM and EFM are important for stabilizing and influencing the MLA (Chen et al., 2016; Xu et al., 2017), we could not see any changes in the MLA parameter, neither the arch shape (i.e., AHI), nor the arch rigidity (i.e., ARI) (Appendix A, Table 1). Nevertheless, the measure of foot posture (i.e., FPI-6) improved. Davis et al. (2021), Miller et al. (2014) and Curtis et al. (2021) all suggest that the MLA deforming mechanisms might be improved by wearing MS. This seems logical due to the altered strike pattern and shoe flexibility in MS compared to cushioned shoes. Nevertheless, for the static MLA parameters (i.e. AHI and navicular height and navicular drop), prior studies also did not find any significant influence of MS (D’AoÛt et al., 2009; Miller et al., 2014). According to these and our studies’ findings, one could discuss if the IFM actually have the strength capacity to increase the height of a fully loaded arch during static stance. Moreover, for an increase in muscle strength to have an effect on arch shape, it must be assumed that the muscles are inherently weak and unable to maintain the arch in the first instance.
Nevertheless, although the foot arch itself seems not to change by wearing MS, the literature suggests that wearing MS seems to influence general foot posture and leads to less foot deformities. D’AoÛt et al. (2009) report that populations wearing MS show shorter and wider feet, as well as a greater foot area. Further, the hallux angle seems to be decreased in habitual use of MS (Barnicot & Hardy, 1955; Shu et al., 2015). One could argue that, as the mid- or forefoot strike pattern applies more pressure to the forefoot, especially to the MTPJ (Bergstra et al., 2015; Chen et al., 2016), it seems logical that static foot posture changes could rather be seen on the frontal plane or the forefoot, than in the sagittal plane (i.e., MLA) alone (Franklin et al., 2015; Kadambande et al., 2006). This hypothesis is supported by prior studies, which found that the IFM growth is mainly seen in the muscles, which are prominent in the forefoot, mainly the short toe flexors, abductor digiti minimi, and abductor hallucis (Chen et al., 2016; Curtis et al., 2021; Holowka et al., 2018; Johnson et al., 2016; Miller et al., 2014; Xu et al., 2017). This could be a potential explanation why in our study, we saw changes in the overall foot posture (measures all three body planes), but not the MLA alone. Further, at a first glance, it seems strange that there were also significant changes in the FPI-6 in the control group at M3. Looking at the descriptive data (Appendix A, Table 1), it becomes obvious that there was only a minor change from (mean ± SD) 24.5 ± 2.9 at M2 to 23.3 ± 3.3 at M3 (in contrast to the MS group (M1: 24.2 ± 2.0, M2: 20.1 ± 2.1)).
Range of motion
For the ROM parameters, there weren no significant interactions between groups over time. Currently, there is conflicting evidence on the effect of MS on foot ROM. Willy and Davis 23 report more dorsiflexion in the ankle and more knee flexion at foot strike, and also Davis, et al. 5 state that due to the elevated heel of a cushioned shoe, the foot is placed in greater plantarflexion at foot strike. In contrast, Miller, et al. 4 report that wearing MS resulted in a decrease in dorsiflexion at foot contact during running. Hollander, et al. 22 also report in their review that MS running goes along with decreased ankle dorsiflexion. Nevertheless, it must be noted that there seems to be a difference between walking and running in MS, as walking probably requires the foot to go through a greater ROM, because of potential heel to toe walking, rather than mid- or forefoot striking during running 11. Yet, no ROM changes, neither locally in the foot nor other DC areas, were found in our study.
Balance during single-leg stance
Balance ability, which is commonly tested when investigating the effect of MS or foot strengthening interventions, can be seen as a functional parameter for foot mobility and stability 18,48. We found a positive effect of wearing MS on balance performance during single-leg stance. Both parameters (i.e., CoP path, CoP EA), decreased from M1 to M2 and even further at M3 (yet, not significantly for CoP path). This is in line with prior studies. Petersen, et al. 13, for example, report an increased local dynamic stability in younger and older adults when walking in MS (measured via motion capture system). Cudejko, et al. 18 found that elderly participants with a history of falls were more stable (reduced CoP range in mm) during standing and walking in MS. In general, walking or running barefoot or in MS seems to be positively correlated with balance skills and consequently lowers the risk of falls 5,8,10,13,18. This improvement in stability is reported to be associated with an increase in IFM strength 8,10.
Muscle strength of the dorsal chain
Although studies frequently reported the positive effects of MS on IFM strength 4,8,11,17, little is known about the influence of MS on motor performance (i.e., sports) in general and more specifically about muscle strength of the DC 22. We assessed this with three different tests, addressing either isolated structures (i.e., hamstrings) or more composite measures of strength of the DC. In our study, the MS intervention had no effect on isolated hamstrings strength. In contrast, for both composite strength measurements of the DC, the Bunkie test and the IPC, test performance seemed to improve from M1 to M2 and was maintained or even slightly more improved from M2 to M3. Yet, there was no significant group over time interaction, which means that performance differed between groups at all time points and both groups improved over time. Therefore, although this study gives first hints for an influence of wearing MS on composite strength measurements of the DC, the results must be interpreted with caution. Nevertheless, these results leads to the assumption that it cannot be an increase in hamstrings strength (alone). The calf muscles, for example, are reported to be more stimulated and therefore strengthened during MS running, which could be explained by the changes in foot strike pattern 5,9,21,24. Snow, et al. 24 report also an increased activity of the gluteus maximus muscle during MS running. Our results highlight the need for physical assessments considering the total DC. Yet, we suggest further investigations on both applied tests (i.e., Bunkie test and IPC), in order to gain further knowledge, e.g. about potential learning mechanisms, as probably observed in our study (i.e., also control group improved from M2 to M3) 41,46,49.
In the following, we discuss two potential mechanisms, which might explain potential effects on DC muscle strength. (1) Altered loading: walking in MS changes walking pattern and biomechanics, which includes an altered foot strike, a decrease of step length and an increase of cadence 27. Participants wearing MS during walking and running adapt to various surfaces by adjusting their overall leg stiffness, which leads to an increase in vertical ground reaction force 5,27,50. Further, the altered foot mechanics (i.e., more natural) also affects more proximal regions of the leg (e.g. the knee) and might lead to altered muscle activation in these body areas, too 5,22,23,25.
(2) Force transmission via linked connective tissue: Recently, the concept of transmission effects via myofascial connections became popular 26,51. Following that, the interest in the human foot, not only as stable basis for other body areas, but also as modifiable part of the DC, linked via connective tissue to influence nondirectly adjacent body areas, increased 52. For wearing MS, it is discussed that the plantar fascia could be modified (i.e., gets thinner) over time due to reaction of increased stimuli and IFM strength increase 5. However, to date the effects of longterm stimuli (especially muscle activation) on the plantar foot sole and its effect on strength along the DC (i.e., transmitting effects) are rare. In contrast, studies proposed that, for example, via stretching or foam rolling on the plantar surface, ROM in remote body areas along the DC could be acutely increased 53–55, whereas a combination of massage, foam rolling, and stretching on the plantar foot structures was shown to acutely decrease performance in the DC 56.
Limitations
One of the main limitations of our study is that, although we hypothesize that some of our parameters might be influenced by IFM strength, we did not measure IFM strength directly. According to Johnson, et al. 17 IFM strength measurement remains challenging as commercial dynamometers are not practicable for testing IFM strength. This is why prior studies usually refer to IFM size changes. Measurement via magnetic resonance imaging is used for that and is expensive and often not accessible. Further, the IFM differentiation is difficult 17. This is why ultrasound imaging is often preferred for measuring IFM size. Yet, also this measurement technique is often not accessible, requires special expertise and refers to muscle size alone, which can also only be seen as an indirect measure of IFM strength 17. Therefore, we decided to apply more functional, indirect measures of IFM strength (i.e., MLA parameter, balance) instead in this study.
For the balance parameters, it is noticeable that the SD of the descriptive data (Appendix A, Table 3) in some cases is markedly different from the MS condition. It could be that this suggests that the individuals within the group may have had very different responses to the MS intervention.
Further, we only asked participants to track and report the steps when they wore the MS. This was kind of a compromise in order to keep efforts for participants as low as possible to increase compliance of reporting. Nevertheless, this does not allow us to report the daily amount of steps taken by the control group or the MS group when not wearing the MS, including the wash-out period. Yet, both groups reported a similar amount of steps/day at baseline (i.e., tracked before study start) (Table 1) and were asked not to make major changes in sports or steps level during the study. This is why we assume that differences in activity levels did not influence our results to a great extent. The self-reported exercise adherence (see 3.1.) seems to match the requirements, considering that participants were asked to perform the intervention on at least 5days/week.
In addition, one could argue that the stimuli via MS in our study were too low, including the intervention period and the steps/day. According to Tudor-Locke, et al. 28 adults taking at least 5,000 steps/day (inclusion criteria) can be classified as non-sedentary. As we wanted to investigate the effects of MS in recreationally active participants and at the same time avoid that participants increase their usual step amount during the study, we gave the instruction to walk ‘up to 5,000 steps in MS’ per day (on at least 5days/week) from week 2–4, which is in accordance to a prior study by Ridge, et al. 11. The intervention period in prior studies ranged from three weeks 57, over eight weeks 11, ten weeks 17, 12 weeks 4 to several months 8,9. Ridge, et al. 11 report that there were already changes in outcome parameters by four weeks and assume that shorter interventions might also be successful. This was also seen in 4-week foot strengthening interventions 31,32. As we wanted to keep compliance high, participants burden low and add an additional wash-out period, we decided for an intervention period of four weeks. In addition, we increased to the 5,000 steps/day one week earlier than in the study by Ridge, et al. 11.