Peak pressure
The purpose of this study was to compare the effect of forefoot cushioning and a metatarsal pad orthosis on the peak pressure in the forefoot of running healthy recreational runners. The principal finding was that the forefoot cushioning orthosis showed a lower peak pressure in the forefoot in comparison to the metatarsal pad orthosis and the control condition. These findings support previous research where it was shown that shock absorbing and cushioning insoles reduce peak pressure in the forefoot during walking and running in military personnel [
34,
35]. Additionally, in the current literature about footwear and orthoses in sports medicine [
48], forefoot cushioning orthoses are recommended for various conditions due to the positive experiences of clinicians but without any support of evidence including plantar pressure measurements.
The mean values of peak pressures for the forefoot area of this study are consistent with the current literature reporting peak pressures in runners without foot orthoses. One study examined the effect of foot type on in-shoe plantar pressure; the resulting pressure [kPa] in the forefoot was 304.6 ± 118.6 (mean ± SD) [
9]. Another study analysed the effect of plantar fasciitis on pain and plantar pressure; the peak pressure [kPa] in the forefoot of the controls was 374.4 ± 96.4 [
49]. A further study compared the plantar loads during treadmill and overground running; peak pressure [kPa] in the forefoot in treadmill running was 350.8 ± 82.3 [
50]. These studies underline the validity of the current forefoot pressure data. Also, the total foot pressure data [kPa] of the mentioned studies above (305 to 381) were comparable to the current study (328 to 399), which further underlines the validity of measured data. Nonetheless, the methodologies of the mentioned literature differ from the present study mainly with respect to the running surface, running speed and running shoe or foot orthosis type. These differences may account for the small variations in total foot and forefoot pressure data in the findings of the studies mentioned above [
17].
In contrast to earlier studies, however, the metatarsal pad used in the present study showed no significant pressure reduction in the forefoot in comparison to the two other conditions. These results differ from those of the current literature demonstrating that in running and walking individuals a metatarsal pad positioned just proximal to the metatarsal heads achieves optimal pressure reduction in the forefoot [
20,
26,
29,
30,
51]. A possible explanation for this discrepancy might be the huge variability among the methodologies used (e.g., characteristics of the participants, footwear, and position of the metatarsal pad) and that the present study was the only one which used a full contact orthosis with an integrated pad (compared to isolated pads without full contact orthoses). In addition, the individual anatomical variations of the position of the metatarsal heads could influence the optimal positioning of metatarsal pads [
31]. This point could not be considered in this study because prefabricated orthoses were used. By using customised orthoses, the metatarsal pads can be positioned individually according to the metatarsal head position. This presumably may lead to different results in pressure reduction.
Comfort
In accordance with the presented results, it has been reported that recreational runners often wear foot orthoses for the treatment of running-related injuries, as a support or comfort device, and potentially to improve performance [
52]. Authors pointed out that comfort is an important characteristic of footwear and may be influenced by impact perception and cushioning [
53]. Hence, the participants in this study had to rate the perceived comfort of the three different conditions to obtain an impression of the individual subjective wear comfort of the orthoses. In this study, participants to a small extent (plus 15 points on ICI) preferred the forefoot cushioning orthosis over the two other conditions. These results are in accordance with the findings of an earlier study, in which healthy participants prioritised contouring or soft orthoses over hard orthoses [
39,
54]. Each of the three tested conditions in this study was rated as comfortable. It can therefore be assumed that orthosis comfort did not influence the running sequences or the pressure measurements negatively.
Limitations
Questions have been raised about the comparability of studies which have been conducted either with prefabricated or customised orthoses [
16,
48]. However, a growing body of literature suggests that there seem to be only minor differences in the effects on plantar pressures between customised and prefabricated orthoses [
25,
55‐
57]. By using prefabricated orthoses, the metatarsal pads could not be placed individually according to the metatarsal head location. This may lead to different results in forefoot pressure compared to customised orthoses with an individually located pad. The rationale of using prefabricated orthoses with identical surface designs (as presented in this study) was to guarantee a standardised measurement setting.
Several authors discussed the transferability of laboratory measurements made on a treadmill to overground running [
50,
58‐
61]. In one study, it was concluded that treadmill running resulted in lower maximum pressures in the medial forefoot and toe regions [
50]. Nevertheless, the treadmill setting allows optimal conditions for standardised measurements [
58,
59]. In order to guarantee the reproducibility of the in-shoe plantar pressure measurement system pedar-x®-System, it is essential to control running speed [
43]. Furthermore, authors pointed out that a familiarisation time of at least 6 min is required prior to data capturing; this recommendation has been respected in the present study [
61]. Another limitation could be that the in-shoe plantar pressure measurement system provides a reliable measure of the vertical loading but no information about shear forces [
32,
42,
62]. Hence, the effect of the 3D-shape of the tested orthoses could not be analysed precisely. However, in this study, the region of interest was the forefoot, which is a nearly flat area of the foot and therefore the “true” vertical component that is measured is assumed to be comparable to the normal force and can be assessed with sufficient accuracy. Moreover, the hypothesis of this study referred to the testing of relative differences in pressure values, which are likely to be more robust than absolute values [
45]. Furthermore, in this study, only the peak pressure was reported as a pressure parameter because of its high correlation with the mean pressure and pressure–time integral [
63‐
65]. The pedar-x®-System is used in foot orthoses and plantar pressure studies because it can be considered as the gold standard in plantar pressure assessment [
24,
25].
The felt layer, which was fixed onto the surface of the orthoses used in this study for blinding reasons, could have reduced the cushioning effect of the forefoot cushioning orthosis. This aspect could not be considered in the results of this study but it can be assumed that it affected the results. Maybe a further reduction of peak pressure could be achieved without the layer that was integrated to blind the forefoot for all participants in this study.
Since the study was limited to runners with a heel-to-toe running pattern, no conclusion can be made for runners with a forefoot strike pattern [
9]. In addition, all the runners participating in this study had normal-arched feet and therefore no statement can be made for runners with other foot postures.
Validity of the insole comfort index is limited because two criteria have not been met [
39]. Firstly, subject-specific repeatability has not been established and secondly, the test has not been repeated over separate sessions [
39]. The comfort rating of the orthoses was only used to exclude apparent discomfort during measurement. It was not a main outcome.
Despite these limitations, this study has considerable strengths to be mentioned. All the participants ran with the same neutral running shoe on a treadmill with a standardised running speed and therefore any speed or shoe-specific effect was avoided [
66]. Considering the influence of foot type on in-shoe plantar pressure [
9], the foot type of all participants was assessed and classified with the NNHt. As mentioned before in the methods section, all the participants in this study had normal-arched feet. Thus, the influence of differences in foot morphology should not have biased the results of this study. Furthermore, to control for possible sex-related differences in gait biomechanics, a heterogeneous group of female and male runners was recruited for this study [
67].