A sample size was prior calculated using G*power (Version 3.1.9.6 for Macintosh, University of Kiel, Germany) for t-tests between two dependent means (matched pairs; effect size d = 0.5, 2 ToMs, α error probability = 0.05, power: 0.95). A minimum group size of 45 individuals was calculated. To account for potential dropouts, a total of 57 participants (sex: 28♀, 29 ♂; age: 48.5 ± 11.8 years; height: 1.73 ± 0.1 m; weight: 79.1 ± 15.2 kg; body mass index: 26.3 ± 4.0 kg/m²) were included in this crossover randomized controlled study. This study was conducted in accordance with the current guidelines of Declaration of Helsinki and was approved by the responsible ethics commission (No. 55). All participants provided written informed consent after receiving a full explanation of the study and agreeing to the participation and publication of the results.

The inclusion criterion was age >30 years, when postural balance starts declining (28, 29). The youngest participant was 30 years old, and the oldest participant was 68 years old. The exclusion criteria were diabetes mellitus, rheumatic diseases, and neurological disorders of the musculoskeletal system. Additionally, individuals with an acute foot or ankle injury in the past eight weeks were excluded. These exclusion criteria were selected because of their potential influence on balance. As this is the first study on SMFO and balance, a healthy sample was selected in order to check whether effects can already be seen here.

SMFOs were manufactured according to the Woltring/Springer and built using ethylene-vinyl acetate (EVA) material with a shore hardness of 35 by the same and experienced orthopedic shoe technician. The medial and lateral elements were grounded to prevent muscle activation or stabilization. In contrast, the toe bar (7.5–8.1 mm) and retrocapital pad (8.7–9.5 mm) were constructed in relation to the shoe size and not individually manufactured. The exact structure and the height ranges of the elements are given in Figure 1.

First, we randomly determined whether a person started an SMFO or NFO. Next, we randomly determined whether static balance was measured with OE or CE. Figure 2 illustrates the exact procedure followed after randomization. Measurements were conducted in the private setting for each participant using neutral shoes (model: Samba, Adidas AG, Herzogenaurach, Germany) with flat soles and without arch support. Static balance was analyzed using a posturographic measurement of 3 × 30 s (OE and CE) on a mobile force plate (model: GP Multisens, version: MS4, go-tec GmbH, Münster, Germany). The plate features 2.304 resistance sensors over a 39 × 39 cm surface (2 sensors per cm²) with a sampling rate of 200 Hz. Data processing and visualization were performed using the GP Manager (version 7, Go-tec GmbH, Münster, Germany) to derive the movement of the center of pressure, known as the area of the ellipse of the sway area (cm²). Out of the three tests performed for each of the conditions [SMFO (OE, CE); NFO (OE, CE)] mean value was used for the statistical analysis. The pressure plate was positioned facing the wall to reduce distraction. The measurements were performed without acoustic disturbances. The participants positioned themselves in a hip-width stance with the same stand width between the tests. The head was in an upright position, looking straight ahead, and the arms were relaxed on their sides. The participants were instructed to stand as still as possible and to visually fixate on a point on a wall during the tests with OE. The investigator and the display were outside of the field of view to ensure that there was no distraction and, in particular, no visual feedback on the movements of the COP.

Statistical analyses were performed in R (version 4.4.2). The results are expressed as mean values ± standard deviations and 95% confidence intervals. Both, the orthoses (SMFO, NFO) and the setting (OE, CE) were seen as repeated measures factors. The various conditions were calculated in one model based common procedure in this discipline (30). Since the data violated normal distribution (31), a LD-F2 design with ANOVA-Type statistics was calculated using the R package nparLD (32) with effect sizes eta². Significant effects were post-hoc tested with a Wilcoxon signed-rank test with Pearson's r effect size. Statistical significance was set at p < 0.05. The outliers were evaluated by experts and found to be systematic. This was also confirmed by the methodological procedure, which always included the best of three attempts for all conditions in the statistics.

From the authors’ perspective, there are several possible explanations for the positive influence of SMFO on balance regulation. The first explanation is based on a changed proprioception do to the fact that the combination and arrangement of the two elements (see Figure 1) causes the toe flexors to stretch, as the metatarsal heads fall into the gap between the toe bar and the retrocapital pad. Consequently, an increased muscle tone resolved via Ia afferents triggered by the muscle spindles (33). This indicated that the activation frequency of these muscular tension sensors increased in the afferent signal chain (33). The neuromuscular consequence is a contraction of the plantar foot muscles to protect them from further increases in stretch (33). In this case, however, as the participants were standing statically on the SMFO, it is probably only an isometric contraction and an increase in tension, as a concentric shortening of the muscle probably cannot be achieved mechanically due to the standing position. Nevertheless, an increase in the tension of the toe flexors may be beneficial for balance regulation. It is known that the toe flexors play an important role here (34), particularly while standing (35). In a further explanatory approach, it is conceivable that this increase in tension on the plantar aponeurosis could pull the calcaneus into a slight retroversion that transmits a similar supportive toning effect via the Ia afferents to the m. gastrocnemius and m. soleus (36), which also play an important role in balance regulation (37, 38).

The following explanation focuses on the toe bar and its primarily effects on mechanoception. This element of the SMFO is said to be beneficial for the pre-tension of the toe flexors and leads to a greater area of contact between toes and SMFO because of its shape. The increased contact surface can increase afferent information density via known mechanoceptors in the skin (20, 39), such as the Pacini corpuscles, Ruffini corpuscles, and Merkel cells. An increase in sensory information to the CNS seems to be a helpful factor, ashas been shown in other studies (19, 20). Furthermore, the toe bar might improve toe grip strength and standing balance, as Nakano et al. have already revealed for a slightly different toe grip bar (40).

Another explanation is the mechanical stimulus by the elements, which is altered by the surface profile compared to the NFO and causes new joint positioning, which could unconsciously have a greater impact on the afferent information from this area (41–43), which having a unspecific but somehow positive effect on balance regulation, in this case, COP control (19, 44).

The last explanation is a combination of the previous approaches. In conclusion, it should be emphasized that the results demonstrate the positive change with the present experimental design and methodology. Other studies are needed to answer this complex neurophysiological question and thus provide an evidence-based explanation. Nonetheless though the exact neurophysiological processes of the SMFO are unclear, this first study confirms the assumption that the SMFO positively influences balance.

To the best of our knowledge, this is the first study on SMFO and their effects on balance. Therefore, further studies must examine other forms of balance and participants. Nevertheless, previous studies have shown that balance can be influenced by the surface texture of the insole (19). These forms of insoles also influence the afferent information within the sensorimotor system. While the SMFO consists of four central elements, all of which have specific and targeted functions (toning and detoning of muscle tension) (15, 16) this form consists of flat, industrially manufactured insoles that only have a very small und unspecific textured surface that is the same at all points of the insole (19). However, based on the design in these cases, primarily via a change in mechanoception through the non-specific surface texture (spikes, pyramids and granulations) of the insole, there were no fundamental changes to the profile with specific and predefined objectives (19). Compared with other reference values for the area of the ellipse (45), the present values are higher on average, which is presumably due to the methodical approach: average value from three attempts, instead of the best value per condition (46).

Kalron, Pasitselsky (47) showed that in 25 patients with MS with an insole containing miniature square pyramids, CE led to immediate improvements in static balance (sway path length and sway rate). A significant observation was also confirmed after a wearing period of 4 weeks. As in the present study, Kalron, Pasitselsky (47) did not find any significant improvements with OE. Hatton, Dixon (48) made similar observations in 50 healthy adults. With OE, static balance (medio-lateral sway) improved significantly, by 9.2% when wearing an insole with pyramidal peaks. The same authors also found a significant improvement in balance (sway velocity) in patients with diabetic neuropathy (49). A textured insole with a nodule design was used, whereby the intervention group with the textured insole improved significantly, by 5%, after a wearing duration of 4 weeks in the static test with OE. Qiu, Cole (50) showed that patients with Parkinson disease significantly improved their static balance (medio-lateral sway) by wearing textured insoles with granulations across the upper surface. Other studies with a similar designs also demonstrated the positive influence of textured insoles with significant results, particularly with CE (51).

All of them are only partially comparable with the present study, as these types of mainly industrially manufactured insoles must be distinguished from the SMFO by their variation of specific manufactured elements. The insoles used in these studies primarily work with an unspecific and consistently structured surface texture, whereas the SMFO uses different elements that lead to mechanical position changes in the feet and lower extremities via a specific profile with the respective objectives (18, 52–54). Treatment with an SMFO is usually based on a specific indication that is also known from biomechanical foot orthosis treatment and is an individual, handcrafted product. For SMFO, the improvement in balance can therefore, be described as a positive side effect, which can make sense in several clinical cases that affect balance regulation, such as multiple sclerosis (47, 55), Parkinson disease (56), or polyneuropathy (49). However, a comparison with the studies presented shows that balance can be influenced by the sole of the foot and the afferent information to the CNS (20). Furthermore, the findings show that even if the heterogeneity of the collected parameters and participants weakens the comparability, that people seem to benefit from the insoles, especially when their eyes are closed.

Nevertheless, some studies have not shown significant improvements in static balance (57–59). However, it is important to note that, thus far, there have not been any opposing trends that would worsen the balance via these approaches (19).

Although the connection between SMFO and balance requires further confirmatory studies, this topic is highly relevant in practice. Demographic changes and the resulting increase in the proportion of older people are some of the reasons for this. The sense of balance declines continuously from middle age onwards (60, 61). This contributes to an increased number of falls among the elderly population (7). In this population group, the declining visual sense also plays a role (62), so that SMFO could presumably be particularly helpful for these people, as the results with CE show. Consequently, SMFOs could support fall prevention in the elderly population, even though improved balance has a positive side effect in the majority of SMFO treatments. The actual reason for SMFO treatment will probably be a specific indication [e.g., pes planovalgus (63)] or corresponding kinematic abnormalities that justify preventive action.

Furthermore, SMFO may be more important in the context of rehabilitation. Owing to its muscle-activating properties (63, 64), it may be of increased interest to clients anyway. Proprioceptive training, which is basically a type of balance training, is also an essential component of many post-traumatic or post-operative rehabilitation programs, as proprioceptive abilities have a stabilizing and therefore protective effect on joint kinematics (4, 12). This training effect might be enhanced by using SMFO.

Since the ability to maintain balance is often functionally limited because the sensorimotor status is deficient (8), SMFO could make a positive contribution to minimizing the risk of functional ankle instability, as the static balance is considered a predictor for ankle injuries (6). Especially if the SMFO is equipped with the lateral and medial spot in an individually adapted form. Ludwig and colleagues (64) have already been able to demonstrate that the lateral spot of a SMFO significantly increases the muscle activity of the m. peroneus longus. As a main foot pronator, the m. peroneus plays an important role in securing the ankle joint against supination strains. Ludwig and colleagues showed that both the integrated electromyographic output increased and that earlier activation of the same muscle was initiated. No lateral and medial spot were used for this study, as the focus was on the two anterior elements (toe bar and retrocapital pad). Nevertheless, the toe bar and the retrocapital pad already appear to improve the sensorimotor status and improving the static balance, which could be beneficial for the ankle stability.

Although there are evidence-based positive correlations between balance and, for example, the likelihood of injury (5, 6, 65), we must await further research to better assess the clinical relevance of a improved sway area for everyday life. Similarly, further research is needed to explicitly assess the difference between OE and CE of the different conditions (e.g., SMFO OE vs. NFO OE). This step was not taken due to the lack of an interaction effect (Table 2). The general difference between OE and CE, as applied here due to the statistical procedure used, is negligible from the authors’ point of view. The primary argument is the difference that the visual sense as one of the most important supporters of the sense of balance is absent (26). Nevertheless, both parts of the study are important, as they reflect everyday life on the one hand (OE) and reduce the focus on internal sensorimotor control circuits on the other (CE) (45).

Despite these positive results, there are some limitations that restrict their generalizability. This is the first study on balance in combination with SMFO. The sample size was appropriate and healthy, but the results cannot be applied to younger people or older people, or to people with specific diseases. It is questionable whether SMFO would be useful for younger people, as their sense of balance might still be sufficiently developed. Balance has several dimensions. Whether these effects can be positively transferred to a dynamic balance needs to be investigated. Furthermore, this study analyzed only short-term effects, and no statements about the long-term effects can be made. Moreover, to calculate the group differences, the mean values of three measurements per condition were determined. This step results in a high coefficient of variation for all groups (46), which should be noted for future studies and interpretation. Limiting yourself to the respective best value could also be a suitable approach. Posturography is often only used with one measurement per condition in practical screenings.

It should be noted that the SMFOs in this study were prefabricated for all shoe sizes by an experienced master craftsman. This means that the retrocapital pads and the toe bars were adjusted to an average shape in relation to the foot size. However, they were not individually manufactured for each participant. However, from the authors’ point of view, however, this should be seen as a positive rather than a negative aspect, as an individual fitting should be more precise.

Follow-up studies should also compare BMFO as a control variable or target the influence of fatigue (66) as a confounding variable in combination with SMFO. It is conceivable that in the case of intentional neuromuscular fatigue, the support of the SMFO could lead to even better effects. Furthermore future studies should add self-reported ankle stability questionnaires to combine the objective results with the subjective perception.