*3.3.1 50% isometric contraction*

An increase in mean EMG activity was observed in the GL and GM muscles for both insole conditions (**Figure 3**). The corresponding increases were smaller for the GL compared to the GM in the CTRL condition, while the mean EMG activity of these two muscles increased similarly when participants were wearing the SSCO. For both conditions and both muscles, the relative changes in mean EMG frequency were negligible (~1%). None of these changes reached statistical significance when comparing across conditions (**Table 3**).


#### **Table 3.**

*Mean relative change in arch index (static and dynamic) and mean EMG activity and frequency (for GL and GM during 50% isometric contraction and for GL, GM, VL, VM during the uphill power walking intervention) from non-fatigued to fatigued (SE: Standard error) (N = 21).*

#### **Figure 3.**

*Mean relative change in mean EMG activity (left) and frequency (right) of the GL and GM muscles from baseline to post-intervention. The error bars represent the standard error resulting from the data of all fatigued subjects (N = 21).*

#### **Figure 4.**

*Mean relative change in mean EMG activity (top) and frequency (bottom) of the GL, GM, VL and VM muscles from first to last minute of the intervention. The error bars represent the standard error resulting from the data of all fatigued subjects (N = 21).*

#### *3.3.2 Uphill power walking intervention*

For both conditions, the mean EMG activity of the GL and GM decreased between the first and last minute of the intervention, while the opposite trend was observed for the VL and GM (**Figure 4** top). The decrease observed in mean EMG activity for the GL and GM was slightly stronger, and the increase in mean EMG activity of the VL was slightly less for the SSCO condition compared to CTRL. No specific difference could be observed when comparing the changes in mean EMG frequency between conditions (**Figure 4** bottom). None of the results reached statistical significance when comparing across conditions (**Table 3**).

#### **4. Discussion**

In this study, a protocol was designed to induce perceived fatigue in the lower limbs of healthy male volunteers. A variety of qualitative and quantitative variables were collected at baseline and post-intervention to assess the impact of perceived fatigue on different metrics. Two insole conditions were assessed and compared including one control (CTRL) and one self-selected customized orthotics (SSCO). The purpose of this comparison was to evaluate the impact of short-term exposure to SSCO on the perception of lower limbs fatigue. This study was motivated by

#### *Impact of Self-Selected Customized Orthotics on Lower Limbs Biomechanics DOI: http://dx.doi.org/10.5772/intechopen.94233*

the fact that, to our knowledge, no investigation had yet presented the potential effect that this new semi-custom insole design may have on the physiology and biomechanical behavior of the lower extremities. In comparison, many reports have highlighted the benefits of customized orthotics, especially regarding their positive impact on balance [2] and pressure distribution [3, 4], which appear to reduce foot pain [1] and improve the quality of life of people with specific lower limb conditions [5–14]. As a result, this study aimed at providing new insights regarding the potential impact of wearing SSCO over a short period of time (sub-hour) on selected variables associated with foot morphology, lower limb muscle activity and perceived fatigue.

In terms of subjective assessment, most of the participants (N = 21) reached the targeted perceived fatigue level within the limited time window defined by the protocol. Among these 21 participants, the mean absolute gain in perceived muscle fatigue was slightly lower when the SSCO were worn. Considering that participants started both trials with the same level of baseline fatigue and that the duration of the intervention was constant across conditions, this indicates that the SSCO helped to slightly reduce the sensation of muscle fatigue during the intervention. In addition, 63% of these participants reported experiencing improvements in foot fatigue when wearing the SSCO. Such insights agree with, see [16], who reported that 68% of their tested population experienced improved foot fatigue after wearing customized orthotics during long periods of standing and walking.

Regarding the relative change in arch index, for both conditions, small differences were observed between left and right foot, as well as between static and dynamic trials. The largest change was found statically on the left foot, where the increase in arch index was slightly lower for the SSCO compared to CTRL. Considering that an increase in arch index indicates a flattening of the arch, such insight may indicate that the integrity of the arch of the left foot was slightly less impacted while wearing the SSCO. Knowing that SSCO were designed to provide additional arch support and if we hypothesize that a flattening of the arch is associated with fatigue (i.e. loss of muscle integrity over time), this result could potentially be linked to the findings of, see [17], who reported that insoles with custom arch support help reduce the impact of fatigue during up- and downhill walking. However, none of the results presented in the current study reached statistical significance. This could be explained by the large standard errors, which is expected to result from the morphological differences between participants (i.e. low vs. high arch). As a result, these results appear challenging to interpret and further investigations should focus on populations with similar arch types to assess whether these standard errors would be decreased.

The mean EMG activity of the GL and GM muscles calculated during the 50% isometric contraction increased between baseline and post-intervention for both conditions. If we hypothesize that the mean EMG activity reflects the number of motor units that is required to perform a certain task, this observation could indicate that, when fatigue occurs, groups of motor units lose their ability to generate a consistent amount of force, therefore leading to more motor units being recruited [23, 24]. The comparison between conditions showed a slightly lower increase in EMG activity for the GM while wearing the SSCO, indicating a potential smaller impact of fatigue on the corresponding muscle. However, the opposite trend was found for the GL during the same task. In addition, the mean EMG activity of these two muscles decreased for both conditions between the first and the last minute of the walking trial, while the opposite trend was found for the VL and VM. Considering the large standard errors and the lack of statistical significance, these findings remain challenging to interpret.

Almost no change was observed with respect to mean EMG frequency during the 50% isometric contraction. However, the mean EMG frequency of the GL and GM muscles increased between the first and last minute of the walking intervention. This indicates that the firing rate of the corresponding motor units increased with fatigue, which is the opposite of the expected behavior reported in the literature [23]. No significant change was observed in terms of mean EMG frequency for the VL and VM muscles throughout the intervention.

These results reflect the complexity of measuring and interpreting long dynamic EMG signals, which can easily be impacted by several external factors such as excessive sweating or skin motion artifacts. In addition, it should be noted that the use of EMG for fatigue assessment has been severely debated in the last few decades, mostly due to the constantly changing definition of the actual fatigue phenomenon. Fatigue was originally broadly defined as the inability to maintain a specific force [25], which became more specific later as the decrease in the force production ability of the neuromuscular system during sustained contractions [26]. Due to the limitations of such definitions with respect to protocol design (i.e. purely static [27]), the definition of fatigue has been evolving throughout the year in order to allow dynamic measurements [28]. However, the scientific community has still not settled on a standard definition of this very subjective concept and its potential assessment, which shows that further research is still needed to elaborate a reliable definition of fatigue and to design consistent protocols to measure it. Nevertheless, this study still resulted in 70% of the individuals experiencing fatigue, with many of those individuals demonstrating less perceived fatigue while wearing SSCO. As a result, the data from this study could be re-evaluated once a more reliable definition of fatigue using EMG is established.

Several limitations should be noted with respect to the design of this study. First, none of the results reached statistical significance, and therefore, every conclusion should be interpreted with care. This may be due to the fact that the sample size was small and included people with slightly different morphologies and physical condition. While all attempts were made to control for such population characteristics, they remain challenging to assess prior to recruitment (i.e. arch type, physical condition with respect to uphill power walking). As a result, large standard errors were observed, which may have been reduced if the sample size had been a bit larger. A large sample size would also have allowed the potential classification of participants based on their morphological characteristics, which should be further explored in future studies.

The short time exposure to the new insole condition (i.e. less than one hour wearing the SSCO) was also a limitation that may not have given enough time for some morphological changes to occur. Future work should inspect the potential long-term impact of wearing such orthotics on the physiological and biomechanical behavior of the lower limbs. It should also be noted that the experiment was not blinded. While participants were unaware of the design and purpose of the SSCO, it was impossible to blind them to the fact that the insoles in their shoes had been changed. To try and limit the potential testing order impact, the session order was randomized among participants.

The very subjective aspect of fatigue and the way it is defined should also be included as an important limitation. In this study, participants were given a specific definition of perceived fatigue (**Table 1**) which was designed to be as intuitive as possible but remains a purely qualitative measure. Finally, the lack of literature regarding the potential impact of SSCO on the physiological and biomechanical behavior of the lower extremities made the direct comparisons with previous studies impossible.
