The results of this study show that four major muscle synergies are used during single-limb stance, i.e., two ankle-dominant synergies, one knee-dominant synergy, and one hip/back-dominant synergy, in an open-eyes as well as in closed-eyes condition. In addition, there is no difference in the recruitment level between the open-eyes and closed-eyes conditions, except for the hip/back synergy, which showed a decreased activation in the closed-eyes compared to the open-eyes condition. At the same time an increase in the ankle balance strategy was found in the closed-eyes compared to the open-eyes condition, confirming the initial hypothesis of this study.

Since the work by Horak and Nashner [42], it is widely recognized the essential role of the ankle for the control of upright posture and for the maintenance of posture when balance is challenged by perturbations of the supporting surface. In these circumstances, muscles around the ankle joint provide the first activation strategy for balance maintenance [14]. In our study, no perturbations were applied to the supporting surface and participants were required to maintain a quiet stance. The key role of the ankle for the control of posture in quiet stance is confirmed by the observation of two ankle-dominant synergies adopted by the participants in this study. The first ankle-dominant synergy (W1) is mainly featured by the tibialis anterior and the soleus muscle activation. The second ankle dominant synergy (W4) is mainly featured by peroneus longus and brevis muscles and gastrocnemius lateralis muscle activations. The two synergies may reflect the activations related to anterior posterior sway and medio-lateral sway, respectively, which may occur during a single-limb stance task. In particular, the co-activation of antagonist muscles, in this case tibialis anterior and soleus, might represent a strategy to cope with reduced base of support, with the aim to reduce movement variability and maintaining stability. Previous studies found an increase in tibialis anterior and soleus muscles activation in particular in older adults to compensate for reduced vision [43] or decreased tendon stiffness [44], and both in children and elderly which showed a diminished postural steadiness when compared with young adults [28].

Literature reports that as difficult the task becomes as higher is the involvement of more proximal joints for the maintenance of balance, in particular the hip [5, 14]. In experimental settings, the difficulty of the task is usually increased by increasing the magnitude of a perturbation, by decreasing the magnitude of the supporting surface or by changing the features of the supporting surface [5, 23, 24, 26]. For example, it has been reported that by moving from a stable to an unstable surface, the angular displacement of the ankle was stable across all the testing condition, with the knee and hip displacement arising when the difficulty of the task was higher [5, 45].

In our study, the difficulty of the task was not modified throughout the experiment and the support surface was not unstable. However, standing on a single limb might be considered as a per se difficult task because of the reduced base of support in comparison to the common double-limbs stance. Usually, when the support base is reduced, a precaution strategy consisting in moving forward the center of mass is adopted to avoid falling backward. This explains the presence of the hip/back muscle synergy (mainly featured by hamstrings and back muscles) adopted by the participants of our study. It should be also mentioned that, in a condition of quiet stance, the co-existence of the hip strategy with the ankle strategy has been reported [45], highlighting that the two strategies are not different entities, but one predominates depending upon the task and conditions of the environment [45]. It is reasonable to think that the participants of the present study used the hip/back synergy to compensate for ankle dorsiflexion used to move forward the center of mass to manage the reduced base of support.

The essential role of quadriceps muscle for balance control during single-limb stance tasks is highlighted by the presence of the knee-dominant synergy (W2) used by the participants in this study. In fact, the knee-dominant synergy was probably used when the ankle synergy was not effective for the maintenance of balance, but the condition did not require yet the involvement of the hip or the back synergy. These results highlight the fine coordination between ankle muscles and quadriceps muscle. It was observed that when the knee-dominant synergy was used, ankle muscles had in general a low activation. This was especially observed in the closed-eyes condition, when the lack of visual information led to an increase in the difficulty of the task. In fact, it was observed a significantly lower activation of the soleus and gastrocnemius muscles when the knee-dominant synergy was used. This observation arises two possible speculations. The first is that the knee synergy is used when the ankle synergy is not sufficient for balance control. The second is that knee-synergy may be effective alone to guarantee stability during single-limb stance in some circumstances. At the same time when ankle-dominant synergies are used, a low activation of the quadriceps is observed in particular when the ankle synergy is featured by evertor muscles activation. This could be explained by the fact that this synergy is mainly used to manage with medio-lateral displacement. This observation is further confirmed by the higher activation of back muscles of the contra-lateral side for back stabilization in the mediolateral direction.

However, despite some differences in the closed- compared to the open-eyes condition, the number of synergies used is the same between the conditions, as well as the level of recruitment. This is in accordance with previous literature reporting the stability of muscle synergies adopted between tasks with the variation of the visual feedback [24, 46]. It has been shown that in general the lack or the disturbance of vision does not affect synergies because during standing postural control mostly relies on proprioceptive feedback [24, 46]. In fact, the results of previous investigations show that proprioceptive disturbance, but not visual disturbances, affected the regulation of muscle synergies [24] and the increase in body sway [46].

The reduction in the recruitment level of the hip/back synergy in closed- compared to the open-eyes condition seems not in accordance with previous literature, reporting a major involvement of proximal joints as the difficulty of the task increases [5, 42, 45]. However, in the present study, an increase in the involvement of the ankle-dominant synergy for balance control has been observed in the closed-eyes condition. This result confirms the initial hypothesis of this study on the increase in muscles activations around the ankle joint. It is likely to think that this modulation aimed at decreasing the degrees of movement to increase stability, was probably sufficient to maintain balance and the use of muscle synergies involving proximal joints and segments was not determinant for the outcome of the task.

The observation of a change in the modulation of some of the muscle activations in the closed-eyes condition is in accordance with previous literature. A decrease in synergistic muscle coherence was observed during double-limb stance in a closed-eyes compared to an open-eyes condition [31], thus showing that the lack of visual feedback and the reliance on other sources of afferent information affects the generation of neural inputs on synergistic muscles. Regarding the results of the present study, it can be thus speculated that the lack of visual information affects the modulation of muscle activation, without altering the type and numbers of synergies adopted. For example, an increase in the ankle balance strategy was found in the closed-eyes compared to the open-eyes condition. It is plausible to think that the lack of the visual feedback led to a sensory reweighting for the control of posture, shifting the sensory information arising from vision with an increased proprioceptive information arising from the ankle joint and ankle movements [47].

Accordingly, the results of this study suggest also that muscle synergies are probably not exclusively managed throughout a feedforward control, but can be modulated with a feedback control based on the signals arising from sensory receptors, with the aim to correct movement errors which may occur in some circumstances. It is likely to think that the maintenance of the single-limb stance in this study was controlled with pre-programmed muscle synergies. However, the difficulty of the task leading to continuous losses and recovery of balance probably needs a continuous movement correction based on a feedback control relying on information arising from sensory receptors. Animal studies have reported organized patterns of muscles activations in response to focal stimulation of the spinal cord [48,49,50,51], thus suggesting that a feedback control may be launched at spinal level in response to specific sensory stimuli to modulate the centrally organized synergy recruitment. It is likely to think that similar patterns may regulate muscle synergies also in humans.

Finally, it should be mentioned that in this study biceps femoris and semitendinosus, which are two-joints muscles, were grouped in the hip/back synergy, and not into the knee synergy. This is related to the fact that as for the nature of the task, hamstrings muscles were more deputed to the hip extension than to knee flexion [52, 53]. At the same time, quadriceps muscle, which is also a two-joints muscle, was grouped only in the knee synergy. This is related to the fact that participants were asked to stand in an upright posture with the hip joint in full extension. In the latter position, the quadriceps (and in particular the RF) has a reduced activation and thus a lower involvement in the hip joint control [54].

To the best of the authors’ knowledge, this is the first study investigating muscle synergies deputed to the maintenance of posture during a single-limb stance task without external perturbations, in an open-eyes and closed-eyes condition. Due to the large use of this kind of task in clinical practice, both for rehabilitation and functional assessment, as well as in sport practice for training and testing, the results of the present study give important information on motor control of this kind of task in healthy individuals. Future studies should investigate muscle synergies also in other populations to investigate the effects of orthopedic and neurologic pathologies on muscle synergies, as well as the effect of rehabilitation and training.

The main limitation of this study is that we recruited only healthy young individuals, and thus the results cannot be generalized to all healthy individuals. Future studies should identify muscle synergies used for single-limb stance also in other age groups. A second limitation of the study was that muscle synergies for the transition between double- and single-limb stance (and vice versa) were not analyzed, thus the results of the present study have to be considered exclusively for steady single-limb stance tasks.

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