This study aimed to investigate the occurrence and the condition of the overflow from upper limb migrating to inferior contralateral supported limb in standing position. Our first hypothesis was confirmed just for the SO muscle although we found a clinical relevance for the other muscles. The higher EMG activity for SO muscle was in the following order. First, the DIR task (with isotonic contraction), followed by DEXT task (free diagonal, but with an extension of the right lower limb), then the DISO task (an isometric diagonal), followed by DP task (free diagonal), and finally, the lower EMG activity was in R task (just a proposed posture without diagonals, but maintained weight support). So, the second hypothesis was also confirmed because the muscle recruitment was superior in PNF diagonals (with the use of resistance) compared to free voluntary movement, but with differences between the muscles.
The results confirm the overflow of muscle activation from the upper limb to the contralateral supported lower limb. That occurrence was mainly during the DIR task (isotonic diagonal) as the condition with a more significant impact on the increase in muscle activity. The use of isometric diagonal is widespread to promote overflow principle (Gardner, 1963; Moore, 1975; Pink, 1981; Gontijo et al., 2012), but the performance of isotonic PNF diagonals can also change muscular activity in distant regions (Pink, 1981; Gontijo et al., 2012; Abreu et al., 2015; Nakada et al., 2018). The present results revealed the importance of the upper limb diagonal (flexion, abduction, and external rotation of the shoulder, an extension of elbow, wrist, and fingers) in the activity of soleus in standing position. We believe this position, in particular, needs stability, which can be acquired by the facilitation of soleus by the overflow. One crucial function of soleus is the contribution to a prolonged biped posture  Baudry, Penzer, & Duchateau, 2014) in a tonic way. In this line of thought, we can assume that phasic activity, provided during the isotonic diagonal in the upper limb, modified muscle activation in a distant muscle with a predominant tonic activity. An important impact of these findings regards to the rehabilitation field, in the retraining of standing position and balance.
Although the other muscles did not show statistical differences, the moderate and high clinical relevance indicates they participated in overflow. Looking at under kinesiology viewpoint, we can say that all muscles contributed to joint stability to support the movement on the left upper limb. To elevate the upper limb above to head (diagonal primitive), it is necessary trunk extension, and that extension needs the trunk and pelvis stability. While the upper limb moves, the stability was provided by gluteus muscles, AD, RA, and TFL muscles . The GMAX help to hip extension and the GMED and AD muscles to maintain the hip in a neutral position, followed by TFL, while the RA helped to keep the trunk and pelvis stability. Muscle activation also was necessary to achieve body stability during movement. First, SO activation produced plantar flexion, and foot moved against the ground. However, the TA activation produced a balance of SO activation and as a result, the ankle stability . Finally, the VMO was activated to avoid increased flexion of the knee, maintained close to an extension .
The results of this study have a clinical application since many patients cannot stay standing in initial phases of recuperation of an orthopedic, traumatic, or neurological condition. The use of the present approach could improve the condition of distal muscles, mainly soleus, facilitating the acquisition of a standing position as soon as possible. Some examples of situations that could benefit from this approach can be postoperative surgery of knee, ankle when the load is released and cases of stroke or traumatic brain injury in acute or subacute phases.
It is possible to observe several limitations in this study. First, the small sample size does not allow us to generalize the results, but indicates the importance of soleus activity, after overflow intervention. Second, there was no study of kinematic and kinetic variables, which would provide a better understanding of the effects of overflow. It can be indicative of future studies. However, a first step, exploring EMG activity, gives us a start in the knowledge of the overflow effects in an important muscle for the stand position.