The most important findings of this study indicate that SR stimulation used as an adjunct therapy to coordination training enhanced postural stability deficits associated with FAI. Subjects participating in six weeks of coordination training with SR stimulation had better postural stability than subjects training without SR stimulation and control subjects at posttest. Furthermore, treatment effects associated with SR stimulation were greater than effects associated with coordination training alone. Of particular importance were improvements in COPvel and M/L COPsd following training with SR stimulation. Faster COPvel and greater M/L COPsd have been indicative of ankle sprain injury in the physically active [15, 16]. Thus, SR stimulation has implications for treating and preventing ankle sprain injury associated with FAI since this stimulation slowed COPvel and reduced M/L COPsd.
Single leg stance postural stability has also improved with SR stimulation applied to the lower extremity of healthy subjects, elderly, and diabetic patients [27–30]. Furthermore, SR stimulation applied during single leg balance has improved postural stability (COPvel) in subjects with FAI when compared to single leg balance without SR stimulation . Our current results indicate that postural stability as measured by COP measures (COPvel, COPsd, COPmax, COParea) can be enhanced following six weeks of coordination training with SR stimulation after the stimulation was removed. These results have clinical significance, as clinicians can rehabilitate individuals with FAI using SR stimulation for several weeks, and then return individuals to full physical activity with enhanced postural stability.
Potential mechanism whereby SR stimulation improved postural stability in this current investigation might be related to improvement in signal detection and enhancement of motor system function. Stochastic resonance stimulation has been reported to act directly on muscle spindle mechanoreceptors or indirectly through cutaneous fusimotor reflexes to enhance signal detection . Enhanced detection of signals related to postural control could have improved postural stability in the SCT group. In addition to affecting the sensory system, SR stimulation has been reported to affect the motor system in the muscle spindle motoneuron synapse by modulating monosynaptic reflexes generated from muscle spindles . This type of SR phenomenon has potential for improving sensorimotor deficits associated with FAI. Arthrogenic muscle inhibition is a sensorimotor deficit associated with FAI, and has been implicated as a causal factor of FAI, as depressed maximal H-reflex to maximal M-wave (H:M) ratios have been associated with FAI . A therapy such as SR stimulation eliciting greater monosynaptic reflexes has implications for improving arthrogenic muscle inhibition by facilitating muscle activation. Thus, greater dynamic ankle joint stability may result from SR stimulation. In our current study, six weeks of coordination training with SR stimulation might have introduced neuroplastic changes that increased muscle activation, thereby improving postural stability.
The results of this current investigation are similar to results reported in other coordination training investigations [21, 22]. Wobble board training with strips of athletic tape applied to the lateral aspect of the foot and ankle of subjects with FAI has improved single leg postural stability (COParea) more than wobble board training without tape after six weeks of training . Proprioception might have improved by athletic tape stimulating cutaneous receptors during wobble board training . In a related investigation to our current study, the effects of SR stimulation on dynamic postural stability (time-to-stabilization) were examined, and the results indicated that coordination training with SR stimulation might enhance dynamic postural stability in subjects with FAI earlier and to a greater extent than coordination training alone after four weeks of training .
Coordination training alone has improved postural stability in subjects with FAI [17–22]. The medium treatment effect (0.37) associated with CCT group's COParea suggests that postural stability improved COParea following coordination training. This medium treatment effect, however, was not as high as the treatment effect (0.63) associated the SCT group's COParea. This higher effect in the SCT group suggests that coordination training with SR stimulation facilitates rehabilitation more than coordination training alone.
Researchers have also reported that coordination training alone has not impacted certain single leg balance COP measures of subjects with FAI [18, 20, 25]. These results concur with our current findings, as the CCT group did not enhance subjects' postural stability to a greater extent than the control group. Additionally, the moderately negative treatment effect associated with the M/L COPsd in the control group indicates that postural stability worsened at posttest. We do not know the reason for this negative treatment effect. Negative treatment effect for the control group indicates that the M/L COPsd was not a valid or reliable measure of postural stability in this study.
Our orthogonal contrast provided a statistical technique to detect a treatment effect of SR stimulation on postural stability. The rationale for using orthogonal contrasts were based on results presented by several researchers, who reported that learning effects were responsible for COP excursion improvements in both balance training and control subjects [18–20]. Additionally, Verhagen et al  did not find group posttest differences between training and control groups. Thus, we believed that differences might not occur between control and CCT group posttest means in this current investigation. The first orthogonal contrast comparing control and CCT groups in our study was established based on this speculation. The second orthogonal contrast examined the effects of SR stimulation compared to the pooled posttest means of control and CCT groups. Our results indicate that coordination training alone did not result in significantly better postural stability than subjects who did not participate in coordination training at posttest. Since differences were not evident, the pooled means of the control and CCT groups were then compared to the SCT group's means to detect treatment effects associated with SR stimulation. Thus, our results indicate that SR stimulation might be used as an alternative therapy to improve postural stability deficits associated with FAI.
Coordination training that enhances postural stability has implications in preventing ankle sprain injury [1, 20, 24]. Alternative therapies that improve postural stability to a greater extent than coordination training alone might also help prevent ankle sprain injury. Coordination training with SR stimulation is one such alternative therapy that can be used clinically to improve postural instabilities associated with FAI. Future research should confirm our findings with a larger sample size and should examine the effects SR stimulation has on the prevention of recurrent ankle sprain injury in physically active individuals with FAI.