In support of our primary hypothesis, a significant improvement in JPS relative to the control condition was found when SR stimulation was applied with a neoprene sleeve in normal subjects during a PWB task. The importance of the combined SR stimulation and sleeve condition is that the clinical application of SR stimulation would undoubtedly be applied by electrodes incorporated into some form of sleeve, brace, or garment. Thus, the current findings show some promise for the clinical application of SR stimulation to enhance knee proprioception. The observation that the improvement in proprioception occurred in the PWB knee is also important in that it suggests the potential for this condition to improve joint positioning in a more highly loaded knee when the knee is typically at greater risk of injury. Our findings correspond with a previous study of SR electrical stimulation of the knee which has shown that postural sway can be reduced during single-legged stance in older adults. Somatosensory information is critical to balance control and this previous study indirectly suggested the potential of SR stimulation to enhance the sensitivity of the somatosensory system and improve knee proprioception.
For the PWB task, our findings that JPS was no different with SR in combination with the sleeve when compared with the sleeve alone were contrary to our hypothesis. However, our mean comparison testing also showed the JPS of the sleeve alone condition and the SR stimulation alone condition were each not different from the control condition in the PWB task. These results highlight the importance of the sleeve in enabling enhancement of knee proprioception with SR stimulation. Past studies have shown that sleeves or braces can enhance knee proprioception in the NWB knee [12, 13]. However similar to our findings, a past study has been unable to demonstrate a significant improvement in knee proprioception with the presence of a sleeve/brace in the WB or loaded knee . It is unclear why the SR stimulation alone was unable to improve JPS, but it may be that the presence of the sleeve increased coupling at the skin-electrode interface during limb movement.
For the NWB task we were unable to detect any improvements in JPS with any of the 4 conditions, and thus were unable to provide support for our hypothesis under NWB conditions. It is unclear if the lack of an effect with the SR stimulation/sleeve condition in the NWB task was a result of us being unable to detect this effect, or if there was truly no such effect. The absence of an effect of SR stimulation with the sleeve in the NWB condition could be because the mechanoreceptors contributing proprioceptive input in the NWB limb were not specifically targeted by the SR stimulation. In addition, a lack of an effect could also suggest that joint tissues may have to be prestressed for the mechanoreceptors residing in them to be more responsive to the SR stimulus. Additionally, since our standard deviation values for the NWB task were greater than the estimated 50% that was set in our priori power analysis, it is possible that a type II error may have occurred. Similar to past studies, our data showed a pattern for the sleeve alone to enhance knee proprioception, however this did not prove to be statistically significant in our study. Birmingham et al.  demonstrated a 1.2 degree decrease in absolute mean error when a sleeve was added during a sitting open kinetic chain exercise in healthy young adults. Herrington et al.  demonstrated a 0.6 degree difference in mean absolute error between the no sleeve and sleeve conditions for subjects seated in a NWB position. Specific to this study, we saw a 0.90 degree difference in mean absolute error when the sleeve was added compared with the control condition. When comparing the absolute error values of the NWB task to the PWB task it can be observed that the errors are larger for the NWB. This pattern agrees with past studies that have shown JPS to be more accurate during WB tasks than for NWB tasks [2, 3]. Investigators have suggested that the improved JPS present in the WB limb is likely due to increased proprioceptive information being available [2, 3, 14]. This augmented proprioceptive information may be coming from adjacent joints or because of enhanced stimulation of mechanoreceptors of the joint of interest when tissues are loaded.
The small magnitude of improvement in JPS with the SR stimulation/sleeve condition in the PWB task may prompt some to question the clinical significance of this effect. While it is difficult to define what magnitude of improvement in error is clinically significant, studies examining the influence of proprioceptive training on knee function provide some indication that the observed difference may prove clinically significant. Tsauo et al. 2008  and Lin et al. 2007  each conducted randomized clinical trials to evaluate the effect of proprioceptive training exercises on knee proprioception and self-reported knee function in patients with knee osteoarthritis. Both studies found improvements in the absolute error of JPS testing of approximately 2 degrees with training. Both studies also reported a significant improvement in self-reported function (WOMAC index) with training that occurred in parallel with the improvement of proprioceptive acuity. These studies are suggestive that minor improvements in proprioception acuity can cause significant changes in function.
An important consideration in interpreting the results of this study is that the improvements seen with the treatment conditions may have been limited by utilizing young, healthy adults. Our regression analyses indicated that larger proprioceptive improvements occurred in individuals with larger initial errors for the control condition. This observation leads us to believe that enhancements in knee proprioception with the SR stimulation/sleeve condition may be greater in a clinical population that has a knee proprioceptive deficit. There are several clinical populations with a knee proprioceptive deficit that could be the focus of future studies with SR stimulation. Knee proprioception is known to be impaired with aging, knee osteoarthritis (OA), and ACL injury [7, 10, 22]. The effect of the SR stimulation/sleeve condition could be examined in each of these populations to determine if improvements in proprioception are greater than those observed in healthy subjects.
While we believe this study is important with valid results, it was not without limitations. Lasting effects of the stimulation may have been a limitation as they could have affected results in subsequent conditions; however the counterbalanced design of our study likely minimized such an effect, and our analysis indicated no effect of the relative location of the control condition within the task sequence. The fitting of the neoprene knee sleeve may have also been a limitation. The sleeve was fitted for each subject based on comfort. Hassan et al.  tested JPS in OA subjects while the subjects wore one of two types of bandages, with one fitting more loosely than the other. They found a significant improvement in proprioception acuity with the looser bandage, but no improvement with the standard fit bandage. Specific to our study, we felt the neoprene sleeve was fit securely enough to provide the necessary support, although the degree of cutaneous mechanoreceptor stimulation may have varied across subjects. The use of a single target angle may have been a limitation as well. It is possible that despite the use of "dummy angles" incorporated throughout the testing sequence, a memorization effect may have remained. Testing joint position sense with only 3 repetitions may have also been a limitation as some studies suggest there should be at least 5 repetitions before stable data can be assumed [23, 24]. Selfe et al. evaluated the effect of increasing the number of test trials in the assessment of knee joint position sense by measuring the progression of means and standard deviations as the trial number increased . Their goal was to determine the point at which the mean and standard deviation could be considered to have stabilized and this was set as the point where the standard deviation changed by less than 5 percent of the cumulative mean. The progression of the means and standard deviations were calculated specific to this study and we found the change in standard deviation was within 5 percent of the cumulative mean after only three trials in all conditions of the NWB task and in all but one condition of the PWB task (E:NS). The stimulation alone (E:NS) condition was not statistically significant. Additionally, we believed conducting 5 repetitions would have extended an already lengthy testing session likely causing the subjects to lose focus during the testing. Similar studies testing knee proprioception have used 3 repetitions [2, 19]. A final limitation may be that the level of stimulus may not have been high enough to elicit activation of the specific mechanoreceptors required for proprioceptive acuity.