Customized Noise-Stimulation Intensity for Bipedal Stability and Unipedal Balance Deficits Associated With Functional Ankle Instability

Context:

Stochastic resonance stimulation (SRS) administered at an optimal intensity could maximize the effects of treatment on balance.

Objective:

To determine if a customized optimal SRS intensity is better than a traditional SRS protocol (applying the same percentage sensory threshold intensity for all participants) for improving double- and single-legged balance in participants with or without functional ankle instability (FAI).

Design:

Case-control study with an embedded crossover design.

Setting:

Laboratory.

Patients or Other Participants:

Twelve healthy participants (6 men, 6 women; age = 22 ± 2 years, height = 170 ± 7 cm, mass = 64 ± 10 kg) and 12 participants (6 men, 6 women; age = 23 ± 3 years, height = 174 ± 8 cm, mass = 69 ± 10 kg) with FAI.

Intervention(s):

The SRS optimal intensity level was determined by finding the intensity from 4 experimental intensities at the percentage sensory threshold (25% [SRS₂₅], 50% [SRS₅₀], 75% [SRS₇₅], 90% [SRS₉₀]) that produced the greatest improvement in resultant center-of-pressure velocity (R-COPV) over a control condition (SRS₀) during double-legged balance. We examined double- and single-legged balance tests, comparing optimal SRS (SRS_opt1) and SRS₀ using a battery of center-of-pressure measures in the frontal and sagittal planes.

Main Outcome Measure(s):

Anterior-posterior (A-P) and medial-lateral (M-L) center-of-pressure velocity (COPV) and center-of-pressure excursion (COPE), R-COPV, and 95th percentile center-of-pressure area ellipse (COPA-95).

Results:

Data were organized into bins that represented optimal (SRS_opt1), second (SRS_opt2), third (SRS_opt3), and fourth (SRS_opt4) improvement over SRS₀. The SRS_opt1 enhanced R-COPV (P ≤ .05) over SRS₀ and other SRS conditions (SRS₀ = 0.94 ± 0.32 cm/s, SRS_opt1 = 0.80 ± 0.19 cm/s, SRS_opt2 = 0.88 ± 0.24 cm/s, SRS_opt3 = 0.94 ± 0.25 cm/s, SRS_opt4 = 1.00 ± 0.28 cm/s). However, SRS did not improve R-COPV over SRS₀ when data were categorized by sensory threshold. Furthermore, SRS_opt1 improved double-legged balance over SRS₀ from 11% to 25% in all participants for the center-of-pressure frontal- and sagittal-plane assessments (P ≤ .05). The SRS_opt1 also improved single-legged balance over SRS₀ from 10% to 17% in participants with FAI for the center-of-pressure frontal- and sagittal-plane assessments (P ≤ .05). The SRS_opt1 did not improve single-legged balance in participants with stable ankles.

Conclusions:

The SRS_opt1 improved double-legged balance and transfers to enhancing single-legged balance deficits associated with FAI.Key Words: chronic ankle instability, noise, postural stability, therapy

Key Points

• Stochastic resonance stimulation can be considered an alternative treatment for balance impairments.
• Stochastic resonance stimulation may be an effective treatment in the early stages of rehabilitation to facilitate immediate balance improvements that may help patients transition to complex postural stability exercises or functional movements.
• A double-legged balance-optimization protocol may be an efficient method to determine a customized optimal stochastic resonance stimulation intensity that will transfer to improving single-legged balance for functional ankle instability.

Functional ankle instability (FAI) is a residual symptom of ankle sprains that often causes the sensation of “giving way” at the ankle and recurrent ankle sprains.¹ In addition, sensorimotor deficits associated with FAI are present as balance impairments.² Postural instabilities are important to identify because poor balance is a predisposing factor of ankle sprain injury.^3–5 Given that balance improvements associated with rehabilitation often take 6 weeks to occur,^6,7 a therapy, such as stochastic resonance stimulation (SRS), that facilitates balance improvements immediately⁸ or more quickly than rehabilitation alone^9,10 would be beneficial for individuals with FAI. Stochastic resonance stimulation is a therapy that introduces subsensory mechanical noise through the skin to enhance the ability of mechanoreceptors to detect and transmit weak signals related to balance.^11–13Natural noise created in the body can promote signal detection by amplifying weak sensory signals.^14,15 This natural noise occurs from external stimuli, physiologic processes, and biomechanics.^14,15 However, this internally generated noise may not be at a high enough level in some individuals to improve signal detection.^14,15 Healthy and injured individuals may benefit from SRS therapy when the level of naturally occurring noise is too low to facilitate signal detection.^14,15 Most evidence has indicated that individuals with and without sensorimotor impairments react similarly to SRS,^9,10,16–20 suggesting that the level of natural noise in the body is low enough for SRS to have positive treatment effects.Interestingly, however, Priplata et al¹⁸ reported that elderly participants had a better response to SRS than young healthy participants because the former used SRS to facilitate sensory signal detection to reduce sway. In addition, SRS improved balance in the elderly participants to within the normal range for young, healthy participants.¹⁸ Sensorimotor impairments are associated with age, and the naturally occurring noise in the elderly participants might not have contributed to signal detection.¹⁸ Thus, SRS corrected these sensorimotor deficits to facilitate balance improvements.¹⁸ Given the findings in the elderly participants,¹⁸ we postulate that the balance response to SRS might be better in individuals with FAI than in healthy individuals because FAI also is associated with sensorimotor deficits. Currently, no evidence exists to demonstrate that SRS produces better balance for FAI than stable ankles. Demonstrating that SRS improves balance more in FAI than stable ankles lends credence to the notion that this therapy enhances sensorimotor function.Recent evidence²¹ has indicated that the sensorimotor dysfunction with FAI may be due to reflex depressions, which can cause excessive sway with single-legged balance. These poor postural reflexes can result from an inability to integrate afferent input and efferent output.²² That is, diminished sensation from the foot and ankle may not detect signals related to postural control, leading to inappropriate muscle contractions that maintain stability. The inability to sense signals to generate adequate postural reflexes suggests that the naturally occurring internal noise is at a level too low to facilitate signal detection. To correct this sensorimotor impairment, SRS can serve as a pedestal to predispose mechanoreceptors to fire in the presence of real sensory signals, especially signals that otherwise would be undetectable.^11–13The traditional method for examining the effects of SRS on balance improvements is to apply the same subsensory intensity to all participants within a research study.^16–20 Subsensory intensities from 25% to 90% have enhanced balance in patients who are healthy, have diabetes, or have had a stroke.^16–20 Researchers¹⁷ also have presented preliminary data indicating that 75% of sensory threshold could be the optimal SRS intensity to affect the degree of balance improvements. This finding was confirmed in a second experiment¹⁷ when this specific SRS intensity was applied to all participants to optimize balance enhancements.Two research groups recently have proposed customizing the intensity of SRS applied to an individual to maximize treatment effects in lieu of applying the same intensity to all participants.^8,23 The rationale for this customized design was deduced from the early work of Collins et al,²⁴ who demonstrated that performance increased to a peak with increasing levels of SRS intensity and then decreased; however, the SRS intensity associated with this optimal intensity was slightly different for participants. In other words, the levels of SRS intensity for improving sensorimotor function must be fine tuned because subsensory intensities that are too low may not improve balance and those that are too high can diminish function.^{8,11,17,23,24} Furthermore, a customized SRS intensity is proposed for minimizing random error in datasets, potentially decreasing washout effects in a group analysis.⁸ Specifically related to FAI, researchers⁸ using 1 of 2 input SRS intensities have demonstrated that 92% of participants with FAI improved their single-legged balance with at least 1 input intensity, whereas 55% of them had impaired balance at the other input intensity. This finding suggests that using 1 intensity for all participants may have masked the treatment effects of SRS if the intensity that impaired balance was used for analysis.⁸ More recently, Mulavara et al²³ found that customizing the SRS intensity applied to an individual was crucial for maximizing balance improvements in healthy participants. These researchers defined an optimal intensity as the stimulus amplitude emitted from the SRS device that best improved balance over a control (no-SRS) condition.²³ By determining this customized optimal intensity for each individual, we speculate that treatment effects associated with SRS will increase compared with the same intensity for all participants.Double-legged balance tests are recommended for determining the treatment effects of SRS on stability.^16–20,23 These bipedal assessments allow individuals to maximize their stability with a wide base of support, providing a reliable means of determining the optimal SRS intensity. This recommended double-legged SRS protocol has not been tested in participants with FAI. Clinically, this protocol may be important to examine with FAI because balance can be assessed quickly when optimizing SRS intensity. Single-legged balance protocols may not be efficient for optimizing SRS intensity because of the number of unsuccessful trials associated with FAI. However, most researchers do not use single-legged balance as a criterion standard for assessing balance deficits associated with FAI² or quantifying treatment effects of SRS on FAI.^8–10 Therefore, for clinical applications, we propose using a double-legged balance protocol to quickly and efficiently optimize SRS intensity and then using this intensity to enhance single-legged balance. This optimization protocol may be more clinically relevant if the intensity for enhancing double-legged balance transfers to improving single-legged balance.Along these lines of clinical effectiveness, we believe that clinicians need to focus on 1 balance outcome measure when optimizing SRS intensity to improve stability. Common balance outcome measures that have improved with SRS over control conditions include sway velocity, excursion, and area.^16–20 Specifically related to FAI, resultant center-of-pressure velocity (COPV) has been used to assess the immediate effects of SRS on single-legged balance.⁸ Other balance measures also have been examined with SRS in participants with FAI, but all use center-of-pressure excursion (COPE) data points to compute the outcome measures (eg, COPV is computed by dividing excursion by time).^9,10 For clinical applicability, we have taken a minimalist approach in our study by selecting resultant COPV as our main outcome measure for the optimization protocol because it has detected balance improvements associated with SRS in participants with FAI.⁸Therefore, the initial purpose of our study was to determine if a customized optimal SRS intensity was better than a traditional SRS protocol (applying the same percentage sensory threshold intensity for all participants) for improving double- and single-legged balance in participants with and without FAI. Using a customized optimal SRS intensity, we wanted to determine (1) if individuals with FAI and individuals with stable ankles responded at different rates, (2) if double-legged balance (as measured by additional center-of-pressure measures) improved more with the optimal intensity than a control condition, and (3) if the optimal intensity for double-legged balance could transfer to improving single-legged balance over a control condition. Our hypotheses included the following: (1) The customized optimal SRS intensity protocol would improve double-legged balance better than the traditional protocol; (2) the treatment response to optimal SRS would be greater in individuals with FAI than in individuals with stable ankles; (3) the optimal intensity would improve double-legged balance more than a control condition; and (4) the optimal intensity would transfer to improving single-legged balance more than a control condition. The results of our study may be clinically relevant because a customized optimal SRS intensity level that maximally improves balance may enhance rehabilitation outcome measures and lead to greater ankle stability.