Addition of transcutaneous electrical nerve stimulation to therapeutic exercise and activities of daily living does not enhance the development of quadriceps strength and voluntary activation: a randomized controlled trial

Abstract

Purpose: Therapeutic exercise (TE) is recommended for improving lower extremity muscle function in individuals with knee osteoarthritis (KOA). Specific rehabilitative efforts are made to enhance quadriceps strength, as quadriceps weakness is a risk factor of KOA onset and is associated with worse physical performance and increased disability in those with KOA. Unfortunately, improving quadriceps strength remains a common clinical problem in those with KOA, as many individuals with KOA are unable to voluntarily activate motor neurons innervating the quadriceps. Transcutaneous electrical stimulation (TENS), applied to the knee, has been found to increase motor neuron pool excitability of the quadriceps in experimental knee joint effusion models. Preliminary evidence from a small laboratory-based study suggests using TENS in combination with TE may improve quadriceps strength and voluntary activation in individuals with KOA compared to TE alone. It remains unknown if TENS applied during supervised TE and activities of daily living will enhance the development of quadriceps strength and voluntary activation in individuals with KOA and voluntary activation deficits. The purpose of our study was to determine if maximal voluntarily isometric contraction (MVIC) strength and voluntarily quadriceps activation could be improved by the addition of TENS to activities of daily living and a progressive supervised TE regimen (TENS+TE). Methods: Ninety individuals with radiographic (Kellgren-Lawrence grading 2-4) and symptomatic KOA (Western Ontario and McMaster University Osteoarthritis Index function subscale score >31%), as well as voluntary quadriceps activation deficits (< 92% on the involved limb, which was determined as the limb causing the most disability in those with bilateral KOA) were included in a phase II, double-blinded randomized controlled trial. Individuals were stratified based on level of voluntarily quadriceps activation and radiographic KOA severity prior to being randomized into 1 of 3 groups including: 1) TENS+TE (n=32, 18 females, 61±7 years old, 30.0±5.7body mass index [BMI]); 2) Sham TENS+TE (n=29, 19 females, 63±7 years old, 29.2±3.3 BMI); or 3) TE Only (n=29, 14 females, 63±8 years old, 27.8±4.5BMI). Quadriceps MVIC was assessed using a HUMAC Norm dynamometer with the knee in 60° of flexion and peak torque values were normalized to body mass (Nm/kg). Voluntary activation was assessed during the MVIC, as described above, using a burst superimposition technique and voluntary activation was expressed as a percent of full activation using the central activation ratio (CAR). Measurements were collected at baseline, following completion of the 4-week intervention (Post 1), as well as 4-weeks following completion of the intervention (Post 2). All individuals received standardized, progressive TE directed by a licensed physical therapist to specifically increase quadriceps strength. The TE intervention consisted of 10 visits scheduled over a 28-day period. Individuals in the TENS+TE group received an active TENS unit used to apply a strong, sensory stimulus to the knee joint and were instructed to use the TENS during all TE sessions, as well as 8 hours per day when they were the most physically active. Individuals in the Sham TENS+TE group were provided the same stimulator designed to emit no current after 20 seconds of initial stimulation. Separate, mixed effects models were used to determine differences between groups over time for MVIC and CAR between baseline and Post 1, as well as baseline and Post 2. We controlled for baseline values, age, sex, BMI at baseline, and number of TE sessions attended for each analysis. Results: Of the 90 total individuals enrolled, 91% and 86% of the sample was retained at the Post 1 and Post 2 follow-up testing time points, respectively. There were no differences in MVIC between groups at Post 1 (P=0.32) or Post 2 (P=0.45), while all groups significantly improved from baseline at Post 1 (P<0.001) and Post 2 (P<0.001; Table 1). Similarly, There were no differences for CAR between groups at Post 1 (P=0.47) or Post 2 (P=0.83), while all groups significantly improved from baseline at Post 1 (P<0.001) and Post 2 (P<0.001). Conclusions: All groups improved MVIC and CAR over the 4-week intervention, which were sustained at 8-weeks; yet TENS did not provide any added benefit for enhancing the effects of TE on MVIC or CAR. Our data suggest standardized, progressive TE was a catalyst for improvement in MVIC and CAR. Previous research has demonstrated only moderate sustainable benefits for physical function and strength following TE. Future work should determine if our inclusion criteria, which specifically selected individuals with low voluntary activation and disability, played a role in identifying individuals who would respond to TE.Tabled 1Mean±Standard Deviation of Quadriceps MVIC and CARQuadriceps Maximal Voluntary Isometric Contraction Normalized to Body Mass (Nm/Kg)BaselinePost 1Post 2TENS + TE1.31±0.521.52±0.58*1.50±0.58*Sham TENS + TE1.28±0.461.51±0.49*1.53±0.48*TE Only1.27±0.521.54±0.59*1.54±0.57*Quadriceps Central Activation Ratio (Percent of full activation)TENS + TE80.54±11.7790.50±10.95*87.47±9.89*Sham TENS + TE78.17±12.1985.92±12.54*85.81±12.45*TE Only77.52±13.8487.47±10.35*86.67±11.76** = Greater than Baseline Open table in a new tab