Disinhibitory Interventions and Voluntary Quadriceps Activation: A Systematic Review

Objective: To determine the effects of various therapeutic interventions on increasing voluntary quadriceps muscle activation. Background: Decreased voluntary quadriceps activation is commonly associated with knee injury. Recently, research has focused on developing specific disinhibitory interventions to improve voluntary quadriceps activation; yet, it remains unknown which interventions are most effective in promoting this improvement. Data Sources: We searched Web of Science from January 1, 1965 through September 27, 2012, using the key words quadriceps activation and transcutaneous electrical nerve stimulation, transcranial magnetic stimulation, cryotherapy, focal joint cooling, joint mobilization, joint mobilisation, joint manipulation, manual therapy, and neuromuscular electrical stimulation. Study Selection: Studies evaluating the effect of disinhibitory interventions on volitional quadriceps activation were used in our review. Standardized effect sizes (Cohen d) and 95% confidence intervals (CIs) were calculated from voluntary quadriceps activation means and standard deviations measured at baseline and at all available postintervention time points from each study. Data Synthesis: Ten studies were grouped into 5 categories based on intervention type: manual therapy (4 studies), transcutaneous electrical nerve stimulation (2 studies), cryotherapy (2 studies), neuromuscular electrical stimulation (2 studies), and transcranial magnetic stimulation (1 study). Transcutaneous electrical nerve stimulation demonstrated the strongest immediate effects (d = 1.03; 95% CI = 0.06, 1.92) and long-term effects (d = 1.93; 95% CI = 0.91, 2.83). Cryotherapy (d = 0.76; 95% CI = −0.13, 1.59) and transcranial magnetic stimulation (d = 0.54; 95% CI = −0.33, 1.37) had moderate immediate effects in improving voluntary quadriceps activation, whereas manual therapy (d = 0.38; 95% CI = −0.35, 1.09) elicited only weak immediate effects. Neuromuscular electrical stimulation produced weak negative to strong positive effects (range of d values = −0.50 to 1.87) over a period of 3 weeks to 6 months. Conclusions: Transcutaneous electrical nerve stimulation demonstrated the strongest and most consistent effects in increasing voluntary quadriceps activation and may be the best disinhibitory intervention for improving the same.