Sonographic Quantification of Pronator Quadratus Activity During Gripping Effort.

OBJECTIVES: The aim of this study was to obtain quantitative data related to the activity of each head of the pronator quadratus based on dynamic sonographic parameters during gripping effort and to assess their relationships with grip and pronation strengths. METHODS: The forearms of 33 healthy volunteers were evaluated at 5 positions of axial rotation (full supination, 45° supination, neutral, 45° pronation, and full pronation). Echogenic intensity ratios and muscle thicknesses of each head of the pronator quadratus were measured from transverse cross-sectional sonograms obtained during maximal grip and release. Grip strengths and maximal isokinetic pronation torques at 90°/s and 360°/s were evaluated as strength parameters and correlated with sonographic measures. RESULTS: Echogenic intensity ratios of both heads significantly decreased during power grip (P ≤ .002). Changes in echogenic intensity ratios of the superficial head were greater when the forearm was pronated compared to supination (P < .001), whereas changes in echogenic intensity ratios of the deep head were constant at all forearm positions. Muscle thicknesses of the superficial head maximally increased at the neutral position during power grip (P < .001), whereas muscle thicknesses of the deep head did not change. There were significant negative correlations between grip strength and echogenic intensity ratios of both heads at all respective forearm positions (P ≤ .048). Pronation torque was significantly correlated with echogenic intensity ratios of the superficial head at all forearm positions and the deep head at 45° and full pronation positions (P ≤ .034). CONCLUSIONS: The data revealed that the superficial head of the pronator quadratus more actively contracts when the forearm is in pronation, whereas the deep head constantly contracts at all positions. This study suggests that both heads of the pronator quadratus also play a role in grip strength, but the superficial head contributes more to pronation strength.