The effects of body position and muscle activation on patellar tendon reflex properties

Our purpose was to examine the effects of body position and a low-intensity voluntary contraction on patellar tendon tap reflex properties.Surface electromyography, torque, and accelerometry signals were obtained from 30 subjects (25.0   ±   4.6 years) during patellar tendon taps. These signals were used to quantify reflex magnitude and the subcomponents of reflex latency for each subject in the upright position (i.e. control), supine position, and during α-γ coactivation (i.e. a low-intensity contraction). The Jendrassik maneuver was also performed to examine any potential benefits beyond that of standard reinforcement.Neither experimental condition significantly altered reflex magnitude. However, the supine body position condition resulted in a significant decrease in reflex latency (p = 0.037) which appears largely attributable to a decreased electromechanical delay. Interestingly, the low-intensity contraction had no effect on any of the latency components.The assessment of reflex latency can be improved by utilizing a supine body position. This effect may be due to the presence of slack within musculotendinous structures during the traditional upright position. A voluntary contraction, however, does not enhance the reflex response beyond that of standard reinforcement. Limitations regarding the use of a light contraction during tendon taps are discussed for future investigations.