Reproducibility of tendon jerk reflexes during a voluntary contraction

Objectives: The present study explored whether testing tendon jerks during voluntary contraction of the test muscle would improve reproducibility by effectively ‘clamping’ the excitability of the motoneurone pool at firing threshold. Methods: Tendon jerk reflexes of soleus, tibialis anterior and vastus lateralis and the soleus H reflex were recorded in 12 healthy subjects at rest and during voluntary contractions of 10–20% of maximum. Recordings were repeated 8–10 times in 5 subjects, in whom reflex symmetry was also determined. Results: Not all tendon jerk reflexes could be recorded at rest, and the variability of latency and amplitude was high. All reflexes could be recorded in each subject during contractions. The latency of tendon jerk reflexes decreased by approximately 2 ms during contractions, but H-reflex latency decreased by only 0.2–0.3 ms. For the tendon jerks, an asymmetry of >3.0 ms at rest and >2.5 ms contracting would be outside 3 SD of the normal mean. In repeat studies, the coefficient of variation of reflex latency was <4% for the tendon jerk. Conclusions: A voluntary contraction could potentiate the tendon jerk by a number of mechanisms, but the most important is probably enhancement of the excitability of the motoneurone pool. The present techniques should increase the value of tendon reflex testing when assessing possible peripheral nerve, plexus and root disturbances.