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 e ffectively 'clamping' the excitability of the motoneurone pool at firing th reshold. Methods: Tendon jerk reflexes of soleus, tibialis anterior and vastus later alis and the soleus H reflex were recorded in 12 healthy subjects at rest a nd during voluntary contractions of 10-20% of maximum. Recordings were repe ated 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 va riability of latency and amplitude was high. All reflexes could be recorded in each subject during contractions. The latency of tendon jerk reflexes d ecreased by approximately 2 ms during contractions, but H-reflex latency de creased by only 0.2-0.3 ms. For the tendon jerks, an asymmetry of >3.0 ms a t 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 increa se the value of tendon reflex testing when assessing possible peripheral ne rve, plexus and root disturbances. (C) 1999 Elsevier Science Ireland Ltd. A ll rights reserved.