Influence of quadriceps conditioning on soleus motoneuron excitability in young and old adults.

In an effort to investigate neuromuscular changes with age, the isometric force-time curve of the Achilles tendon-tap reflex (ATR) was measured in 10 college age (M = 20.9 yr) and 10 healthy active old subjects (M = 74.3 yr). In an effort to investigate spinal interneuronal pathways, the tendon-jerk was also conditioned with a tap to the ipsilateral or contralateral patellar tendon. The conditioning stimulus preceded the ATR by 25, 40, 55, 70, 85, 100, 115, 130, or 145 ms. Three trials were elicited at each conditioning interval plus three unilateral trials, for 30 trials per experimental session. Results indicated that the force production of the unilateral Achilles tendon-tap reflex was significantly reduced and the half-relaxation times were significantly lengthened in the old subjects. Moreover, ipsilateral conditioning produced short-latency facilitation and long-latency inhibition to the triceps surae in young subjects, whereas the same conditioning produced only a delayed long-latency inhibition in the old subjects. Similarly, the contralateral conditioning produced short-latency facilitation in the young subjects, with no changes observed in the old subjects. It is concluded that unilateral Achilles tendon-tap responses are different for the two groups, and that the reflex recovery profiles for the two groups are different. Several neurophysiological mechanisms are proposed to contribute to these differences.