Maximal motor unit discharge rates in the quadriceps muscles of older weight lifters

Although the existence of "neural factors" is regularly cited as an importa nt contributor to muscular strength, we have little specific knowledge rega rding the existence of such neural factors or how they contribute to the ex pression of muscular force. Purpose: The present investigation sought to as sess maximal motor unit discharge rates in older, highly resistance-trained adults to determine whether maximal motor unit discharge rates might be on e such neural contributor to maximal strength production. Methods: Subjects consisted of seven well-trained older weight lifters (ages 67-79 yr) and f ive untrained age-matched older adults. While subjects performed 50 and 100 % maximal voluntary knee extensor contractions (MVC), recordings from group s of motor units were obtained from the rectus femoris muscle by using an i ndwelling electrode. Off-line analysis was performed to identify individual motor unit firing occurrences and to compute maximal motor unit discharge rates. Results: As expected, knee extension strength in the trained weight lifters (367.0 +/- 72.0 N) was significantly greater than that in the contr ol subjects (299.9 +/- 35.9 N; P < 0.05). Motor unit discharge rates were s imilar in the two subject groups at the 50% MVC force level (P > 0.05), but maximal (100% MVC) motor unit discharge rate in the weight lifters (23.8 /- 7.71 pps) was significantly greater than that in the age-marched control s (19.1 +/- 6.29 pps; P < 0.05). Conclusion: Motor unit discharge rates may comprise an important neural factor contributing to maximal strength in ol der adults.