Motor-unit discharge rates in maximal voluntary contractions of three human muscles

F. Bellemare, J. J. Woods, R. Johansson and B. Bigland-Ritchie

Single motor-unit firing rates have been recorded during maximal voluntary contractions using tungsten microelectrodes. Over 300 units from four subjects were sampled from each of three muscles. These were the biceps brachii, adductor pollicis, and soleus, chosen because of known differences in their fiber-type composition and contractile properties. In all cases the contraction maximality was assured by delivering single supramaximal shocks during the voluntary contractions. All motor units were deemed to have already been fully activated if no additional force resulted. Thus for each muscle, the firing rates elicited by a maximal voluntary effort are sufficient to generate a fully fused tetanus in each motor unit. For the biceps brachii and adductor pollicis muscles, the mean firing rates (+/- SD) were 31.1 +/- 10.1 and 29.9 +/- 8.6 Hz, respectively, while for soleus they were only 10.7 +/- 2.9 HZ. For each muscle the firing rates distribution covered approximately a four-fold range about the mean value. The mean firing rates for each muscle varied roughly in proportion to their respective twitch contraction and half relaxation times. These contractile time measurements for both biceps brachii and adductor pollicis agreed well with the mean values reported for human fast-twitch motor units, while those for soleus fell in the range observed for human slow-twitch units. An argument is presented that suggests that, in response to voluntary effort, the range of discharge rates of each motor-unit pool is limited to those only just sufficient to produce maximum force in each motor unit. This suggestion is based on the relationship between the range of motor-unit firing frequencies observed during maximum voluntary contractions, their range of contraction times, and the stimulation frequencies required for maximum force generation. The implications of this hypothesis for motor control are discussed.








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

Applied Sciences
Medicine & Science in Sports & Exercise. 31(11):1638, November 1999.
LEONG, BENTLEY; KAMEN, GARY; PATTEN, CAROLYNN; BURKE, JEANMARIE R.

Abstract:
LEONG, B., G. KAMEN, C. PATTEN, and J. R. BURKE. Maximal motor unit discharge rates in the quadriceps muscles of older weight lifters. Med. Sci. Sports Exerc., Vol. 31, No. 11, pp. 1638-1644, 1999. Although the existence of "neural factors" is regularly cited as an important contributor to muscular strength, we have little specific knowledge regarding the existence of such neural factors or how they contribute to the expression of muscular force.

Purpose: The present investigation sought to assess maximal motor unit discharge rates in older, highly resistance-trained adults to determine whether maximal motor unit discharge rates might be one such neural contributor to maximal strength production.

Methods: Subjects consisted of seven well-trained older weight lifters (ages 67-79 yr) and five untrained age-matched older adults. While subjects performed 50 and 100% maximal voluntary knee extensor contractions (MVC), recordings from groups of motor units were obtained from the rectus femoris muscle by using an indwelling 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 control subjects (299.9 +/- 35.9 N;P < 0.05). Motor unit discharge rates were similar 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-matched controls (19.1 +/- 6.29 pps;P < 0.05).

Conclusion: Motor unit discharge rates may comprise an important neural factor contributing to maximal strength in older adults.












Adaptations in maximal motor unit discharge rate to strength training in young and older adults

Carolynn Patten, PhD 1 2 *, Gary Kamen, PhD 2, Daniel M. Rowland, MS 2
1Rehabilitation Research & Development Center, VA Palo Alto Health Care System, 3801 Miranda Avenue, Palo Alto, California 94304, USA
2Department of Exercise Science, University of Massachusetts, Amherst, Massachusetts, USA
email: Carolynn Patten (patten@rrd.stanford.edu)

Abstract
Six young (mean = 23 years) and 6 older (mean = 76 years) adults participated in isometric resistance training 5 days/week for 6 weeks. The task involved isometric fifth finger abduction. Maximal motor unit discharge rates (MUDRs) were obtained from the abductor digiti minimi of each hand at 0, 2, 14, and 42 days of training using a quadrifilar needle electrode and automatic spike recognition software. In agreement with previous findings, maximal MUDR at baseline was significantly lower in older adults (P < 0.001), averaging 51.5 (±17.13) HZ in young and 43.3 (±14.88) HZ in older adults. In response to resistance training, maximal voluntary force increased 25% in young and 33% in older subjects (P < 0.001). Maximal MUDR increased significantly (11% young, 23% older) on day 2 [F(3,36) = 2.58, P < 0.05], but in older subjects returned to baseline levels thereafter. These adaptations in abductor digiti minimi MUDR suggest a two-part response to strengthening fifth finger abduction: early disinhibition followed by altered MU activation.© 2001 John Wiley & Sons, Inc. Muscle Nerve 24:542-550, 2001