Adarq.org
Performance Area => Peer Reviewed Studies Discussion => Topic started by: adarqui on June 04, 2009, 06:13:58 pm
-
All conclusions of studies will be listed in this original post (TABLE OF SUMMARIES) for quick reference.
Post any study regarding MUDR / Rate coding on strength, explosiveness, and performance.
1. Motor-unit discharge rates in maximal voluntary contractions of three human muscles
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.
2. Maximal motor unit discharge rates in the quadriceps muscles of older weight lifters.
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).
3. Adaptations in maximal motor unit discharge rate to strength training in young and 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.
4. Quadriceps muscle strength, contractile properties, and motor unit firing rates in young and old men
Thus, the substantial age-related weakness in this muscle does not seem to be related to changes in neural drive. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 1094-1103, 1999
5. Fatigue of submaximal static contractions.
The single unit EMG recordings suggest that, in sustained and repeated submaximal contractions, muscle contractile failure is compensated by recruitment of additional motor units rather than by rate coding of those already active. During intermittent contractions large increases in the surface EMG were associated with only modest increases in firing rates. In sustained contractions when the EMG was held constant the discharge rates declined in parallel with the force. In constant force contractions involving about 35% muscle contractile failure no changes in discharge rates were seen despite substantial increases in EMG.
-
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
-
Quadriceps muscle strength, contractile properties, and motor unit firing rates in young and old men
Abstract
Changes with age in the voluntary static and dynamic strength of the quadriceps muscle group have been well characterized, and the importance of the muscle group for locomotion and independent living have been highlighted in both normal human aging and in clinical studies. Surprisingly few studies of this muscle group have described age-related changes in voluntary activation ability using twitch interpolation and changes in stimulated contractile properties, and none have assessed the influence of old age on motor unit firing rates. We compared in 13 young (mean age 26 years) and 12 old (mean age 80 years) men the voluntary isometric strength, stimulated contractile properties, and average steady state motor unit firing rates in the quadriceps muscle. Maximum voluntary contraction (MVC) force and twitch tension were 50% lower in the old men, but contractile speed was only 10% slower than in the young men. There was no difference in the ability of either group to activate the quadriceps to a high degree (94-96%). At all isometric force levels tested (10%, 25%, 50%, 75%, and 100% MVC), there were no differences in mean motor unit firing rates. In both groups, the range of firing rates was similar and not large (8 Hz at 10% MVC and 26 Hz at MVC). Thus, the substantial age-related weakness in this muscle does not seem to be related to changes in neural drive. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 1094-1103, 1999
Fatigue of submaximal static contractions.
Bigland-Ritchie B, Cafarelli E, Vøllestad NK.
Experiments are described which suggest that the loss of force generating capacity seen during fatigue from intermittent, submaximal voluntary contractions of the quadriceps muscle cannot be explained by any of the usual factors thought to be responsible for fatigue. During the first 30 min of intermittent contractions at 30% MVC the force generated periodically by a brief test train of 50 Hz stimulation and by brief maximal voluntary contractions both declined by 50%. Yet no significant changes were seen in the muscle lactate, ATP or phosphocreatine. Glycogen depletion was confined only to the type I and type IIA fibres, with less than 10% totally depleted. The depletion patterns indicated that the type IIAB and type IIB motor units were not recruited during the first 30 min. The central nervous system appeared to remain capable of generating full muscle activation since the force from maximal voluntary efforts declined in parallel with that from 50 Hz stimulation. We suggest that, in this type of fatigue, the loss of force may be largely due to impaired excitation/contraction coupling. This possibility is supported by the disproportionate depression of the twitches recorded between contractions compared with that from 50 Hz stimulation (low frequency fatigue). The single unit EMG recordings suggest that, in sustained and repeated submaximal contractions, muscle contractile failure is compensated by recruitment of additional motor units rather than by rate coding of those already active. During intermittent contractions large increases in the surface EMG were associated with only modest increases in firing rates. In sustained contractions when the EMG was held constant the discharge rates declined in parallel with the force. In constant force contractions involving about 35% muscle contractile failure no changes in discharge rates were seen despite substantial increases in EMG.