Biomedical subjects

Gary Kamen

Publications and source records attributed to Gary Kamen.

14 recordsLinked to original sources

PubMed2006

Modulation of motor unit firing rates during a complex sinusoidal force task in young and older adults.

This study compared motor unit rate coding and muscular force control in the first dorsal interosseous muscle of older (n = 11, mean 72.3 yr) and young (n = 12, mean 18.7 yr) adults. Rate coding during a sinusoidal isometric force-matching task was evaluated using spectral analysis of the time-varying changes in firing rate. The task required force modulations to match a trajectory comprising the sum of 0.15- and 0.45-Hz sine waves. Based on the amplitude of spectral peaks at 0.15 and 0.45 Hz, the amplitude of force modulation was similar in young and older adults at both frequencies (F = 1.9, P = 0.17). Force modulation gain (FMG) was computed as the ratio of the amplitude of force modulation to the amplitude of firing rate modulation. To account for rate coding differences related to the properties of the motoneuron, recruitment threshold force was used as a covariate in age-group comparisons. At both task frequencies, firing rate was modulated with less amplitude (F = 0 14, P < 0.001) and FMG was greater (F = 0 27, P < 0.001) in the older adults. In its transformation of neural input to mechanical output, muscle is known to act as a low-pass filter. Compared with modulation at 0.15 Hz, less change in force per change in firing rate at 0.45 Hz (lower FMG; F = 0 67, P < 0.001), independent of age group, is consistent with this filtering effect. Our conclusion is that there is a reduced amplitude of firing rate modulation in older adults.

Action Potentials↗

PubMed2006

Doublet discharges in motoneurons of young and older adults.

The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 +/- 3.56 yr) and eight older (74.1 +/- 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges.

Adult↗

PubMed2006

Motor-evoked potentials following imagery and limb disuse.

This article investigated whether mental imagery training could reduce the detrimental effects produced by limb disuse. Subjects learned a task requiring fifth finger abduction. Motor-evoked potentials (MEPs) were obtained from the abductor digiti minimi muscle using transcranial magnetic stimulation. The arm was casted and the experimental group then practiced using imagery. After seven days, variable error scores increased. The immobilization treatment decreased MEP scores in both groups, with no difference between groups. It is concluded that limb immobilization produces a decrement in the area of motor cortex dedicated to this muscle as a result of disuse. Imagery failed to ameliorate the disuse-related changes.

Adolescent↗

PubMed2006

Neural adaptations to resistive exercise: mechanisms and recommendations for training practices.

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation. An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated. There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation. Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force. Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the integrity of the joint. Thus far, it is not clear what the CNS will optimise: force production or joint integrity. The following recommendations are made by the authors based on the existing literature. Motor learning theory and imagined contractions should be incorporated into strength-training practice. Static contractions at greater muscle lengths will transfer across more joint angles. Submaximal eccentric contractions should be used when there are issues of muscle pain, detraining or limb immobilisation. The reversal of antagonists (antagonist-to-agonist) proprioceptive neuromuscular facilitation contraction pattern would be useful to increase the rate of tension development in older adults, thus serving as an important prophylactic in preventing falls. When evaluating the neural changes induced by strength training using EMG recording, antagonist EMG activity should always be measured and evaluated.

Adaptation, Physiological↗

PubMed2005

Evidence of self-sustained motoneuron firing in young and older adults.

Motoneurons demonstrate a type of self-sustained firing behavior that seems to be produced by a prolonged period of depolarization caused by intrinsic long-term changes in the motoneuron. Such self-sustained firing behavior has previously been reported in human motor units. The purpose of the present study was to investigate the occurrence of self-sustained firing behavior in older adults. Eight young (mean age 24 yrs) and eight older (mean age 73 yrs) individuals participated in the investigation. While subjects produced light dorsiflexion contractions, a brief vibration stimulus was applied to the tibialis anterior muscle. Motor unit recordings were also obtained from the tibialis anterior muscle. Self-sustained firing behavior was evidenced by the appearance of new motor unit recruitment following vibration, even as the motor units that fired before the vibratory stimulus maintained a steady firing rate. The proportion of motor units exhibiting self-sustained firing activity was similar in both young and older adults (approx. 23% of trials). We conclude that self-sustained firing behavior is a ubiquitous phenomenon that does not seem to be affected by the aging process.

Action Potentials↗

PubMed2005

Decreases in motor unit firing rate during sustained maximal-effort contractions in young and older adults.

Previous studies have suggested that older adults may be more resistant to muscular fatigue than young adults. We sought to determine whether motor unit firing rate might be a factor that determines the response to fatiguing exercise in young and older subjects. Motor unit recordings and muscular forces were obtained from the tibialis anterior (TA) muscle of 11 young and 8 older individuals. Maximal voluntary force was first measured during maximal-effort dorsiflexion contractions. Each subject then performed a series of 15 maximal isometric contractions, with each contraction lasting 30s. A 10-s rest period separated the fatiguing contractions. As a result of the fatiguing exercise, both subject groups demonstrated a significant loss in maximal force. The force decline was less in the older adults (20.4%) than in the young adults (33.8%). As expected, prior to muscle fatigue, maximal firing rates in the TA muscle were greater in the young (28.1+/-5.8 imp/s) than in the older adults (22.3+/-4.8 imp/s). The decrease in motor unit firing rate with fatigue was also greater in the young adults (34.9%), than in the older adults (22.0%). These results suggest that the greater fatigue-resistance exhibited by older individuals might be explained by the fact that the decline in motor unit firing rate during fatigue is greater in young persons than it is in older adults.

Action Potentials↗

PubMed2005

Aging, resistance training, and motor unit discharge behavior.

Researchers have alluded the existence of "neural factors" in the expression and development of muscular strength. Candidate neural factors including motor unit recruitment, rate coding, double firing, and motor unit synchronization are discussed in this review. Aging is generally accompanied by lower motor unit discharge rates. However, both young and older adults exhibit rapid changes in muscular strength with repeated strength testing. These strength changes occur with concomitant albeit transient increase in motor unit discharge rate. These and other neural factors may contribute to the initial increase in muscular strength observed during the early phases of resistance exercise training.

Adaptation, Physiological↗

PubMed2004

Enhanced motor unit rate coding with improvements in a force-matching task.

These data describe improved modulation of discharge rates (rate coding) of first dorsal interosseous motor units throughout the acquisition of a complex force-matching skill involving isometric index finger abduction. In each of 15 consecutive trials, subjects attempted to match their force to a trajectory consisting of the sum of two sine waves (0.15 and 0.5 Hz) and random oscillations (overall mean force level approximately 20% MVC). Reductions in root-mean-square (RMS) error of each subject's force relative to the trajectory indicated substantial improvements in force-matching ability (F = 33.8, p < 0.001). With the acquisition of this new skill, there was increased amplitude modulation of muscular force near both dominant frequencies of the force-matching trajectory (F = 10.6, p = 0.008). The standard deviation and coefficient of variation of motor unit inter-spike intervals both decreased with improved performance indicating a general reduction in the amplitude of firing rate modulations (SD: F = 18.69, p = 0.001; CV: F = 43.6, p < 0.001). After skill acquisition, there was decreased firing rate modulation outside of the two dominant frequencies and increased amplitude of firing rate modulation at the higher of the two dominant frequencies (0.5 Hz, F = 8.23, p = 0.015). These findings indicate that improved precision of rate coding was a contributor to the acquisition of the new force-matching task. That the change in rate coding was frequency dependent suggests that factors other than frequency coding may contribute to the improved force matching at 0.15 Hz.

Adult↗

PubMed2004

Neuromotor issues in human performance: introduction.

Two current approaches frequently implemented to understand mechanisms involved in motor control are presented in a series of papers. One approach aims at understanding what specific neural mechanisms are responsible for motor behavior Such neural mechanisms might be operative at any of several sites in the nervous system, including spinal sites capable of complex reflex organization, as well as supraspinal sites involved in integrating multiple sensory and motor processes. Alternative approaches involve the assessment of kinematic and other variables used to describe the behavior using dynamical systems theory. Such an approach allows the researcher to explore relationships among groups of variables in, for example, multijoint control. In the series of papers that follow, the value of both of these approaches is discussed using examples from tasks that involve both rhythmic activities (running and cycling) and discrete movement (weight lifting and golf).

Exercise↗

PubMed2004

Neural issues in the control of muscular strength.

During the earliest stages of resistance exercise training, initial muscular strength gains occur too rapidly to be explained solely by muscle-based mechanisms. However; increases in surface-based EMG amplitude as well as motor unit discharge rate provide some insight to the existence of neural mechanisms in the earliest phases of resistance exercise training. Moreover, other phenomena such as the cross transfer of strength following unilateral exercise and neural changes following limb immobilization also support a role for neural control in muscular strength. While studies to date have focused on the phenomenon of neural change, future studies will need to focus on the many motoneuron influences as well as intrinsic motoneuronal properties that may be responsible for these neural adaptations.

Electromyography↗

PubMed2004

Training-related adaptations in motor unit discharge rate in young and older adults.

BACKGROUND: This study evaluated changes in motor unit (MU) firing rates in young and older adults during exercise training. METHODS: Vastus lateralis MU discharge rates were measured in 8 young and 7 older adults. Each participant performed isometric knee extension contractions at 10%, 50%, and 100% of maximal voluntary contraction or effort (MVC) on two separate occasions. Participants then completed a 6-week resistance exercise training protocol. RESULTS: Significant increases in maximal force were observed as early as 1 week after the first baseline testing session, and these were accompanied by increases in the MU discharge rate. Exercise training increased maximal voluntary force by 29% and 36% in the young and older adults, respectively. Motor unit discharge rates at 100% of maximal effort were significantly greater in the young (24.7 pps) than in the older adults (17.8 pps). Furthermore, the young adults also exhibited significantly greater discharge rates at 50% MVC, but there were no differences at the 10% force level. Maximal MU discharge rates increased during the 1-week period separating the two initial testing sessions. After the 6-week training period, maximal MU discharge rates were 15% higher for the young adults and 49% higher for the older adults. No changes in discharge rates were observed for either group at 10% or 50% MVC after exercise training. CONCLUSIONS: The early increase in maximal MU discharge rate with repeated maximal force assessment may comprise an important neural mechanism mediating early, rapid gains in muscular force capability.

Adaptation, Physiological↗

PubMed2004

Reliability of motor-evoked potentials during resting and active contraction conditions.

PURPOSE: To determine the reliability of motor-evoked potentials (MEP) obtained using transcranial magnetic stimulation (TMS) in the first dorsal interosseous (FDI) and biceps brachii muscles. METHODS: Fourteen college subjects attended the laboratory on three separate days. TMS was used to obtain MEP with the subject relaxed (resting condition) at stimulation intensities of 70%, 85%, and 100% of maximal stimulator output. MEP were also obtained during four active contraction conditions involving contractions of 25%, 50%, 75%, and 100% of maximal effort (MVC). Reliability was measured using an intraclass correlation analysis of variance (ANOVA) design. RESULTS: In the resting condition, substantial increases in MEP amplitude were observed for both muscles from day 1 to day 2. Intraclass reliability estimates were higher for the biceps muscle (ICC = 0.95-0.99) than for the FDI muscle (ICC = 0.60-0.81). During the active conditions, the greatest MEP were observed at 25% and 50% MVC, with smaller MEP at 75% and 100% MVC. Intraclass correlations in the active condition were approximately 0.63-0.73. CONCLUSIONS: : Moderate to good reliability of MEP amplitude in the biceps and FDI muscles can be obtained using TMS in both resting and active contraction conditions.

Adolescent↗

PubMed2003

Hemispheric differences in the relationship between corticomotor excitability changes following a fine-motor task and motor learning.

Motor performance induces a postexercise increase in corticomotor excitability that may be associated with motor learning. We investigated whether there are hemispheric differences in the extent and/or time course of changes in corticomotor excitability following a manipulation task (Purdue pegboard) and their relationship with motor performance. Single- and paired-pulse (3 ms) transcranial magnetic stimulation (TMS) was used to assess task-induced facilitation of the muscle evoked potential (MEP) and intracortical inhibition (ICI) for three intrinsic hand muscles acting on digits 1, 2, and 5. Fifteen right-handed subjects performed three 30-s pegboard trials with left or right hand in separate sessions. TMS was applied to contralateral motor cortex before and after performance. Number of pegs placed was higher with the right hand, and performance improved (motor learning) with both hands over the three trials. MEP facilitation following performance was short-lasting (<15 min), selective for muscles engaged in gripping the pegs, and of similar magnitude in left and right hands. ICI was reduced immediately following performance with the right hand, but not the left. The extent of MEP facilitation was positively correlated with motor learning for the right hand only. We conclude that the pegboard task induces a selective, short-lasting change in excitability of corticospinal neurons controlling intrinsic hand muscles engaged in the task. Only left hemisphere changes were related to motor learning. This asymmetry may reflect different behavioral strategies for performance improvement with left and right upper limb in this task or hemispheric differences in the control of skilled hand movements.

Adolescent↗

PubMed2003

Comparison of MRI with EMG to study muscle activity associated with dynamic plantar flexion.

This study compared magnetic resonance imaging (MRI) and surface electromyography (EMG) to evaluate the effect of knee angle upon plantar flexion activity in the triceps surae muscles [medial & lateral gastrocnemius (MG, LG) and the soleus (SOL)]. Two weight & height matched groups performed identical protocols, twelve (6M, 6F) in the MRI group, twelve (8M, 4F) in the EMG group. Subjects plantar flexed dynamically for 2 min at 25% of 1-repetition maximum voluntary contraction (1-RM). Exercise was performed with the knee extended (0 degrees flexion), flexed (90 degrees ), and partially flexed (45 degrees ). In the MRI group spin-echo images were acquired before and immediately following each exercise session. T(2) times, calculated at rest and after exercise by fitting the echoes to a monoexponential decay pattern with a least-squares algorithm, were compared with EMG data. In the EMG group a bipolar electrode was used to collect samples were from the MG, LG, SOL, and anterior tibialis (TA) during exercise at each knee angle, MRI also examined the peroneus (PER). At 0 degrees flexion MRI demonstrated a significant post-exercise T(2) increase in the MG (p < or = 0.001), LG (p < or = 0.001), and PER (p < or = 0.01), with no T(2) change in the SOL or TA. At 90 degrees flexion there was a significant T(2) increase in the SOL (p < or = 0.001) with no significant T(2) change in the MG, LG, PER, or TA. At 45 degrees T(2) increased significantly in the SOL (p < or = 0.001) and LG (p < or = 0.05), but not the MG, PER, or TA. EMG produced similar results with the exception that there was significant activity in the TA during the relaxation cycle of the 90 degrees protocol. We conclude that: 1) Soleus activity is measurable by MRI; and 2) MRI and EMG produce similar results from different physiological sources, and are therefore complementary tools for evaluating muscle activity.

Adult↗