PubMed Health⌕ Search

Biomedical subjects

John G Semmler

Publications and source records attributed to John G Semmler.

11 recordsLinked to original sources
PubMed2006

Motor unit synchronization measured by cross-correlation is not influenced by short-term strength training of a hand muscle.

The purpose of the study was to quantify the strength of motor unit synchronization and coherence from pairs of concurrently active motor units before and after short-term (4-8 weeks) strength training of the left first dorsal interosseous (FDI) muscle. Five subjects (age 24.8 +/- 4.3 years) performed a training protocol three times/week that consisted of six sets of ten maximal isometric index finger abductions, whereas three subjects (age 27.3 +/- 6.7 years) acted as controls. Motor unit activity was recorded from pairs of intramuscular electrodes in the FDI muscle with two separate motor unit recording sessions obtained before and after strength training (trained group) or after 4 weeks of normal daily activities that did not involve training (control group). The training intervention resulted in a 54% (45.2 +/- 8.3 to 69.5 +/- 13.8 N, P = 0.001) increase in maximal index finger abduction force, whereas there was no change in strength in the control group. A total of 163 motor unit pairs (198 single motor units) were examined in both subject groups, with 52 motor unit pairs obtained from 10 recording sessions before training and 51 motor unit pairs from 10 recording sessions after training. Using the cross-correlation procedure, there was no change in the strength of motor unit synchronization following strength training (common input strength index; 0.71 +/- 0.41 to 0.67 +/- 0.43 pulses/s). Furthermore, motor unit coherence z scores at low (0-10 Hz; 3.9 +/- 0.3 before to 4.4 +/- 0.4 after) or high (10-30 Hz; 1.7 +/- 0.1 before to 1.9 +/- 0.1 after) frequencies were not influenced by strength training. These motor unit data indicate that increases in strength following several weeks of training a hand muscle are not accompanied by changes in motor unit synchronization or coherence, suggesting that these features of correlated motor unit activity are not important in the expression of muscle strength.

Action Potentials↗
PubMed2006

Training adaptations in the behavior of human motor units.

The purpose of this brief review is to examine the neural adaptations associated with training, by focusing on the behavior of single motor units. The review synthesizes current understanding on motor unit recruitment and rate coding during voluntary contractions, briefly describes the techniques used to record motor unit activity, and then evaluates the adaptations that have been observed in motor unit activity during maximal and submaximal contractions. Relatively few studies have directly compared motor unit behavior before and after training. Although some studies suggest that the voluntary activation of muscle can increase slightly with strength training, it is not known how the discharge of motor units changes to produce this increase in activation. The evidence indicates that the increase is not attributable to changes in motor unit synchronization. It has been demonstrated, however, that training can increase both the rate of torque development and the discharge rate of motor units. Furthermore, both strength training and practice of a force-matching task can evoke adaptations in the discharge characteristics of motor units. Because the variability in discharge rate has a significant influence on the fluctuations in force during submaximal contractions, the changes produced with training can influence motor performance during activities of daily living. Little is known, however, about the relative contributions of the descending drive, afferent feedback, spinal circuitry, and motor neuron properties to the observed adaptations in motor unit activity.

Action Potentials↗
PubMed2006

Diminished task-related adjustments of common inputs to hand muscle motor neurons in older adults.

The purpose of this study was to quantify correlated motor unit activity during isometric, shortening and lengthening contractions of a hand muscle in older adults. Thirteen old subjects (69.6+/-5.9 years, six women) lifted and lowered a light load with abduction-adduction movements of the index finger over 10 degrees using 6-s shortening and lengthening contractions of the first dorsal interosseus muscle. The task was repeated 10-20 times while activity in 23 pairs of motor units was recorded with intramuscular electrodes. The data were compared with 23 motor-unit pairs in 15 young (25.9+/-4.6 years, five women) subjects obtained using a similar protocol in a previous study. Correlated motor unit activity was quantified using time-domain (synchronization index; Common Input Strength) and frequency-domain (coherence) analyses for the same motor-unit pairs. For all contractions, there was no difference with age for the strength of motor-unit synchronization, although age-related differences were observed for synchronous peak widths (young, 17.6+/-7.4 ms; old, 13.7+/-4.9 ms) and motor-unit coherence at 6-9 Hz (z score for young, 3.0+/-1.8; old, 2.2+/-1.5). Despite increased synchrony during lengthening contractions and narrower peak widths for shortening contractions in young subjects, there was no difference in the strength of motor unit synchronization (CIS approximately 0.8 imp/s), or the width of the synchronous peak (approximately 14 ms) during the three tasks in old subjects. Furthermore, no significant differences in motor-unit coherence were observed between tasks at any frequency for old adults. These data suggest that the strategy used by the central nervous system to control isometric, shortening, and lengthening contractions varies in young adults, but not old adults. The diminished task-related adjustments of common inputs to motor neurons are a likely consequence of the neural adaptations that occur with advancing age.

Adult↗
PubMed2005

Age-related differences in corticospinal control during functional isometric contractions in left and right hands.

The purpose of the study was to examine age-related differences in electromyographic (EMG) responses to transcranial magnetic stimulation (TMS) during functional isometric contractions in left and right hands. EMG responses were recorded from the first dorsal interosseus muscle following TMS in 10 young (26.6 +/- 1.3 yr) and 10 old (67.6 +/- 2.3 yr) right-handed subjects. Muscle evoked potentials (MEPs) and silent-period durations were obtained in the left and right hands during index finger abduction, a precision grip, a power grip, and a scissor grip, while EMG was held constant at 5% of maximum. For all tasks, MEP area was 30% (P < 0.001) lower in the left hand of old compared with young subjects, whereas there was no age difference in the right hand. The duration of the EMG silent period was 14% (P < 0.001) shorter in old (150.3 +/- 2.9 ms) compared with young (173.9 +/- 3.0 ms) subjects, and the age differences were accentuated in the left hand (19% shorter, P < 0.001). For all subjects, the largest MEP area (10-12% larger) and longest EMG silent period (8-19 ms longer) were observed for the scissor grip compared with the other three tasks, and the largest task-dependent change in these variables was observed in the right hand of older adults. These differences in corticospinal control in the left and right hands of older adults may reflect neural adaptations that occur throughout a lifetime of preferential hand use for skilled (dominant) and unskilled (nondominant) motor tasks.

Adult↗
PubMed2004

Motor-unit activity differs with load type during a fatiguing contraction.

Despite a similar rate of change in average electromyographic (EMG) activity, previous studies have observed different rates of change in mean arterial pressure, heart rate, perceived exertion, and fluctuations in motor output during the performance of fatiguing contractions that involved different types of loads. To obtain a more direct measure of the motor output from the spinal cord, the purpose of this study was to compare the discharge characteristics of the same motor unit in biceps brachii during the performance of two types of fatiguing contractions. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task) for a prescribed duration. The force and position tasks were performed in random order at a target force equal to 3.5 +/- 2.1% (mean +/- SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of the isolated motor unit. Each subject maintained the two tasks for an identical duration (161 +/- 96 s) at a mean target force of 22.2 +/- 13.4% MVC (range: 3-49% MVC). The dependent variables included the discharge characteristics of the same motor unit in biceps brachii, fluctuations in motor output (force or acceleration), mean arterial pressure, heart rate, and rating of perceived exertion. Despite similar increases in the amplitude of the averaged EMG (% MVC) for the elbow flexor muscles during both tasks (P = 0.60), the rates of increase in mean arterial pressure (P < 0.001), rating of perceived exertion (P = 0.023), and fluctuations in motor output (P = 0.003) were greater during the position task compared with the force task. Consistent with these differences, mean discharge rate declined at a greater rate during the position task (P = 0.03), and the coefficient of variation for discharge rate increased only during the position task (P = 0.02). Furthermore, more motor units were recruited during the position task compared with the force task (P = 0.01). These findings indicate that despite a comparable net muscle torque, the rate of increase in the motor output from the spinal cord was greater during the position task.

Action Potentials↗
PubMed2004

Motor-unit coherence and its relation with synchrony are influenced by training.

The purpose of the study was to quantify the strength of motor-unit coherence from the left and right first dorsal interosseous muscles in untrained, skill-trained (musicians), and strength-trained (weightlifters) individuals who had long-term specialized use of their hand muscles. The strength of motor-unit coherence was quantified from a total of 394 motor-unit pairs in 13 subjects using data from a previous study in which differences were found in the strength of motor-unit synchronization depending on training status. In the present study, we found that the strength of motor-unit coherence was significantly greater in the left compared with the right hand of untrained right-handed subjects with the largest differences observed between 21 and 24 Hz. The strength of motor-unit coherence was lower in both hands of skill-trained subjects (21-27 Hz) and the right (skilled) hand of untrained subjects (21-24 Hz), whereas the largest motor-unit coherence was observed in both hands of strength-trained subjects (3-9 and 21-27 Hz). A strong curvilinear association was observed between motor-unit synchronization and the integral of coherence at 10-30 Hz in all motor-unit pairs (r2 = 0.77), and was most pronounced in strength-trained subjects (r2 = 0.90). Furthermore, this association was accentuated when using synchronization data with broad peaks (>11 ms), suggesting that the 10- to 30-Hz coherence is due to oscillatory activity in indirect branched common inputs. The altered coherence with training may be due to an interaction between cortical inhibition and the number of direct common inputs to motor neurons in skill- or strength-trained hands.

Adolescent↗
PubMed2003

Mechanisms that contribute to differences in motor performance between young and old adults.

This paper examines the physiological mechanisms responsible for differences in the amplitude of force fluctuations between young and old adults. Because muscle force is a consequence of motor unit activity, the potential mechanisms include both motor unit properties and the behavior of motor unit populations. The force fluctuations, however, depend not only on the age of the individual but also on the muscle group performing the task, the type and intensity of the muscle contraction, and the physical activity status of the individual. Computer simulations and experimental findings performed on tasks that involved single agonist and antagonist muscles suggest that differences in force fluctuations are not attributable to motor unit twitch force, motor unit number, or nonuniform activation of the agonist muscle, but that they are influenced by the variability and common modulation of motor unit discharge in both the agonist and antagonist muscles. Because the amplitude of the force fluctuations does not vary linearly with muscle activation, these results suggest that multiple mechanisms contribute to the differences in force fluctuations between young and old adults, although the boundary conditions for each mechanism remain to be determined.

Adult↗
PubMed2003

Motor-unit coherence during isometric contractions is greater in a hand muscle of older adults.

The purpose of this study was to quantify the strength of motor-unit coherence from the first dorsal interosseus muscle in young and old adults using data obtained in a previous study, where no differences in motor-unit synchronization between the two groups were observed. The strength of motor-unit coherence was quantified from 47 motor-unit pairs in 11 young adults (age 24.1 +/- 4.1 yrs) and from 48 motor-unit pairs in 14 old adults (age 70.4 +/- 5.9 yrs). The strength of motor-unit coherence was greater in old adults, particularly at low frequencies of 5-9 Hz (85% greater in old adults at 5 Hz). In addition, the older adults expressed an extra oscillation at approximately 12-13 Hz that was not present in the young subjects. These data demonstrate that common oscillatory inputs to motor neurons (motor-unit coherence) are enhanced in older adults despite no age-related difference in the strength of shared inputs (synchronization). Furthermore, the data emphasize that measures of motor-unit synchronization and coherence highlight different features of the same common input, and a coherence analysis may be a more sensitive tool to characterize shared input to motor neurons.

Adult↗
PubMed2002

Motor unit synchronisation is enhanced during slow lengthening contractions of a hand muscle.

This study examined the strength of motor unit synchronisation based on time- and frequency-domain measures during postural, shortening and lengthening contractions of a hand muscle in young adults. Single motor unit activity was recorded with intramuscular electrodes in the left first dorsal interosseus muscle as the subject held the index finger at a constant position while supporting a light load for 2-5 min. The subject then performed slow (1.7 deg s(-1)) shortening and lengthening contractions to lift and lower the load. The movement required subjects to perform 10-25 constant-velocity contractions with the index finger over a 10 deg range of motion by using 6 s shortening and lengthening contractions. Individual discharge times were obtained from 23 pairs of motor units in 14 subjects to assess the strength of motor unit synchronisation and coherence during the three tasks. The strength of motor unit synchronisation was approximately 50 % greater during the lengthening contractions compared with the postural and shortening contractions, and the width of the central synchronous peak in the cross-correlation histogram was approximately 4 ms narrower during shortening contractions. These findings reveal that there is an increase in common input to motoneurones during lengthening contractions and a greater relative contribution of direct common inputs to motoneurones during shortening contractions compared with postural tasks. Furthermore, the amount of motor unit coherence in the low-frequency band (2-12 Hz) was reduced during shortening contractions compared with postural and lengthening contractions. These data indicate that the timing of inputs received by the motoneurones innervating the first dorsal interosseus of young adults differs during postural, shortening and lengthening contractions against a light load.

Adult↗
PubMed2002

Motor unit synchronization and neuromuscular performance.

The acute and chronic plasticity of motor unit synchronization suggests that it must represent a deliberate strategy for neuromuscular activation. The most likely functional role of motor unit synchronization is to increase the rate of force development during rapid contractions or as a mechanism to coordinate the activity of multiple muscles in synergy.

Action Potentials↗