scholarly journals Discharge Characteristics of Biceps Brachii Motor Units at Recruitment When Older Adults Sustained an Isometric Contraction

2011 ◽  
Vol 105 (2) ◽  
pp. 571-581 ◽  
Author(s):  
Michael A. Pascoe ◽  
Matthew R. Holmes ◽  
Roger M. Enoka
Keyword(s):  
Older Adults ◽  
Young Adults ◽  
Isometric Contraction ◽  
Biceps Brachii ◽  
Motor Units ◽  
Discharge Characteristics ◽  
Recruitment Threshold ◽  
Old Adults ◽  
Threshold Force ◽  
Target Force

The purpose of this study was to compare the discharge characteristics of motor units recruited during an isometric contraction that was sustained with the elbow flexor muscles by older adults at target forces that were less than the recruitment threshold force of each isolated motor unit. The discharge times of 27 single motor units were recorded from the biceps brachii in 11 old adults (78.8 ± 5.9 yr). The target force was set at either a relatively small (6.6 ± 3.7% maximum) or large (11.4 ± 4.5% maximum) difference below the recruitment threshold force and the contraction was sustained until the motor unit was recruited and discharged action potentials for about 60 s. The time to recruitment was longer for the large target-force difference ( P = 0.001). At recruitment, the motor units discharged repetitively for both target-force differences, which contrasts with data from young adults when motor units discharged intermittently at recruitment for the large difference between recruitment threshold force and target force. The coefficient of variation (CV) for the first five interspike intervals (ISIs) increased from the small (18.7 ± 7.9) to large difference (35.0 ± 10.2%, P = 0.008) for the young adults, but did not differ for the two target force differences for the old adults (26.3 ± 14.7 to 24.0 ± 13.1%, P = 0.610). When analyzed across the discharge duration, the average CV for the ISI decreased similarly for the two target-force differences ( P = 0.618) in old adults. These findings contrast with those of young adults and indicate that the integration of synaptic input during sustained contractions differs between young and old adults.

scholarly journals Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

2013 ◽  
Vol 109 (4) ◽  
pp. 1055-1064 ◽  
Author(s):  
Michael A. Pascoe ◽  
Jeffrey R. Gould ◽  
Roger M. Enoka
Keyword(s):  
Young Adults ◽  
Motor Unit ◽  
Action Potentials ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Age Groups ◽  
Motor Units ◽  
Old Adults ◽  
Target Force ◽  
Force Difference

The purpose of the study was to compare the discharge characteristics of biceps brachii motor units of young and old adults when they performed steady, submaximal contractions while the arm supported different inertial loads. Young (28 ± 4 yr; n = 16) and old (75 ± 4 yr; n = 14) adults performed steady contractions with the elbow flexors at target forces set at either small (11.7 ± 4.4% maximum) or large (17.8 ± 6.5% maximum) differences below the recruitment threshold force of the motor unit ( n = 40). The task was to maintain an elbow angle at 1.57 rad until the motor unit was recruited and discharged action potentials for ∼120 s. Time to recruitment was longer for the larger target force difference (187 ± 227 s vs. 23 ± 46 s, P < 0.001). Once recruited, motor units discharged action potentials either repetitively or intermittently, with a greater proportion of motor units exhibiting the repetitive pattern for old adults. Discharge rate at recruitment and during the steady contraction was similar for the two target force differences for old adults but was greater for the small target force difference for young adults. Discharge variability was similar at recruitment for the two age groups but less for the old adults during the steady contraction. The greatest difference between the present results and those reported previously when the arm pulled against a rigid restraint was that old adults modulated discharge rate less than young adults across the two contraction intensities for both load types.

scholarly journals Estimates of persistent inward currents are reduced in upper limb motor units of older adults

2021 ◽  
Author(s):  
Altamash S Hassan ◽  
Melissa E Fajardo ◽  
Mark Cummings ◽  
Laura Miller McPherson ◽  
Francesco Negro ◽  
...  
Keyword(s):  
Older Adults ◽  
Young Adults ◽  
Biceps Brachii ◽  
Gain Control ◽  
Motor Units ◽  
Older Individuals ◽  
Triceps Brachii ◽  
Persistent Inward Currents ◽  
Firing Rates ◽  
Inward Currents

Aging is a natural process that causes alterations in the neuromuscular system, which contribute to weakness and reduced quality of life. Reduced firing rates of individual motor units (MUs) likely contribute to weakness, but the mechanisms underlying reduced firing rates are not clear. Persistent inward currents (PICs) are crucial for the initiation, gain control, and maintenance of motoneuron firing, and are directly proportional to the level of monoaminergic input. Since the concentration of monoamines (i.e. serotonin and norepinephrine) are reduced with age, we sought to determine if estimates of PICs are reduced in older (>60 years old) compared to young adults (<35 years old). We decomposed MU spike trains from high-density surface electromyography over the biceps brachii and triceps brachii during isometric ramp contractions to 20% of maximum. Estimates of PICs (i.e. ΔF) were computed using the paired MU analysis technique. Regardless of the muscle, peak firing rates of older adults were reduced by ~1.6 pulses per second (pps) (P = 0.0292), and ΔF was reduced by ~1.9 pps (P < 0.0001), compared to young adults. We further found that age predicted ΔF in older adults (P = 0.0261), resulting in a reduction of ~1pps per decade, but there was no relationship in young adults (P = 0.9637). These findings suggest that PICs are reduced in older adults, and, further, age is a significant predictor of estimates of PICs in older adults. Reduced PIC magnitude represents one plausible mechanism for reduced firing rates and weakness in older individuals.

Older Adults Use a Unique Strategy to Lift Inertial Loads With the Elbow Flexor Muscles

2000 ◽  
Vol 83 (4) ◽  
pp. 2030-2039 ◽  
Author(s):  
Andrew E. Graves ◽  
Kurt W. Kornatz ◽  
Roger M. Enoka
Keyword(s):  
Young Adults ◽  
Elbow Joint ◽  
Muscle Activation ◽  
Angular Displacement ◽  
Elbow Flexor ◽  
Lengthening Contractions ◽  
Old Adults ◽  
Flexor Muscles ◽  
Elbow Flexor Muscles ◽  
Target Force

The purpose of this study was to determine the effect of age on the ability to exert steady forces and to perform steady flexion movements with the muscles that cross the elbow joint. An isometric task required subjects to exert a steady force to match a target force that was displayed on a monitor. An anisometric task required subjects to raise and lower inertial loads so that the angular displacement around the elbow joint matched a template displayed on a monitor. Steadiness was measured as the coefficient of variation of force and as the normalized standard deviation of wrist acceleration. For the isometric task, steadiness as a function of target force decreased similarly for old adults and young adults. For the anisometric task, steadiness increased as a function of the inertial load and there were significant differences caused by age. Old adults were less steady than young adults during both shortening and lengthening contractions with the lightest loads. Furthermore, old adults were least steady when performing lengthening contractions. These behaviors appear to be associated with the patterns of muscle activation. These results suggest that different neural strategies are used to control isometric and anisometric contractions performed with the elbow flexor muscles and that these strategies do not change in parallel with advancing age.

Memory Skill Acquisition for Young and Old Adults: Does Training Order Affect Learning?

1994 ◽  
Vol 38 (2) ◽  
pp. 155-159 ◽  
Author(s):  
Jennifer L. Clark ◽  
Wendy A. Rogers
Keyword(s):  
Older Adults ◽  
Young Adults ◽  
Skill Acquisition ◽  
Search Task ◽  
Memory Search ◽  
Old Adults ◽  
Older Subjects ◽  
Memory Skill ◽  
Order Of Training ◽  
Strategy I

The purpose of the present experiment was to identify the effects of altering the order of training for a memory search task in old and young adults. We provided subjects with extensive practice on consistently mapped (CM) and variably mapped (VM) versions of a memory search task. Half of the subjects in each age group received CM training followed by VM training and the other half received VM first followed by CM. Based on previous findings (Fisk, Rogers, and Giambra, 1990), in which older adults did not switch to a more efficient search strategy (i. e., from serial exhaustive to serial self-terminating) we predicted that older subjects who received VM training first would not adopt the most efficient strategy on subsequent CM training compared to old adults who received the CM training first. The results supported our prediction: namely, the comparison slopes were shallower (i. e., more efficient) for the older adults who received CM training first, relative to those who received VM training prior to the CM training. Order of practice did not significantly affect the performance of the young adults. These data have important implications for the development of training programs in which subjects will be required to learn several task components.

scholarly journals 95 Accuracy of Off the Shelf Activity Trackers in Ambulatory Settings in Young and Old Adults

2019 ◽  
Vol 48 (Supplement_3) ◽  
pp. iii1-iii16
Author(s):  
John Barton ◽  
Suzanne Timmons ◽  
Salvatore Tedesco ◽  
Marco Sica ◽  
Colum Crowe ◽  
...  
Keyword(s):  
Older Adults ◽  
Young Adults ◽  
Video Recording ◽  
Real Life ◽  
Poor Performance ◽  
Percentage Error ◽  
Old Adults ◽  
Activity Trackers ◽  
Household Activities ◽  
Gold Standards

Abstract Background Wrist-worn activity trackers have experienced a tremendous growth lately. Robust studies of the comparative accuracy of currently available, mainstream trackers, in young adults versus older adults are still scarce in literature. This study explores the performance of ten trackers estimating steps, travelled distance, and heart-rate measurements against gold-standards in two cohorts of young and old adults. Methods Overall, 38 subjects completed a structured protocol involving walking tasks, simulated household activities, and sedentary activities, including less standardised activities, such as dusting, vacuuming, or playing cards, in order to simulate real-life scenarios. Both wrist-mounted and chest/waist-mounted devices were considered. Gold-standards included treadmill, waist-mounted pedometer, ECG-based chest strap, direct observation or video recording according to the activity and parameter. Results Every tracker shows a decreasing accuracy with slower walking speed, which resulted in a significant step under-counting. Large mean absolute percentage error (MAPE) was displayed by every monitor at slower walking speeds. During household activities, the MAPE in young adults climbing up/down-stairs ranged from 3.91-11.41% and 4.34-11.92% (dominant and non-dominant arm), respectively. However, for the same activities older adults displayed a larger MAPE, at 8.38-19.3% and 10.06-19.01%, respectively. Chest-worn or waist-worn devices had more uniform performance. However, unstructured activities (dusting, vacuuming, playing cards), and accuracy in people using a walking aid represent a challenge for all consumer-level trackers as evidenced by large MAPE. Poor performance in travelled distance estimation was also evident during walking at low speeds and household activities for both cohorts. Conclusion This study shows a number of limitations to current, mainstream consumer-level wrist-based activity trackers, requiring caution if adopted in healthcare, whether clinical or research. This study demonstrates the particular deficits in commercial devices for use in an aging population, and provides some indications on how to best measure these health parameters in this population.

Arithmetic Stroop Interference as a Function of Age: Maintenance and Modification of Automatic Processes

1989 ◽  
Vol 33 (3) ◽  
pp. 169-173 ◽  
Author(s):  
Wendy A. Rogers ◽  
Arthur D. Fisk
Keyword(s):  
Older Adults ◽  
Young Adults ◽  
Stroop Interference ◽  
Automatic Process ◽  
Practical Training ◽  
Automatic Processes ◽  
Arithmetical Operation ◽  
Old Adults ◽  
Multiplication Tables ◽  
Young Subjects

This experiment investigated whether well-learned “automatic” processes remain stable as a function of age, as well as whether the ability to modify automatic processes is disrupted for older adults. We used an arithmetic “Stroop” task. Nineteen young (mean 22) and 19 old adults (mean 75) participated in three sessions for a total of 450 trials. The young subjects had faster verification times, overall, than the old adults. Both young and old subjects showed significant Stroop interference. These results support the hypothesis that automatic processes, in this case access of addition and multiplication tables, are maintained for old adults. Furthermore, both groups reduced their RT with practice. For the young adults, there was a decrease in interference with practice suggesting that they were learning to inhibit the automatic process of performing the arithmetical operation. However, the old adults showed no significant decrease in interference, which implies that they were impaired in their ability to inhibit automatic processes, even when those processes interfered with performance. Theoretical and practical training implications are discussed.

Different Neural Adjustments Improve Endpoint Accuracy With Practice in Young and Old Adults

2007 ◽  
Vol 97 (5) ◽  
pp. 3340-3350 ◽  
Author(s):  
Evangelos A. Christou ◽  
Brach Poston ◽  
Joel A. Enoka ◽  
Roger M. Enoka
Keyword(s):  
Young Adults ◽  
Motor Output ◽  
Peak Force ◽  
Absolute Difference ◽  
Antagonist Muscle ◽  
Old Adults ◽  
Time To Peak ◽  
Output Variability ◽  
Motor Output Variability ◽  
Target Force

The purpose of the study was to determine the practice-induced adjustments in the motor-output variability and the agonist–antagonist activity that accompanied improvements in endpoint accuracy of goal-directed isometric contractions in young and old adults. Young and old adults performed 100 trials that involved accurately matching the peak of a force trajectory (25% maximum) to a target force in 150 ms. Endpoint accuracy was quantified as the absolute difference between the target and the peak force and time-to-peak force. Motor-output variability was expressed as the SDs of the force trajectory, peak force, and time-to-peak force. The force and time errors differed between the two groups initially, but after 35 practice trials the errors were similar for the two groups. Reductions in force endpoint error were predicted by decreases in the variability of the force trajectory for both groups, adaptations in the agonist (first dorsal interosseus) and antagonist (second palmar interosseus) EMG for young adults, and adaptations only for the agonist EMG for old adults. Reductions in time endpoint error were predicted by increases in the SD of time-to-peak force and a longer delay to the peak EMG of the antagonist muscle for young adults, but by decreases in the SDs of time-to-peak force and force trajectory and a shorter delay to the peak EMG of the antagonist muscle for the old adults. The findings indicate that the neural adjustments underlying the improvement in endpoint accuracy with practice differed for young and old adults.

scholarly journals Modulation of motor unit activity in biceps brachii by neuromuscular electrical stimulation applied to the contralateral arm

2015 ◽  
Vol 118 (12) ◽  
pp. 1544-1552 ◽  
Author(s):  
Ioannis G. Amiridis ◽  
Diba Mani ◽  
Awad Almuklass ◽  
Boris Matkowski ◽  
Jeffrey R. Gould ◽  
...  
Keyword(s):  
Electrical Stimulation ◽  
Motor Unit ◽  
Unit Activity ◽  
Pulse Width ◽  
Biceps Brachii ◽  
Neuromuscular Electrical Stimulation ◽  
Motor Units ◽  
Elbow Flexors ◽  
Discharge Characteristics ◽  
Low Threshold

The purpose of the study was to determine the influence of neuromuscular electrical stimulation (NMES) current intensity and pulse width applied to the right elbow flexors on the discharge characteristics of motor units in the left biceps brachii. Three NMES current intensities were applied for 5 s with either narrow (0.2 ms) or wide (1 ms) stimulus pulses: one at 80% of motor threshold and two that evoked contractions at either ∼10% or ∼20% of maximal voluntary contraction (MVC) force. The discharge times of 28 low-threshold (0.4–21.6% MVC force) and 16 high-threshold (31.7–56.3% MVC force) motor units in the short head of biceps brachii were determined before, during, and after NMES. NMES elicited two main effects: one involved transient deflections in the left-arm force at the onset and offset of NMES and the other consisted of nonuniform modulation of motor unit activity. The force deflections, which were influenced by NMES current intensity and pulse width, were observed only when low-threshold motor units were tracked. NMES did not significantly influence the discharge characteristics of tracked single-threshold motor units. However, a qualitative analysis indicated that there was an increase in the number of unique waveforms detected during and after NMES. The findings indicate that activity of motor units in the left elbow flexors can be modulated by NMES current and pulse width applied to right elbow flexors, but the effects are not distributed uniformly to the involved motor units.

Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction

1994 ◽  
Vol 76 (6) ◽  
pp. 2411-2419 ◽  
Author(s):  
S. J. Garland ◽  
R. M. Enoka ◽  
L. P. Serrano ◽  
G. A. Robinson
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Human Subjects ◽  
Absolute Threshold ◽  
Motor Units ◽  
Single Motor ◽  
Threshold Force ◽  
Single Motor Units ◽  
Discharge Rates

The activity of 50 single motor units was recorded in the biceps brachii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on motor unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval on the discharge rates of single motor units during a fatiguing contraction. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contraction, whereas the discharge rates of most newly recruited units were either constant or increased slightly. The absolute threshold forces of recruitment and derecruitment decreased, and the variability of interimpulse intervals increased after the fatigue task. The change in motor unit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be predicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units declined despite an increase in the excitatory drive to the motoneuron pool during the fatigue task.

Eccentric Muscle Damage Has Variable Effects on Motor Unit Recruitment Thresholds and Discharge Patterns in Elbow Flexor Muscles

2009 ◽  
Vol 102 (1) ◽  
pp. 413-423 ◽  
Author(s):  
Tamara J. Dartnall ◽  
Nigel C. Rogasch ◽  
Michael A. Nordstrom ◽  
John G. Semmler
Keyword(s):  
Muscle Damage ◽  
Motor Unit ◽  
Eccentric Exercise ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Recruitment Threshold ◽  
Flexor Muscles ◽  
Elbow Flexor Muscles

The purpose of this study was to determine the effect of eccentric muscle damage on recruitment threshold force and repetitive discharge properties of low-threshold motor units. Ten subjects performed four tasks involving isometric contraction of elbow flexors while electromyographic (EMG) data were recorded from human biceps brachii and brachialis muscles. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; 3) motor unit recruitment threshold task; and 4) minimum motor unit discharge rate task. These tasks were performed on three separate days before, immediately after, and 24 h after eccentric exercise of elbow flexor muscles. MVC force declined (42%) immediately after exercise and remained depressed (29%) 24 h later, indicative of muscle damage. Mean motor unit recruitment threshold for biceps brachii was 8.4 ± 4.2% MVC, ( n = 34) before eccentric exercise, and was reduced by 41% (5.0 ± 3.0% MVC, n = 34) immediately after and by 39% (5.2 ± 2.5% MVC, n = 34) 24 h after exercise. No significant changes in motor unit recruitment threshold were observed in the brachialis muscle. However, for the minimum tonic discharge rate task, motor units in both muscles discharged 11% faster (10.8 ± 2.0 vs. 9.7 ± 1.7 Hz) immediately after ( n = 29) exercise compared with that before ( n = 32). The minimum discharge rate variability was greater in brachialis muscle immediately after exercise (13.8 ± 3.1%) compared with that before (11.9 ± 3.1%) and 24 h after exercise (11.7 ± 2.4%). No significant changes in minimum discharge rate variability were observed in the biceps brachii motor units after exercise. These results indicate that muscle damage from eccentric exercise alters motor unit recruitment thresholds for ≥24 h, but the effect is not the same in the different elbow flexor muscles.

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