scholarly journals Neuromuscular changes of the aged human hamstrings

2018 ◽  
Vol 120 (2) ◽  
pp. 480-488 ◽  
Author(s):  
Eric A. Kirk ◽  
Kevin J. Gilmore ◽  
Charles L. Rice
Keyword(s):  
Motor Unit ◽  
Contractile Function ◽  
Age Groups ◽  
Biceps Femoris ◽  
Surface Emg ◽  
Muscle Group ◽  
Voluntary Activation ◽  
Discharge Rates ◽  
Age Related ◽  
Motor Unit Discharge

Despite the life-long importance for posture and locomotion, neuromuscular properties of the hamstrings muscle have not been explored with adult aging. The purpose of this study was to assess and compare age-related effects on contractile function, spinal motor neuron output expressed as motor unit (MU) discharge rates in the hamstrings of 11 young (26 ± 4 yr) and 10 old (80 ± 5 yr) men. Maximal voluntary isometric contractions (MVC), stimulated contractile properties, and surface and intramuscular electromyography (EMG) from submaximal to MVC were recorded in the biceps femoris (BF) and semimembranosus-semitendinosus (SS) muscles. MVC torque was ~50% less in the old with both age groups attaining ≥93% mean voluntary activation. Evoked twitches in the old were ~50% lower in amplitude and >150% longer in duration compared with those in the young. At successive voluntary contractions of 25, 50, and 100% MVC, MU discharge rates were up to 45% lower in old, with no differences in relative submaximal surface EMG between age groups. Furthermore, the old had significantly lower MU discharge rates in the SS at all contraction intensities compared with the BF muscle. Men in their 8th to 10th decades of life demonstrate substantially lower strength and MU discharge rates in this functionally important large lower limb muscle group, with greater age-related effect on discharge rates in the medial hamstrings. These findings, compared with those in other muscles studied, highlight that the neuromuscular properties of limb muscles, and indeed within functionally similar portions of a muscle group, are not all affected equally by the aging process. NEW & NOTEWORTHY In the hamstrings, we found that both contractile function and motor unit discharge rates across the range of voluntary intensities were lower in the old. The differences in discharge rates due to age were greater in the medial hamstrings muscle group compared with the lateral hamstrings. Compared with previous studies, these results highlight that not all muscles are affected equally by aging and there may be compartmental differences within functionally similar muscles.

scholarly journals Age-related differences in performance and fatigability during an isometric quadriceps intermittent fatigue test

2021 ◽  
Author(s):  
Giorgio Varesco ◽  
Eric Luneau ◽  
Léonard Féasson ◽  
Thomas Lapole ◽  
Vianney Rozand
Keyword(s):  
Older Adults ◽  
Contractile Function ◽  
Age Groups ◽  
Femoral Nerve ◽  
Voluntary Activation ◽  
Knee Extensors ◽  
Men And Women ◽  
Very Old ◽  
Old Adults ◽  
Age Related

AbstractThe aim of the present study was to investigate age-related differences in fatigability induced by an isometric quadriceps intermittent fatiguing test in young (<35 years old), old (>60 years old) and very old (>80 years old) men and women. Maximal force loss, contractile function and voluntary activation of the knee extensors were evaluated throughout an isometric fatiguing test using femoral nerve magnetic stimulations. Older adults performed more contractions (index of relative performance) than young (P = 0.046) and very old adults (P = 0.007), without differences between young and very old adults. Total work (absolute performance) was greater for young and old adults compared to very old adults (P < 0.001), without differences between young and old adults. At exhaustion, force loss was greater for young (−28 ± 9%) compared to old adults (−19 ± 8%), but not very old adults (−23 ± 8%). The response to femoral nerve stimulation decreased similarly at exhaustion for the three age groups, indicating similar alteration in contractile function with age. No impairment in voluntary activation was observed. Impairments in neuromuscular parameters were similar for men and women. This study showed that older adults were less fatigable than young adults during an isometric intermittent fatiguing task of the knee extensors. This greater fatigue resistance was not maintained in very old adults independent of sex. Fatigability at exhaustion was likely due to impairments in contractile function for the three age groups.

Discharge Rate of Sternohyoid Motor Units Activated With Surface EMG Feedback

2009 ◽  
Vol 101 (2) ◽  
pp. 624-632 ◽  
Author(s):  
Dario Farina ◽  
Deborah Falla
Keyword(s):  
Motor Unit ◽  
Motor Neurons ◽  
Discharge Rate ◽  
Maximum Level ◽  
Motor Units ◽  
Surface Emg ◽  
Discharge Rates ◽  
Average Maximum ◽  
Motor Unit Discharge ◽  
Voluntary Contractions

We analyzed individual motor units of the sternohyoid muscle with the aim of characterizing their minimum and maximum discharge rates and their variability in discharge during voluntary contractions. Surface EMG signals were recorded with an array of eight electrodes from the sternohyoid muscle of seven healthy men (age: 30.2 ± 3.5 yr). The multichannel surface EMG signals were displayed as feedback for the subjects who identified and modulated the activity of one target motor unit in 30-s contractions during which the discharge rate was increased from minimum to maximum (ramp contraction), sustained at maximum level (sustained), or increased in brief bursts (burst). During the ramp contractions, the minimum average discharge rate over epochs of 1 s was 11.6 ± 1.5 pulses per second (pps) and the maximum 57.0 ± 5.7 pps ( P < 0.001). During the sustained contractions, the motor unit discharge rate decreased from 65.5 ± 8.4 pps at the beginning to 52.9 ± 7.6 pps at the end of the contraction ( P < 0.05). The coefficient of variation for the interspike interval during the sustained contractions was 40.2 ± 9.8% and a large percentage of discharges had instantaneous rates >50 pps (52.2 ± 12.5%) and >100 pps (8.0 ± 1.2%), with peak values >150 pps. During the burst contractions, the instantaneous discharge rate reached average maximum values of 97.6 ± 36.8 pps. The observed discharge rates and their variability are higher than those reported for limb muscles, which may be due to large synaptic input and noise received by these motor neurons.

scholarly journals Human motor unit characteristics of the superior trapezius muscle with age-related comparisons

2019 ◽  
Vol 122 (2) ◽  
pp. 823-832
Author(s):  
Eric A. Kirk ◽  
Kevin J. Gilmore ◽  
Daniel W. Stashuk ◽  
Timothy J. Doherty ◽  
Charles L. Rice
Keyword(s):  
Motor Neuron ◽  
Motor Unit ◽  
Compound Muscle Action Potential ◽  
Age Groups ◽  
Stability Parameters ◽  
Discharge Rates ◽  
Age Related ◽  
Lower Amplitude ◽  
Human Motor ◽  
The Impact

Current understanding of human motor unit (MU) control and aging is mostly derived from hand and limb muscles that have spinal motor neuron innervations. The aim here was to characterize and test whether a muscle with a shared innervation supply from brainstem and spinal MU populations would demonstrate similar age-related adaptations as those reported for other muscles. In humans, the superior trapezius (ST) muscle acts to elevate and stabilize the scapula and has primary efferent supply from the spinal accessory nerve (cranial nerve XI) located in the brainstem. We compared electrophysiological properties obtained from intramuscular and surface recordings between 10 young (22–33 yr) and 10 old (77–88 yr) men at a range of voluntary isometric contraction intensities (from 15 to 100% of maximal efforts). The old group was 41% weaker with 43% lower MU discharge frequencies compared with the young (47.2 ± 9.6 Hz young and 26.7 ± 5.8 Hz old, P < 0.05) during maximal efforts. There was no difference in MU number estimation between age groups (228 ± 105 young and 209 ± 89 old, P = 0.33). Furthermore, there were no differences in needle detected near fiber (NF) stability parameters of jitter or jiggle. The old group had lower amplitude and smaller area of the stimulated compound muscle action potential and smaller NF MU potential area with higher NF counts. Thus, despite age-related ST weakness and lower MU discharge rates, there was minimal evidence of MU loss or compensatory reinnervation. NEW & NOTEWORTHY The human superior trapezius (ST) has shared spinal and brainstem motor neuron innervation providing a unique model to explore the impact of aging on motor unit (MU) properties. Although the ST showed higher MU discharge rates compared with most spinally innervated muscles, voluntary strength and mean MU rates were lower in old compared with young at all contraction intensities. There was no age-related difference in MU number estimates with minimal electrophysiological evidence of collateral reinnervation.

Contraction-induced potentiation of human motor unit discharge and surface EMG activity

1988 ◽  
Vol 20 (4) ◽  
pp. 391-395 ◽  
Author(s):  
SHUJI SUZUKI ◽  
KOJI KAIYA ◽  
SHIROH WATANABE ◽  
ROBERT S. HUTTON
Keyword(s):  
Motor Unit ◽  
Surface Emg ◽  
Emg Activity ◽  
Unit Discharge ◽  
Motor Unit Discharge ◽  
Human Motor

scholarly journals Stroke increases ischemia-related decreases in motor unit discharge rates

2018 ◽  
Vol 120 (6) ◽  
pp. 3246-3256 ◽  
Author(s):  
Spencer A. Murphy ◽  
Francesco Negro ◽  
Dario Farina ◽  
Tanya Onushko ◽  
Matthew Durand ◽  
...  
Keyword(s):  
Motor Unit ◽  
Time Constant ◽  
Motor Units ◽  
Unit Discharge ◽  
Firing Rates ◽  
Sensory Pathways ◽  
Discharge Rates ◽  
Motor Unit Firing ◽  
Motor Unit Discharge ◽  
Density Surface

Following stroke, hyperexcitable sensory pathways, such as the group III/IV afferents that are sensitive to ischemia, may inhibit paretic motor neurons during exercise. We quantified the effects of whole leg ischemia on paretic vastus lateralis motor unit firing rates during submaximal isometric contractions. Ten chronic stroke survivors (>1 yr poststroke) and 10 controls participated. During conditions of whole leg occlusion, the discharge timings of motor units were identified from decomposition of high-density surface electromyography signals during repeated submaximal knee extensor contractions. Quadriceps resting twitch responses and near-infrared spectroscopy measurements of oxygen saturation as an indirect measure of blood flow were made. There was a greater decrease in paretic motor unit discharge rates during the occlusion compared with the controls (average decrease for stroke and controls, 12.3 ± 10.0% and 0.1 ± 12.4%, respectively; P < 0.001). The motor unit recruitment thresholds did not change with the occlusion (stroke: without occlusion, 11.68 ± 5.83%MVC vs. with occlusion, 11.11 ± 5.26%MVC; control: 11.87 ± 5.63 vs. 11.28 ± 5.29%MVC). Resting twitch amplitudes declined similarly for both groups in response to whole leg occlusion (stroke: 29.16 ± 6.88 vs. 25.75 ± 6.78 Nm; control: 38.80 ± 13.23 vs 30.14 ± 9.64 Nm). Controls had a greater exponential decline (lower time constant) in oxygen saturation compared with the stroke group (stroke time constant, 22.90 ± 10.26 min vs. control time constant, 5.46 ± 4.09 min; P < 0.001). Ischemia of the muscle resulted in greater neural inhibition of paretic motor units compared with controls and may contribute to deficient muscle activation poststroke. NEW & NOTEWORTHY Hyperexcitable inhibitory sensory pathways sensitive to ischemia may play a role in deficient motor unit activation post stroke. Using high-density surface electromyography recordings to detect motor unit firing instances, we show that ischemia of the exercising muscle results in greater inhibition of paretic motor unit firing rates compared with controls. These findings are impactful to neurophysiologists and clinicians because they implicate a novel mechanism of force-generating impairment poststroke that likely exacerbates baseline weakness.

scholarly journals Motor unit behavior during submaximal contractions following six weeks of either endurance or strength training

2010 ◽  
Vol 109 (5) ◽  
pp. 1455-1466 ◽  
Author(s):  
Carolina Vila-Chã ◽  
Deborah Falla ◽  
Dario Farina
Keyword(s):  
Strength Training ◽  
Endurance Training ◽  
Motor Unit ◽  
Conduction Velocity ◽  
Training Programs ◽  
Control Group ◽  
Unit Discharge ◽  
Task Failure ◽  
Discharge Rates ◽  
Motor Unit Discharge

The study investigated changes in motor output and motor unit behavior following 6 wk of either strength or endurance training programs commonly used in conditioning and rehabilitation. Twenty-seven sedentary healthy men (age, 26.1 ± 3.9 yr; mean ± SD) were randomly assigned to strength training (ST; n = 9), endurance training (ET; n = 10), or a control group (CT; n = 8). Maximum voluntary contraction (MVC), time to task failure (isometric contraction at 30% MVC), and rate of force development (RFD) of the quadriceps were measured before ( week 0), during ( week 3), and after a training program of 6 wk. In each experimental session, surface and intramuscular EMG signals were recorded from the vastus medialis obliquus and vastus lateralis muscles during isometric knee extension at 10 and 30% MVC. After 6 wk of training, MVC and RFD increased in the ST group (17.5 ± 7.5 and 33.3 ± 15.9%, respectively; P < 0.05), whereas time to task failure was prolonged in the ET group (29.7 ± 13.4%; P < 0.05). The surface EMG amplitude at 30% MVC force increased with training in both groups, but the training-induced changes in motor unit discharge rates differed between groups. After endurance training, the motor unit discharge rate at 30% MVC decreased from 11.3 ± 1.3 to 10.1 ± 1.1 pulses per second (pps; P < 0.05) in the vasti muscles, whereas after strength training it increased from 11.4 ± 1.2 to 12.7 ± 1.3 pps ( P < 0.05). Finally, motor unit conduction velocity during the contractions at 30% MVC increased for both the ST and ET groups, but only after 6 wk of training ( P < 0.05). In conclusion, these strength and endurance training programs elicit opposite adjustments in motor unit discharge rates but similar changes in muscle fiber conduction velocity.

An integrative model of motor unit activity during sustained submaximal contractions

2010 ◽  
Vol 108 (6) ◽  
pp. 1550-1562 ◽  
Author(s):  
Jakob L. Dideriksen ◽  
Dario Farina ◽  
Martin Baekgaard ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Unit Activity ◽  
Discharge Rate ◽  
Motor Units ◽  
Compartment Model ◽  
Integrative Model ◽  
Motor Unit Activity ◽  
Discharge Rates ◽  
Motor Unit Discharge ◽  
Experimental Findings

The purpose of the study was to expand a model of motor unit recruitment and rate coding ( 30 ) to simulate the adjustments that occur during a fatiguing contraction. The major new components of the model were the introduction of time-varying parameters for motor unit twitch force, recruitment, discharge rate, and discharge variability, and a control algorithm that estimates the net excitation needed by the motoneuron pool to maintain a prescribed target force. The fatigue-induced changes in motor unit activity in the expanded model are a function of changes in the metabolite concentrations that were computed with a compartment model of the intra- and extracellular spaces. The model was validated by comparing the simulation results with data available from the literature and experimentally recorded in the present study during isometric contractions of the first dorsal interosseus muscle. The output of the model was able to replicate a number of experimental findings, including the time to task failure for a range of target forces, the changes in motor unit discharge rates, the skewness and kurtosis of the interspike interval distributions, discharge variability, and the discharge characteristics of newly recruited motor units. The model output provides an integrative perspective of the adjustments during fatiguing contractions that are difficult to measure experimentally.

Muscle CSA, Force Production, and Activation of Leg Extensors during Isometric and Dynamic Actions in Middle-Aged and Elderly Men and Women

1998 ◽  
Vol 6 (3) ◽  
pp. 232-247 ◽  
Author(s):  
Keijo Häkkinen ◽  
Markku Alen ◽  
Mauri Kallinen ◽  
Mikel Izquierdo ◽  
Kirsi Jokelainen ◽  
...  
Keyword(s):  
Isometric Force ◽  
Age Groups ◽  
Voluntary Activation ◽  
Maximal Strength ◽  
Power Performance ◽  
Time Duration ◽  
Cross Sectional ◽  
Men And Women ◽  
Age Related ◽  
Leg Extensors

Forty-two healthy men and women in two age groups (40 and 70 years) were examined for muscle cross-sectional area (CSA), maximal voluntary bilateral isometric force, force-time characteristics, maximal concentric 1 RM. and power performance of the leg extensors in a sitting position, squat jump, and standing long-jump. The results suggested that the decline in maximal strength with increasing age is related to the decline in muscle CSA; however, particularly in older women, the force/CSA ratio may also be lowered. Explosive force seems to decrease with increasing age even more than maximal strength. suggesting that muscle atrophy with aging is greater in fast-twitch fibers. The voluntary activation of the agonist and antagonist muscles seems to vary depending on the type of muscle action and/or velocity and time duration of the action in both age groups but to a greater extent in older people. There appears to be an age-related increase in antagonist co activation. especially in dynamic explosive movements.

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