Motor unit firing rates of the gastrocnemii during maximal and sub-maximal isometric contractions in young and old men

Jakobi, J. M., and E. Cafarelli. Neuromuscular drive and force production are not altered during bilateral contractions. J. Appl. Physiol. 84(1): 200–206, 1998.—Several investigators have studied the deficit in maximal voluntary force that is said to occur when bilateral muscle groups contract simultaneously. A true bilateral deficit (BLD) would suggest a significant limitation of neuromuscular control; however, some of the data from studies in the literature are equivocal. Our purpose was to determine whether there is a BLD in the knee extensors of untrained young male subjects during isometric contractions and whether this deficit is associated with a decreased activation of the quadriceps, increased activation of the antagonist muscle, or an alteration in motor unit firing rates. Twenty subjects performed unilateral (UL) and bilateral (BL) isometric knee extensions at 25, 50, 75, and 100% maximal voluntary contraction. Total UL and BL force (Δ3%) and maximal rate of force generation (Δ2.5%) were not significantly different. Total UL and BL maximal vastus lateralis electromyographic activity (EMG; 2.7 ± 0.28 vs. 2.6 ± 0.24 mV) and coactivation (0.17 ± 0.02 vs. 0.20 ± 0.02 mV) were also not different. Similarly, the ratio of force to EMG during submaximal UL and BL contractions was not different. Analysis of force production by each leg in UL and BL conditions showed no differences in force, rate of force generation, EMG, motor unit firing rates, and coactivation. Finally, assessment of quadriceps activity with the twitch interpolation technique indicated no differences in the degree of voluntary muscle activation (UL: 93.6 ± 2.51 Hz, BL: 90.1 ± 2.43 Hz). These results provide no evidence of a significant limitation in neuromuscular control between BL and UL isometric contractions of the knee extensor muscles in young male subjects.

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Motor unit firing rates and contractile properties in tibialis anterior of young and old men

Journal of Applied Physiology ◽

10.1152/jappl.1999.87.2.843 ◽

1999 ◽

Vol 87 (2) ◽

pp. 843-852 ◽

Cited By ~ 199

Author(s):

Denise M. Connelly ◽

Charles L. Rice ◽

Martin R. Roos ◽

Anthony A. Vandervoort

Keyword(s):

Motor Unit ◽

Tibialis Anterior ◽

Tibialis Anterior Muscle ◽

Young Men ◽

Contractile Properties ◽

Firing Rates ◽

Twitch Contraction ◽

Age Related ◽

Motor Unit Firing ◽

Old Men

The effects of aging on motoneuron firing rates and muscle contractile properties were studied in tibialis anterior muscle by comparing results from six young (20.8 ± 0.8 yr) and six old men (82.0 ± 1.7 yr). For each subject, data were collected from repeated tests over a 2-wk period. Contractile tests included maximal voluntary contraction (MVC) with twitch interpolation and stimulated twitch contractions. The old men had 26% lower MVC torque ( P < 0.01) than did the young men, but percent activation was not different (99.1 and 99.3%, respectively). Twitch contraction durations were 23% longer ( P < 0.01) in the old compared with the young men. During a series of repeated brief steady-state contractions at 10, 25, 50, 75, and 100% MVC, motor unit firing rates were recorded. Results from ∼950 motor unit trains in each subject group indicated that at all relative torque levels mean firing rates were 30–35% lower ( P < 0.01) in the old subjects. Comparisons between young and old subjects’ mean firing rates at each of 10%, 50%, and MVC torques and their corresponding mean twitch contraction duration yielded a range of moderate-to-high correlations ( r = −0.67 to −0.84). That lower firing rates were matched to longer twitch contraction durations in the muscle of old men, and relatively higher firing rates were matched with shorter contraction times from the young men, indirectly supports the neuromuscular age-related remodeling principle.

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Quadriceps muscle strength, contractile properties, and motor unit firing rates in young and old men

Muscle & Nerve ◽

10.1002/(sici)1097-4598(199908)22:8<1094::aid-mus14>3.0.co;2-g ◽

1999 ◽

Vol 22 (8) ◽

pp. 1094-1103 ◽

Cited By ~ 170

Author(s):

Martin R. Roos ◽

Charles L. Rice ◽

Denise M. Connelly ◽

Anthony A. Vandervoort

Keyword(s):

Muscle Strength ◽

Motor Unit ◽

Quadriceps Muscle ◽

Contractile Properties ◽

Firing Rates ◽

Quadriceps Muscle Strength ◽

Motor Unit Firing ◽

Old Men

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Instability of motor unit firing rates during prolonged isometric contractions in human masseter

Brain Research ◽

10.1016/0006-8993(91)90467-a ◽

1991 ◽

Vol 549 (2) ◽

pp. 268-274 ◽

Cited By ~ 21

Author(s):

M.A. Nordstrom ◽

T.S. Miles

Keyword(s):

Motor Unit ◽

Isometric Contractions ◽

Firing Rates ◽

Motor Unit Firing

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Caffeine increases time to fatigue by maintaining force and not by altering firing rates during submaximal isometric contractions

Journal of Applied Physiology ◽

10.1152/japplphysiol.00937.2004 ◽

2005 ◽

Vol 99 (3) ◽

pp. 1056-1063 ◽

Cited By ~ 53

Author(s):

B.M. Meyers ◽

E. Cafarelli

Keyword(s):

Motor Unit ◽

Repeated Measures ◽

Muscle Activation ◽

Isometric Contractions ◽

Double Blind ◽

Single Motor Unit ◽

Instantaneous Rate ◽

Firing Rates ◽

Motor Unit Firing ◽

Placebo Capsule

Caffeine increases time to fatigue [limit of endurance (Tlim)] during submaximal isometric contractions without altering whole muscle activation or neuromuscular junction transmission. We used 10 male volunteers in a randomized, double-blind, repeated-measures experiment to examine single motor unit firing rates during intermittent submaximal contractions and to determine whether administering caffeine increased Tlim by maintaining higher firing rates. On 2 separate days, subjects performed intermittent 50% maximal voluntary contractions of the quadriceps to Tlim, 1 h after ingesting a caffeine (6 mg/kg) or placebo capsule. Average motor unit firing rates recorded with tungsten microelectrodes were constant for the duration of contractions. Caffeine increased average Tlim by 20.5 ± 8.1% ( P < 0.05) compared with placebo conditions. This increase was due to seven subjects, termed responders, who increased Tlim significantly. Two other subjects showed no response, and a third had a shorter Tlim. Neither the increased Tlim nor the responders' performance could be explained by alterations in firing rates or other neuromuscular variables. However, the amplitude of the evoked twitch and its maximal instantaneous rate of relaxation did not decline to the same degree in the caffeine trial of the responders; this resulted in values 20 and 30% higher at the time point matching the end of the placebo trial ( P < 0.05). The amplitude of the evoked twitch and the maximal instantaneous rate of relaxation were linearly correlated (caffeine r = 0.72, placebo r = 0.80, both P < 0.001), suggesting that the increase in Tlim may be partially explained by caffeine's effects on calcium reuptake and twitch force.

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Effects Of Resistance Training On Maximal Motor Unit Firing Rates In Young And Older Males

Medicine & Science in Sports & Exercise ◽

10.1249/01.mss.0000536499.08903.69 ◽

2018 ◽

Vol 50 (5S) ◽

pp. 429-430

Author(s):

Phuong L. Ha ◽

Garrett M. Hester ◽

Ryan J. Colquhoun ◽

Mitchel A. Magrini ◽

Zachary K. Pope ◽

...

Keyword(s):

Resistance Training ◽

Motor Unit ◽

Firing Rates ◽

Motor Unit Firing ◽

Older Males

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Stroke increases ischemia-related decreases in motor unit discharge rates

Journal of Neurophysiology ◽

10.1152/jn.00923.2017 ◽

2018 ◽

Vol 120 (6) ◽

pp. 3246-3256 ◽

Cited By ~ 2

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.

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