scholarly journals Lack of increased rate of force development after strength training is explained by specific neural, not muscular, motor unit adaptations

2021 ◽  
Author(s):  
Alessandro Del Vecchio ◽  
Andrea Casolo ◽  
Jakob Lund Dideriksen ◽  
Per Aagaard ◽  
Francesco Felici ◽  
...  
Keyword(s):  
Strength Training ◽  
Motor Unit ◽  
Discharge Rate ◽  
Force Production ◽  
Population Data ◽  
Motor Units ◽  
Rate Of Force Development ◽  
Force Development ◽  
Maximal Force ◽  
Contraction Speed

While maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data acquired from surface electromyography during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by ~4 spikes/s) when reaching the plateau phase (~150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

Why humans are stronger but not faster after isometric strength training: specific neural, not muscular, motor unit adaptations

2021 ◽  
Author(s):  
A. Del Vecchio ◽  
A. Casolo ◽  
J. Dideriksen ◽  
P. Aagaard ◽  
F. Felici ◽  
...  
Keyword(s):  
Strength Training ◽  
Motor Unit ◽  
Discharge Rate ◽  
Force Production ◽  
Population Data ◽  
Motor Units ◽  
Isometric Strength ◽  
Maximal Force ◽  
Contraction Speed ◽  
Rapid Generation

AbstractWhile maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by ∼4 spikes/s) when reaching the plateau phase (∼150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

Motor unit discharge behavior in older adults during maximal-effort contractions

1995 ◽  
Vol 79 (6) ◽  
pp. 1908-1913 ◽  
Author(s):  
G. Kamen ◽  
S. V. Sison ◽  
C. C. Du ◽  
C. Patten
Keyword(s):  
Older Adults ◽  
Motor Unit ◽  
Discharge Rate ◽  
Force Production ◽  
Motor Units ◽  
Older Individuals ◽  
Unit Discharge ◽  
Maximal Force ◽  
Motor Unit Discharge ◽  
Common Observation

A reduction in maximal force production is a common observation in older individuals. In an effort to determine whether aging is accompanied by reductions in central motoneuron drive limiting motor performance, motor unit discharge records were obtained from seven young (21–33 yr) and seven older (> 67 yr) adults. Informed consent was obtained from all subjects. The task required the subject to perform a maximal abduction of the second digit under isometric conditions. Motor unit potentials in the first dorsal interosseous were monitored by using a selective four-wire needle electrode and identified off-line with the aid of a Dantec electromyograph. The maximal discharge rate in the older adults (31.1 impulses/s) was significantly smaller (P < 0.05) than that in the younger subjects (50.9 impulses/s). These findings suggest that reductions in maximal force capability in older adults are partially due to an impaired ability to fully drive the surviving motor units.

scholarly journals Interfacing Spinal Motor Units in Non-Human Primates Identifies a Principal Neural Component for Force Control Constrained by the Size Principle

2021 ◽  
Author(s):  
Alessandro Del Vecchio ◽  
Rachael H. A. Jones ◽  
Ian S. Schofield ◽  
Thomas M Kinfe ◽  
Jaime Ibáñez ◽  
...  
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Rate Of Force Development ◽  
High Rate ◽  
Common Input ◽  
Size Principle ◽  
Flexible Control ◽  
Force Development ◽  
Orderly Fashion

ABSTRACTMotor units convert the last neural code of movement into muscle forces. The classic view of motor unit control is that the central nervous system sends common synaptic inputs to motoneuron pools and that motoneurons respond in an orderly fashion dictated by the size principle. This view however is in contrast with the large number of dimensions observed in motor cortex which may allow individual and flexible control of motor units. Evidence for flexible control of motor units may be obtained by tracking motor units longitudinally during the performance of tasks with some level of behavioural variability. Here we identified and tracked populations of motor units in the brachioradialis muscle of two macaque monkeys during ten sessions spanning over one month during high force isometric contractions with a broad range of rate of force development (1.8 – 38.6 N·m·s-1). During the same sessions we recorded intramuscular EMG signals from 16 arm muscles of both limbs and elicited the full recruitment through neural stimulation of the median and deep radial nerves. We found a very stable recruitment order and discharge characteristics of the motor units over sessions and contraction trials. The small deviations from orderly recruitment were observed between motor units with close recruitment thresholds, and only during high rate of force development. Moreover, we also found that one component explained more than ~50% of the motor unit discharge rate variance, and that the remaining components could be described as a time-shifted version of the first, as it could be predicted from the interplay between the size principle of recruitment and one common input. In conclusion, our results show that motoneurons recruitment is determined by the interplay of the size principle and common input and that this recruitment scheme is not violated over time nor by the speed of the contractions.

scholarly journals Neural and muscular determinants of maximal rate of force development

2020 ◽  
Vol 123 (1) ◽  
pp. 149-157 ◽  
Author(s):  
Jakob L. Dideriksen ◽  
Alessandro Del Vecchio ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Maximum Rate ◽  
Rapid Development ◽  
Motor Units ◽  
Rate Of Force Development ◽  
Time Range ◽  
Limiting Factor ◽  
Force Development ◽  
Twitch Contraction ◽  
Discharge Rates

The ability to produce rapid forces requires quick motor unit recruitment, high motor unit discharge rates, and fast motor unit force twitches. The relative importance of these parameters for maximum rate of force development (RFD), however, is poorly understood. In this study, we systematically investigated these relationships using a computational model of motor unit pool activity and force. Across simulations, neural and muscular properties were systematically varied in experimentally observed ranges. Motor units were recruited over an interval starting from contraction onset (range: 22–233 ms). Upon recruitment, discharge rates declined from an initial rate (range: 89–212 pulses per second), with varying likelihood of doublet (interspike interval of 3 ms; range: 0–50%). Finally, muscular adaptations were modeled by changing average twitch contraction time (range: 42–78 ms). Spectral analysis showed that the effective neural drive to the simulated muscle had smaller bandwidths than the average motor unit twitch, indicating that the bandwidth of the motor output, and thus the capacity for explosive force, was limited mainly by neural properties. The simulated RFD increased by 1,050 ± 281% maximal voluntary contraction force per second from the longest to the shortest recruitment interval. This effect was more than fourfold higher than the effect of increasing the initial discharge rate, more than fivefold higher than the effect of increasing the chance of doublets, and more than sixfold higher than the effect of decreasing twitch contraction times. The simulated results suggest that the physiological variation of the rate by which motor units are recruited during ballistic contractions is the main determinant for the variability in RFD across individuals. NEW & NOTEWORTHY An important limitation of human performance is the ability to generate explosive movements by means of rapid development of muscle force. The physiological determinants of this ability, however, are poorly understood. In this study, we show using extensive simulations that the rate by which motor units are recruited is the main limiting factor for maximum rate of force development.

The differential effect of metabolic alkalosis on maximum force and rate of force development during repeated, high-intensity cycling

2013 ◽  
Vol 115 (11) ◽  
pp. 1634-1640 ◽  
Author(s):  
Jason C. Siegler ◽  
Paul W. M. Marshall ◽  
Sean Raftry ◽  
Cristy Brooks ◽  
Ben Dowswell ◽  
...  
Keyword(s):  
Sodium Bicarbonate ◽  
High Intensity ◽  
Differential Effect ◽  
Force Production ◽  
Rate Of Force Development ◽  
Maximum Force ◽  
Whole Body ◽  
Peak Power Output ◽  
Force Development ◽  
Maximal Force

The purpose of this investigation was to assess the influence of sodium bicarbonate supplementation on maximal force production, rate of force development (RFD), and muscle recruitment during repeated bouts of high-intensity cycling. Ten male and female ( n = 10) subjects completed two fixed-cadence, high-intensity cycling trials. Each trial consisted of a series of 30-s efforts at 120% peak power output (maximum graded test) that were interspersed with 30-s recovery periods until task failure. Prior to each trial, subjects consumed 0.3 g/kg sodium bicarbonate (ALK) or placebo (PLA). Maximal voluntary contractions were performed immediately after each 30-s effort. Maximal force (Fmax) was calculated as the greatest force recorded over a 25-ms period throughout the entire contraction duration while maximal RFD (RFDmax) was calculated as the greatest 10-ms average slope throughout that same contraction. Fmax declined similarly in both the ALK and PLA conditions, with baseline values (ALK: 1,226 ± 393 N; PLA: 1,222 ± 369 N) declining nearly 295 ± 54 N [95% confidence interval (CI) = 84–508 N; P < 0.006]. RFDmax also declined in both trials; however, a differential effect persisted between the ALK and PLA conditions. A main effect of condition was observed across the performance time period, with RFDmax on average higher during ALK (ALK: 8,729 ± 1,169 N/s; PLA: 7,691 ± 1,526 N/s; mean difference between conditions 1,038 ± 451 N/s, 95% CI = 17–2,059 N/s; P < 0.048). These results demonstrate a differential effect of alkalosis on maximum force vs. maximum rate of force development during a whole body fatiguing task.

scholarly journals The relative strength of common synaptic input to motor neurons is not a determinant of the maximal rate of force development in humans

2019 ◽  
Vol 127 (1) ◽  
pp. 205-214 ◽  
Author(s):  
Alessandro Del Vecchio ◽  
Deborah Falla ◽  
Francesco Felici ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Motor Neurons ◽  
Synaptic Input ◽  
Discharge Rate ◽  
Rate Of Force Development ◽  
Force Generation ◽  
Unit Discharge ◽  
Force Development ◽  
Motor Unit Synchronization ◽  
Motor Unit Discharge

Correlation between motor unit discharge times, often referred to as motor unit synchronization, is determined by common synaptic input to motor neurons. Although it has been largely speculated that synchronization should influence the rate of force development, the association between the degree of motor unit synchronization and rapid force generation has not been determined. In this study, we examined this association with both simulations and experimental motor unit recordings. The analysis of experimental motor unit discharges from the tibialis anterior muscle of 20 healthy individuals during rapid isometric contractions revealed that the average motor unit discharge rate was associated with the rate of force development. Moreover, the extent of motor unit synchronization was entirely determined by the average motor unit discharge rate ( R > 0.7, P < 0.0001). The simulation model demonstrated that the relative proportion of common synaptic input received by motor neurons, which determines motor unit synchronization, does not influence the rate of force development ( R = 0.03, P > 0.05). Nonetheless, the estimates of correlation between motor unit spike trains were significantly correlated with the rate of force generation ( R > 0.8, P < 0.0001). These results indicate that the average motor unit discharge rate, but not the degree of motor unit synchronization, contributes to most of the variance of human contractile speed among individuals. In addition, estimates of correlation between motor unit discharge times depend strongly on the number of identified motor units and therefore are not indicative of the strength of common input. NEW & NOTEWORTHY It is commonly assumed that motor unit synchronization has an impact on the rate of force development of a muscle. Here we present computer simulations and experimental data of human tibialis anterior motor units during rapid contractions that show that motor unit synchronization is not a determinant of the rate of force production. This conclusion clarifies the neural determinants of rapid force generation.

The effect of rate of force development on maximal force production: acute and training-related aspects

2007 ◽  
Vol 99 (6) ◽  
pp. 605-613 ◽  
Author(s):  
Andreas Holtermann ◽  
Karin Roeleveld ◽  
Beatrix Vereijken ◽  
Gertjan Ettema
Keyword(s):  
Force Production ◽  
Rate Of Force Development ◽  
Force Development ◽  
Maximal Force ◽  
And Training

Pain-induced changes in motor unit discharge depend on recruitment threshold and contraction speed

2021 ◽  
Author(s):  
Eduardo Martinez-Valdes ◽  
Francesco Negro ◽  
Michail Arvanitidis ◽  
Dario Farina ◽  
Deborah Falla
Keyword(s):  
Discharge Rate ◽  
Tibialis Anterior Muscle ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Inhibitory Effect ◽  
Voluntary Contraction ◽  
Recruitment Threshold ◽  
Contraction Speed ◽  
Discharge Rates ◽  
Hypertonic Saline Injection

At high forces, the discharge rates of lower and higher threshold motor units (MU) are influenced in a different way by muscle pain. These differential effects may be particularly important for performing contractions at different speeds since the proportion of lower and higher threshold MUs recruited varies with contraction velocity. We investigated whether MU discharge and recruitment strategies are differentially affected by pain depending on their recruitment threshold (RT), across a range of contraction speeds. Participants performed ankle dorsiflexion sinusoidal-isometric contractions at two frequencies (0.25Hz and 1Hz) and two modulation amplitudes [5% and 10% of the maximum voluntary contraction (MVC)] with a mean target torque of 20%MVC. High-density surface electromyography recordings from the tibialis anterior muscle were decomposed and the same MUs were tracked across painful (hypertonic saline injection) and non-painful conditions. Torque variability, mean discharge rate (MDR), DR variability (DRvar), RT and the delay between the cumulative spike train and the resultant torque output (neuromechanical delay, NMD) were assessed. The average RT was greater at faster contraction velocities (p=0.01) but was not affected by pain. At the fastest contraction speed, torque variability and DRvar were reduced (p<0.05) and MDR was maintained. Conversely, MDR decreased and DRvar and NMD increased significantly during pain at slow contraction speeds (p<0.05). These results show that reductions in contraction amplitude and increased recruitment of higher threshold MUs at fast contraction speeds appears to compensate for the inhibitory effect of nociceptive inputs on lower threshold MUs, allowing the exertion of fast submaximal contractions during pain.

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