Resistance Training: Cortical, Spinal, and Motor Unit Adaptations

2005 ◽  
Vol 30 (3) ◽  
pp. 328-340 ◽  
Author(s):  
Lisa Griffin ◽  
Enzo Cafarelli
Keyword(s):  
Resistance Training ◽  
Motor Unit ◽  
Cortical Excitability ◽  
Voluntary Contraction ◽  
Force Output ◽  
Single Motor Unit ◽  
Neural Adaptations ◽  
Maximal Force ◽  
Early Stages ◽  
Discharge Rates

During the first few weeks of isometric resistance training there is an increase in maximal muscle force output that cannot be accounted for by muscle hypertrophy. Early on, researchers postulated the existence of neural adaptations to training primarily through the use of surface electromyographic recordings. More recent evidence also suggests that increased excitation may occur at the cortical levels following short-term resistance training. Alterations in synergistic activation and reductions in antagonist activation are neural factors that have been identified as changing during the early stages of resistance training which could contribute to maximal force generation. Neural adaptations that occur during the ramp-up phase of isometric contraction include decreases in motor unit recruitment thresholds, increased motor unit discharge rates, and increases in double discharges. An increase in the maximal rate of force development also occurs during the early stages of resistance training, but whether the neural mechanisms associated with the increase in the rate of rise are also associated with the increase in maximal force has not been elucidated. More work is needed to examine the integration of changes in cortical and spinal excitability with single motor unit firing patterns during this simple form of exercise before we can extend our understanding to different types of training. Key words: strength training, neural adaptation, H-reflex, maximal voluntary contraction, cortical excitability

Vastus lateralis surface and single motor unit EMG following submaximal shortening and lengthening contractions

2008 ◽  
Vol 33 (6) ◽  
pp. 1086-1095 ◽  
Author(s):  
Teatske M. Altenburg ◽  
Cornelis J. de Ruiter ◽  
Peter W.L. Verdijk ◽  
Willem van Mechelen ◽  
Arnold de Haan
Keyword(s):  
Motor Unit ◽  
Muscle Activation ◽  
Discharge Rate ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Knee Extensors ◽  
Single Motor Unit ◽  
Discharge Rates ◽  
Active Motor

A single shortening contraction reduces the force capacity of muscle fibers, whereas force capacity is enhanced following lengthening. However, how motor unit recruitment and discharge rate (muscle activation) are adapted to such changes in force capacity during submaximal contractions remains unknown. Additionally, there is limited evidence for force enhancement in larger muscles. We therefore investigated lengthening- and shortening-induced changes in activation of the knee extensors. We hypothesized that when the same submaximal torque had to be generated following shortening, muscle activation had to be increased, whereas a lower activation would suffice to produce the same torque following lengthening. Muscle activation following shortening and lengthening (20° at 10°/s) was determined using rectified surface electromyography (rsEMG) in a 1st session (at 10% and 50% maximal voluntary contraction (MVC)) and additionally with EMG of 42 vastus lateralis motor units recorded in a 2nd session (at 4%–47%MVC). rsEMG and motor unit discharge rates following shortening and lengthening were normalized to isometric reference contractions. As expected, normalized rsEMG (1.15 ± 0.19) and discharge rate (1.11 ± 0.09) were higher following shortening (p < 0.05). Following lengthening, normalized rsEMG (0.91 ± 0.10) was, as expected, lower than 1.0 (p < 0.05), but normalized discharge rate (0.99 ± 0.08) was not (p > 0.05). Thus, muscle activation was increased to compensate for a reduced force capacity following shortening by increasing the discharge rate of the active motor units (rate coding). In contrast, following lengthening, rsEMG decreased while the discharge rates of active motor units remained similar, suggesting that derecruitment of units might have occurred.

scholarly journals Sex differences in motor unit discharge rates at maximal and submaximal levels of force output

2020 ◽  
Vol 45 (11) ◽  
pp. 1197-1207
Author(s):  
J. Greig Inglis ◽  
David A. Gabriel
Keyword(s):  
Sex Differences ◽  
Motor Unit ◽  
Discharge Rate ◽  
Maximum Voluntary Contraction ◽  
Voluntary Contraction ◽  
Force Output ◽  
Neural Drive ◽  
Discharge Rates ◽  
Intramuscular Electrodes ◽  
Motor Unit Discharge

This study evaluated potential sex differences in motor unit (MU) behaviour at maximal and submaximal force outputs. Forty-eight participants, 24 females and 24 males, performed isometric dorsiflexion contractions at 20%, 40%, 60%, 80%, and 100% of a maximum voluntary contraction (MVC). Tibialis anterior electromyography was recorded both by surface and intramuscular electrodes. Compared with males, females had a greater MU discharge rate (MUDR) averaged across all submaximal intensities (Δ 0.45 pps, 2.56%). Males exhibited greater increases in MUDR above 40% MVC, surpassing females at 100% MVC (p’s < 0.01). Averaged across all force outputs, females had a greater incidence of doublet and rapid discharges and a greater percentage of MU trains with doublet and rapid (5–10 ms) discharges (Δ 75.55% and 61.48%, respectively; p’s < 0.01). A subset of males (n = 8) and females (n = 8), matched for maximum force output, revealed that females had even greater MUDR (Δ 1.38 pps, 7.47%) and percentage of MU trains with doublet and rapid discharges (Δ 51.62%, 56.68%, respectively; p’s < 0.01) compared with males at each force output, including 100% MVC. Analysis of the subset of strength-matched males and females suggest that sex differences in MU behaviour may be a result of females needing to generate greater neural drive to achieve fused tetanus. Novelty Females had higher MUDRs and greater percentage of MU trains with doublets across submaximal force outputs (20%–80% MVC). Differences were even greater for a strength matched subset. Differences in motor unit behaviour may arise from musculoskeletal differences, requiring greater neural drive in females.

scholarly journals Motor unit rate coding is severely impaired during forceful and fast muscular contractions in individuals post stroke

2013 ◽  
Vol 109 (12) ◽  
pp. 2947-2954 ◽  
Author(s):  
Li-Wei Chou ◽  
Jacqueline A. Palmer ◽  
Stuart Binder-Macleod ◽  
Christopher A. Knight
Keyword(s):  
Motor Unit ◽  
Force Production ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Paretic Limb ◽  
Discharge Rates ◽  
Rate Coding ◽  
Motor Unit Action ◽  
Unit Action ◽  
Rehabilitation Strategies

Information regarding how motor units are controlled to produce forces in individuals with stroke and the mechanisms behind muscle weakness and movement slowness can potentially inform rehabilitation strategies. The purpose of this study was to describe the rate coding mechanism in individuals poststroke during both constant ( n = 8) and rapid ( n = 4) force production tasks. Isometric ankle dorsiflexion force, motor unit action potentials, and surface electromyography were recorded from the paretic and nonparetic tibialis anterior. In the paretic limb, strength was 38% less and the rate of force development was 63% slower. Linear regression was used to describe and compare the relationships between motor unit and electromyogram (EMG) measures and force. During constant force contractions up to 80% maximal voluntary contraction (MVC), rate coding was compressed and discharge rates were lower in the paretic limb. During rapid muscle contractions up to 90% MVC, the first interspike interval was prolonged and the rate of EMG rise was less in the paretic limb. Future rehabilitation strategies for individuals with stroke could focus on regaining these specific aspects of motor unit rate coding and neuromuscular activation.

scholarly journals The knowns and unknowns of neural adaptations to resistance training

2020 ◽  
Author(s):  
Jakob Škarabot ◽  
Callum G. Brownstein ◽  
Andrea Casolo ◽  
Alessandro Del Vecchio ◽  
Paul Ansdell
Keyword(s):  
Resistance Training ◽  
Motor Unit ◽  
Force Production ◽  
Intracortical Inhibition ◽  
Short Term ◽  
Neural Adaptations ◽  
Non Invasive ◽  
Reticulospinal Tract ◽  
Large Motor ◽  
Potential Substrate

AbstractThe initial increases in force production with resistance training are thought to be primarily underpinned by neural adaptations. This notion is firmly supported by evidence displaying motor unit adaptations following resistance training; however, the precise locus of neural adaptation remains elusive. The purpose of this review is to clarify and critically discuss the literature concerning the site(s) of putative neural adaptations to short-term resistance training. The proliferation of studies employing non-invasive stimulation techniques to investigate evoked responses have yielded variable results, but generally support the notion that resistance training alters intracortical inhibition. Nevertheless, methodological inconsistencies and the limitations of techniques, e.g. limited relation to behavioural outcomes and the inability to measure volitional muscle activity, preclude firm conclusions. Much of the literature has focused on the corticospinal tract; however, preliminary research in non-human primates suggests reticulospinal tract is a potential substrate for neural adaptations to resistance training, though human data is lacking due to methodological constraints. Recent advances in technology have provided substantial evidence of adaptations within a large motor unit population following resistance training. However, their activity represents the transformation of afferent and efferent inputs, making it challenging to establish the source of adaptation. Whilst much has been learned about the nature of neural adaptations to resistance training, the puzzle remains to be solved. Additional analyses of motoneuron firing during different training regimes or coupling with other methodologies (e.g., electroencephalography) may facilitate the estimation of the site(s) of neural adaptations to resistance training in the future.

scholarly journals Recruitment Order of Motor Units in Human Vastus Lateralis Muscle Is Maintained During Fatiguing Contractions

2003 ◽  
Vol 90 (5) ◽  
pp. 2919-2927 ◽  
Author(s):  
Alexander Adam ◽  
Carlo J. De Luca
Keyword(s):  
Motor Unit ◽  
Time Course ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Vastus Lateralis Muscle ◽  
Force Output ◽  
Similar Time ◽  
Lateralis Muscle ◽  
Recruitment Order

Motor-unit firing patterns were studied in the vastus lateralis muscle of five healthy young men [21.4 ± 0.9 (SD) yr] during a series of isometric knee extensions performed to exhaustion. Each contraction was held at a constant torque level, set to 20% of the maximal voluntary contraction at the beginning of the experiment. Electromyographic signals, recorded via a quadrifilar fine wire electrode, were processed with the precision decomposition technique to identify the firing times of individual motor units. In repeat experiments, whole-muscle mechanical properties were measured during the fatigue protocol using electrical stimulation. The main findings were a monotonic decrease in the recruitment threshold of all motor units and the progressive recruitment of new units, all without a change of the recruitment order. Motor units from the same subject showed a similar time course of threshold decline, but this decline varied among subjects (mean threshold decrease ranged from 23 to 73%). The mean threshold decline was linearly correlated ( R2 ≥ 0.96) with a decline in the elicited peak tetanic torque. In summary, the maintenance of recruitment order during fatigue strongly supports the notion that the observed common recruitment adaptations were a direct consequence of an increased excitatory drive to the motor unit pool. It is suggested that the increased central drive was necessary to compensate for the loss in force output from motor units whose muscle fibers were actively contracting. We therefore conclude that the control scheme of motor-unit recruitment remains invariant during fatigue at least in relatively large muscles performing submaximal isometric contractions.

scholarly journals Vastus lateralis single motor unit EMG at the same absolute torque production at different knee angles

2009 ◽  
Vol 107 (1) ◽  
pp. 80-89 ◽  
Author(s):  
T. M. Altenburg ◽  
A. de Haan ◽  
P. W. L. Verdijk ◽  
W. van Mechelen ◽  
C. J. de Ruiter
Keyword(s):  
Motor Unit ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Knee Extensors ◽  
Emg Activity ◽  
Single Motor Unit ◽  
Recruitment Threshold ◽  
Single Motor ◽  
Discharge Rates ◽  
Isometric Torque

Single motor unit electromyographic (EMG) activity of the knee extensors was investigated at different knee angles with subjects ( n = 10) exerting the same absolute submaximal isometric torque at each angle. Measurements were made over a 20° range around the optimum angle for torque production (AngleTmax) and, where feasible, over a wider range (50°). Forty-six vastus lateralis (VL) motor units were recorded at 20.7 ± 17.9 %maximum voluntary contraction (%MVC) together with the rectified surface EMG (rsEMG) of the superficial VL muscle. Due to the lower maximal torque capacity at positions more flexed and extended than AngleTmax, single motor unit recruitment thresholds were expected to decrease and discharge rates were expected to increase at angles above and below AngleTmax. Unexpectedly, the recruitment threshold was higher ( P < 0.05) at knee angles 10° more extended (43.7 ± 22.2 N·m) and not different ( P > 0.05) at knee angles 10° more flexed (35.2 ± 17.9 N·m) compared with recruitment threshold at AngleTmax (41.8 ± 21.4 N·m). Also, unexpectedly the discharge rates were similar ( P > 0.05) at the three angles: 11.6 ± 2.2, 11.6 ± 2.1, and 12.3 ± 2.1 Hz. Similar angle independent discharge rates were also found for 12 units ( n = 5; 7.4 ± 5.4 %MVC) studied over the wider (50°) range, while recruitment threshold only decreased at more flexed angles. In conclusion, the similar recruitment threshold and discharge behavior of VL motor units during submaximal isometric torque production suggests that net motor unit activation did not change very much along the ascending limb of the knee-angle torque relationship. Several factors such as length-dependent twitch potentiation, which may contribute to this unexpected aspect of motor control, are discussed.

Fine-wire recordings of flexor hallucis brevis motor units up to maximal voluntary contraction reveal a flexible, nonrigid mechanism for force control

2020 ◽  
Vol 123 (5) ◽  
pp. 1766-1774
Author(s):  
J. Aeles ◽  
L. A. Kelly ◽  
Y. Yoshitake ◽  
A. G. Cresswell
Keyword(s):  
Full Range ◽  
Maximum Voluntary Contraction ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Motor Unit Action Potential ◽  
Single Motor Unit ◽  
Discharge Rates ◽  
Motor Unit Action ◽  
First Time ◽  
Unit Action

We recorded for the first time single motor unit action potential trains in the flexor hallucis brevis, a short toe muscle, over the full range of maximum voluntary contraction. Its motor units are recruited up to very high (98%) recruitment thresholds with a substantial range of discharge rates. We further show high variability with crossover of discharge rates as a function of recruitment threshold both between participants and between motor units within participants.

scholarly journals The Effects of Spinal Manipulation on Motor Unit Behavior

2021 ◽  
Vol 11 (1) ◽  
pp. 105
Author(s):  
Lucien Robinault ◽  
Aleš Holobar ◽  
Sylvain Crémoux ◽  
Usman Rashid ◽  
Imran Khan Niazi ◽  
...  
Keyword(s):  
Motor Unit ◽  
Conduction Velocity ◽  
Spinal Manipulation ◽  
Maximum Voluntary Contraction ◽  
Force Production ◽  
Movement Control ◽  
Position Sense ◽  
Joint Position Sense ◽  
Voluntary Contraction ◽  
Passive Movement

Over recent years, a growing body of research has highlighted the neural plastic effects of spinal manipulation on the central nervous system. Recently, it has been shown that spinal manipulation improved outcomes, such as maximum voluntary force and limb joint position sense, reflecting improved sensorimotor integration and processing. This study aimed to further evaluate how spinal manipulation can alter neuromuscular activity. High density electromyography (HD sEMG) signals from the tibialis anterior were recorded and decomposed in order to study motor unit changes in 14 subjects following spinal manipulation or a passive movement control session in a crossover study design. Participants were asked to produce ankle dorsiflexion at two force levels, 5% and 10% of maximum voluntary contraction (MVC), following two different patterns of force production (“ramp” and “ramp and maintain”). A significant decrease in the conduction velocity (p = 0.01) was observed during the “ramp and maintain” condition at 5% MVC after spinal manipulation. A decrease in conduction velocity suggests that spinal manipulation alters motor unit recruitment patterns with an increased recruitment of lower threshold, lower twitch torque motor units.

Sign in / Sign up

Export Citation Format

Share Document