Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men

Low-load blood flow restriction reduces time-to-minimum single motor unit discharge rate

Resistance training with low loads in combination with blood flow restriction (BFR) facilitates increases in muscle size and strength comparable with high-intensity exercise. We investigated the effects of BFR on single motor unit discharge behavior throughout a sustained low-intensity isometric contraction. Ten healthy individuals attended two experimental sessions: one with, the other without, BFR. Motor unit discharge rates from the tibialis anterior (TA) were recorded with intramuscular fine-wire electrodes throughout the duration of a sustained fatigue task. Three 5-s dorsiflexion maximal voluntary contractions (MVC) were performed before and after the fatigue task. Each participant held a target force of 20% MVC until endurance limit. A significant decrease in motor unit discharge rate was observed in both the non-BFR condition (from 13.13 ± 0.87 Hz to 11.95 ± 0.43 Hz, P = 0.03) and the BFR condition (from 12.95 ± 0.71 Hz to 10.9 ± 0.75 Hz, P = 0.03). BFR resulted in significantly shorter endurance time and time-to-minimum discharge rates and greater end-stage motor unit variability. Thus, low-load BFR causes an immediate steep decline in motor unit discharge rate that is greater than during contractions performed without BFR. This shortened neuromuscular response of time-to-minimum discharge rate likely contributes to the rapid rate of neuromuscular fatigue observed during BFR.

Time-related changes in firing rates are influenced by recruitment threshold and twitch force potentiation in the first dorsal interosseous

NEW FINDINGS

What is the central question of this study? The influences of motor unit recruitment threshold and twitch force potentiation on the changes in firing rates during steady-force muscular contractions are not well understood. What is the main finding and its importance? The behaviour of motor units during steady force was influenced by recruitment threshold, such that firing rates decreased for lower-threshold motor units but increased for higher-threshold motor units. In addition, individuals with greater changes in firing rates possessed greater twitch force potentiation. There are contradictory reports regarding changes in motor unit firing rates during steady-force contractions. Inconsistencies are likely to be the result of previous studies disregarding motor unit recruitment thresholds and not examining firing rates on a subject-by-subject basis. It is hypothesized that firing rates are manipulated by twitch force potentiation during contractions. Therefore, in this study we examined time-related changes in firing rates at steady force in relationship to motor unit recruitment threshold in the first dorsal interosseous and the influence of twitch force potentiation on such changes in young versus aged individuals. Subjects performed a 12 s steady-force contraction at 50% maximal voluntary contraction, with evoked twitches before and after the contraction to quantify potentiation. Firing rates, in relationship to recruitment thresholds, were determined at the beginning, middle and end of the steady force. There were no firing rate changes for aged individuals. For the young, firing rates decreased slightly for lower-threshold motor units but increased for higher-threshold motor units. Twitch force potentiation was greater for young than aged subjects, and changes in firing rates were correlated with twitch force potentiation. Thus, individuals with greater increases in firing rates of higher-threshold motor units and decreases in lower-threshold motor units possessed greater twitch force potentiation. Overall, changes in firing rates during brief steady-force contractions are dependent on recruitment threshold and explained in part by twitch force potentiation. Given that firing rate changes were measured in relationship to recruitment threshold, this study illustrates a more complete view of firing rate changes during steady-force contractions.

Age-related differences in twitch properties and muscle activation of the first dorsal interosseous

OBJECTIVE

To examine twitch force potentiation and twitch contraction duration, as well as electromyographic amplitude (EMG_RMS) and motor unit mean firing rates (MFR) at targeted forces between young and old individuals in the first dorsal interosseous (FDI). Ultrasonography was used to assess muscle quality.

METHODS

Twenty-two young (YG) (age=22.6±2.7years) and 14 older (OD) (age=62.1±4.7years) individuals completed conditioning contractions at 10% and 50% maximal voluntary contraction, (MVC) during which EMG_RMS and MFRs were assessed. Evoked twitches preceded and followed the conditioning contractions. Ultrasound images were taken to quantify muscle quality (cross-sectional area [CSA] and echo intensity [EI]).

RESULTS

No differences were found between young and old for CSA, pre-conditioning contraction twitch force, or MFRs (P>0.05). However, OD individuals exhibited greater EI and contraction duration (P<0.05), and EMG_RMS (YG=35.4±8.7%, OD=43.4±13.2%; P=0.034). Twitch force potentiation was lower for OD (0.311±0.15N) than YG (0.619±0.26N) from pre- to post-50% conditioning contraction (P<0.001).

CONCLUSIONS

Lower levels of potentiation with elongated contraction durations likely contributed to greater muscle activation during the conditioning contractions in the OD rather than altered MFRs. Ultrasonography suggested age-related changes in muscle structure contributed to altered contractile properties in the OD.

SIGNIFICANCE

Greater muscle activation requirements can have negative implications on fatigue resistance at low to moderate intensities in older individuals.

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

Motor unit properties from three synergistic muscles during ramp isometric elbow extensions

Many tasks require synergistic activation of muscles that possess different architectural, mechanical, and neural control properties. However, investigations of the motor unit (MU) mechanisms which modulate force are mostly restricted to individual muscles and low forces. To explore the pattern of MU recruitment and discharge behavior among three elbow extensors (lateral and long heads of the triceps brachii, and anconeus) during ramp isometric contractions, recruitment thresholds of 77 MUs in five young men were determined and corresponding MU discharge rates were tracked in 1-s epochs over forces ranging from 0 to 75 % of maximal voluntary isometric force (MVC). Across all forces, MUs in the lateral head discharged at higher rates than those in the anconeus (p < 0.001, Δ = 0.23). When all MUs were considered, recruitment thresholds in the long head of the triceps brachii were higher than the lateral head (p < 0.05, Δ = 0.70) with a trend (p = 0.08, Δ = 0.48) for higher recruitment thresholds in the long head compared with the anconeus. Together, these data indicate a potential mechanical disadvantage of the long head of the triceps brachii at 0° shoulder flexion. However, among low-threshold MUs (<10 % MVC), recruitment thresholds were lower in the anconeus than in both heads of the triceps brachii consistent with the expected twitch contractile and fiber type differences among these muscles. These findings illustrate the importance of considering synergistic relations among muscles used for a coordinated task, and the sensitivity of synergies to muscle architectural, mechanical, and possibly specific synaptic input factors.

Decreased motor unit discharge rate in the potentiated human tibialis anterior muscle

AIM

The purpose of this study was to examine the influence of post-activation potentiation (PAP), the transient increase in low-frequency isometric force observed after muscle activity, on motor unit discharge rates measured during submaximal contractions.

METHODS

A quadrifilar needle electrode was inserted into the tibialis anterior muscle to determine discharge rate of individual motor units while monopolar electrodes were used to monitor the root-mean-square (RMS) and mean power frequency (MPF) of the surface EMG signal. Control (unpotentiated) and experimental (potentiated) measures were obtained during a 5 s voluntary contraction at 50% of maximal. In between these measures, subjects performed a 10 s maximal voluntary contraction (MVC) to induce PAP.

RESULTS

All subjects data are reported as means ± SEM (n = 10). Compared to baseline values measured prior to the MVC, isometric twitch force measured immediately after the MVC was increased by 260 ± 16% (day 3). Motor unit discharge rate in the potentiated tibialis anterior muscle decreased by approx. 10%, from 20.3 ± 0.8 (before) to 18.3 ± 0.99 pps (P = 0.01) (after). Moreover, the MPF was decreased by approx. 9% (from 58.1 ± 2.84 to 53.6 ± 2.85 Hz; P = 0.01) in the potentiated tibialis anterior. On the other hand, consistent with the absence of fatigue during the MVC, the RMS signal was not altered in the potentiated tibialis anterior (0.29 ± 0.03 vs. 0.33 ± 0.04 mV; P = 0.07).

CONCLUSION

Motor unit discharge rates determined during a brief, submaximal contraction were decreased in the potentiated human tibialis anterior muscle.

Differential age-related changes in motor unit properties between elbow flexors and extensors

AIM

Healthy adult ageing of the human neuromuscular system is comprised of changes that include atrophy, weakness and slowed movements with reduced spinal motor neurone output expressed by lower motor unit discharge rates (MUDRs). The latter observation has been obtained mostly from hand and lower limb muscles. The purpose was to determine the extent to which elbow flexor and extensor contractile properties, and MUDRs in six old (83 +/- 4 years) and six young (24 +/- 1 years) men were affected by age, and whether any adaptations were similar for both muscle groups.

METHODS

Maximal isometric voluntary contraction (MVC), voluntary activation, twitch contractile properties, force-frequency relationship and MUDRs from sub-maximal to maximal intensities were assessed in the elbow flexors and extensors.

RESULTS

Both flexor and extensor MVCs were significantly (P < 0.05) less (approximately 42% and approximately 46% respectively) in the old than in the young. Contractile speeds and the force-frequency relationship did not show any age-related differences (P > 0.05). For the elbow flexors contraction duration was approximately 139 ms and for the extensors it was approximately 127 ms for both age groups (P > 0.05). The mean MUDRs from 25% MVC to maximum were lower (approximately 10% to approximately 36%) in the old than in the young (P < 0.01). These age-related differences were larger for biceps (Cohen's d = 8.25) than triceps (Cohen's d = 4.79) brachii.

CONCLUSION

Thus, at least for proximal upper limb muscles, mean maximal MUDR reductions with healthy adult ageing are muscle specific and not strongly related to contractile speed.

Caloric Restriction Does Not Offset Age-Associated Changes in the Biophysical Properties of Motoneurons

Age-associated changes in neuromuscular function may be due to a loss of motor neurons as well as changes in their biophysical properties. Neuronal damage imposed by reactive oxygen species may contribute to age-related deficits in CNS function. Thus we hypothesized that aging would alter the functional properties of motoneurons and that caloric-restriction would offset these changes. Intracellular recordings were made from lumbar motoneurons of old Fisher Brown Norway (FBN) fed ad libitum (oldAL, 30.8 ± 1.3 mo) or on a fortified calorie-restricted diet from 14 wk of age (oldCR, 31.0 ± 1.8 mo). Basic and rhythmic firing properties recorded from these aged motoneurons (MNs) were compared with properties recorded from young FBN controls (young, 8.4 ± 4.6 mo). Compared with young MNs, old MNs had a 104% greater ( P < 0.001) afterhyperpolarization potential (AHP), a 21.1% longer AHP half-decay time ( P < 0.05), 28.7% lower rheobase ( P < 0.001), 49.7% greater ( P < 0.001) input resistance, 21.1% ( P < 0.0001) less spike frequency adaptation, lower minimal (30.2%, P < 0.0001) and maximal (16.7%, P < 0.0001) steady-state firing frequencies, a lower (35.5%, P < 0.0001) frequency-current slope, and an increased incidence of persistent inward current. Because basic properties became more diverse in old MNs and the slope of the frequency-current relationship, which is normally similar for high- and low-threshold MNs, was lower in the old group, we conclude that aging alters the biophysical properties of MNs in a fashion that cannot be simply attributed to a loss of high-threshold MNs. Surprisingly, caloric restriction, which is known to attenuate aging-associated changes in hindlimb muscles, had no effect on the progress of aging in the innervating MNs.

Postactivation potentiation of short tetanic contractions is differently influenced by stimulation frequency in young and elderly adults

The purpose of this study was to examine the effects of postactivation potentiation (PAP) on the torque and rate of torque development for contractions evoked by short trains of stimuli at different frequencies, in young and elderly adults. Individual mechanical contributions to each electrical stimulus within trains were also analysed. Single pulse, and two- (PT2) and three-pulse trains (PT3) delivered at 20, 50, 80 and 100 Hz were evoked before and during a 10 min period after a 6-s conditioning MVC. The results show that PAP of the torque for PT2 decreased with the increase of the stimulation frequency for young (from 184.5 to 140.4% of control values) and elderly (from 140.5 to 109.6%). Regardless of the stimulation frequency, the peak of PAP was greater in young than in elderly and occurred immediately after the conditioning MVC but was delayed for the 100 Hz condition in elderly adults. For PT3, the results were similar although the extent of PAP was less. The PAP of the mechanical contributions within the trains also decreased with the augmentation of the stimulation frequency. For most of the frequencies above 20 Hz, the peak of PAP for each mechanical contribution was delayed by 1 min after the conditioning MVC. These results indicate an age- and frequency-related PAP saturation of the successive mechanical contributions within a train of stimuli that decrease with time. The functional implication of the findings is that PAP effect is lower and delayed at high compared with low activation rate.

Branched EMG electrodes for stable and selective recording of single motor unit potentials in humans

Branched surface EMG electrodes are bipolar electrodes with the hot signal pole referenced to two or more short-circuited leading-off surfaces. This technique provides stable recording of single motor unit potentials during real movements, up to maximal muscle contractions. The selective characteristic of branched electrodes is based on the same principles as the double differential detection system and spatial filtering technique proposed later. Equi-weight calculations to assess the selectivity of different electrode types and their position are used. The main advantage of branched electrodes, especially high stability, is achieved by the wire electrode version. The design, manufacture, implementation, and application of wire electrodes are discussed in detail. During recording of motor unit potentials, electrodes are positioned subcutaneously over the muscle fascia. This positioning maximizes electrode stability. Appropriate orientation of the electrode relative to the muscle architecture ensures adequate selectivity for single motor unit recordings. Branched electrodes require ordinary EMG equipment (two or even one amplifier).