Effects of tonic vibration reflex on motor unit recruitment in human wrist extensor muscles

Excitation of natural spinal reflex loops in the sensory-motor control of hand prostheses

Sensory feedback for prosthesis control is typically based on encoding sensory information in specific types of sensory stimuli that the users interpret to adjust the control of the prosthesis. However, in physiological conditions, the afferent feedback received from peripheral nerves is not only processed consciously but also modulates spinal reflex loops that contribute to the neural information driving muscles. Spinal pathways are relevant for sensory-motor integration, but they are commonly not leveraged for prosthesis control. We propose an approach to improve sensory-motor integration for prosthesis control based on modulating the excitability of spinal circuits through the vibration of tendons in a closed loop with muscle activity. We measured muscle signals in healthy participants and amputees during different motor tasks, and we closed the loop by applying vibration on tendons connected to the muscles, which modulated the excitability of motor neurons. The control signals to the prosthesis were thus the combination of voluntary control and additional spinal reflex inputs induced by tendon vibration. Results showed that closed-loop tendon vibration was able to modulate the neural drive to the muscles. When closed-loop tendon vibration was used, participants could achieve similar or better control performance in interfaces using muscle activation than without stimulation. Stimulation could even improve prosthetic grasping in amputees. Overall, our results indicate that closed-loop tendon vibration can integrate spinal reflex pathways in the myocontrol system and open the possibility of incorporating natural feedback loops in prosthesis control.

The Efficacy of Whole-Body Vibration in Managing Postburn Victims' Complications: A Systematic Review

Burn injury is a complicated traumatic event with both systemic and local consequences. These complications include long periods of bed rest, pain, muscle weakness, vitamin D deficiency, and bone mineral density loss. Whole-body vibration demonstrated effectiveness in improving muscle power and bone mineral density in various musculoskeletal populations. This systematic review of randomized controlled trials aims to assess evidence for the effectiveness of using whole-body vibration on postburn survivors with at least 1% total body surface area on sensory and motor outcomes. A systematic search was conducted across six databases, which are, PubMed, Cochrane, PEDro, Scopus, Web of Science, and Google Scholar, from inception till March 2022. Retrieved studies were screened by title and abstract and full-text in two stages using Rayyan web-based. Included studies were assessed for risk of bias using the Cochrane risk of bias tool ROB 2.0. Six randomized controlled trials with 203 participants were included. Five of the included studies demonstrated an overall high risk of bias. Compared to conventional physiotherapy programs, whole-body vibration demonstrated improvement in functional mobility and balance using timed up and go and Biodex balance assessment, respectively. However, there were no differences between whole-body vibration and conventional physiotherapy program alone in bone mineral density and muscle power. Although the current evidence of whole-body vibration is limited, whole-body vibration combined with traditional physical therapy programs may improve functional mobility and balance in postburn survivors compared to physical therapy programs alone.

Acute effects of blood flow restriction with whole-body vibration on sprint, muscle activation and metabolic accumulation in male sprinters

Purpose: The aim of this study was to explore the acute effects of Blood Flow Restriction Training (BFRT), Whole-Body Vibration (WBV), and BFRT + WBV on the 20 m sprint, muscle activation, and metabolic accumulation in male sprinters.

Method: Sixteen male sprinters randomly performed BFRT, WBV, or BFRT + WBV interventions with 72 h intervals. Electromyography (EMG) signals were collected before and during interventions. Fingertip blood was taken before, immediately after, and 15 min after the intervention. 20 m sprint was performed before and 3 min after the intervention.

Results: 1) 0–10m and 0–20 m sprint performance were significantly improved after WBV and BFRT + WBV interventions (p < 0.05), 0–20 m sprint performance was significantly improved after all three interventions (p < 0.05), 2) After BFRT + WBV intervention, the EMG amplitude of the vastus lateralis and soleus were significantly improved. Greater increases in EMG activity of the tibialis anterior muscle (p < 0.05)and blood lactate (p < 0.05)were observed following BFRT intervention compared to BFRT + WBV intervention.

Conclusion: For sprint performance, BFRT and WBV had similar post-activation enhancement effects to BFRT + WBV, and the metabolic accumulation immediately following the BFRT were higher than that following BFRT + WBV in male sprinters.

Exploring the comparative adequacy of a unimanual and a bimanual stimulus-response setup for use with three-alternative choice response time tasks

Research often conceptualises complex social factors as being distinct binary categories (e.g., female vs male, feminine vs masculine). While this can be appropriate, the addition of an 'overlapping' category (e.g., non-binary, gender neutral) can contextualise the 'binary', both for participants (allowing more complex conceptualisations of the categories than the 'either/or' conceptualisation in binary tasks), and for the results (by providing a neutral baseline for comparison). However, it is not clear what the best response setup for such a task would be. In this study, we explore this topic through comparing a unimanual (N = 34) and a bimanual response setup (N = 32) for use with a three-alternative choice response time task. Crucially, one of the stimulus categories ('mixed') was composed of stimulus elements from the other two stimulus categories used in that task (Complex Task). A reference button task was included to isolate the motoric component of response registration (Simple Task). The results of the simple task indicated lower motoric costs for the unimanual compared to the bimanual setup. However, when statistically controlling for these motoric costs in the complex task, the bimanual setup had a lower error rate and faster response times than the unimanual setup. Further, in the complex task error rates and response times were higher for the mixed than the matched stimuli, indicating that responding to mixed stimuli is more challenging for encoding and/or decision making processes. This difference was more pronounced in the unimanual than the bimanual setup. Taken together these results indicate that the unimanual setup is more adequate for the reference button task, whereas the intricacy of overlapping categories in the complex task is better contained in the bimanual setup, i.e. when some response alternatives are allocated to one hand and other alternatives to the other hand.

Motor Unit Discharge Patterns in Response to Focal Tendon Vibration of the Lower Limb in Cats and Humans

High-frequency vibration of the tendon provides potent activation of Ia afferents time-locked to the stimulation frequency and provides excitatory ionotropic activation of homonymous motor pools. In cats, the evoked motor unit discharge is constrained to discharge at integer multiples of the vibration frequency, resulting in a probability of discharge that is highly punctuated. Here we quantify the robustness of this punctuated response in the cat and evaluate whether it is present in the human. Soleus electromyography (EMG) was collected from eight cats using 64 channel electrodes during three modes of motoneuron activation. First, tendon vibration parameters were modified. Second, secondary reflex inputs are applied concurrently with tendon vibration. Third, the state of the spinal cord was altered through pharmacological or surgical manipulations. Analogous surface high-density EMG was collected from the lower leg of six humans during both vibration evoked and matched volitional contractions. Array EMG signals from both the cat and human were decomposed into corresponding motor unit action potential spike trains, and the punctuation in discharge was quantified. In the cat, regardless of vibration parameters, secondary synaptic drive, and state of spinal circuitry, focal tendon vibration evoked punctuated motor unit discharge. However, in the human lower limb, the vibration-evoked contractions do not produce punctuated motor unit discharge.

Ia EPSPs in rat spinal motoneurons are potentiated after a 5-week whole-body vibration

Whole-body vibration (WBV) is often applied as an alternative method for strength training or to prevent muscle force decrease. Previous studies indicated that WBV induced: 1) changes in the contractile parameters predominantly of fast motor units, 2) higher motoneuron excitability, and 3) higher motoneuron firing rates at lower stimulus intensities compared with the control. In this study, we evaluated the influence of WBV on Ia monosynaptic input from muscle spindles because the tonic vibration reflex is responsible for the enhancement of muscle activity observed after WBV. The aim was to answer the question of whether repeated activation of muscle spindles during WBV may result in altered synaptic excitation of motoneurons. WBV was performed on adult male Wistar rats, 5 days per week, for 5 weeks, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Fast-type medial gastrocnemius motoneurons were investigated intracellularly in deeply anesthetized animals in the experimental (n=7, 34 motoneurons) and control (n=7, 32 motoneurons) groups. Monosynaptic Ia EPSPs were evoked by electrical stimulation of afferent fibers from the synergistic lateral gastrocnemius and soleus muscles. Data were analyzed using a mixed linear model. WBV induced an increase of the mean EPSP amplitude by 28% (P=0.025), correlated with the resting membrane potential and input resistance, and a shortening of the mean EPSP rise time by 11% (P=0.012). The potentiation of synaptic excitation of motoneurons indicates that WBV may support rehabilitation or training processes aimed at increasing muscle strength on the basis of increased motoneuronal drive.

Effects of whole-body vibration training frequency on neuromuscular performance: a randomized controlled study

This study compared the efficacy of two whole-body vibration (WBV) protocols with equal training volume and different frequency of training sessions/week on body composition and physical fitness. Sixty male air force cadets (age: 20.5±1.4 years) were randomly assigned to a lower frequency (Lf-WBV), a higher frequency (Hf-WBV) or a control group (CG). The training volume was equated (20 training sessions) between the two WBV groups, but the number of weekly training sessions was different. The Lf-WBV group trained three times per week, the Hf-WBV group trained five times per week, while the CG did not perform any training. Each training session, for both groups, included 10 sets x 1 min with 1 min rest of WBV on a synchronous vibration platform (25-35 Hz, 4-6 mm). Body composition, flexibility, maximal strength, 30 m sprint time, squat jump (SJ) and countermovement jump (CMJ) performance, Wingate test performance, and 20 m shuttle run performance were evaluated before and after training. Only the Hf-WBV group demonstrated an increase in flexibility (+7%; p<0.01; d=0.33) and maximal strength (+10%; p=0.016; d= 0.59), and a significant reduction in fat mass (-6.2%; p<0.01; d=0.21). SJ performance improved to a similar extent in both Hf-WBV and Lf-WBV groups (+7%; p<0.01; d=0.32). There were no changes in any parameter in the CG. In conclusion, only the condensed weekly WBV protocol was effective in improving body composition, flexibility, lower limb strength and power in young active individuals. The WBV protocol with the higher dispersion of training sessions per week (lower training frequency/week) improved SJ performance but did not have any effect on body composition, flexibility, or maximal strength.

Low Gain Servo Control During the Kohnstamm Phenomenon Reveals Dissociation Between Low-Level Control Mechanisms for Involuntary vs. Voluntary Arm Movements

The Kohnstamm phenomenon is a prolonged involuntary aftercontraction following a sustained voluntary isometric muscle contraction. The control principles of the Kohnstamm have been investigated using mechanical perturbations, but previous studies could not dissociate sensorimotor responses to perturbation from effects of gravity. We induced a horizontal, gravity-independent Kohnstamm movement around the shoulder joint, and applied resistive or assistive torques of 0.5 Nm after 20° angular displacement. A No perturbation control condition was included. Further, participants made velocity-matched voluntary movements, with or without similar perturbations, yielding a 2 × 3 factorial design. Resistive perturbations produced an increase in agonist electromyography (EMG), in both Kohnstamm and voluntary movements, while assistive perturbations produced a decrease. While overall Kohnstamm EMGs were greater than voluntary EMGs, the EMG responses to perturbation, when expressed as a percentage of unperturbed EMG activity, were significantly smaller during Kohnstamm movements than during voluntary movements. The results suggest that the Kohnstamm aftercontraction involves a central drive, coupled with low-gain servo control by a negative feedback loop between afferent input and a central motor command. The combination of strong efferent drive with low reflex gain may characterize involuntary control of postural muscles. Our results question traditional accounts involving purely reflexive mechanisms of postural maintenance. They also question existing high-gain, peripheral accounts of the Kohnstamm phenomenon, as well as accounts involving a central adaptation interacting with muscle receptors via a positive force feedback loop.

Kv1.2 Channels Promote Nonlinear Spiking Motoneurons for Powering Up Locomotion

Spinal motoneurons are endowed with nonlinear spiking behaviors manifested by a spike acceleration whose functional significance remains uncertain. Here, we show in rodent lumbar motoneurons that these nonlinear spiking properties do not rely only on activation of dendritic nifedipine-sensitive L-type Ca2+ channels, as assumed for decades, but also on the slow inactivation of a nifedipine-sensitive K+ current mediated by Kv1.2 channels that are highly expressed in axon initial segments. Specifically, the pharmacological and computational inhibition of Kv1.2 channels occluded the spike acceleration of rhythmically active motoneurons and the correlated slow buildup of rhythmic motor output recorded at the onset of locomotor-like activity. This study demonstrates that slow inactivation of Kv1.2 channels provides a potent gain control mechanism in mammalian spinal motoneurons and has a behavioral role in enhancing locomotor drive during the transition from immobility to steady-state locomotion.

Vibrations transmitted from human hands to upper arm, shoulder, back, neck, and head

Some powered hand tools can generate significant vibration at frequencies below 25 Hz. It is not clear whether such vibration can be effectively transmitted to the upper arm, shoulder, neck, and head and cause adverse effects in these substructures. The objective of this study is to investigate the vibration transmission from the human hands to these substructures. Eight human subjects participated in the experiment, which was conducted on a 1-D vibration test system. Unlike many vibration transmission studies, both the right and left hand-arm systems were simultaneously exposed to the vibration to simulate a working posture in the experiment. A laser vibrometer and three accelerometers were used to measure the vibration transmitted to the substructures. The apparent mass at the palm of each hand was also measured to help in understanding the transmitted vibration and biodynamic response. This study found that the upper arm resonance frequency was 7-12 Hz, the shoulder resonance was 7-9 Hz, and the back and neck resonances were 6-7 Hz. The responses were affected by the hand-arm posture, applied hand force, and vibration magnitude. The transmissibility measured on the upper arm had a trend similar to that of the apparent mass measured at the palm in their major resonant frequency ranges. The implications of the results are discussed.

RELEVANCE TO INDUSTRY

Musculoskeletal disorders (MSDs) of the shoulder and neck are important issues among many workers. Many of these workers use heavy-duty powered hand tools. The combined mechanical loads and vibration exposures are among the major factors contributing to the development of MSDs. The vibration characteristics of the body segments examined in this study can be used to help understand MSDs and to help develop more effective intervention methods.

Local vibration inhibits H-reflex but does not compromise manual dexterity and does not increase tremor

The present work aimed at investigating the effects of local vibration on upper limb postural and kinetic tremor, on manual dexterity and on spinal reflex excitability. Previous studies have demonstrated a decrease in spinal reflex excitability and in force fluctuations in the lower limb but an increase in force fluctuation in the upper limbs. As hand steadiness is of vital importance in many daily-based tasks, and local vibration may also be applied in movement disorders, we decided to further explore this phenomenon. Ten healthy volunteers (26±3years) were tested for H reflex, postural and kinetic tremor and manual dexterity through a Purdue test. EMG was recorded from flexor carpi radialis (FCR) and extensor digitorum communis (EDC). Measurements were repeated at baseline, after a control period during which no vibration was delivered and after vibration. Intervention consisted in holding for two minutes a vibrating handle (frequency 75Hz, displacement∼7mm), control consisted in holding for two minutes the same handle powered off. Reflex excitability decreased after vibration whilst postural tremor and manual dexterity were not affected. Peak kinetic tremor frequency increased from baseline to control measurements (P=0.002). Co-activation EDC/FCR increased from control to vibration (P=0.021). These results show that two minutes local vibration lead to a decrease in spinal excitability, did not compromise manual dexterity and did not increase tremor; however, in contrast with expectations, tremor did not decrease. It is suggested that vibration activated several mechanisms with opposite effects, which resulted in a neutral outcome on postural and kinetic tremor.

Effects of real and sham whole-body mechanical vibration on spinal excitability at rest and during muscle contraction

We examined the effects of whole-body mechanical vibration (WBV) on indices of motoneuronal excitability at rest and during muscle contraction in healthy humans. Real and sham WBV at 30 Hz had no effect on reflexes measured during muscle contraction. Real WBV at 30 and 50 Hz depressed the H-reflex ∼45%. These depressions diminished across the five inter-bout rest intervals. The depression converted to 27% and 7% facilitation over the 15-min long recovery period following real WBV at 30 and 50 Hz, respectively. The depression, measured during the inter-bout rest, correlated r = 0.48 (P = 0.007) with the subsequent facilitation, measured during the follow-up. The depression produced by sham vs real WBV was significant but less (23%), recovered faster, and the facilitation was absent in the 15-min long follow-up period. WBV produced time-varying depression followed by facilitation of the H-reflex at rest. A lack of change in volitional wave suggests that WBV did not affect the efferent neural drive.

Effects of string stiffness on muscle fatigue after a simulated tennis match

We tested the influence of string stiffness on the occurrence of forearm muscle fatigue during a tennis match. Sixteen tennis players performed two prolonged simulated tennis matches with low-stiffness or high-stiffness string. Before and immediately after exercise, muscle fatigability was evaluated on the forearm muscles during a maximal intermittent gripping task. Groundstroke ball speeds and the profile of acceleration of the racquet frame at collision were recorded during each match. The peak-to-peak amplitude of acceleration and the resonant frequency of the frame were significantly greater with high- (5060 ± 1892 m/s(2) and 204 ± 29 Hz, respectively) than with low-stiffness string (4704 ± 1671 m/s(2) and 191 ± 16 Hz, respectively). The maximal and the averaged gripping forces developed during the gripping task were significantly reduced after the tennis match with high- (-15 ± 14%, and -22 ± 14%, respectively), but not with low-stiffness string. The decrease of ball speed during the simulated matches tended to be greater with high- than with low-stiffness string (P = .06). Hence, playing tennis with high-stiffness string promotes forearm muscle fatigue development, which could partly contribute to the groundstroke ball speed decrement during the game.

Whole-body vibration versus eccentric training or a wait-and-see approach for chronic Achilles tendinopathy: a randomized clinical trial

STUDY DESIGN

Randomized clinical trial.

OBJECTIVES

To test the hypothesis that whole-body vibration training results in greater improvements in symptoms and pain, structural changes, and muscle flexibility and strength of the triceps surae muscle-tendon unit than those achieved with eccentric training or with a wait-and-see approach.

BACKGROUND

The potential use of vibration training for the treatment of Achilles tendinopathy has not been explored.

METHODS

Fifty-eight patients (mean age, 46.0 years) with Achilles tendinopathy were randomly assigned to a 12-week intervention using whole-body vibration training, eccentric training, or a wait-and-see approach. Pain, tendon structure and path, and muscle flexibility and strength were assessed at baseline and follow-up, and compared using mixed-factor analyses of variance.

RESULTS

Pain improvements at the midsection of the tendon were greater in the vibration- and eccentric-training groups than in the wait-and-see group (mean difference from the vibration-training group, -18.0; 95% confidence interval [CI]: -35.0, -1.1; mean difference from the eccentric-training group, -27.0; 95% CI: -50.9, -3.1). Improvements in pain at the musculotendinous junction were greater in the eccentric-training group than in the other groups (mean difference from the vibration-training group, -31.4; 95% CI: -60.7, -2.0; mean difference from the wait-and-see group, -50.2; 95% CI: -82.3, -18.1). Improvements in most participants were achieved in the vibration-training group, followed by the eccentric-training group.

CONCLUSION

Vibration training may be an alternative or a complementary treatment in patients who do not respond well to eccentric training, especially in those with insertional pain.

LEVEL OF EVIDENCE

Therapy, level 2b-.

The influence of a 5-wk whole body vibration on electrophysiological properties of rat hindlimb spinal motoneurons

The study aimed at determining the influence of a whole body vibration (WBV) on electrophysiological properties of spinal motoneurons. The WBV training was performed on adult male Wistar rats, 5 days a week, for 5 wk, and each daily session consisted of four 30-s runs of vibration at 50 Hz. Motoneuron properties were investigated intracellularly during experiments on deeply anesthetized animals. The experimental group subjected to the WBV consisted of seven rats, and the control group of nine rats. The WBV treatment induced no significant changes in the passive membrane properties of motoneurons. However, the WBV-evoked adaptations in excitability and firing properties were observed, and they were limited to fast-type motoneurons. A significant decrease in rheobase current and a decrease in the minimum and the maximum currents required to evoke steady-state firing in motoneurons were revealed. These changes resulted in a leftward shift of the frequency-current relationship, combined with an increase in slope of this curve. The functional relevance of the described adaptive changes is the ability of fast motoneurons of rats subjected to the WBV to produce series of action potentials at higher frequencies in a response to the same intensity of activation. Previous studies proved that WBV induces changes in the contractile parameters predominantly of fast motor units (MUs). The data obtained in our experiment shed a new light to possible explanation of these results, suggesting that neuronal factors also play a substantial role in MU adaptation.

Influence of whole-body vibration on biodistribution of the radiopharmaceutical [99mTc]methylene diphosphonate in Wistar rats

PURPOSE

The radionuclide bone scan is the basis of skeletal nuclear medicine imaging. Bone scintigraphy is a highly sensitive method for indicating disease in bone. Mechanical stimulation in the manner of whole-body vibration (WBV) appears beneficial to the maintenance and/or enhancement of skeletal mass in individuals. The aim of this work was to evaluate the effect of WBV on the biodistribution of the radiopharmaceutical [99mTc]methylene diphosphonate (99mTc-MDP) in Wistar rats.

MATERIALS AND METHODS

In the biodistribution analysis, animals were anesthetized with sodium thiopental, the radiopharmaceutical (99m)Tc-MDP was administered via ocular plexus and after 10 min the animals were submitted to vibration of 20 Hz (1 min) in an oscillatory platform. Following, the animals were sacrificed, the organs were isolated, the radioactivity determined in a well counter, and the percentages of radioactivity per gram (%ATI/g) in the organs were calculated. An unpaired t-test following Welch test (p < 0.05) was done for statistical analysis of the results.

RESULTS

The biodistribution was significantly (p < 0.05) decreased in kidney, bone, lung, stomach, prostate and bowel.

CONCLUSION

The analysis of the results indicates that the vibration could produce metabolic alterations with influence in the uptake of the radiopharmaceutical 99mTc-MDP in bone, stomach, bowel, prostate, kidney and bladder.

Acute effect of whole-body vibration on power, one-repetition maximum, and muscle activation in power lifters

The purpose of this study was to investigate the acute effect of whole-body vibration with a frequency of 50 Hz (WBV(50Hz)) on peak power in squat jump (SJ), 1 repetition maximum (1RM) in parallel squat, and electromyography (EMG) activity and compare them with no-vibration conditions in power lifters. Twelve national level male power lifters (age 24 ± 5 years, body mass 110 ± 24 kg, height 179 ± 7 cm) tested peak power in SJ and 1RM in parallel squat while they were randomly exposed to WBV(50Hz) or to no vibration. These tests were performed in a Smith Machine. Peak power output was higher while performed with a WBV(50Hz) compared with the no-WBV condition (p < 0.05). This increase in power output was accompanied by higher EMG starting values and EMG peak values of the investigated thigh muscles during WBV(50Hz) (p < 0.05). There was no difference between adding WBV(50Hz) and no-vibration conditions in 1RM parallel squat. In conclusion, the results of this study suggest that the application of WBV(50Hz) acutely increases peak power output during SJ in well strength trained individuals such as power lifters. This increase in power was accompanied by an increased EMG activity in the quadriceps muscles. However, in 1RM parallel squat, there was no difference between WBV50Hz and no-vibration conditions. Therefore, adding WBV(50Hz) has no acute additive effect on 1RM parallel squat in power lifters and, based on the present findings, may thus not be recommended in the training to improve 1RM in power lifters. However, WBV(50Hz) seems to have an acute additive effect on peak power output and may be used in well strength trained individuals for whom a high power output is important for performance.

Effects of whole body vibration on motor unit recruitment and threshold

Whole body vibration (WBV) has been suggested to elicit reflex muscle contractions but this has never been verified. We recorded from 32 single motor units (MU) in the vastus lateralis of 7 healthy subjects (34 ± 15.4 yr) during five 1-min bouts of WBV (30 Hz, 3 mm peak to peak), and the vibration waveform was also recorded. Recruitment thresholds were recorded from 38 MUs before and after WBV. The phase angle distribution of all MUs during WBV was nonuniform (P < 0.001) and displayed a prominent peak phase angle of firing. There was a strong linear relationship (r = -0.68, P < 0.001) between the change in recruitment threshold after WBV and average recruitment threshold; the lowest threshold MUs increased recruitment threshold (P = 0.008) while reductions were observed in the higher threshold units (P = 0.031). We investigated one possible cause of changed thresholds. Presynaptic inhibition in the soleus was measured in 8 healthy subjects (29 ± 4.6 yr). A total of 30 H-reflexes (stimulation intensity 30% Mmax) were recorded before and after WBV: 15 conditioned by prior stimulation (60 ms) of the antagonist and 15 unconditioned. There were no significant changes in the relationship between the conditioned and unconditioned responses. The consistent phase angle at which each MU fired during WBV indicates the presence of reflex muscle activity similar to the tonic vibration reflex. The varying response in high- and low-threshold MUs may be due to the different contributions of the mono- and polysynaptic pathways but not presynaptic inhibition.

Spectral properties of H-reflex recordings after an acute bout of whole-body vibration

Although research supports the use of whole-body vibration (WBV) to improve neuromuscular performance, the mechanisms for these improvements remain unclear. The purpose of this study was to identify the effect of WBV on the spectral properties of electrically evoked H-reflex recordings in the soleus (SOL) muscle. The H-reflex recordings were measured in the SOL muscle of 20 participants before and after a bout of WBV. The H-reflexes were evoked every 15 seconds for 150 seconds after WBV. A wavelet procedure was used to extract spectral data, which were then quantified with a principle components analysis. Resultant principle component scores were used for statistical analysis. The analysis extracted 1 principle component associated with the intensity of the myoelectric spectra and 1 principle component associated with the frequency. The scores of the principle component that were related to the myoelectric intensity were smaller at 30 and 60 milliseconds after WBV than before WBV. The WBV transiently decreased the intensity of myoelectric spectra during electrically evoked contractions, but it did not influence the frequency of the spectra. The decrease in intensity likely indicates a smaller electrically evoked muscle twitch response, whereas the lack of change in frequency would indicate a similar recruitment pattern of motor units before and after WBV.

Superimposed vibration confers no additional benefit compared with resistance training alone

Eighteen participants (22-43 years) were randomly allocated to one of two groups: resistance training combined with vibration (VIB; five males, four females) or resistance training alone (CON; five males, four females). Each participant trained three sessions per week (three sets of 10 seated calf raises against a load, which was increased progressively from 75% of one repetition maximum (1RM) to 90% 1RM for 4 weeks. For the VIB group, a vibratory stimulus (30 Hz, 2.5 mm amplitude) was applied to the soles of the feet by a vibration platform. The two groups did not differ significantly with respect to the total amount of work performed during training. Both groups showed a significant increase in maximum voluntary contraction and 1RM (P<0.01) with training. There were no significant changes in measures that assessed the rate at which force was developed. Countermovement jump height increased for the CON (P<0.01) but not for the VIB group. Comparisons between the groups revealed that they did not differ significantly from one another with respect to any measure of performance, before or following training. It appears that vibration superimposed upon resistance training does not alter or augment the increase in strength induced by resistance training alone.

Functioning of peripheral Ia pathways in infants with typical development: responses in antagonist muscle pairs

In muscle responses of proprioceptive origin, including the stretch/tendon reflex (T-reflex), the corresponding reciprocal excitation and irradiation to distant muscles have been described from newborn infants to older adults. However, the functioning of other responses mediated primarily by Ia-afferents has not been investigated in infants. Understanding the typical development of these multiple pathways is critical to determining potential problems in their development in populations affected by neurological disease, such as spina bifida or cerebral palsy. Hence, the goal of the present study was to quantify the excitability of Ia-mediated responses in lower limb muscles of infants with typical development. These responses were elicited by mechanical stimulation applied to the distal tendons of the gastrocnemius-soleus (GS), tibialis anterior (TA) and quadriceps (QAD) muscles of both legs in twelve 2- to 10-month-old infants and recorded simultaneously in antagonist muscle pairs by surface EMG. Tendon taps alone elicited responses in either, both or neither muscle. The homonymous response (T-reflex) was less frequent in the TA than the GS or QAD muscle. An 80 Hz vibration superimposed on tendon taps induced primarily an inhibition of monosynaptic responses; however, facilitation also occurred in either muscle of the recorded pair. These responses were not influenced significantly by age or gender. Vibration alone produced a tonic reflex response in the vibrated muscle (TVR) and/or the antagonist muscle (AVR). However, for the TA muscle the TVR was more frequently elicited in older than younger infants. High variability was common to all responses. Overall, the random distribution and inconsistency of muscle responses suggests that the gain of Ia-mediated feedback is unstable. We propose that during infancy the central nervous system needs to learn to set stable feedback gain, or destination of proprioceptive assistance, based on their use during functional movements. This will tailor the neuromuscular connectivity to support adaptive motor behaviors.

Acute effects of various whole-body vibration frequencies on lower-body power in trained and untrained subjects

The purpose of this study was to investigate the acute effects of whole-body vibration (WBV) with different vibration frequencies on power production during squat jump (SJ) and countermovement jump (CMJ) with submaximal external loads in strength trained and untrained subjects. Subjects were randomly exposed to WBV with frequencies of 20, 35, or 50 Hz (amplitude: 3 mm), or no vibration. Peak average power during SJ and CMJ was assessed on a Smith machine while standing on a vibration platform. Both the trained and untrained group increased peak average power during SJ at an WBV frequency of 50 Hz (6.8 +/- 1.9 and 7.3 +/- 1.7%, respectively; p < 0.05). This increase was larger than in the other test conditions, in which no changes occurred (p < 0.05). Untrained subjects increased peak average power during CMJ with 4.4 +/- 1.3% (p < 0.05) while vibrating at a frequency of 50 Hz, but there was no difference for the strength trained subjects. Furthermore, there was no difference in peak average power in CMJ and SJ while vibrating at frequencies of 20 and 35 Hz compared with no vibration in either of the groups. In conclusion, WBV with a frequency of 50 Hz increases peak average power in both trained and untrained subjects, whereas vibration frequencies of 20 and 35 Hz do not have this effect. Thus, if the purpose of using WBV is to increase the stimulus to the neuromuscular system to a greater extent than traditional explosive strength/power training, the WBV frequency should be 50 Hz and the exercises should be explosive and submaximally loaded (like traditional explosive strength/power training).

The effects of vibration on explosive and reactive strength when applying individualized vibration frequencies

The aim of this study was to assess the effects of 8 weeks of whole-body vibrations on explosive and reactive leg strength. Thirty-three physically active students took part in the study and were randomly assigned to an individualized-vibration group, a fixed-vibration group or a control group. The frequency of vibration was set to 30 Hz for the fixed-vibration group, whereas the frequency for the individualized-vibration group was determined by monitoring the participants' EMGrms activity. The participants in the two vibration groups were exposed three times a week for 8 weeks to a series of 10 x 1-min whole-body vibrations with a 1-min pause between series of vibrations and a 4-min pause after the first five series of vibrations. Jump height in the squat jump increased significantly in all three groups (by 11% for the individualized-vibration group, p=0.001; by 3% for the fixed-vibration group, p=0.011; and by 2% for the control group, p=0.006), but countermovement jump height was not affected. In continuous rebound jumps by the individualized-vibration group, jumping height increased by 22% (p=0.006) and power increased by 18% (p=0.002). The results of this study suggest that the use of an individualized vibration frequency produces a greater response from the neuromuscular system and is more beneficial than vibrations at a fixed pre-selected frequency.

Differential effects of whole body vibration durations on knee extensor strength

The effectiveness and optimality of whole body vibration (WBV) duration on muscular strength is yet to be determined. Hence the aim of this study was to investigate the effects of three different durations of continuous WBV exposure on isometric right knee extensor strength measured pre and post exposure. The study involved 12 trained male subjects (age 23.7+/-4.2 years, height 1.82+/-0.06m, weight 81.8+/-15.5kg). Pre and post knee extensor strength was measured using the Biodex System 3. Peak and mean torques were recorded over three maximal 2s contractions with 10s intervals. All subjects completed three interventions of WBV lasting 2, 4, or 6min, in a balanced randomized order. Whole body vibration was performed on the Galileo machine set at 26Hz with peak-to-peak amplitude of 4mm. We found significant interaction (durationxpre-post) effects for both peak and mean torque. Two minutes of WBV provided a significantly different (p<0.05) effect (peak torque +3.8%, mean torque +3.6%) compared to 4min (-2.7% and -0.8%, respectively), and compared to 6min (-6.0% and -5.2%, respectively), while 4min produced significantly different results compared to 6min for peak torque measurements only. Two minutes of WBV produced an improvement in isometric right knee extension strength compared to 4 and 6min, both of which produced strength decreases. Nevertheless, the mechanisms and optimal dose-response character of vibration exposure remain unclear.

The effects of whole-body vibration on upper- and lower-body EMG during static and dynamic contractions

Whole-body vibration (WBV) training uses a vertically oscillating platform and reports suggest that this perturbation elicits reflexive muscle contractions that augment muscle activity and contribute to increased strength. No WBV study has measured both upper- and lower-body muscle activation. The purpose of this study was to determine the optimal WBV stimulus (frequency × amplitude) to increase electromyography (EMG) in upper- and lower-body muscles for three distinctive unloaded actions: isometric semi-squat, dynamic leg squats, and static and dynamic bilateral bicep curls. Surface EMG was measured for the vastus lateralis (VL), biceps femoris (BF), biceps brachii (BB), and triceps brachii (TB) in 10 recreationally active male university students (24.4 ± 2.0 years; mean ± SD) when WBV was administered at 2 and 4 mm and at 25, 30, 35, 40, and 45 Hz. EMG changes are reported as the difference between WBV and no WBV EMG root mean square expressed as a percentage of maximum voluntary exertion (%MVE). In static semi-squat, WBV increased muscle activity 2.9%–6.7% in the VL and 0.8%–1.2% in the BF. During dynamic squatting, WBV increased muscle activity in the VL by 3.7%–8.7% and in the BF by 0.4%–2.0%. In a static biceps curl, WBV had no effect on BB EMG, but did increase TB activity 0.3%–0.7%. During dynamic biceps curls, WBV increased BB EMG activity by 0.6%–0.8% and TB activity by 0.2%–1.0%. The higher WBV amplitude (4 mm) and frequencies (35, 40, 45 Hz) resulted in the greatest increases in EMG activity.

Variation in Neuromuscular Responses during Acute Whole-Body Vibration Exercise

PURPOSE

Leg muscle strength and power are increased after whole-body vibration (WBV) exercise. These effects may result from increased neuromuscular activation during WBV; however, previous studies of neuromuscular responses during WBV have not accounted for motion artifact.

METHODS

Sixteen healthy adults performed a series of static and dynamic unloaded squats with and without two different directions of WBV (rotational vibration, RV; and vertical vibration, VV; 30 Hz; 4 mmp-p). Activation of unilateral vastus lateralis, biceps femoris, gastrocnemius, and tibialis anterior was recorded using EMG. During RV and VV, increases in EMG relative to baseline were compared over a range of knee angles, contraction types (concentric, eccentric, isometric), and squatting types (static, dynamic).

RESULTS

After removing large, vibration-induced artifacts from EMG data using digital band-stop filters, neuromuscular activation of all four muscles increased significantly (P<or=0.05) during RV and VV. Average responses of the extensors were significantly greater during RV than VV, whereas responses of the tibialis anterior were significantly greater during VV than RV. For all four muscles, responses during static squatting were greater than or equal to responses during dynamic squatting, whereas responses during eccentric contractions were equal to or smaller than responses during concentric and isometric contractions. Neuromuscular responses of vastus lateralis, gastrocnemius, and tibialis anterior were affected by knee angle, with greatest responses at small knee angles.

CONCLUSIONS

Motion artifacts should be removed from EMG data collected during WBV. We propose that neuromuscular responses during WBV may be modulated by leg muscle cocontraction as a postural control strategy and/or muscle tuning by the CNS intended to minimize soft-tissue vibration.

Effect of vibration training in maximal effort (70% 1RM) dynamic bicep curls

PURPOSE

To examine (i) the acute effect of direct vibration on neuromuscular performance with a maximal-effort dynamic resistance exercise and (ii) the acute residual effect of direct vibration training both with and without the resistance exercise.

METHODS

Fourteen subjects were exposed to four training conditions in random order: exercise with vibration (E + V); exercise with sham vibration (E + SV); no exercise with vibration (NE + V); and no exercise with sham vibration (NE + SV). The exercise comprised three sets of maximal-effort bicep curls with a load of 70% 1RM. A portable vibrator was strapped onto the skin over the bicep tendon to apply vibration with an amplitude and frequency of 1.2 mm and 65 Hz. Elbow joint angle and bicep EMG were measured both during training and in pre- and posttraining tests. Angular velocity, moment, power, and bicep root mean squared value of EMG (EMG(rms)) and mean power frequency of EMG (EMG(mpf)) were determined for the concentric phase. Interday reliability ranged from 0.69 to 0.99.

RESULTS

During training (acute effect) vibration did not enhance mean angular velocity (1.5 vs 1.5 rad.s(-1), P = 0.86), peak angular velocity (2.7 vs 2.7 rad.s(-1), P = 0.90), mean moment (27.3 vs 27.4 N.m, P = 0.83), peak moment (39.8 vs 39.4 N.m, P = 0.53), mean power (40.3 vs 41.1 W, P = 0.72), peak power (91.9 vs 90.2 W, P = 0.77), or bicep EMG(rms) (73.9 vs 71.9, P = 0.78). Similarly, after training (acute residual effect) there was no enhancement from vibration in the mechanical and EMG output when the muscle was trained or was rested (P > 0.05).

CONCLUSION

These findings suggest that direct vibration, with an amplitude of 1.2 mm and frequency of 65 Hz, applied to the bicep muscle tendon, does not enhance neuromuscular performance in maximal-effort contractions during or immediately after training.

Reflex gain of muscle spindle pathways during fatigue

There are conflicting observations of the effects of fatigue on the sensitivity of large diameter Ia afferents. Our goal was to characterize any fatigue-related changes in the spinal reflex pathways during fatigue. Manipulation of the Ia afferent response by vibration and tendon tap, in which the motor neuron pool is modulated by both short- and long-loop activation from muscle spindles, were elicited before and after a fatigue task. The fatigue task consisted of intermittent submaximal and maximal voluntary contractions (MVCs). Percent voluntary activation fell from 98.75% MVC to 80.92% MVC following the fatigue task as measured by the twitch interpolation technique. Voluntary contractions of the same force profile as the force produced by 30 s of vibration were produced by having participants (n = 10) follow the trajectory on a computer monitor, before and after the fatigue task. Recruitment thresholds (RTs) of voluntarily activated units showed no change during fatigue; however, units activated via the reflex pathway were recruited approximately 30% sooner during fatigue (P < 0.05). The ratio of the electrical-to-mechanical response of the tendon tap increased significantly with fatigue. Our findings of decreased RTs in response to vibration and increased EMG activity during the tendon tap following the fatigue task indicate that Ia afferent input to the motoneuron pool was increased. The decrease in MVC force indicates that during this time the descending drive was compromised. These results provide evidence that the gain of the gamma loop is increased during fatigue, indicating possible peripheral neural compensation to the motor neuron pool in order to preserve force output.

Effects of vibration and resistance training on neuromuscular and hormonal measures

The aim was to study whether whole body vibration (WBV) combined with conventional resistance training (CRT) induces a higher increase in neuromuscular and hormonal measures compared with CRT or WBV, respectively. Twenty-eight young men were randomized in three groups; squat only (S), combination of WBV and squat (S+V) and WBV only (V). S+V performed six sets with eight repetitions with corresponding eight repetition maximum (RM) loads on the vibrating platform, whereas S and V performed the same protocol without WBV and resistance, respectively. Maximal isometric voluntary contraction (MVC) with electromyography (EMG) measurements during leg press, counter movement jump (CMJ) measures (mechanical performance) including jump height, mean power (Pmean), peak power (Ppeak) and velocity at Ppeak (Vppeak) and acute hormonal responses to training sessions were measured before and after a 9-week training period. ANOVA showed no significant changes between the three groups after training in any neuromuscular variable measured [except Pmean, S higher than V (P<0.05)]. However, applying t tests within each group revealed that MVC increased in S and S+V after training (P<0.05). Jump height, Pmean and Ppeak increased only in S, concomitantly with increased Vppeak in all groups (P<0.05). Testosterone increased during training sessions in S and S+V (P<0.05). Growth hormone (GH) increased in all groups but S+V showed higher responses than S and V (P<0.05). Cortisol increased only in S+V (P<0.05). We conclude that combined WBV and CRT did not additionally increase MVC and mechanical performance compared with CRT alone. Furthermore, WBV alone did not increase MVC and mechanical performance in spite of increased GH.

Acute whole body vibration training increases vertical jump and flexibility performance in elite female field hockey players

OBJECTIVE

To quantify the acute effect of whole body vibration (WBV) training on arm countermovement vertical jump (ACMVJ), grip strength, and flexibility performance.

METHODS

Eighteen female elite field hockey players each completed three interventions of WBV, control, and cycling in a balanced random manner. WBV was performed on a Galileo machine (26 Hz) with six different exercises being performed. For the control, the same six exercises were performed at 0 Hz, whilst cycling was performed at 50 W. Each intervention was 5 min in duration with ACMVJ, grip strength, and flexibility measurements being conducted pre and post intervention.

RESULTS

There was a positive interaction effect (intervention x pre-post) of enhanced ACMVJ (p<0.001) and flexibility (p<0.05) parameters following WBV; however no changes were observed after the control and cycling interventions. There was no interaction effect for grip strength following the three interventions.

CONCLUSIONS

Acute WBV causes neural potentiation of the stretch reflex loop as shown by the improved ACMVJ and flexibility performance. Additionally, muscle groups less proportionally exposed to vibration do not exhibit physiological changes that potentiate muscular performance.

Whole-Body-Vibration Training Increases Knee-Extension Strength and Speed of Movement in Older Women

OBJECTIVES

To investigate the effects of 24 weeks of whole-body-vibration (WBV) training on knee-extension strength and speed of movement and on counter-movement jump performance in older women.

DESIGN

A randomized, controlled trial.

SETTING

Exercise Physiology and Biomechanics Laboratory, Leuven, Belgium.

PARTICIPANTS

Eighty-nine postmenopausal women, off hormone replacement therapy, aged 58 to 74, were randomly assigned to a WBV group (n=30), a resistance-training group (RES, n=30), or a control group (n=29).

INTERVENTION

The WBV group and the RES group trained three times a week for 24 weeks. The WBV group performed unloaded static and dynamic knee-extensor exercises on a vibration platform, which provokes reflexive muscle activity. The RES group trained knee-extensors by performing dynamic leg-press and leg-extension exercises increasing from low (20 repetitions maximum (RM)) to high (8RM) resistance. The control group did not participate in any training.

MEASUREMENTS

Pre-, mid- (12 weeks), and post- (24 weeks) isometric strength and dynamic strength of knee extensors were measured using a motor-driven dynamometer. Speed of movement of knee extension was assessed using an external resistance equivalent to 1%, 20%, 40%, and 60% of isometric maximum. Counter-movement jump performance was determined using a contact mat.

RESULTS

Isometric and dynamic knee extensor strength increased significantly (P<.001) in the WBV group (mean+/-standard error 15.0+/-2.1% and 16.1+/-3.1%, respectively) and the RES group (18.4+/-2.8% and 13.9+/-2.7%, respectively) after 24 weeks of training, with the training effects not significantly different between the groups (P=.558). Speed of movement of knee extension significantly increased at low resistance (1% or 20% of isometric maximum) in the WBV group only (7.4+/-1.8% and 6.3+/-2.0%, respectively) after 24 weeks of training, with no significant differences in training effect between the WBV and the RES groups (P=.391; P=.142). Counter-movement jump height enhanced significantly (P<.001) in the WBV group (19.4+/-2.8%) and the RES group (12.9+/-2.9%) after 24 weeks of training. Most of the gain in knee-extension strength and speed of movement and in counter-movement jump performance had been realized after 12 weeks of training.

CONCLUSION

WBV is a suitable training method and is as efficient as conventional RES training to improve knee-extension strength and speed of movement and counter-movement jump performance in older women. As previously shown in young women, it is suggested that the strength gain in older women is mainly due to the vibration stimulus and not only to the unloaded exercises performed on the WBV platform.

Strength increase after whole-body vibration compared with resistance training

PURPOSE

The aim of this study was to investigate and to compare the effect of a 12-wk period of whole-body vibration training and resistance training on human knee-extensor strength.

METHODS

Sixty-seven untrained females (21.4 +/- 1.8 yr) participated in the study. The whole-body vibration group (WBV, N = 18) and the placebo group (PL, N = 19) performed static and dynamic knee-extensor exercises on a vibration platform. The acceleration of the vibration platform was between 2.28 g and 5.09 g, whereas only 0.4 g for the PL condition. Vibration (35-40 Hz) resulted in increased EMG activity, but the EMG signal remained unchanged in the PL condition. The resistance-training group (RES, N = 18) trained knee extensors by dynamic leg-press and leg-extension exercises (10-20 RM). All training groups exercised 3x wk-1. The control group (CO, N = 12) did not participate in any training. Pre- and postisometric, dynamic, and ballistic knee-extensor strength were measured by means of a motor-driven dynamometer. Explosive strength was determined by means of a counter-movement jump.

RESULTS

Isometric and dynamic knee-extensor strength increased significantly (P < 0.001) in both the WBV group (16.6 +/- 10.8%; 9.0 +/- 3.2%) and the RES group (14.4 +/- 5.3%; 7.0 +/- 6.2%), respectively, whereas the PL and CO group showed no significant (P > 0.05) increase. Counter-movement jump height enhanced significantly (P < 0.001) in the WBV group (7.6 +/- 4.3%) only. There was no effect of any of the interventions on maximal speed of movement, as measured by means of ballistic tests.

CONCLUSIONS

WBV, and the reflexive muscle contraction it provokes, has the potential to induce strength gain in knee extensors of previously untrained females to the same extent as resistance training at moderate intensity. It was clearly shown that strength increases after WBV training are not attributable to a placebo effect.

Ia Afferent input alters the recruitment thresholds and firing rates of single human motor units

Vibration of the patellar tendon recruits motor units in the knee extensors via excitation of muscle spindles and subsequent Ia afferent input to the alpha-motoneuron pool. Our first purpose was to determine if the recruitment threshold and firing rate of the same motor unit differed when recruited involuntarily via reflex or voluntarily via descending spinal pathways. Although Ia input is excitatory to the alpha-motoneuron pool, it has also been shown paradoxically to inhibit itself. Our second purpose was to determine if vibration of the patellar tendon during a voluntary knee extension causes a change in the firing rate of already recruited motor units. In the first protocol, 10 subjects voluntarily reproduced the same isometric force profile of the knee extensors that was elicited by vibration of the patellar tendon. Single motor unit recordings from the vastus lateralis (VL) were obtained with tungsten microelectrodes and unitary behaviour was examined during both reflex and voluntary knee extensions. Recordings from 135 single motor units showed that both recruitment thresholds and firing rates were lower during reflex contractions. In the second protocol, 7 subjects maintained a voluntary knee extension at 30 N for approximately 40-45 s. Three bursts of patellar tendon vibration were superimposed at regular intervals throughout the contraction and changes in the firing rate of already recruited motor units were examined. A total of 35 motor units were recorded and each burst of superimposed vibration caused a momentary reduction in the firing rates and recruitment of additional units. Our data provide evidence that Ia input modulates the recruitment thresholds and firing rates of motor units providing more flexibility within the neuromuscular system to grade force at low levels of force production.

Intrinsic activation of human motoneurons: reduction of motor unit recruitment thresholds by repeated contractions

The main purpose of this study was to examine whether facilitation of human motor unit recruitment by repeated voluntary contractions is mediated, in part, by time and activity-dependent increases in the intrinsic excitability of the parent motoneuron. To do this, pairs of tibialis anterior or soleus motor units were recorded during slowly increasing and then decreasing voluntary contractions. The firing rate of the lower-threshold motor unit of the pair (control unit) was used as a measure of effective synaptic excitation (i.e., drive) to the motoneurons. This rate was used to estimate the recruitment threshold of the higher-threshold unit of the pair (test unit). The test unit was repeatedly recruited and de-recruited in a series of contractions, and the interval between the de-recruitment and re-recruitment of the test unit (interactivation interval) was systematically varied between 0.6 and 60 s. An increase in intrinsic excitability of a unit was considered to have occurred if the level of estimated synaptic input (as measured by the firing rate of the control motor unit) needed to recruit a unit was reduced. At short interactivation intervals (1-2 s), the control unit firing frequency was significantly lower when the test unit was recruited on the second contraction, compared with the first (by 3.9 Hz or a 64% reduction). This suggested that the intrinsic excitability of the test motor unit had increased during the second contraction because it could be recruited at a much lower level of estimated synaptic drive. Longer interaction intervals (2-6 s) produced less recruitment facilitation. At even longer interactivation intervals (>6 s) there was no significant facilitation (time constant of effect was 4.8 s). In some motor units, the effect of this short-term facilitation appeared to be so pronounced that it resulted in reversing the order of de-recruitment with the other initially lower-threshold motor units. Such reversals were occasionally observed for orderly re-recruitment. The time course and behavior of the observed short-term facilitation of motor unit discharge was qualitatively similar to the warm-up phenomenon of plateau potentials seen in motoneurons of reduced preparations (e.g., 4-6 s). The possibility of warm-up contributing to the time and activity-dependent facilitation of human motor unit recruitment is discussed.

Activation patterns of hindlimb motor units in the awake rat and their relation to motoneuron intrinsic properties

The activity of hindlimb motor units from the lateral gastrocnemius and tibialis anterior muscles in the awake rat was compared during locomotion and during slow, sinusoidal muscle stretch. The majority of units were activated with high initial frequencies and often commenced firing with an initial doublet or triplet, even when activated by slow muscle stretch. The high firing rates at recruitment occurred without jumps in the firing rates of other concurrently activated units, the firing rate profiles of which were used as a measure of the net synaptic drive onto the motoneuronal pool. This suggested that the sharp recruitment jumps were not due to an abrupt increase in synaptic drive but rather due to intrinsic properties of the motoneuron. In addition, motor units that were activated phasically by the muscle stretch fired more action potentials during muscle shortening than during muscle lengthening, resulting in rightwardly skewed, asymmetrical firing profiles. In contrast, when the same units fired tonically during the imposed muscle stretch, the frequency profiles were modulated symmetrically and no nonlinearities were observed. Tonically firing units were modulated symmetrically throughout a wide range of firing frequencies, and discrete jumps in rate (i.e., bistable firing) were not observed. The sharp recruitment jumps during locomotion and muscle stretch are proposed to have resulted from the additional depolarization produced by the activation of plateau potentials at recruitment. Likewise, the sustained activation of plateaus subsequent to recruitment may have produced the prolonged firing of the motor units during sinusoidal muscle stretch.

How different afferent inputs control motoneuron discharge and the output of the motoneuron pool

In theory, there are at least two distinct mechanisms by which afferent inputs could alter motoneuron discharge and shape the output of a motoneuron pool: either by delivering synaptic current to the motoneurons' somata ('classic' synaptic transduction); or by altering the motoneurons' voltage-sensitive conductances (neuromodulation). Recent work has confirmed the operation of both of these mechanisms. It has been shown that the effect of a 'classic' synaptic input on motoneuron firing rate is predicted by the product of the effective synaptic current and the slope of the motoneuron's frequency-current relation. It has also been shown that neuromodulators can alter both the slope of a motoneuron's frequency-current relation and its threshold for repetitive firing. It is argued here, however, that when two or more sources of synaptic input are activated concurrently, the distinction between these two mechanisms is blurred. Computer simulations of motoneuron and motor pool behavior have proved extremely useful in understanding these processes.