The effects of acute and prolonged muscle vibration on the function of the muscle spindle’s reflex arc

Abstract

Purpose:?Localized mechanical vibration, applied directly to a muscle, is known to have powerful, duration-dependent effects on the muscle spindle’s reflex arc. Here, the conditioning of the function of the spindle reflex arc via vibration was examined with considerations for use as a non-invasive, sensorimotor research tool.

Methods:?Muscle spindle function was examined with patellar tendon taps prior to and following exposure to muscle vibration applied to the quadriceps femoris for acute (<5?s) and prolonged (20?min) durations. Surface electromyography (sEMG), torque, and accelerometry signals were obtained during the taps to quantify various measures of reflex magnitude and latency.

Results:?Our findings suggest that acute vibration had no effect on normalized reflex torque or sEMG amplitude (p?>?0.05), but increased total reflex latency (p?=?0.022). Alternatively, prolonged vibration reduced normalized reflex torque and sEMG amplitude (p?<?0.001), and increased reflex latency (p?<?0.001).

Conclusions:?Our findings support the use of prolonged vibration as a practical means to decrease the function of the muscle spindle’s reflex arc. Overall, this suppressive effect was evident in the majority of subjects, but the extent was variable. This approach could potentially be used to help delineate the muscle spindle’s role in various sensory or motor tasks in which more direct measures are not feasible. Acute vibration, however, did not potentiate muscle spindle function as hypothesized. Rather, our results suggest that acute vibration increased total reflex latency. Accordingly, potential mechanical and neurophysiological mechanisms are discussed.     

Relationship between short latency stretch reflex and fascicle behavior in the soleus muscle in vivo

Objectives:Stretch reflex responses were considered to be affected by the velocity of muscle fiber lengthening and angular velocity. However, the results of previous studies in vivo and in vitro are inconsistent in this regard. The purpose of the present study was to investigate the effects of the velocity of fascicle lengthening on the amplitude of the stretch reflex for each trial with a high angular velocity and wide range of motion.Methods:Thirteen healthy men volunteered for this study. While the ankle was passively moved from 100 to 80 deg at five different angular velocities (100, 200, 300, 500, and 600 deg⋅s^-1), the velocity of fascicle lengthening in the soleus muscle was measured using ultrasonography. In addition, the amplitude of the short latency stretch reflex in the soleus muscle was also measured.Results:As angular velocity increased, the amplitude of the stretch reflex and velocity of fascicle lengthening significantly increased (both p<0.001). For each trial in all subjects, the amplitude of the stretch reflex was not correlated with the velocity of fascicle lengthening at any of the angular velocities.Conclusion:In conclusion, the stretch reflex size is not related to the fascicle behavior in each trial.     

Effects of different vibration frequencies on spinal cord reflex circuits and thermoalgesic perception

Objectives:We studied the effect of different vibration frequencies on spinal cord excitability and heat pain perception. We hypothesized that the effects of vibration on spinal cord reflexes, and, also those on heat pain perception, depend on vibration frequency.Methods:In 9 healthy subjects, we applied vibration over the tibialis anterior muscle at three different frequencies (50, 150, or 250 Hz) on spinal cord reflex excitably, tested with the H reflex and the T wave in the soleus muscle, as well as on sensory and pain perception, tested by measuring warm perception (WT) and heat pain perception thresholds, (HPT) in sites rostral and caudal to vibration. Exams were carried out before, during, and after vibration.Results:The amplitude of the H reflex and T wave significantly decreased during vibration in comparison to baseline. Low frequencies (50 and 150Hz) induced greater reflex suppression than high frequency (250Hz). No significant changes were observed on WT and HPT.Conclusions:The effects of vibratory stimulation can be summarized as frequency-related suppression of the spinal cord excitability without an effect on warm and heat pain perception. The present results may help to design vibration-related interventions intended to diminish spinal cord reflex excitability in spastic patients.     

Neuromuscular performance of lower limbs during voluntary and reflex activity in power- and endurance-trained athletes

Neural, mechanical and muscle factors influence muscle force production. This study was, therefore, designed to compare possible differences in the function of the neuromuscular system among differently adapted subjects. A group of 11 power-trained athletes and 10 endurance-trained athletes volunteered as subjects for this study. Maximal voluntary isometric force and the rate of force production of the knee extensor and the plantar flexor muscles were measured. In addition, basic reflex function was measured in the two experimental conditions. The power athletes produced higher voluntary forces (P<0.01-0.001) with higher rates for force production (P<0.001) by both muscle groups measured. Unexpectedly, however, no differences were noticed in the electromyogram time curves between the groups. During reflex activity, the endurance group demonstrated higher sensitivity to the mechanical stimuli, i.e. the higher reflex amplitude caused a higher rate of reflex force development, and the reflex amplitude correlated with the averaged angular velocity. The differences in the isometric conditions could be explained by obviously different muscle fibre distribution, by different amounts of muscle mass, by possible differences in the force transmission from individual myofibrils to the skeletal muscle and by specificity of training. In addition, differences in nervous system structure and muscle spindle properties could explain the observed differences in reflex activity between the two groups.     

Gravitational force modulates muscle activity during mechanical oscillation of the tibia in humans

Mechanical oscillation (vibration) is an osteogenic stimulus for bone in animal models and may hold promise as an anti-osteoporosis measure in humans with spinal cord injury (SCI). However, the level of reflex induced muscle contractions associated with various loads (g force) during limb segment oscillation is uncertain. The purpose of this study was to determine whether certain gravitational loads (g forces) at a fixed oscillation frequency (30 Hz) increases muscle reflex activity in individuals with and without SCI. Nine healthy subjects and two individuals with SCI sat with their hip and knee joints at 90° and the foot secured on an oscillation platform. Vertical mechanical oscillations were introduced at 0.3, 0.6, 1.2, 3 and 5g force for 20 s at 30 Hz. Non-SCI subjects received the oscillation with and without a 5% MVC background contraction. Peak soleus and tibialis anterior (TA) EMG were normalized to M-max. Soleus and TA EMG were <2.5% of M-max in both SCI and non-SCI subjects. The greatest EMG occurred at the highest acceleration (5g). Low magnitude mechanical oscillation, shown to enhance bone anabolism in animal models, did not elicit high levels of reflex muscle activity in individuals with and without SCI. These findings support the g force modulated background muscle activity during fixed frequency vibration. The magnitude of muscle activity was low and likely does not influence the load during fixed frequency oscillation of the tibia.     

The effects of whole-body vibration on EMG activity of the lower body muscles in supine static bridge position

Objectives:The purpose of the current study was to firstly examine the effects of different whole-body vibration (WBV) frequencies in the lower-body muscles when applied simultaneously during a bridge exercise. Secondly, determine if there were any sex differences in the lower-body muscles of WBV during the bridge.Methods:Seven females and 7 males completed 2 familiarization and 1 test sessions. In the test session participants were randomized to complete one 30 s bout of a bridge exercise for 3 separate condition s followed by 3-min of rest. The 3 conditions (a) No-WBV (without WBV); (b) WBV-30 (30 Hz, low amplitude); (c) WBV-50 (50 Hz, low amplitude) were performed on a WBV platform. Muscle activity of the biceps femoris (BF), semitendinosus (ST), gluteus maximus (Gmax), multifidus muscle (MF) muscles were measured.Results:Muscle activity was increased with WBV in the BF and ST muscles at WBV-30 and WBV-50 conditions (p<0.05) vs. no-WBV. During No-WBV and WBV-50 conditions, males had a higher biceps femoris activity compared to females for (p<0.05) 45 and 27 %, respectively; however, during all conditions females had a high level of Gmax activity (57%) than males (p<0.05).Conclusion:Additional vibration at 30 and 50 Hz during the bridge exercise could be a useful method to enhance hamstring muscle activity.     

Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle.

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 +/- 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials (P < 0.01) and by 38% during the Recovery trials (P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials (P < 0.05) and by 28% during the Recovery trials (P < 0.01). The discharge rate of motor units increased from 10.3 +/- 2.7 pulses/s (pps) before vibration to 12.2 +/- 3.1 pps (P < 0.01) during the After trials and to 11.9 +/- 2.6 pps during the Recovery trials (P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.     

Cross-training effect of chronic whole-body vibration exercise: a randomized controlled study

Abstract

Purpose: To determine whether unilateral leg whole-body vibration (WBV) strength training induces strength gain in the untrained contralateral leg muscle. The secondary aim was to determine the potential role of spinal neurological mechanisms regarding the effect of WBV exercise on contralateral strength training.

Materials and Methods: Forty-two young adult healthy volunteers were randomized into two groups: WBV exercise and Sham control. An isometric semi-squat exercise during WBV was applied regularly through 20 sessions. WBV training was applied to the right leg in the WBV group and the left leg was isolated from vibration. Sham WBV was applied to the right leg of participants in the Control group. Pre- and post-training isokinetic torque and reflex latency of both quadricepses were evaluated.

Results: The increase in the strength of right (vibrated) knee extensors was 9.4?±?10.7% in the WBV group (p?=?.001) and was 1.2?±?6.6% in the Control group (p?=?.724). The left (non-vibrated) extensorsvibrated) knee extensors w4?±?8.4% in the WBV group (p?=?.038), whereas it decreased by 1.4?±?7.0% in the Control (p?=?.294). The strength gains were significant between the two groups. WBV induced the reflex response of the quadriceps muscle in the vibrated ipsilateral leg and also in the non-vibrated contralateral leg, though with a definite delay. The WBV-induced muscle reflex (WBV-IMR) latency was 22.5?±?7.7?ms for the vibrated leg and 39.3?±?14.6?ms for the non-vibrated leg.

Conclusions: Chronic WBV training has an effect of the cross-transfer of strength to contralateral homologous muscles. The WBV-induced muscular reflex may have a role in the mechanism of cross-transfer strength.     

The reflex mechanism underlying the neuromuscular effects of whole-body vibration: Is it the tonic vibration reflex?

Objectives:Whole-body vibration (WBV) is applied to the sole of the foot, whereas local mechanical vibration (LMV) is applied directly to the muscle or tendon. The time required for the mechanical stimulus to reach the muscle belly is longer for WBV. Therefore, the WBV-induced muscular reflex (WBV-IMR) latency may be longer than the tonic vibration reflex (TVR) latency. The aim of this study was to determine whether the difference between WBV-IMR and TVR latencies is due to the distance between the vibration application point and the target muscle.Methods:Eight volunteers participated in this study. The soleus reflex response was recorded during WBV, LMVs, and tendon tap. LMVs were applied to the Achilles tendon and sole of the foot. The latencies were calculated using the cumulative averaging technique.Results:The latency (33.4±2.8 ms) of the soleus reflex induced by the local foot vibration was similar to the soleus TVR latency (30.9±3.2 ms) and T-reflex (32.0±2.4 ms) but significantly shorter than the latency of the soleus WBV-IMR (42.3±3.4 ms) (F_(3,21)=27.46, p=0.0001, partial η²=0.797).Conclusions:The present study points out that the neuronal circuitries of TVR and WBV-IMR are different.     

Unpleasant visual stimuli increase the excitability of spinal motor neurons

Abstract

Purpose: In physical therapy for post-stroke patients, we often experience cases in which unpleasant emotions cause abnormal muscle tonus. Previously, we suggested that the magnitude of spinal motor neuron excitability was correlated with the grade of muscle tonus. Therefore, spinal motor neuron excitability was considered to be a useful index to evaluate the influence of unpleasant emotions on muscle tonus. In this study, we investigated whether unpleasant emotions evoked by visual stimuli affected the excitability of spinal motor neurons.

Materials and Methods: The F-waves, an indicator of spinal motor neuron excitability, were measured in 19 healthy adult volunteers. Firstly, for the rest trial, F-waves were measured during relaxation to determine the baseline of spinal motor neuron excitability. Following the rest trial, the unpleasant trial was conducted in which F-waves were measured while the subjects viewed an unpleasant picture for 1?min. After the unpleasant trial, F-waves were measured during relaxation. For the control condition, F-waves were measured while the subjects viewed a neutral picture instead of the unpleasant picture. The recorded F-wave data were analysed for persistence and the F/M amplitude ratio.

Results: Persistence and the F/M amplitude ratio were significantly greater during the unpleasant trial than during the rest trial. In the control condition, there was no significant difference in persistence and the F/M amplitude ratio compared with the three trials.

Conclusions: Our findings indicate that unpleasant emotions may affect spinal motor neuron excitability. Therefore, learning how to control emotions should be important aspect of physical therapy.     

Altered neuromuscular activity and postural stability during standing balance tasks in persons with non-specific neck pain

ObjectivesTo compare neck, trunk, and lower extremity muscle activity in standing in persons with neck pain (NP) to healthy controls and determine associations with postural sway.MethodsParticipants included 25 persons with NP and 25 controls. Surface electromyography was recorded bilaterally from neck (sternocleidomastoid, SCM; splenius capitis, SC; upper trapezius, UT), trunk (erector spinae, ES), and lower extremity (rectus femoris, RF; biceps femoris, BF; tibialis anterior, TA; medial gastrocnemius, GN) muscles. Postural sway was measured using a force platform in narrow stance with eyes open/closed, on firm/soft surfaces.ResultsCompared to controls, the NP group demonstrated higher activity in all muscles, except UT and had higher amplitude ratios for neck muscles (SCM, SC) for all tasks (p < .05). No between-group difference was found in amplitude ratios for lower extremity muscles, except for GN. Lower extremity activity was moderately correlated with larger postural sway for both groups (r = 0.41–0.66, p < .05). There were no correlations between sway and neck and trunk muscle activity (p > .05).ConclusionIncreased muscle activity with NP is associated with increased postural sway. Both groups used similar postural control strategies, but the increased neck activity in the NP group is likely related to the NP disorder rather than postural instability.     

Organization of meissner corpuscles in the glabrous skin of monkey and cat

Abstract

Purpose: Motor imagery, the process of imagining a physical action, has been shown to facilitate the excitability of spinal anterior horn cells. In the acute phase after a stroke, the excitability of spinal anterior horn cells is significantly reduced, which leads to motor deficits. This loss of movement can be prevented by increasing the excitability of spinal anterior horn cells immediately following an injury. Motor imagery is an effective method for facilitating the excitability of spinal anterior horn cells in patients with impaired movement; however, the optimal duration for motor imagery is unclear.

Materials and Methods: To investigate time-dependent changes in spinal anterior horn cell excitability during motor imagery, healthy adult participants were recruited to measure the F-wave, an indicator of anterior horn cell excitability. F-waves were measured from participants at baseline, during motor imagery, and post-motor imagery. During motor imagery, participants imagined isometric thenar muscle activity at 50% maximum voluntary contraction for 5?min. F-waves were measured at 1, 3, and 5?min after beginning motor imagery and analysed for persistence and F/M amplitude ratio.

Results: Persistence and F/M amplitude ratios at 1- and 3-min after motor imagery initiation were significantly greater than at baseline. The persistence and F/M amplitude ratio at 5-min after motor imagery initiation, however, was comparable to baseline levels.

Conclusion: Therefore, 1 to 3?min of motor imagery is likely sufficient to facilitate the excitability of spinal anterior horn cells.     

Influence of motor imagery of isometric flexor hallucis brevis activity on the excitability of spinal neural function

Purpose: This study aimed to validate the preliminary steps of motor image voluntary training in patients who are prone to falling as toe flexion muscle strength decreases.

Materials and methods: We recorded the F-wave in 30 healthy subjects (20 men, 10 women; mean age, 22.5?±?2.1?years). First, in a resting condition, the muscle was relaxed during the F-wave recording. Subsequently, the motion of the left flexor hallucis brevis muscle is photographed. F-waves were recorded immediately and at 5, 10, and 15?min after motor imagery. The amplitude of the F/M ratio and persistence were measured. The intervention group watched the exercise task video used for F-wave measurement daily for 1?month, whereas the non-intervention group did not. The second measurement was performed 1?month later in each group.

Results: In the first measurement of the amplitude of the F/M ratio in both intervention and non-intervention groups, the image condition was significantly increased compared with the resting condition, but there was no significant difference in persistence. A significant decrease in the amplitude of the F/M ratio after image conditioning was observed in the second measurement of the intervention group.

Conclusion: Although spinal nerve function excitement was enhanced during motor imagery, movement suppression was promoted, and spinal nerve excitability was suppressed when repeating the simple task. In the future, gradually upscaling the difficulty level of the toe flexion motor task used in the motor image may be necessary to prevent falls.     

The effects of muscle vibration on gait control: a review

Abstract

Background: The purpose of the review is to summarize the literature surrounding the use of muscle vibration as it relates to modifying human gait.

Methods: After a brief introduction concerning historical uses and early research identifying the effect of vibration on muscle activation, we reviewed 32 articles that used muscle vibration during walking. The review is structured to address the literature within four broad categories: the effect of vibration to ‘trigger’ gait-like lower limb motions, the effect of vibration on gait control of healthy individuals and individuals with clinical conditions in which gait disorders are a prominent feature, and the effect of vibration training protocols on gait.

Results: The acute effects of vibration during gait involving healthy participants is varied. Some authors reported differences in segmental kinematic and spatiotemporal measures while other authors reported no differences in these outcome measures. The literature involving participants with clinical conditions revealed that vibration consistently had a significant impact on gait, suggesting vibration may be an effective rehabilitation tool. All of the studies that used vibration therapy over time reported significant improvement in gait performance.

Conclusions: This review highlights the difficulties in drawing definitive conclusions as to the impact of vibration on gait control, partly because of differences in walking protocols, site of vibration application, and outcome measures used across different investigative teams. It is suggested that the development of common investigative methodologies and outcome measures would accelerate the identification of techniques that may provide optimal rehabilitation protocols for individuals experiencing disordered gait control.     

Greater subchondral vBMD at the tibia is observed between 1 and 5 years of anterior cruciate ligament injury

Objectives:This study aimed to determine if differences exist in tibial subchondral bone and muscle imbalances between individuals with and without an Anterior Cruciate Ligament (ACL) repair within the past 1 to 5 years (median 3 years).Methods:Fifteen individuals (ages 18-23 years) that had a unilateral ACL repair with no contralateral knee injuries and 15 age- and sex-matched controls (no prior knee injuries) were recruited to participate. Subchondral bone was measured using peripheral quantitative computed tomography (pQCT) distal to the tibial plateau. Muscle force, power, and force efficiency were measured using single leg jumps performed on a force platform.Results:Within subject analysis showed a greater subchondral vBMD in the injured versus uninjured legs of cases (278±11 mg/cm³ and 258±6 mg/cm³, respectively, mean±SD, p=0.01). Subchondral vBMD was greater on the injured leg of cases than controls (267±8 mg/cm³ and 237±8 mg/cm³, respectively, marginal mean±SE, p=0.01). No differences were observed between cases and controls for muscle force, power, or force efficiency.Conclusions:Greater subchondral bone mineral density was observed in participants between 1- and 5-years post-op. Given the results of this study and the known long-term effects of ACL injuries, future research must continue to focus on the prevention of these injuries.     

Tendon reflex is suppressed during whole-body vibration

In this study we have investigated the effect of whole body vibration (WBV) on the tendon reflex (T-reflex) amplitude. Fifteen young adult healthy volunteer males were included in this study. Records of surface EMG of the right soleus muscle and accelerometer taped onto the right Achilles tendon were obtained while participant stood upright with the knees in extension, on the vibration platform. Tendon reflex was elicited before and during WBV. Subjects completed a set of WBV. Each WBV set consisted of six vibration sessions using different frequencies (25, 30, 35, 40, 45, 50 Hz) applied randomly. In each WBV session the Achilles tendon was tapped five times with a custom-made reflex hammer. The mean peak-to-peak (PP) amplitude of T-reflex was 1139.11 ± 498.99 µV before vibration. It decreased significantly during WBV (p < 0.0001). The maximum PP amplitude of T-reflex was 1333 ± 515 μV before vibration. It decreased significantly during WBV (p < 0.0001). No significant differences were obtained in the mean acceleration values of Achilles tendon with tapping between before and during vibration sessions. This study showed that T-reflex is suppressed during WBV. T-reflex suppression indicates that the spindle primary afferents must have been pre-synaptically inhibited during WBV similar to the findings in high frequency tendon vibration studies.     

Optimizing neuromuscular electrical stimulation for hand opening

Aim: To investigate the effect of introducing an interphase interval to a biphasic pulse on force production and muscle fatigue, during stimulation of the wrist and finger extensors and to determine whether the IPI effect on force is dependent on electrode position.

Methods: Electrically-induced contraction forces of the wrist and finger extensors were measured in 15 healthy subjects undergoing stimulation. These forces were assessed with interphase interval settings at 0, 100, and 200μs, with both electrodes located just distal to common extensor origin (proximal placement) or with the distal electrode placed over the extensor and abductor policies longus muscles (distal placement). The degree of discomfort related to stimulation sensation was evaluated using a numeric rating scale. Muscle fatigue was measured during proximal placement.

Results: Under both electrode locations, introduction of 100 or 200?μs interphase interval enhanced force production; yet, only the 100μs interphase interval increased force without increasing discomfort. Additionally, stimulation sensation was more comfortable with proximal placement. Introducing interphase interval significantly increased the muscle force output during a repetitive stimulation fatigue protocol.

Conclusions: When using neuromuscular electrical stimulation to activate the wrist and finger extensors, clinicians should consider locating both stimulating electrodes proximally over the extensor surface of the forearm and apply a 100?µs interphase interval to a biphasic pulse. Future research that should establish these findings in individuals with various pathologies, especially in patients with residual hand spasticity.     

Acute exposure to microgravity does not influence the H-reflex with or without whole body vibration and does not cause vibration-specific changes in muscular activity

PurposeMany potential countermeasures for muscle and bone loss caused by exposure to microgravity require an uncompromised stretch reflex system. This is especially true for whole body vibration (WBV), as the main source of the neuromuscular activity during WBV has been attributed to stretch reflexes. A priori, it cannot be assumed that reflexes and Ia afferent transmission in particular have the same characteristics in microgravity as in normal gravity (NG). Therefore, the purpose of the study was to compare Ia afferent transmission in microgravity and NG and to assess how microgravity affects muscle activity during WBV.MethodsIn 14 participants, electromyographic activity of four leg muscles as well as Hoffmann-reflexes were recorded during NG and microgravity induced by parabolic flights.ResultsThe size of the Hoffmann-reflex was reduced during WBV, but did not differ during acute exposure to microgravity compared to NG. The influence of the gravity conditions on the electromyographic activity did not change depending on the vibration condition.ConclusionsAs far as the electromyographic activity of the recorded leg muscles is concerned, the effect of WBV is the same in microgravity as in NG. Moreover, Ia afferent transmission does not seem to be affected by acute exposure to microgravity when subjects are loaded with body weight and postural sway is minimized.     

Altered reflex sensitivity after repeated and prolonged passive muscle stretching.

Experiments were carried out to test the effect of prolonged and repeated passive stretching (RPS) of the triceps surae muscle on reflex sensitivity. The results demonstrated a clear deterioration of muscle function immediately after RPS. Maximal voluntary contraction, average electromyographic activity of the gastrocnemius and soleus muscles, and zero crossing rate of the soleus muscle (recorded from 50% maximal voluntary contraction) decreased on average by 23.2, 19.9, 16.5, and 12.2%, respectively. These changes were associated with a clear immediate reduction in the reflex sensitivity; stretch reflex peak-to-peak amplitude decreased by 84. 8%, and the ratio of the electrically induced maximal Hoffmann reflex to the maximal mass compound action potential decreased by 43. 8%. Interestingly, a significant (P < 0.01) reduction in the stretch-resisting force of the measured muscles was observed. Serum creatine kinase activity stayed unaltered. This study presents evidence that the mechanism that decreases the sensitivity of short-latency reflexes can be activated because of RPS. The origin of this system seems to be a reduction in the activity of the large-diameter afferents, resulting from the reduced sensitivity of the muscle spindles to repeated stretch.     

Thoracic 9 Spinal Transection-Induced Model of Muscle Spasticity in the Rat: A Systematic Electrophysiological and Histopathological Characterization

The development of spinal hyper-reflexia as part of the spasticity syndrome represents one of the major complications associated with chronic spinal traumatic injury (SCI). The primary mechanism leading to progressive appearance of muscle spasticity is multimodal and may include loss of descending inhibitory tone, alteration of segmental interneuron-mediated inhibition and/or increased reflex activity to sensory input. Here, we characterized a chronic thoracic (Th 9) complete transection model of muscle spasticity in Sprague-Dawley (SD) rats. Isoflurane-anesthetized rats received a Th9 laminectomy and the spinal cord was transected using a scalpel blade. After the transection the presence of muscle spasticity quantified as stretch and cutaneous hyper-reflexia was identified and quantified as time-dependent changes in: i) ankle-rotation-evoked peripheral muscle resistance (PMR) and corresponding electromyography (EMG) activity, ii) Hoffmann reflex, and iii) EMG responses in gastrocnemius muscle after paw tactile stimulation for up to 8 months after injury. To validate the clinical relevance of this model, the treatment potency after systemic treatment with the clinically established anti-spastic agents baclofen (GABA_B receptor agonist), tizanidine (α₂-adrenergic agonist) and NGX424 (AMPA receptor antagonist) was also tested. During the first 3 months post spinal transection, a progressive increase in ankle rotation-evoked muscle resistance, Hoffmann reflex amplitude and increased EMG responses to peripherally applied tactile stimuli were consistently measured. These changes, indicative of the spasticity syndrome, then remained relatively stable for up to 8 months post injury. Systemic treatment with baclofen, tizanidine and NGX424 led to a significant but transient suppression of spinal hyper-reflexia. These data demonstrate that a chronic Th9 spinal transection model in adult SD rat represents a reliable experimental platform to be used in studying the pathophysiology of chronic spinal injury-induced spasticity. In addition a consistent anti-spastic effect measured after treatment with clinically effective anti-spastic agents indicate that this model can effectively be used in screening new anti-spasticity compounds or procedures aimed at modulating chronic spinal trauma-associated muscle spasticity.