Experimental Muscle Pain Impairs the Synergistic Modular Control of Neck Muscles

A motor task can be performed via different patterns of muscle activation that show regularities that can be factorized in combinations of a reduced number of muscle groupings (also referred to as motor modules, or muscle synergies). In this study we evaluate whether an acute noxious stimulus induces a change in the way motor modules are combined to generate movement by neck muscles. The neck region was selected as it is a region with potentially high muscular redundancy. We used the motor modules framework to assess the redistribution of muscular activity of 12 muscles (6 per side) in the neck region of 8 healthy individuals engaged in a head and neck aiming task, in non-painful conditions (baseline, isotonic saline injection, post pain) and after the injection of hypertonic saline into the right splenius capitis muscle. The kinematics of the task was similar in the painful and control conditions. A general decrease of activity was noted for the injected muscle during the painful condition together with an increase or decrease of the activity of the other muscles. Subjects did not adopt shared control strategies (motor modules inter subject similarity at baseline 0.73±0.14); the motor modules recorded during the painful condition could not be used to reconstruct the activation patterns of the control conditions, and the painful stimulus triggered a subject-specific redistribution of muscular activation (i.e., in some subjects the activity of a given muscle increased, whereas in other subjects it decreased with pain). Alterations of afferent input (i.e., painful stimulus) influenced motor control at a multi muscular level, but not kinematic output. These findings provide new insights into the motor adaptation to pain.

Experimental muscle pain increases variability of neural drive to muscle and decreases motor unit coherence in tremor frequency band

It has been observed that muscle pain influences force variability and low-frequency (<3 Hz) oscillations in the neural drive to muscle. In this study, we aimed to investigate the effect of experimental muscle pain on the neural control of muscle force at higher frequency bands, associated with afferent feedback (alpha band, 5-13 Hz) and with descending cortical input (beta band, 15-30 Hz). Single-motor unit activity was recorded, in two separate experimental sessions, from the abductor digiti minimi (ADM) and tibialis anterior (TA) muscles with intramuscular wire electrodes, during isometric abductions of the fifth finger at 10% of maximal force [maximum voluntary contraction (MVC)] and ankle dorsiflexions at 25% MVC. The contractions were repeated under three conditions: no pain (baseline) and after intramuscular injection of isotonic (0.9%, control) and hypertonic (5.8%, painful) saline. The results showed an increase of the relative power of both the force signal and the neural drive at the tremor frequency band (alpha, 5-13 Hz) between the baseline and hypertonic (painful) conditions for both muscles (P < 0.05) but no effect on the beta band. Additionally, the strength of motor unit coherence was lower (P < 0.05) in the hypertonic condition in the alpha band for both muscles and in the beta band for the ADM. These results indicate that experimental muscle pain increases the amplitude of the tremor oscillations because of an increased variability of the neural control (common synaptic input) in the tremor band. Moreover, the concomitant decrease in coherence suggests an increase in independent input in the tremor band due to pain.

Can neck exercises enhance the activation of the semispinalis cervicis relative to the splenius capitis at specific spinal levels?

The deep cervical extensor, semispinalis cervicis, displays changes in behaviour and structure in people with chronic neck pain yet there is limited knowledge on how activation of this muscle can be emphasized during training. Using intramuscular electromyography (EMG), this study investigated the activity of the deep semispinalis cervicis and the superficial splenius capitis muscle at two spinal levels (C2 and C5) in ten healthy volunteers during a series of neck exercises: 1. Traction and compression, 2. Resistance applied in either flexion or extension at the occiput, at the level of the vertebral arch of C1 and of C4, and 3. Maintaining the neck in neutral while inclined on the elbows, with and without resistance at C4. The ratio between semispinalis cervicis and the splenius capitis EMG amplitude was quantified as an indication of whether the exercise could emphasize the activation of the semispinalis cervicis muscle relative to the splenius capitis. Manual resistance applied in extension over the vertebral arch emphasized the activation of the semispinalis cervicis relative to the splenius capitis at the spinal level directly caudal to the site of resistance (ratio: 2.0 ± 1.1 measured at C5 with resistance at C4 and 2.1 ± 1.2 measured at C2 with resistance at C1). This study confirmed the possibility of emphasizing the activation of the semispinalis cervicis relative to the splenius capitis which may be relevant for targeted exercise interventions for this deep extensor muscle. Further studies are required to investigate the clinical efficacy of these exercises for people with neck pain.

Predictors of outcome following a short multimodal rehabilitation program for patients with whiplash associated disorders

BACKGROUND

Patients with whiplash associated disorders (WAD) may present with physical and psychological symptoms which persist long after the initial onset of pain. Several studies have shown that therapeutic exercise for motor and sensorimotor control combined with manual therapy in a multimodal rehabilitation (MMR) program is effective at improving pain and disability in patients with neck disorders. To date, no studies have investigated which self-reported physical or psychological symptoms are predictive of response to this MMR program.

AIM

To determine which baseline features can predict outcome following a 3-week MMR program in patients with WAD.

DESIGN

Observational prospective cohort study.

SETTING

Outpatient rehabilitation clinic.

POPULATION

Thirty-seven patients aged >18 years with a diagnosis of WAD grade II or III.

METHODS

The MMR program included manual therapy, motor control and sensorimotor control training according to the clinical impairments of each patient. Patients were assessed before and after treatment for their physical and psychological symptoms by means of self-reported questionnaires. Regression models were estimated with pain intensity, disability and post-traumatic stress symptoms (PTSS) as outcomes.

RESULTS

After treatment, patients exhibited significant improvements in all evaluated outcomes (all P<0.01). Regression models accounting for 35% and 36% of the variance in pain intensity outcomes included average pain intensity over the previous week and pain catastrophizing as significant predictors. Disability and pain catastrophizing were predictors of changes in disability following the MMR program explaining 49% of the variance in the model. Furthermore, higher PTSS at baseline was a significant predictor of PTSS after treatment, explaining 55% of the variance in the model.

CONCLUSION

Improved outcomes on pain intensity, disability and PTSS following a MMR program could be partially predicted based on the patient's initial presentation.

CLINICAL REHABILITATION IMPACT

This knowledge may assist clinicians in predicting outcome following a MMR program inclusive of specific exercise therapy and manual therapy in patients with WAD.

Inter-and intra-tester reliability of a battery of cervical movement control dysfunction tests

BACKGROUND

Apart from the cranio-cervical flexion test and the deep neck flexor endurance test, evidence related to reliability of cervical movement control dysfunction tests is lacking.

OBJECTIVES

This study investigated the inter- and intra-tester reliability of a battery of cervical movement control dysfunction tests and the effect of clinician experience on reliability in 15 patients with chronic neck pain and 17 non-neck pain controls. In addition, it explored whether impaired performance on this battery of tests was more frequently observed in the neck pain group.

DESIGN

Inter and intra-tester reliability study.

METHOD

Participants were videotaped while performing a battery of nine active cervical movement control dysfunction tests. Two physiotherapists, with different levels of experience, independently rated all tests on two occasions two weeks apart. They were masked to participants' neck pain or non-neck pain status.

RESULTS

Inter-tester reliability for the complete battery of tests was substantial (κ = 0.69; 95% CI: 0.62, 0.76). Intra-rater reliability values for the expert (κ = 0.86; 95% CI: 0.79, 0.92) and novice (κ = 0.76; 95% confidence intervals (CI): 0.68, 0.84) were overall comparable suggesting that novices can achieve good accuracy with the battery of tests if trained. The frequency of impaired performances in cervical movement control dysfunction tests was low and comparable between groups. Only two tests achieved a greater number of impaired ratings in the patient group.

CONCLUSIONS

Although reliable, further research in larger neck pain populations is required to explore this battery of tests, in order to establish their diagnostic accuracy for identifying clinically relevant cervical movement control dysfunction.

Reliability, construct and discriminative validity of clinical testing in subjects with and without chronic neck pain

Background

The reliability of clinical tests for the cervical spine has not been adequately evaluated. Six cervical clinical tests, which are low cost and easy to perform in clinical settings, were tested for intra- and inter-examiner reliability, and two performance tests were assessed for test-retest reliability in people with and without chronic neck pain. Moreover, construct and between-group discriminative validity of the tests were examined.

Methods

Twenty-one participants with chronic neck pain and 21 asymptomatic participants were included. Intra- and inter-reliability were evaluated for the Cranio-Cervical Flexion Test (CCFT), Range of Movement (ROM), Joint Position Error (JPE), Gaze Stability (GS), Smooth Pursuit Neck Torsion Test (SPNTT), and neuromuscular control of the Deep Cervical Extensors (DCE). Test-retest reliability was assessed for Postural Control (SWAY) and Pressure Pain Threshold (PPT) over tibialis anterior, infraspinatus and the C3-C4 segment.

Results

Intraclass Correlation Coefficient (ICC) for intra- and inter-examiner reliability was highest for ROM (range: 0.80 to 0.94), DCE (0.75 to 0.90) and CCFT (0.63 to 0.86). JPE had the lowest ICC (0.02 to 0.66). Intra- and inter-reliability for GS and SPNTT showed kappa ranging from 0.66 to 0.92, and 0.57 to 0.78 (prevalence adjusted), respectively. For the test-retest study, ICC was 0.83 to 0.89 for PPT and 0.39 to 0.79 for SWAY. Construct validity was satisfactory for all tests, except JPE. Significant between group discriminative validity was found for CCFT, ROM, GS, SPNTT and PPT, however, differences were within the limits of the minimal detectable change.

Conclusions

The majority of the tests evaluated showed satisfactory reliability and construct validity supporting their use in the clinical evaluation of patients with chronic neck pain.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-408) contains supplementary material, which is available to authorized users.

Effect of manual therapy techniques on headache disability in patients with tension-type headache. Randomized controlled trial

BACKGROUND

Tension-type headache (TTH) is the most common type of primary headache however there is no clear evidence as to which specific treatment is most effective or whether combined treatment is more effective than individual treatments.

AIM

To assess the effectiveness of manual therapy techniques, applied to the suboccipital region, on aspects of disability in a sample of patients with tension-type headache.

DESIGN

Randomized Controlled Trial.

SETTING

Specialized centre for headache treatment.

POPULATION

Seventy-six (62 women) patients (age: 39.9 ± 10.9 years) with episodic chronic TTH.

METHODS

Patients were randomly divided into four treatment groups: 1) suboccipital soft tissue inhibition; 2) occiput-atlas-axis manipulation; 3) combined treatment of both techniques; 4) control. Four sessions were applied over 4 weeks and disability was assessed before and after treatment using the Headache Disability Inventory (HDI). Headache frequency, severity and the functional and emotional subscales of the questionnaire were assessed. Photophobia, phonophobia and pericranial tenderness were also monitored.

RESULTS

Headache frequency was significantly reduced with the manipulative and combined treatment (P<0.05), and the severity and functional subscale of the HDI changed in all three treatment groups (P<0.05). Manipulation treatment also reduced the score on the emotional subscale of the HDI (P<0.05). The combined intervention showed a greater effect at reducing the overall HDI score compared to the group that received suboccipital soft tissue inhibition and to the control group (both P<0.05). In addition, photophobia, phonophobia and pericranial tenderness only improved in the group receiving combined therapy (P<0.05).

CONCLUSION

When given individually, suboccipital soft tissue inhibition and occiput-atlas-axis manipulation resulted in changes in different parameters related to the disability caused by TTH. However, when the two treatments were combined, effectiveness was noted for all aspects of disability and other symptoms including photophobia, phonophobia and pericranial tenderness.

CLINICAL REHABILITATION IMPACT

Although individual manual therapy treatments showed a positive change in headache features, measures of photophobia, photophobia and pericranial tenderness only improved in the group that received the combined treatment suggesting that combined treatment is the most appropriate for symptomatic relief of TTH.

Reorganization of muscle synergies during multidirectional reaching in the horizontal plane with experimental muscle pain

Muscle pain induces a complex reorganization of the motor strategy which cannot be fully explained by current theories. We tested the hypothesis that the neural control of muscles during reaching in the presence of nociceptive input is determined by a reorganization of muscle synergies with respect to control conditions. Muscle pain was induced by injection of hypertonic saline into the anterior deltoid muscle of eight men. Electromyographic (EMG) signals were recorded from 12 upper limb muscles as subjects performed a reaching task before (baseline) and after the injection of hypertonic (pain) saline, and after the pain sensation vanished. The EMG envelopes were factorized in muscle synergies, and activation signals extracted for each condition. Nociceptive stimulation resulted in a complex muscle reorganization without changes in the kinematic output. The anterior deltoid muscle activity decreased in all subjects while the changes in other muscles were subject specific. Three synergies sufficed to describe the EMG patterns in each condition, suggesting that reaching movements remain modular in the presence of experimental pain. Muscle reorganization in all subjects was accompanied by a change in the activation signals compatible with a change in the central drive to muscles. One, two or three synergies were shared between the baseline and painful conditions, depending on the subject. These results indicate that nociceptive stimulation may induce a reorganization of modular control in reaching. We speculate that such reorganization may be due to the recruitment of synergies specific to the painful condition.

Deterministic accessory spinal movement in functional tasks characterizes individuals with low back pain

OBJECTIVES

To apply a novel method to assess the characteristics of spinal movement in subjects with low back pain (LBP) in a functional task.

METHODS

17 subjects suffering from chronic non-specific LBP (average pain intensity: 1.8±1.6), and 17 age and gender matched controls performed a repetitive lifting task. Spinal movement was recorded using a novel sensor strip with 12 angle sensors recording the spinal dynamics in evenly spaced (25mm) locations along the spine. Recurrence quantification analysis was applied to different components of the angles to assess the structure of its variability.

RESULTS

Mechanically, the LBP and control group performed the task similarly. Reported pain increased in the LBP group, yet task-related angular movement was not different. However, the percentage of determinism for the accessory angular movement (movement variability not directly related to task execution) was significantly higher for the LBP group, indicating a more deterministic (less random) structure of the muscle activation pattern variability.

CONCLUSION

The structure of the variability of spinal movement differs in subjects with chronic non-specific LBP.

SIGNIFICANCE

The determinism of accessory spinal movement may be a useful measure for evaluation of movement impairment in LBP and for monitoring rehabilitation effects.

Delayed onset of vastii muscle activity in response to rapid postural perturbations following eccentric exercise: a mechanism that underpins knee pain after eccentric exercise?

BACKGROUND

Appropriate timing of activity of the vastus medialis obliqus (VMO) and vastus lateralis (VL) muscles is a key factor for proper tracking of the patella in the trochlear groove during knee extension. This study investigates the relative timing of activation of the VMO and VL muscles during unexpected perturbations performed before and after eccentric exercise.

METHODS

Surface electromyography signals were recorded from the VMO and VL muscles of the right leg in 11 healthy men during rapid postural perturbations performed at baseline, immediately after eccentric exercise of the quadriceps, and at 24 and 48 h after exercise. Participants stood on a moveable platform during which eight randomised postural perturbations were performed (4 repetitions of 2 perturbation types: 8 cm forward slides, 8 cm backward slides).

RESULTS

Before the eccentric exercise, the onset of VMO activity was significantly earlier than the VL muscle (average for both forward and backward perturbations: VMO 39.0±7.1 ms; VL 43.7±7.9 ms). However, the onset of VMO activity was significantly later compared with VL muscle immediately after eccentric exercise and this remained 24 and 48 h after eccentric exercise (average across all postexercise sessions and perturbation directions: VMO 72.3±11.1 ms; VL 56.0±8.2 ms; p<0.05).

CONCLUSIONS

The onset of VMO-VL activity in response to rapid destabilising perturbations is altered immediately after eccentric exercise and during eccentric exercise-induced muscle soreness up to 48 h later. These observations may help explain the high prevalence of knee disorders after high intensity eccentric exercise.

Effectiveness of an 8-week exercise programme on pain and specificity of neck muscle activity in patients with chronic neck pain: a randomized controlled study

BACKGROUND

Although exercise can be effective for relief of neck pain, little is known about the effect of exercise on the neural control of neck muscles.

METHODS

A randomized controlled trial was conducted on 46 women with chronic neck pain to investigate the immediate effectiveness of an 8-week exercise programme on pain and directional specificity of neck muscle activity. At baseline, the patients completed questionnaires including the neck disability index (NDI) and performed a circular contraction of their head in the horizontal plane at 15 N force, with continuous change in force direction in the range 0-360°. Electromyography (EMG) was recorded from the sternocleidomastoid (SCM) and splenius capitis (SCap) muscles. Tuning curves of the EMG amplitude were computed, which depicts muscle activity over a range of force directions. The mean point of the tuning curves defined a directional vector, which determined the specificity of muscle activity. Patients were randomly assigned either to a training or control group.

RESULTS

A significant between-group difference in the change in NDI was observed. A reduction in NDI was observed following training (pre: 18.2 ± 7.4; post: 14.1 ± 6.5; p < 0.01) but not for the control group (pre: 17.5 ± 6.3; post: 16.6 ± 7.4). The training group showed higher specificity of muscle activity post-intervention (pre: 18.6 ± 9.8%, post: 24.7 ± 14.3%; p < 0.05), whereas no change occurred for the control group (pre: 19.4 ± 11.9%, post: 18.2 ± 10.1%).

CONCLUSION

An exercise programme that aims to enhance motor control of the cervical spine improves the specificity of neck muscle activity and reduces pain and disability in patients with neck pain.

Adjustments in motor unit properties during fatiguing contractions after training

OBJECTIVE

The objective of the study was to investigate the effect of strength and endurance training on muscle fiber membrane properties and discharge rates of low-threshold motor units of the vasti muscles during fatiguing contractions.

METHODS

Twenty-five sedentary healthy men (age (mean ± SD) = 26.3 ± 3.9 yr) were randomly assigned to one of three groups: strength training, endurance training, or a control group. Conventional endurance and strength training was performed 3 d·wk⁻¹, during a period of 6 wk. Motor unit conduction velocity and EMG amplitude of the vastus medialis obliquus and lateralis muscles and biceps femoris were measured during sustained isometric knee extensions at 10% and 30% of the maximum voluntary contraction before and immediately after training.

RESULTS

After 6 wk of training, the reduction in motor unit conduction velocity during the sustained contractions at 30% of the maximum voluntary force occurred at slower rates compared with baseline (P < 0.05). However, the rate of decrease was lower after endurance training compared with strength training (P < 0.01). For all groups, motor unit discharge rates declined during the sustained contraction (P < 0.001), and their trend was not altered by training. In addition, the biceps femoris-vasti coactivation ratio declined after the endurance training.

CONCLUSIONS

Short-term strength and endurance training induces alterations of the electrophysiological membrane properties of the muscle fiber. In particular, endurance training lowers the rate of decline of motor unit conduction velocity during sustained contractions more than strength training.

Recruitment of motor units in two fascicles of the semispinalis cervicis muscle

This study investigated the behavior of motor units in the semispinalis cervicis muscle. Intramuscular EMG recordings were obtained unilaterally at levels C2 and C5 in 15 healthy volunteers (8 men, 7 women) who performed isometric neck extensions at 5%, 10%, and 20% of the maximal force [maximum voluntary contraction (MVC)] for 2 min each and linearly increasing force contractions from 0 to 30% MVC over 3 s. Individual motor unit action potentials were identified. The discharge rate and interspike interval variability of the motor units in the two locations did not differ. However, the recruitment threshold of motor units detected at C2 (n = 16, mean ± SD: 10.3 ± 6.0% MVC) was greater than that of motor units detected at C5 (n = 92, 6.9 ± 4.3% MVC) (P < 0.01). A significant level of short-term synchronization was identified in 246 of 307 motor unit pairs when computed within one spinal level but only in 28 of 110 pairs of motor units between the two levels. The common input strength, which quantifies motor unit synchronization, was greater for pairs within one level (0.47 ± 0.32) compared with pairs between levels (0.09 ± 0.07) (P < 0.05). In a second experiment on eight healthy subjects, interference EMG was recorded from the same locations during a linearly increasing force contraction from 0 to 40% MVC and showed significantly greater EMG amplitude at C5 than at C2. In conclusion, synaptic input is distributed partly independently and nonuniformly to different fascicles of the semispinalis cervicis muscle.

Changes in H reflex and V wave following short-term endurance and strength training

This study examined the effects of 3 wk of either endurance or strength training on plasticity of the neural mechanisms involved in the soleus H reflex and V wave. Twenty-five sedentary healthy subjects were randomized into an endurance group ( n = 13) or strength group ( n = 12). Evoked V-wave, H-reflex, and M-wave recruitment curves, maximal voluntary contraction (MVC), and time-to-task-failure (isometric contraction at 40% MVC) of the plantar flexors were recorded before and after training. Following strength training, MVC of the plantar flexors increased by 14.4 ± 5.2% in the strength group ( P < 0.001), whereas time-to-task-failure was prolonged in the endurance group (22.7 ± 17.1%; P < 0.05). The V wave-to-maximal M wave (V/Mmax) ratio increased significantly (55.1 ± 28.3%; P < 0.001) following strength training, but the maximal H wave-to-maximal M wave (Hmax/Mmax) ratio remained unchanged. Conversely, in the endurance group the V/Mmax ratio was not altered, whereas the Hmax/Mmax ratio increased by 30.8 ± 21.7% ( P < 0.05). The endurance training group also displayed a reduction in the H-reflex excitability threshold while the H-reflex amplitude on the ascending limb of the recruitment curve increased. Strength training only elicited a significant decrease in H-reflex excitability threshold, while H-reflex amplitudes over the ascending limb remained unchanged. These observations indicate that the H-reflex pathway is strongly involved in the enhanced endurance resistance that occurs following endurance training. On the contrary, the improvements in MVC following strength training are likely attributed to increased descending drive and/or modulation in afferents other than Ia afferents.

Non-uniform muscle adaptations to eccentric exercise and the implications for training and sport

Due to the variations in morphological and architectural characteristics of fibers within a skeletal muscle, regions of a muscle may be differently affected by eccentric exercise. Although eccentric exercise may be beneficial for increasing muscle mass and can be beneficial for the treatment of tendinopathies, the non-uniform effect of eccentric exercise results in regional muscle damage and as a consequence, non-uniform changes in muscle activation. This regional muscle weakness can contribute to muscle strength imbalances and may potentially alter the load distribution on joint structures, increasing the risk of injury. In this brief review, the non-uniform effects of eccentric exercise are reviewed and their implications for training and sport are considered.

Eccentric exercise and delayed onset muscle soreness of the quadriceps induce adjustments in agonist-antagonist activity, which are dependent on the motor task

This study investigates the effects of eccentric exercise and delayed onset muscle soreness (DOMS) of the quadriceps on agonist–antagonist activity during a range of motor tasks. Ten healthy volunteers (age, mean ± SD, 24.9 ± 3.2 years) performed maximum voluntary contractions (MVC) and explosive isometric contractions of the knee extensors followed by isometric contractions at 2.5, 5, 10, 15, 20, and 30% MVC at baseline, immediately after and 24 h after eccentric exercise of the quadriceps. During each task, force of the knee extensors and surface EMG of the vasti and hamstrings muscles were recorded concurrently. Rate of force development (RFD) was computed from the explosive isometric contraction, and the coefficient of variation of the force (CoV) signal was estimated from the submaximal contractions. Twenty-four hours after exercise, the subjects rated their perceived pain intensity as 4.1 ± 1.2 (score out of 10). The maximum RFD and MVC of the knee extensors was reduced immediately post- and 24 h after eccentric exercise compared to baseline (average across both time points: 19.1 ± 17.1% and 11.9 ± 9.8% lower, respectively, P < 0.05). The CoV for force during the submaximal contractions was greater immediately after eccentric exercise (up to 66% higher than baseline, P < 0.001) and remained higher 24 h post-exercise during the presence of DOMS (P < 0.01). For the explosive and MVC tasks, the EMG amplitude of the vasti muscles decreased immediately after exercise and was accompanied by increased antagonist EMG for the explosive contraction only. On the contrary, reduced force steadiness was accompanied by a general increase in EMG amplitude of the vasti muscles and was accompanied by increased antagonist activity, but only at higher force levels (>15% MVC). This study shows that eccentric exercise and subsequent DOMS of the quadriceps reduce the maximal force, rate of force development and force steadiness of the knee extensors, and is accompanied by different adjustments of agonist and antagonist muscle activities.

Low-frequency oscillations of the neural drive to the muscle are increased with experimental muscle pain

We investigated the influence of nociceptive stimulation on the accuracy of task execution and motor unit spike trains during low-force isometric contractions. Muscle pain was induced by infusion of hypertonic saline into the abductor digiti minimi muscle of 11 healthy men. Intramuscular EMG signals were recorded from the same muscle during four isometric contractions of 60-s duration at 10% of the maximal force [maximal voluntary contraction (MVC)] performed before injection (baseline), after injection of isotonic (control) or hypertonic saline (pain), and 15 min after pain was no longer reported. Each contraction was preceded by three 3-s ramp contractions from 0% to 10% MVC. The low-frequency oscillations of motor unit spike trains were analyzed by the first principal component of the low-pass filtered spike trains [first common component (FCC)], which represents the effective neural drive to the muscle. Pain decreased the accuracy of task performance [coefficient of variation (CoV) for force: baseline, 2.8 ± 1.8%, pain, 3.9 ± 1.8%; P < 0.05] and reduced motor unit discharge rates [11.6 ± 2.3 pulses per second (pps) vs. 10.7 ± 1.7 pps; P < 0.05]. Motor unit recruitment thresholds (2.2 ± 1.2% MVC vs. 2.4 ± 1.6% MVC), interspike interval variability (18.4 ± 4.9% vs. 19.1 ± 5.4%), strength of motor unit short-term synchronization [common input strength (CIS) 1.02 ± 0.44 vs. 0.83 ± 0.22], and strength of common drive (0.47 ± 0.08 vs. 0.47 ± 0.06) did not change across conditions. The FCC signal was correlated with force (R = 0.45 ± 0.06), and the CoV for FCC increased in the painful condition (5.69 ± 1.29% vs. 7.83 ± 2.61%; P < 0.05). These results indicate that nociceptive stimulation increased the low-frequency variability in synaptic input to motoneurons.

Delayed-onset muscle soreness alters the response to postural perturbations

INTRODUCTION

Eccentric contractions induce muscle fiber damage that is associated with delayed-onset muscle soreness and an impaired ability of the muscle to generate voluntary force. Pain and pathophysiological changes within the damaged muscle can delay or inhibit neuromuscular responses at the injured site, which is expected to have an effect on reflex activity of the muscle.

PURPOSE

The aim of the study was to investigate the reflex activity of knee muscles to rapid destabilizing perturbations, before, immediately after, and 24 and 48 h after eccentric exercise.

METHODS

Bipolar surface EMG signals were recorded from 10 healthy men with seven pairs of electrodes located on the knee extensor muscles (vastus medialis, rectus femoris, and vastus lateralis) and knee flexor muscles (the medial and lateral heads of the hamstring and the medial and lateral heads of gastrocnemius) of the right leg during rapid perturbations.

RESULTS

The maximal voluntary contraction force decreased by 24% ± 4.9% immediately after exercise and remained reduced by 21.4% ± 4.1% at 24 h and by 21.6% ± 9.9% at 48 h after exercise with respect to baseline. During the postexercise postural perturbations, the EMG average rectified value of the knee extensor muscles was significantly lower than baseline (P < 0.001). Moreover, the decrease in average rectified value over time during postexercise sustained contractions was greatest compared with the session before exercise (P < 0.0001).

CONCLUSIONS

Reflex activity in leg muscles elicited by rapid destabilizing perturbations is reduced after exercise-induced muscle soreness.

The relative timing of trunk muscle activation is retained in response to unanticipated postural-perturbations during acute low back pain

The purpose of this study was to assess the activation of the erector spinae (ES) and external oblique (EO) in response to unanticipated, bi-directional postural perturbations before and after the induction of acute low back pain (LBP) in healthy individuals. An experimental session consisted of a baseline, control, and an acute LBP condition. For the control and acute LBP condition, isotonic or hypertonic saline (HS), respectively, was injected into the right ES muscle. In each condition, participants stood on a moveable platform during which 32 randomized postural perturbations (8 repetitions of 4 perturbation types: 8 cm anterior slides, 8 cm posterior slides, 10° anterior tilts, and 10° posterior tilts) with varying inter-perturbation time intervals were performed over a period of 4–5 min. Bilateral surface electromyography (EMG) was recorded from the ES and EO in addition to subjective pain records. During the acute LBP condition: (1) the onset time of the ES and EO was delayed for the forward and backward sliding perturbations (P < 0.05); (2) EMG amplitude was reduced bilaterally for all perturbations (P < 0.05); (3) the order of activation and interval between the onset times of the ES and EO were unaltered and (4) ES, but not EO, activity was adjusted to account for the directional differences between the perturbations. This study revealed that re-establishment of posture and balance was a result of the individuals’ ability to rapidly modulate ES with respect to EO activity and that the bi-directional postural responses, although shifted in time and amplitude, retained temporal features in the presence of acute LBP.

Motor unit behavior during submaximal contractions following six weeks of either endurance or strength training

The study investigated changes in motor output and motor unit behavior following 6 wk of either strength or endurance training programs commonly used in conditioning and rehabilitation. Twenty-seven sedentary healthy men (age, 26.1 ± 3.9 yr; mean ± SD) were randomly assigned to strength training (ST; n = 9), endurance training (ET; n = 10), or a control group (CT; n = 8). Maximum voluntary contraction (MVC), time to task failure (isometric contraction at 30% MVC), and rate of force development (RFD) of the quadriceps were measured before ( week 0), during ( week 3), and after a training program of 6 wk. In each experimental session, surface and intramuscular EMG signals were recorded from the vastus medialis obliquus and vastus lateralis muscles during isometric knee extension at 10 and 30% MVC. After 6 wk of training, MVC and RFD increased in the ST group (17.5 ± 7.5 and 33.3 ± 15.9%, respectively; P < 0.05), whereas time to task failure was prolonged in the ET group (29.7 ± 13.4%; P < 0.05). The surface EMG amplitude at 30% MVC force increased with training in both groups, but the training-induced changes in motor unit discharge rates differed between groups. After endurance training, the motor unit discharge rate at 30% MVC decreased from 11.3 ± 1.3 to 10.1 ± 1.1 pulses per second (pps; P < 0.05) in the vasti muscles, whereas after strength training it increased from 11.4 ± 1.2 to 12.7 ± 1.3 pps ( P < 0.05). Finally, motor unit conduction velocity during the contractions at 30% MVC increased for both the ST and ET groups, but only after 6 wk of training ( P < 0.05). In conclusion, these strength and endurance training programs elicit opposite adjustments in motor unit discharge rates but similar changes in muscle fiber conduction velocity.

Effect of delayed-onset muscle soreness on muscle recovery after a fatiguing isometric contraction

An increase to above-baseline levels of electromyography (EMG) mean power spectral frequency (MPF) has been observed previously during muscle recovery following fatiguing contractions and has been explained by membrane hyperpolarization due to increased activation of the Na+-K+ pump. It is hypothesized that this membrane mechanism is impaired by muscle fiber damage following eccentric exercise. Thus, the aim of the study was to investigate surface EMG signal characteristics during recovery from fatigue after eccentric exercise. Ten healthy subjects performed sustained isometric knee extensions at 40% of the maximal torque (MVC) until task failure before, immediately after and 24 and 48 h after eccentric exercise. Bipolar surface EMG signals were recorded from six locations over the quadriceps during the sustained isometric contraction and during 3-s long contractions at 40% MVC separated by 1-min intervals for 15 min (recovery). Before the eccentric exercise, MPF of EMG signals increased to values above baseline during recovery from the fatiguing isometric contraction (P<0.001), whereas immediately after and 24 and 48 h after the eccentric task, MPF was lower than baseline during the entire recovery period (P<0.01). In conclusion, delayed-onset muscle soreness abolished the supranormal increase in EMG MPF following recovery from fatigue.

Effect of pain on the modulation in discharge rate of sternocleidomastoid motor units with force direction

OBJECTIVE

To compare the behavior of sternocleidomastoid motor units of patients with chronic neck pain and healthy controls.

METHODS

Nine women (age, 40.4+/-3.5 yr) with chronic neck pain and nine age- and gender-matched healthy controls participated. Surface and intramuscular EMG were recorded from the sternocleidomastoid muscle bilaterally as subjects performed isometric contractions of 10-s duration in the horizontal plane at a force of 15 N in eight directions (0-360 degrees ; 45 degrees intervals) and isometric contractions at 15 and 30 N force with continuous change in force direction in the range 0-360 degrees . Motor unit behavior was monitored during the 10-s contractions and the subsequent resting periods.

RESULTS

The mean motor unit discharge rate depended on the direction of force in the control subjects (P<0.05) but not in the patients. Moreover, in three of the nine patients, but in none of the controls, single motor unit activity continued for 8.1+/-6.1s upon completion of the contraction. The surface EMG amplitude during the circular contraction at 15N was greater for the patients (43.5+/-54.2 microV) compared to controls (16.9+/-14.9 microV; P<0.05).

CONCLUSIONS

The modulation in discharge rate of individual motor units with force direction is reduced in the sternocleidomastoid muscle in patients with neck pain, with some patients showing prolonged motor unit activity when they were instructed to rest.

SIGNIFICANCE

These observations suggest that chronic neck pain affects the change in neural drive to muscles with force direction.

Comparison between the degree of motor unit short-term synchronization and recurrence quantification analysis of the surface EMG in two human muscles

OBJECTIVE

To verify if non-linear recurrence analysis of the surface EMG is a suitable tool for assessing motor unit short-term synchronization.

METHODS

Surface and intramuscular EMG signals were recorded from the abductor digiti minimi and vastus medialis muscles of 12 and 10 healthy men, respectively, during isometric contractions. In the abductor digiti minimi, EMG signals were additionally recorded after a contraction sustained for 1min at 50% of the maximal force. In both muscles, percent of determinism (%DET) was estimated from the surface EMG and common input strength (CIS) index was computed from motor unit recordings.

RESULTS

For both muscles, CIS did not correlate with %DET (abductor digiti minimi: R(2)=0.11, P=0.12; vastus medialis: R(2)=0.04, P=0.56). Although the values of CIS for the vastus medialis were lower than those of the abductor digiti minimi (P<0.001), the %DET values did not differ between the two muscles (71.6+/-5.5% vs 66.9+/-8.7%; P=0.12).

CONCLUSION

The variable %DET extracted from the surface EMG is a poor indicator of the degree of motor unit short-term synchronization.

SIGNIFICANCE

The study provides a systematic evaluation of a technique previously proposed for the estimation of a clinically relevant characteristic of motor unit behavior.

Neuromuscular efficiency of the sternocleidomastoid and anterior scalene muscles in patients with chronic neck pain

PURPOSE

This study compared the neuromuscular efficiency (NME) of the sternocleidomastoid (SCM) and anterior scalene (AS) muscles between 20 chronic neck pain patients and 20 asymptomatic controls.

METHOD

Myoelectric signals were recorded from the sternal head of SCM and the AS muscles as subjects performed sub-maximal isometric cervical flexion contractions at 25 and 50% of the maximum voluntary contraction (MVC). The NME was calculated as the ratio between MVC and the corresponding average rectified value of the EMG signal. Ultrasonography was used to measure subcutaneous tissue thickness over the SCM and AS to ensure that differences did not exist between groups.

RESULTS

For both the SCM and AS muscles, NME was shown to be significantly reduced in patients with neck pain at 25% MVC (p<0.05). Subcutaneous tissue thickness over the SCM and AS muscles was not different between groups.

CONCLUSIONS

Reduced NME in the superficial cervical flexor muscles in patients with neck pain may be a measurable altered muscle strategy for dysfunction in other muscles. This aberrant pattern of muscle activation appears to be most evident under conditions of low load. NME, when measured at 25% MVC, may be a useful objective measure for future investigation of muscle dysfunction in patients with neck pain.