No Difference in Hip Muscle Volumes and Fatty Infiltration in Those With Hip-Related Pain Compared to Controls.

Background: Little is known about muscle morphology in people with hip-related pain, without signs of femoro-acetabular impingement syndrome (FAIS). Identifying changes in hip muscle volume, fatty infiltrate and establishing relationships between muscle volume and strength, may provide insight into potential early treatment strategies. Purposes: To: (i) compare the volumes and fatty infiltrate of gluteus maximus, gluteus medius, gluteus minimis, tensor fascia latae and quadratus femoris between symptomatic and less-symptomatic sides of participants with hip-related pain; (ii) compare the volumes and fatty infiltrate of hip muscles between healthy controls and symptomatic participants; and (iii) explore relationships of hip muscle volumes to muscle strength and patient-reported outcome measures in people with hip-related pain. Study Design: Cross-sectional study. Methods: Muscle volume and fatty infiltrate (from magnetic resonance imaging), hip muscle strength, patient-reported symptoms, function and quality of life (QOL) were determined for 16 participants with hip-related pain (no clinical signs of FAIS; 37±9 years) and 15 controls (31±9 years). Using One Way Analysis of Co-Variance tests, muscle volume and fatty infiltrate was compared between the symptomatic and less-symptomatic sides in participants with hip-related pain as well as between healthy controls and symptomatic participants. In addition, hip muscle volume was correlated with hip muscle strength, hip-reported symptoms, function and QOL. Results: No differences in all the studied muscle volumes or fatty infiltrate were identified between the symptomatic and less-symptomatic hips of people with hip-related pain; or between people with and without hip-related pain. Greater GMED volume on the symptomatic side was associated with less symptoms and better function and QOL (ρ=0.522-0.617) for those with hip-related pain. Larger GMAX volume was associated with greater hip abduction and internal rotation strength, larger GMED volume was associated with greater hip extension strength, and larger QF volume was associated with greater hip abduction strength (rho=0.507-0.638). Conclusion: People with hip-related pain and no clinical signs of FAIS have hip muscle volumes that are not significantly different than those of matched pain-free controls or their less-symptomatic hip. Larger GMED muscle volume was associated with fewer symptoms and greater strength. Level of evidence: Level 3a.

The Relationship of Ultrasound Measurements of Muscle Deformation With Torque and Electromyography During Isometric Contractions of the Cervical Extensor Muscles.

OBJECTIVES: Speckle tracking analysis (STA) of ultrasound (US) images quantifies the longitudinal deformation of a region of muscle tissue to provide a mechanical measure of muscle activity. As the validity and reliability of this method has not yet been adequately assessed, the aim of this study was to determine the validity and reliability of STA in the dorsal neck muscles during isometric neck extension contractions. METHODS: Twenty volunteers performed 3 repetitions of isometric neck extension in a dynamometer at 10%, 20%, 40%, 60%, and 80% of maximal voluntary torque while US and surface electromyography (EMG) data were recorded. Speckle tracking analysis was then used to calculate measurements of muscle deformation. The relationship among torque, muscle deformation (separate for each muscle and summed), and EMG was evaluated using linear regressions and Spearman's correlation coefficients. The reliability of EMG and muscle deformation was determined using intraclass correlation coefficients (ICCs). RESULTS: Muscle deformation in 4 of the 5 muscles when examined separately and collectively was significantly related to torque (P < .05); however, the relationship was weak (r2 = 0.03-0.18). In contrast, a strong linear relationship was observed between torque and EMG (P < .001, r2 = 0.83). Poor to moderate reliability of muscle deformation measures (ICC: 0.02-0.69) was found compared with EMG, which was highly reliable (ICC: 0.67-0.90). CONCLUSION: These results suggest that the validity of US STA measurements for isometric contractions of the dorsal neck muscles is questionable. Further investigation into this method is required before it can be used as a tool to measure muscle activity.

Experimental Pain Decreases Corticomuscular Coherence in a Force- But Not a Position-Control Task.

Differences in neural drive could explain variation in adaptation to acute pain between postural and voluntary motor actions. We investigated whether cortical contributions, quantified by corticomuscular coherence, are affected differently by acute experimental pain in more posturally focused position-control tasks and voluntary focused force-control tasks. Seventeen participants performed position- and force-control contractions with matched loads (10% maximum voluntary contraction) before and during pain (injection of hypertonic saline into the infrapatellar fat pad of the knee). Surface electromyography (EMG) of right knee extensor and flexor muscles was recorded. Electroencephalography (EEG) was recorded using a 128-channel sensor net. Corticomuscular coherence was calculated between 4 EEG electrodes that approximated the contralateral motor cortical area, and EMG. Coherence, EEG, EMG, and target performance accuracy were compared between task types and pain states. Before pain, coherence EEG and EMG did not differ between tasks. During pain, EMG increased in both tasks, but the force-control task showed greater pain interference (decreased coherence, higher EEG frequencies, and increased force fluctuations). Neural substrates of motor performance of postural functions are changed uniquely by experimental pain, which might be explained by differences in cortical demands. Our results provide new insights into the mechanisms of motor adaptations during acute pain. PERSPECTIVE: Understanding of the mechanisms underlying adaptations to motor function in acute pain is incomplete. Experimental work almost exclusively focuses on voluntary motor actions, but these adaptations may be inappropriate for postural actions. Our results show less pain-related interference in brain activity and its relationship to muscle activation during position-control tasks.

Clinical features of people with hip-related pain, but no clinical signs of femoroacetabular impingement syndrome.

OBJECTIVES: Identifying impairments in hip range of motion (ROM) and muscle strength in people with hip-related pain, without signs of femoro-acetabular impingement syndrome (FAIS). STUDY AIMS: to determine if hip strength and ROM i) differs between the symptomatic and less-symptomatic hip of people with hip-related pain; and between people with hip-related pain and healthy controls; and ii) are associated with hip-related Quality of Life and pain. DESIGN: Cross-sectional study. SETTING: University laboratory. PARTICIPANTS: Thirty participants with hip-related pain who were awaiting hip arthroscopy (22 women; age = 37 ±â€¯10yrs), 32 healthy controls (19 women; age = 30 ±â€¯10yrs). MAIN OUTCOME MEASURES: Hip ROM, muscle strength, pain intensity and the Hip disability and Osteoarthritis Outcome Score (HOOS) were assessed. RESULTS: Less hip flexion ROM (p = 0.004), and extension (p = 0.004), abduction (p = 0.001) and internal rotation (IR) (p = 0.048) strength were measured on the symptomatic compared to non-symptomatic side. Hip-related pain participants had lower hip abduction strength (p = 0.045), and less flexion (p < 0.001), IR (p = 0.027) and external rotation (ER) (p = 0.019) ROM compared to controls. Less ER ROM (p = 0.03-0.04), and greater abduction (p = 0.03-0.04) and adduction strength (p = 0.02) were associated with better patient reported outcome measures. CONCLUSIONS: Specific impairments in hip ROM and strength were identified in people with hip-related pain but no FAIS.

Perceived task complexity of trunk stability exercises.

BACKGROUND: Perceived task complexity can impact participation in an exercise programme and the level of skill acquisition resulting from participation. Although trunk stability exercises are commonly included in the management of people with low back pain, potential differences in perceived task complexity between those exercises have not been investigated previously. OBJECTIVE: To investigate the perceived task complexity following first time instruction of two common stability exercises: the abdominal brace and abdominal hollow. DESIGN: Cross-sectional. METHODS: Twenty-four naïve healthy participants received instruction in the performance of an abdominal brace and an abdominal hollow with feedback. Participants rated their perceived task complexity (mental, physical, and temporal demand, performance, effort, frustration) for each exercise on the NASA-Task Load Index. RESULTS: The abdominal hollow was associated with higher perceived mental demand than the abdominal brace (p = 0.01), and required more time to learn (p < 0.01). The abdominal brace was associated with greater mental demand and frustration when performed after the abdominal hollow than before. CONCLUSIONS: This study has provided the first evidence for differences in perceived task complexity between two commonly used trunk stability exercises. Those differences in perceived task complexity may influence the selection of exercises intended to enhance the robustness of spinal stability.

Cortical activity differs between position- and force-control knee extension tasks.

Neural control differs between position- and force-control tasks as evident from divergent effects of fatigue and pain. Unlike force-control tasks, position-control tasks focus on a postural goal to maintain a joint angle. Cortical involvement is suggested to be less during postural control, but whether this differs between position- and force-control paradigms remains unclear. Coherence estimates the functional communication between spatially distinct active regions within the cortex (cortico-cortical coherence; CCC) and between the cortex and muscles (corticomuscular coherence; CMC). We investigated whether cortical involvement differed between force-control and more posturally focused, position-control tasks. Seventeen adults performed position- and force-control knee extensor efforts at a submaximal load (10 % maximum voluntary contraction). Surface electromyography was recorded from the right knee extensor and flexor muscles and brain activity using electroencephalography (EEG). CCC and CMC in the beta (13-30 Hz) and gamma (30-45 Hz) frequency bands were calculated between combinations of intra- and inter-hemispheric pairs of electrodes, and between four EEG electrodes that approximated the left motor cortical area, and right knee extensor EMG, respectively. Differences in EEG power and muscle activity were also calculated. CCC was greater across distributed regions in the force-control task. Beta EEG power in the left hemisphere was higher for the position-control task. Although averaged CMC data differed between tasks, there was no task difference for individual CMC data. Muscle activity and force did not differ between tasks. The results demonstrate differential cortical contributions to control force- versus position-control tasks. This might contribute to differences in performance outcomes of these tasks that have been shown previously.

Experimental pain has a greater effect on single motor unit discharge during force-control than position-control tasks.

OBJECTIVE: When matching target force during pain, single motor unit (SMU) discharge is modified in a manner thought to redistribute load in painful tissue. This adaptation might not be appropriate when maintaining joint posture against an external load. We compared changes in SMU discharge rate of knee extensor muscles in a force-control and a position-control task during pain. METHODS: Thirteen healthy adults (31±6years) performed position- and force-control contractions using matched loads in non-pain and pain states. Pain was induced by injection of hypertonic saline into the infrapatellar fat pad. Intramuscular and surface electromyography of knee extensor and flexor muscles was recorded. RESULTS: When considering the discharge of a select population of SMUs that were recorded across all conditions performed on the same day, there was a decrease in mean discharge rate, and this was smaller in the position- than force-control task for the same SMUs. A similar tendency was observed for SMUs recorded on different days. However, gross agonist muscle activity (which incorporates SMUs that are not included in the discharge rate analysis because they were not present in all conditions) increased in both tasks during pain. Gross antagonist muscle EMG only increased in the force-control task. CONCLUSION: The effect of pain on muscle activity appears unique to the contraction type, with less influence during position- than force-control tasks. SIGNIFICANCE: Simplistic theories of pain adaptation of movement during voluntary efforts cannot be extrapolated to more postural functions. This has implications for understanding movement changes that may underpin persistence/recurrence of pain and the management of musculoskeletal pain.

Comparison of location, depth, quality, and intensity of experimentally induced pain in 6 low back muscles.

INTRODUCTION: The pattern of pain originating from experimentally induced low back pain appears diffuse. This may be because sensory information from low back muscles converges, sensory innervation extends over multiple vertebral levels, or people have difficulty accurately representing the painful location on standardized pain maps. OBJECTIVE: The aim of this study was to provide insight into the perception of pain from noxious stimulation of a range of low back muscles using novel depth and location measures. METHODS: Hypertonic saline (1 mL, 7% NaCl) was injected into bellies of longissimus (LO), quadratus lumborum (QL), superficial multifidus (SM), and deep multifidus (DM) at the level of the fourth lumbar vertebrae (L4) and in SM and DM at L5 using ultrasound guidance over 6 sessions. Fifteen participants reported depth, location, intensity, size, and descriptive quality of pain throughout the painful period (∼14 min). RESULTS: Pain was reported deeper (P<0.04) for DML4/L5 compared with SML4/L5, LO and QL; more cranial for LO compared with DML4 and QL (P<0.01); more lateral for LO compared with DML4 (P<0.02); and more lateral for QL compared with all other muscles at L4 (P<0.0001). Pain intensity was higher in DML4/L5 than all other muscles (P<0.04) for ∼3 minutes. Descriptive qualities varied slightly between muscles. DISCUSSION: Depth and lateral position may be the most critical descriptors to determine the source of acute lumbar muscular pain. Overlapping regions of pain may be explained by convergence of receptive fields, innervation of multifidus fascicles at multiple lumbar segments, and convergence of sensory input from different muscles to the same sensory cell bodies as demonstrated in the lumbar spine of animal preparations.

Does movement variability increase or decrease when a simple wrist task is performed during acute wrist extensor muscle pain?

PURPOSE: The goal of complex tasks can be maintained despite variability in the movements of the multiple body segments involved in the task (VAR(elements)). This variability increases in acute pain and may enable the nervous system to search for less painful/injurious movement options. It is unclear whether VAR(elements) increases when pain challenges simple tasks with fewer movement options, yet maintain successful attainment of the goal. We hypothesised that during acute pain related to a simple movement: (1) the task goal would be maintained; (2) VAR(elements) would be increased; and (3) if VAR(elements) increased during pain, it would decrease over time. METHODS: Movements of the right wrist/forearm were recorded with a three-dimensional motion analysis system and during a repetitive radial-ulnar deviation task between two target angle ranges (the task goal). We measured success of attaining the goal (repetitions that reached the target range and total absolute error in degrees), and variability in the motion of wrist flexion-extension and forearm pronation-supination (VAR(elements)). Fourteen healthy participants performed the task in one session before, during, and after wrist extensor muscle pain induced with hypertonic saline, and in another session without pain. RESULTS: The task goal was maintained during acute pain. However, VAR(elements) in other motion planes either reduced (pronation-supination) or did not change (flexion-extension). Thus, variability of task elements is constrained, rather than increased, in simple tasks. CONCLUSIONS: These data suggest the nervous system adapts simple tasks with limited degrees of freedom by reduction of VAR(elements) rather than the increase observed for more complex tasks.

Effect of pain location on spatial reorganisation of muscle activity.

INTRODUCTION: We aimed to determine whether the changes in muscle activity (in terms of both gross electromyography (EMG) and motor unit (MU) discharge characteristics) observed during pain are spatially organized with respect to pain location within a muscle which is the main contributor of the task. METHODS: Surface and fine-wire EMG was recorded during matched low-force isometric plantarflexion from soleus (from four quadrants with fine-wire EMG and from the medial/lateral sides with surface EMG), both gastrocnemii heads, peroneus longus, and tibialis anterior. Four conditions were tested: two control conditions that each preceded contractions with pain induced in either the lateral (Pain(L)) or medial (Pain(M)) side of soleus. RESULTS: Neither the presence (p = 0.28) nor location (p = 0.19) of pain significantly altered gross muscle activity of any location (lateral/medial side of soleus, gastrocnemii, peroneus longus and tibialis anterior). Group data from 196 MUs show redistribution of MU activity throughout the four quadrants of soleus, irrespective of pain location. The significant decrease of MU discharge rate during pain (p < 0.0001; Pain(L): 7.3 ± 0.9-6.9 ± 1.1 Hz, Pain(M): 7.0 ± 1.1 to 6.6 ± 1.1 Hz) was similar for all quadrants of the soleus (p = 0.43), regardless of the pain location (p = 0.98). There was large inter-participant variation in respect to the characteristics of the altered MU discharge with pain. CONCLUSION: Results from both surface and fine-wire EMG recordings do not support the hypothesis that muscle activity is reorganized in a simple systematic manner with respect to pain location.

Changes in constraint of proximal segments effects time to task failure and activity of proximal muscles in knee position-control tasks.

OBJECTIVE: Maintenance of a limb position against external load (position-control) fails earlier (time to task failure: TTF) than maintenance of identical force against rigid restraint (force-control). Although possibly explained by physiological differences between contractions, we investigated whether less constraint of movements in other planes and proximal segments (commonly less in position-control tasks) shortens TTF. METHODS: Seventeen adults (32±7 years) contracted knee extensor muscles to task failure in a position-control task, with and without constraint of motion in other planes and proximal segments, and a force-control task with constraints. Electromyography of knee extensors, their antagonist and hip muscles was recorded with force/position. RESULTS: TTF was shorter for position-control without (161±55 s) than with constraint (184±51 s). Despite identical constraint, TTF was shorter in position- than force-control (216±56 s). Muscle activity and position variability at failure was greater without constraint. CONCLUSION: Constraint of motion of proximal segments and other planes increases position-control TTF with less muscle activity and variability. As TTF differed between force- and position-control, despite equivalent constraint, other factors contribute to shorter position-control TTF. SIGNIFICANCE: Results clarify that differences in the TTF between position- and force-control tasks are partly explained by unmatched restriction of motion in other planes and proximal segments.

The effect of pain on training-induced plasticity of the corticomotor system.

UNLABELLED: Pain is thought to interfere with training-induced plasticity of corticomotor pathways. Although this implies direct interference with plastic processes, it may be explained by compromised performance in the training task during pain. Repeated finger movements can induce plasticity and change the amplitude/direction of acceleration of finger movement evoked by transcranial magnetic stimulation (TMS). We hypothesized that if pain interferes with plasticity, acceleration of finger movement would not change when the training task was painful, despite control of training task performance. TMS was applied over the optimal scalp site to evoked index finger abduction movements in nine participants. Participants then trained finger adduction with feedback of finger acceleration for three 8-min sessions, in three conditions on separate days. CONDITIONS: first dorsal interosseus (FDI) pain and control (no-pain), with injection of 5% and 0.9% hypertonic saline, respectively, into FDI; and remote pain (5% saline injection into infrapatellar fat pad). Peak acceleration of TMS-evoked finger movement and amplitude of motor evoked potentials (MEPs) in FDI were measured at baseline, between training sessions, and at three 5-min intervals after training ceased. Plastic change was observed (reduced TMS evoked peak finger acceleration in the abduction direction) after motor training during control and FDI pain, but not during the remote pain. There was no change in FDI MEPs in any conditions. These data do not support direct effects of nociceptive input (pain) on training-induced plasticity of corticomotor pathways. Remote pain may compromise learning due to distraction from the training task or other complex central pain processes.

Experimentally induced low back pain from hypertonic saline injections into lumbar interspinous ligament and erector spinae muscle.

Injection of hypertonic saline into back muscles or ligaments can induce acute low back pain (LBP). However, no study has systematically investigated pain characteristics from these structures. Further, induced muscle pain can change with stretching and contraction, which is problematic for studies into the effect of pain on sensorimotor control. However, it is unclear whether this occurs with experimental ligament pain. In separate sessions, 10 healthy volunteers received a single bolus injection of hypertonic (0.2ml, 5% NaCl) or isotonic saline (0.3ml, 0.9% NaCl) into L4/5 interspinous ligament, or hypertonic saline into the left paraspinal muscle. Pain intensity, size and duration were recorded, and a body chart was completed for each injection. Changes in pain intensity and size with stretching or back muscle contractions were also assessed during muscle and ligament pain. Injection of hypertonic saline into the interspinous ligament produced central LBP that was longer in duration and greater in intensity and size compared to hypertonic saline injection into lumbar paraspinal muscles. Isotonic saline injection into the interspinous ligament yielded mild pain that was short-lasting (<2min). Intensity and size of muscle pain reduced with stretching and contraction, whereas these tasks did not affect ligament pain. Surprisingly, some participants pointed to a location of pain that was 1-2 segments above or below the injected level. The results highlight that injection into the interspinous ligament elicits consistent pain that is not influenced by trunk movements. These findings support the implementation of this experimental ligament pain model in research.

Changes in motor unit recruitment strategy during pain alters force direction.

Motor unit (MU) recruitment is altered (decreased discharge rate and cessation of discharge in some units, and recruitment of new units) in force-matched contractions during pain compared to contractions performed before pain. As MU's within a motoneurone pool have different force direction properties we hypothesised that altered MU recruitment during experimental knee pain would change the force vector (total force (F(T)): amplitude and angle) generated by the quadriceps. Force was produced at two levels during 1 × 60-s and 3 × 10-s isometric contractions of knee extensors, and recorded by two force transducers at right angles. This enabled calculation of both F(E) (extension force) and F(T). MU recruitment was recorded from the medial and lateral vastii with four fine-wire electrodes. Pain was induced by hypertonic saline injection in the infra-patella fat pad. Nine subjects matched F(E) and six subjects also matched both medial and lateral forces (F(T)) before and during pain. Changes in MU discharge pattern (decreased discharge rate (P<0.001), complete cessation of firing, and recruitment of new units) during pain were associated with a ∼5° change in absolute force angle. As force angle changed in both directions (left/right) for individual subjects with pain there was no change in average F(T) amplitude between conditions. When both medial and lateral forces were matched MU discharge rate decreased (P<0.001) with pain, but, fewer units ceased firing or were newly recruited during pain. Change in motoneurone recruitment during pain alters direction of muscle force. This may be a strategy to avoid pain or protect the painful part.

Motoneurone recruitment is altered with pain induced in non-muscular tissue.

Motoneurone discharge rate is reduced despite the maintenance of force when pain is induced via injection of hypertonic saline into muscle. Two aspects require consideration. First, hypertonic saline may have direct effects on axons other than small diameter pain fibres including the motoneurones that innervate the painful muscle. Second, it is unclear how force is maintained, when motoneurone discharge rate is decreased. We aimed to determine; (1) if motoneurone discharge rate is reduced during force-matched tasks when pain is induced in non-muscle tissue (to exclude direct effects on motoneurones) and (2) if the reduction of discharge rate is associated with additional changes in motoneurone recruitment over multiple muscle regions. Motoneurone discharge was recorded in the quadriceps with eight pairs of fine-wire electrodes. Seven subjects performed 30-s low-level, force-matched contractions before and during anterior knee pain, which was induced by a bolus (0.25ml) injection of 5% hypertonic saline into the infra-patellar fat pad. In total, 119 motor units were identified. Of these, 34 were identified both before and during pain. The discharge rate of these units decreased during pain from 8.9(1.5) to 7.2(1.4)Hz (P<0.0001). In addition, 31 units were recruited in the no-pain condition but not during pain, when 53 new units were recruited. These changes coincided with a large variability in gross muscle activity measures between muscle regions. These data confirm that motoneurone recruitment is altered when direct effects of saline on motoneurones are excluded. Recruitment of additional motor units may explain force maintenance despite reduced discharge rate of some units.

Impaired neuromuscular function during isometric, shortening, and lengthening contractions after exercise-induced damage to elbow flexor muscles.

The purpose of this study was to examine the effect of exercise-induced damage of the elbow flexor muscles on steady motor performance during isometric, shortening, and lengthening contractions. Ten healthy individuals (age 22+/-4 yr) performed four tasks with the elbow flexor muscles: a maximum voluntary contraction, a one repetition maximum (1 RM), an isometric task at three joint angles (short, intermediate, and long muscle lengths), and a constant-load task during slow (approximately 7 degrees/s) shortening and lengthening contractions. Task performance was quantified as the fluctuations in wrist acceleration (steadiness), and electromyography was obtained from the biceps and triceps brachii muscles at loads of 10, 20, and 40% of 1 RM. Tasks were performed before, immediately after, and 24 h after eccentric exercise that resulted in indicators of muscle damage. Maximum voluntary contraction force and 1-RM load declined by approximately 45% immediately after exercise and remained lower at 24 h ( approximately 30% decrease). Eccentric exercise resulted in reduced steadiness and increased biceps and triceps brachii electromyography for all tasks. For the isometric task, steadiness was impaired at the short compared with the long muscle length immediately after exercise (P<0.01). Furthermore, despite no differences before exercise, there was reduced steadiness for the shortening compared with the lengthening contractions after exercise (P=0.01), and steadiness remained impaired for shortening contractions 24 h later (P=0.01). These findings suggest that there are profound effects for the performance of these types of fine motor tasks when recovering from a bout of eccentric exercise.

Eccentric exercise increases EMG amplitude and force fluctuations during submaximal contractions of elbow flexor muscles.

The purpose of this study was to determine the effect of eccentric exercise on the ability to exert steady submaximal forces with muscles that cross the elbow joint. Eight subjects performed two tasks requiring isometric contraction of the right elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at four submaximal target forces (5, 20, 35, 50% MVC) while electromyography (EMG) was recorded from elbow flexor and extensor muscles. These tasks were performed before, after, and 24 h after a period of eccentric (fatigue and muscle damage) or concentric exercise (fatigue only). MVC force declined after eccentric exercise (45% decline) and remained depressed 24 h later (24%), whereas the reduced force after concentric exercise (22%) fully recovered the following day. EMG amplitude during the submaximal contractions increased in all elbow flexor muscles after eccentric exercise, with the greatest change in the biceps brachii at low forces (3-4 times larger at 5 and 20% MVC) and in the brachialis muscle at moderate forces (2 times larger at 35 and 50% MVC). Eccentric exercise resulted in a twofold increase in coactivation of the triceps brachii muscle during all submaximal contractions. Force fluctuations were larger after eccentric exercise, particularly at low forces (3-4 times larger at 5% MVC, 2 times larger at 50% MVC), with a twofold increase in physiological tremor at 8-12 Hz. These data indicate that eccentric exercise results in impaired motor control and altered neural drive to elbow flexor muscles, particularly at low forces, suggesting altered motor unit activation after eccentric exercise.

Influence of tooth clench on the soleus H-reflex.

The Hoffmann (H) reflex is elicited by electrical stimulation of a mixed nerve and is used to measure the excitability of the spindle-motoneuron synapse. Recent investigations have indicated a positive correlation between increases in bite force and H-reflex facilitation. However, these investigations did not examine the H-reflex in detail or the possible role of periodontal mechanoreceptors (PMRs) in this facilitation. The current investigation was performed to determine whether PMRs play a role in H-reflex facilitation during tooth clench (TC). The H-reflex was elicited in the soleus muscle of human subjects while bite level was maintained at rest (0 N), 40 N, 80 N and maximal TC. The front teeth that contributed to the (40 N and 80 N) bite force were then locally anaesthetised (LA), and the protocol was repeated. The current data suggest that the effect of TC on the H-reflex amplitude in the human limb muscles is variable from one subject to the next. Statistical analysis has shown that the H-reflex was significantly smaller during the rest condition than during the 80 N bite (p<0.05) in both non-LA and LA conditions. Since LA did not alter the response, our results do not support that the PMRs play a major role in the facilitation of distal muscle activity.

The role of periodontal mechanoreceptors in mastication.

The aim of this review is to discuss what is known about the reflex control of the human masticatory system by the periodontal mechanoreceptors and to put forward a method for standardised investigation. To deliver mechanical stimulus in a reproducible way, the following precautions are suggested: the stimulus should be brought into secure contact with the area of stimulation, and slack between the probe and the area to be stimulated should be taken up by the application of a preload. It is also important to ensure that there is minimal simultaneous activation of receptor systems other than the periodontal mechanoreceptors. It is also necessary to standardise the method for recording and analysing the response.

A review of the H-reflex and M-wave in the human triceps surae.

The soleus is the most commonly used muscle for H-reflex studies in humans, while limited comparable data have been produced from the gastrocnemii muscles. This article reviews the fundamental differences between the structure and function of the human soleus and gastrocnemii muscles, including recent data published about their complex innervation zones. Protocols for eliciting, recording, and assessing the H-reflex and M-wave magnitude in the human triceps surae are also discussed.