Deep and superficial cervical muscles respond differently to unstable motor skill tasks.

Biomechanical modelling and physiological studies suggest that various spinal muscle layers differ in their contribution to spine movement and stiffness. This study aimed to investigate the activation of deep and superficial muscles in stable and unstable task conditions. Nine healthy participants performed a task of controlling a metal ball on a plate fixed to the head in seated position. In unstable tasks, visual feedback was provided by mirrors to move the ball to the centre of the plate by small head movements and maintain the position for 3 s. Task difficulty was adjusted in a stepwise progression of difficulty using five surfaces with materials of decreasing resistance. In the stable condition, the ball was fixed to the plate's centre. EMG was recorded with surface (sternocleidomastoid, anterior scalenes, upper trapezius) and fine-wire electrodes (rectus capitis posterior major, obliquus inferior, multifidus, semispinalis cervicis, splenius capitis). The outcome variable was root mean square (RMS) EMG during the part of the task when the ball was maintained in the centre position. Results revealed greater cervical muscle activity in the unstable than stable conditions (p < 0.001, ηp2 = 0.746). Control of deep and superficial cervical muscles differed (p = 0.003, ηp2 = 0.354). Deep cervical muscle activity was greater with unstable tasks, but did not differ with task difficulty. In contrast, superficial cervical muscle activity increased in a stepwise manner with increasing challenge. These results support the notion that the central nervous system uses different strategies for control of deep versus superficial muscle layers of the cervical spine in association with instability.

Oxygenation, EMG and position sense during computer mouse work. Impact of active versus passive pauses.

We investigated the effects of active versus passive pauses implemented during computer mouse work on muscle oxygenation and EMG of the forearm extensor carpi radialis muscle, and on wrist position sense. Fifteen healthy female subjects (age: 19-24 years) performed a 60-min mouse-operated computer task, divided into three 20 min periods, on two occasions separated by 3-6 days. On one occasion a passive pause (subjects resting) was implemented at the end of each 20-min period, and on another occasion an active pause (subjects performed a number of high intensity extensions of the forearm) was implemented. Also at the end of each 20-min period, test contractions were conducted and subjective ratings of fatigue and stress were obtained. Another parameter of interest was total haemoglobin calculated as the summation of oxy-and deoxy-haemoglobin, since it reflects blood volume changes. The most interesting findings were an overall increasing trend in total haemoglobin throughout the mouse work (P<0.001), and that this trend was greater for the active pause as compared to the passive pause (P<0.01). These data were accompanied by an overall increase in oxygen saturation (P<0.001), with a tendency, albeit not significant, toward a higher increase for the active pause (P=0.13). EMG amplitude and median frequency tended to decrease (P=0.08 and 0.05, respectively) during the mouse work but was not different between pause types. Borg ratings of forearm fatigue showed an overall increase during the activity (P<0.001), but the perceptions of stress did not change. Position sense did not change due to the mouse work for either pause type. While increasing trends were found for both pause types, the present study lends support to the hypothesis of an enhancement in oxygenation and blood volume for computer mouse work implemented with active pauses. However, a presumption of an association between this enhancement and attenuated fatigue during the mouse work was not supported.

Responses of muscle spindles in feline dorsal neck muscles to electrical stimulation of the cervical sympathetic nerve.

Previous studies performed in jaw muscles of rabbits and rats have demonstrated that sympathetic outflow may affect the activity of muscle spindle afferents (MSAs). The resulting impairment of MSA information has been suggested to be involved in the genesis and spread of chronic muscle pain. The present study was designed to investigate sympathetic influences on muscle spindles in feline trapezius and splenius muscles (TrSp), as these muscles are commonly affected by chronic pain in humans. Experiments were carried out in cats anesthetized with alpha-chloralose. The effect of electrical stimulation (10 Hz for 90 s or 3 Hz for 5 min) of the peripheral stump of the cervical sympathetic nerve (CSN) was investigated on the discharge of TrSp MSAs (units classified as Ia-like and II-like) and on their responses to sinusoidal stretching of these muscles. In some of the experiments, the local microcirculation of the muscles was monitored by laser Doppler flowmetry. In total, 46 MSAs were recorded. Stimulation of the CSN at 10 Hz powerfully depressed the mean discharge rate of the majority of the tested MSAs (73%) and also affected the sensitivity of MSAs to sinusoidal changes of muscle length, which were evaluated in terms of amplitude and phase of the sinusoidal fitting of unitary activity. The amplitude was significantly reduced in Ia-like units and variably affected in II-like units, while in general the phase was affected little and not changed significantly in either group. The discharge of a smaller percentage of tested units was also modulated by 3-Hz CSN stimulation. Blockade of the neuromuscular junctions by pancuronium did not induce any changes in MSA responses to CSN stimulation, showing that these responses were not secondary to changes in extrafusal or fusimotor activity. Further data showed that the sympathetically induced modulation of MSA discharge was not secondary to the concomitant reduction of muscle blood flow induced by the stimulation. Hence, changes in sympathetic outflow can modulate the afferent signals from muscle spindles through an action exerted directly on the spindles, independent of changes in blood flow. It is suggested that such an action may be one of the mechanisms mediating the onset of chronic muscle pain in these muscles in humans.

Effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense.

The present study investigated the effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense in the upper extremity. Twenty-four healthy subjects (12 males and 12 females) performed a 45-min standardized mouse-operated computer task on two occasions. The task consisted of painting rectangles that were presented on the screen. On one occasion, time pressure and precision demands were imposed (more demanding task, MDT), whereas, on the other occasion, no such restraints were added (less demanding task, LDT). The order of the two task versions was randomized. Tissue oxygen saturation in the trapezius and extensor carpi radialis muscles was recorded throughout, and the position-matching ability of the wrist was measured before and after the tasks. In addition, measurements of autonomic nervous system reactivity and subjective ratings of tenseness and physical fatigue were obtained. Performance was measured in terms of the number of rectangles that were painted during the task. During MDT, oxygen saturation in extensor carpi radialis decreased (P < 0.05) compared to LDT. These data were paralleled by increased electrodermal activity (P < 0.05), skin blood flow (P < 0.05), ratings of tenseness and fatigue (P < 0.01), and increased performance (P < 0.01) during MDT. Females exhibited lower oxygen saturation than males, during rest as well as during the computer tasks (P < 0.01). Wrist repositioning error increased following LDT as compared to MDT (P < 0.05). In conclusion, computer mouse work under time pressure and precision demands caused a decrease in forearm muscle oxygenation, but did not affect wrist position sense accuracy. We attribute our changes in oxygenation more to increased oxygen consumption as a result of enhanced performance, than to vasoconstriction.

Effects of sympathetic stimulation on the rhythmical jaw movements produced by electrical stimulation of the cortical masticatory areas of rabbits.

The somatomotor and sympathetic nervous systems are intimately linked. One example is the influence of peripheral sympathetic fibers on the discharge characteristics of muscle spindles. Since muscle spindles play important roles in various motor behaviors, including rhythmic movements, the working hypothesis of this research was that changes in sympathetic outflow to muscle spindles can change rhythmic movement patterns. We tested this hypothesis in the masticatory system of rabbits. Rhythmic jaw movements and EMG activity induced by long-lasting electrical cortical stimulation were powerfully modulated by electrical stimulation of the peripheral stump of the cervical sympathetic nerve (CSN). This modulation manifested itself as a consistent and marked reduction in the excursion of the mandibular movements (often preceded by a transient modest enhancement), which could be attributed mainly to corresponding changes in masseter muscle activity. These changes outlasted the duration of CSN stimulation. In some of the cortically evoked rhythmic jaw movements (CRJMs) changes in masticatory frequency were also observed. When the jaw-closing muscles were subjected to repetitive ramp-and-hold force pulses, the CRMJs changed characteristics. Masseter EMG activity was strongly enhanced and digastric EMG slightly decreased. This change was considerably depressed during CSN stimulation. These effects of CSN stimulation are similar in sign and time course to the depression exerted by sympathetic activity on the jaw-closing muscle spindle discharge. It is suggested that the change in proprioceptive information induced by an increase in sympathetic outflow (a) has important implications even under normal conditions for the control of motor function in states of high sympathetic activity, and (b) is one of the mechanisms responsible for motor impairment under certain pathological conditions such as chronic musculoskeletal head-neck disorders, associated with stress conditions.

Vertical posture and head stability in patients with chronic neck pain.

OBJECTIVE: To evaluate postural performance and head stabilization of patients with chronic neck pain. DESIGN: A single-blind comparative group study. SUBJECTS: Patients with work-related chronic neck pain (n = 9), with chronic whiplash associated disorders (n = 9) and healthy subjects (n = 16). METHODS: During quiet standing in different conditions (e.g. 1 and 2 feet standing, tandem standing, and open and closed eyes) the sway areas and the ability to maintain the postures were measured. The maximal peak-to-peak displacement of the centre of pressure and the head translation were analysed during predictable and unpredictable postural perturbations. RESULTS: Patients with chronic neck pain, in particular those with whiplash-associated disorders, showed larger sway areas and reduced ability to successfully execute more challenging balance tasks. They also displayed larger sway areas and reduced head stability during perturbations. CONCLUSION: The results show that disturbances of postural control in chronic neck pain are dependent on the aetiology, and that it is possible to quantify characteristic postural disturbances in different neck pain conditions. It is suggested that the dissimilarities in postural performance are a reflection of different degrees of disturbances of the proprioceptive input to the central nervous system and/or of the central processing of such input.

Effects on physical performance and pain from three dynamic training programs for women with work-related trapezius myalgia.

To compare training programs for women with trapezius myalgia regarding physical performance and pain, 102 women were randomized to strength, endurance, co-ordination and non-training groups. Before and after the intervention, static strength and dynamic muscular endurance in shoulder muscles were measured on a Cybex II dynamometer. Muscle activity in shoulder muscles was monitored via surface EMG. The signal amplitude ratio between the active and passive phase of repeated contractions indicated the ability to relax. Pain at present, pain in general and pain at worst were measured on visual analogue scales. After training, within group comparisons showed that the training groups rated less pain, and in the strength training group ratings of pain at worst differed from the non-training group. Using the non-training group as a reference, static strength increased in the strength and endurance training groups and muscular endurance in all training groups. The study indicates that regular exercises with strength, endurance or co-ordination training of neck/shoulder muscles might alleviate pain for women with work-related trapezius myalgia.

Replication and discrimination of limb movement velocity.

It is well known that proprioception is composed of the senses of movement and position. Whereas tests of position sense are quite commonly used, tests of the acuity in perception of movement velocity are scarce. In the present study we examined some novel tests for assessing the sense of limb movement velocity, involving replication and discrimination of single-joint movement velocity. Specifically, we investigated: (1) whether replication of limb movement velocity is more accurate following active criterion movements as compared to passive; (2) whether antagonist muscle contraction during passive limb movement enhances velocity discrimination; (3) how criterion movement velocity influences response accuracy; (4) the relationship between movement velocity and movement extent during velocity replication; and (5) whether subjects really base discrimination of velocities on perceived velocity. Sixteen healthy subjects participated in four tests (I-IV). For each test, horizontal abductions were performed about the right glenohumeral joint from the sagittal plane. The subjects were required to actively replicate the velocity of either an active (Test I) or passive (Test II) criterion movement, or judge whether a passive/semipassive (passive during antagonist muscle contraction) movement was faster or slower than a previous passive/semipassive criterion movement (Test III/IV). The results revealed higher response accuracy for Test I compared to Test II and for slower movements compared to faster, but no difference in response accuracy between Test III and IV. For velocity discrimination, the analysis revealed that the subjects based their judgment on the difference between criterion and comparison velocity rather than time or extent cues.

Position sense testing: influence of starting position and type of displacement.

OBJECTIVE: To assess the effect of starting position, target position, and various types of limb displacement on repositioning tests commonly used for clinical evaluation of rehabilitation. SETTING: Controlled laboratory environment. PARTICIPANTS: Sixteen healthy volunteer subjects. MAIN OUTCOME MEASURE: Absolute error, ie, the absolute difference between target and replicate positions. DESIGN: Each subject performed four testing procedures consisting of different types of limb displacement (active, passive, and passive during antagonist muscle contraction). For each procedure, horizontal movements were performed ipsilaterally about the right glenohumeral joint from one intermediate starting position (40 degrees ) and two extreme starting positions (0 degrees and 80 degrees ). Four fixed target positions (16 degrees , 32 degrees, 48 degrees , 64 degrees ) were presented for each starting position. The subjects were required to replicate target position after returning to the respective starting position. RESULTS: Lower repositioning errors occurred with active displacement procedures compared with passive, and with the intermediate starting position compared with the extreme. Target position, however, had no effect on repositioning errors. CONCLUSIONS: Starting position and type of displacement should be considered in interpretations and comparisons of data from clinical studies.

Position sense acuity is diminished following repetitive low-intensity work to fatigue in a simulated occupational setting.

Repetitive work to fatigue is soundly associated with work-related musculoskeletal disorders (WMSD), although the underlying mechanisms are poorly understood. In the present study, we tested the hypothesis that fatiguing work leads to proprioceptive deficits, which can be an initiating factor for the occurrence of WMSD. Thus, the position sense of the shoulder was determined for 13 males and 13 females before and after performing repetitive low-intensity arm work to fatigue in a simulated occupational setting. From a starting position of 45 degrees to the sagittal plane, position sense tests consisted of subjects attempting to actively reproduce target positions of horizontal movements to 15 degrees and 30 degrees (shoulder adduction) and to 60 degrees and 75 degrees (shoulder abduction). An analysis of variance revealed that the absolute error was significantly increased following fatigue for the subjects as a group (P < 0.001). Furthermore, females had an overall higher error than males (P < 0.01). No difference in error was detected for the shorter movements versus the longer movements. However, the overall absolute error for adduction was significantly higher than for abduction (P < 0.001). The results of the present study support the hypothesis of diminished proprioceptive acuity following low-intensity work to fatigue. A reduction in position sense acuity could lead to impairment in motor control, which would further impact on position sense. Thus, a vicious cycle may be activated that might result in WMSD. The poorer position sense acuity observed for females may contribute to the explanation of why females demonstrate a higher incidence of WMSD than males.

Reliability of position sense testing assessed with a fully automated system.

Position sense testing has increased as a tool for augmenting evaluation of joint injury. In the present study, we investigated the inter-day reliability for four different types of position sense tests using a fully automated system. The tests included (1) passive presentation/active replication, (2) passive presentation/passive replication, (3) semi-passive presentation/semi-passive replication (where semi-passive denotes passive movement during antagonist muscle contraction), and (4) active presentation/active replication. The absolute difference between presented target and replicated position was used as a measure of position sense accuracy. Ten healthy subjects who were blindfolded and seated with the arm in a moveable rig performed the tests on two occasions, separated by 3-4 days. For each type of position sense test, horizontal abduction from a starting position of 0 degrees (relative to the sagittal plane) to target positions of 32 degrees and 64 degrees, and horizontal adduction from a starting position of 80 degrees to 48 degrees and 16 degrees were conducted. A two-way ANOVA revealed no differences in absolute error between days or between testing procedures. However, intra-class correlations (ICC), which are most often used to express test-retest reliability, were moderate at best, ranging from 0.40 to 0.61 for the four types of position sense tests. Hence, the present study indicates that the ability of repositioning tests to detect alterations in proprioceptive function is limited, suggesting that their use in clinical evaluation be approached with prudence.

Localized muscle fatigue decreases the acuity of the movement sense in the human shoulder.

PURPOSE: The purpose of this study was to investigate alterations in the movement sense acuity during localized muscle fatigue in the human dominant shoulder. METHODS: Fourteen healthy volunteers (8 males and 6 females) with a mean age 23 +/- 2 yr participated in the study. The subjects' ability to discriminate movement velocity relative to a reference velocity imposed over the dominant shoulder was tested following two experimental conditions: 1) Light exercise (LE), repetitive isokinetic horizontal flexion/extensions at the shoulder, ranging from 85 degrees to 20 degrees relative to the frontal plane, at 10% of maximal voluntary contraction (MVC) and 2) Hard exercise (HE), same movements as in LE, but performing MVC to fatigue. RESULTS: The results showed that subjects had a lower probability of distinguishing between different movement velocities following HE as compared with those during the LE condition (P < 0.001). When genders were compared, female subjects had a lower probability of distinguishing correctly than male subjects (P < 0.001). CONCLUSIONS: The acuity of the movement sense in the dominant shoulder is reduced in the presence of shoulder muscle fatigue. The possible influence of muscle fatigue via peripheral muscle receptors on movement sense is discussed.

A neural network approach to real-time spike discrimination during simultaneous recording from several multi-unit nerve filaments.

A multi-channel, real-time, unsupervised spike discriminator was developed in order to reconstruct single spike trains from several simultaneously recorded multi-unit nerve filaments. The program uses a Self Organising Map (SOM) algorithm for the classification of the spikes. In contrast to previous similar techniques, the described method is made for use on a PC, and the method may thus be implemented at relatively low cost. In order to test the accuracy of the program, a robustness test was performed, where noise with different RMS levels was superimposed on the spikes. Furthermore, the maximal classification rate was determined. The program is easy to use, since the only manual inputs needed are the voltage threshold for spike detection, and the number of units present in each recorded nerve filament.

A method for analysis of encoding of stimulus separation in ensembles of afferents.

This paper describes a new method for the analysis of ensemble coding in populations of receptor afferents. The method is based on principal component analysis (PCA) combined with algorithms for calculation of stimulus separation (discrimination). It allows a quantification of the ability of ensembles of afferents to discriminate between stimuli of different intensity, and it can also be used to calculate to which extent each afferent in an ensemble contributes to the discrimination. Since the emphasis is put on discrimination of stimuli rather than on similarities between the representations of the stimulus in the ensemble response and the stimulus itself, it involves no implicit assumption about the unknown decoding mechanisms in the CNS. Examples of results obtained with analysis of and comparison of simultaneously recorded and pooled sequentially recorded populations of primary muscle spindle afferents are given. These results support some of the general arguments in favour of the ensemble coding theory, and indicate that the results of studies made on populations of simultaneously recorded receptor afferents may differ considerably from results obtained in studies on pooled sequentially recorded afferents. In contrast to when ensemble responses are based on parallel recordings, population responses constructed from sequential recordings will be distorted as a result of unavoidable temporal variations in physiological conditions.

Influences on the gamma-muscle spindle system from muscle afferents stimulated by increased intramuscular concentrations of bradykinin and 5-HT.

There is evidence that static muscular contractions induce a release of bradykinin (BK) in the working muscle, and that increased concentration of BK and 5-HT in a muscle increases the discharge rate of a subpopulation of group III and group IV muscular afferents. It is also known that activity in group III and IV muscle afferents may activate gamma-motoneurones to both homonymous and heteronymous muscles. The aim of the present study was to investigate whether increased concentration of BK and 5-HT in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding muscles, via fusimotor reflexes. The experiments were made on six cats anaesthetised with alpha-chloralose. The triceps surae (GS) and the posterior biceps and semitendinosus (PBSt) muscles were subjected to sinusoidal stretches. Simultaneous recordings of 2-11 MSAs from these muscles were made and the mean rate of firing and the modulation for each MSA were determined. Responses of 47 MSAs (26 PBSt and 21 GS) were recorded. The responsiveness of the MSAs to injections of BK (9-100 mg/ml, 0.5-1.0 ml) and 5-HT (25-150 mg/ml, 0.5-1.0 ml) was 89% and 83%, respectively, for injections into the arterial supply of the ipsilateral GS muscle, and 84% and 40% respectively for injections to the contralateral GS muscle. Of 10 secondarLy MSAs, only one was unresponsive to BK injections, while several MSAs responded to both ipsilateral and contralateral BK injections.(ABSTRACT TRUNCATED AT 250 WORDS)

Influences on the gamma-muscle-spindle system from contralateral muscle afferents stimulated by KCl and lactic acid.

The aim of the present study was to investigate whether increased concentrations of lactic acid and potassium chloride (KCl) in contralateral muscles can influence the sensitivity of primary and secondary muscle spindle afferents (MSAs) from ipsilateral extensor and flexor muscles. The experiments were performed on 7 cats anaesthetised with alpha-chloralose. Recordings were made simultaneously from 2-12 single MSAs from the triceps surae (GS) and/or the posterior biceps and semitendinosus muscles (PBSt). The mean rate of firing and the amplitude of a fitted sine curve of MSA responses to sinusoidal stretching of the receptor bearing muscles were determined. Responses of 42 primary MSAs (17 from PBSt and 25 from GS) were recorded. On 33 of these, clear-cut alterations in sinusoidal response were evoked by injection of 1 ml KCl (200-400 mM) or 1 ml lactic acid (20-50 mM) into the arterial supply of the contralateral GS or PBSt muscles. Six out of 8 secondary MSAs showed sizeable effects to increased intramuscular concentrations of KCl and/or lactic acid (3 from PBSt and 3 from GS). On both primary and secondary MSAs, from GS as well as from PBSt muscles, the large majority of effects were excitatory. All effects on secondary MSAs were compatible with reflex actions on static fusimotor neurones, whereas on primary MSAs different types of reflex responses were observed (i.e. pure static, pure dynamic and mixtures of static and dynamic fusimotor actions). Stimuli related alterations in MSA responses were completely abolished when the contralateral GS or PBSt nerves were anaesthetised.(ABSTRACT TRUNCATED AT 250 WORDS)

Influences on the gamma-muscle-spindle system from muscle afferents stimulated by increased intramuscular concentrations of arachidonic acid.

There is evidence that static muscular contractions induce a release of arachidonic acid (AA) in the working muscle and that increased concentration of AA in a muscle increases the discharge rate of a subpopulation of groups III and IV muscular afferents. It is also known that activity in groups III and IV muscle afferents may activate gamma-motoneurones to both homo- and heteronymous muscles. The aim of the present study was to investigate if increased concentration of AA in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding muscles, via fusimotor reflexes. The experiments were made on five cats anaesthetized with alpha-chloralose. The triceps surae (GS) and the posterior biceps and semitendinosus (PBSt) muscles were subjected to sinusoidal stretches. Simultaneous recordings of 2-12 MSAs from these muscles were made and the mean rate of firing and the modulation for each MSA were determined. Responses of 36 MSAs (17 PBSt and 19 GS) were recorded. The responsiveness of the MSAs to injections of AA (0.3-1.0 mg; 0.3-1 ml) was 86% (n = 36) for injections into the arterial supply of the ipsilateral GS muscle and 45% (n = 20) for injections to the contralateral GS muscle. Out of 14 secondary MSAs, only one was unresponsive to ipsilateral AA injections while two of eight were unresponsive to contralateral AA injection. The majority of responses were compatible with predominantly static or mixed dynamic and static fusimotor activation. None of the effects were compatible with inhibition of fusimotor activity. The duration of the effects were usually 2-4 min. However, on some occasions the elevations in MSA activity persisted for up to 1 h. Local anaesthesia of the nerve to the injected muscle always abolished the effects of the injections and control injections of the solution in which the AA was dissolved were ineffective in changing the MSA responses. I.v. injections occasionally induced effects on the MSAs, but such effects were significantly different from those caused by close arterial muscle injections. Thus, increased concentration of AA may excite primary and secondary MSAs from homo- as well as heteronymous muscles, including contralateral muscles, most probably via fusimotor reflexes evoked by activity in chemosensitive muscle afferents.

A multichannel hook electrode for simultaneous recording of up to 12 nerve filaments.

This paper describes a simple multichannel hook electrode specially adapted for simultaneous recording from several dorsal root filaments in the cat. The electrode consists of a 3-mm-thick PVC plate and accommodates 12 silver wire hooks placed in notches in the plate. It has a semicircular shape, and the notches are milled radially in the plate. With this electrode we have made successful recordings of single-unit activity from 12 muscle afferents for periods of up to 8 h. In contrast to more rigid and complicated devices, e.g., developed to assess activity of populations of individual neurones from single-channel recordings of multi-unit activity, it also permits selection of the types of afferents to be recorded. The use of this simple device and an appropriate number of amplifiers and window discriminators can greatly enhance the quality of studies on populations of afferents in animals and may also decrease the number of laboratory animals needed.