Neck pain patterns and subgrouping based on weekly SMS-derived trajectories.

BACKGROUND: Neck and low back pain represent dynamic conditions that change over time, often with an initial improvement after the onset of a new episode, followed by flare-ups or variations in intensity. Pain trajectories were previously defined based on longitudinal studies of temporal patterns and pain intensity of individuals with low back pain. In this study, we aimed to 1) investigate if the defined patterns and subgroups for low back pain were applicable to neck pain patients in chiropractic practice, 2) explore the robustness of the defined patterns, and 3) investigate if patients within the various patterns differ concerning characteristics and clinical findings. METHODS: Prospective cohort study including 1208 neck pain patients from chiropractic practice. Patients responded to weekly SMS-questions about pain intensity and frequency over 43 weeks. We categorized individual responses into four main patterns based on number of days with pain and variations in pain intensity, and subdivided each into four subgroups based on pain intensity, resulting in 16 trajectory subgroups. We compared baseline characteristics and clinical findings between patterns and between Persistent fluctuating and Episodic subgroups. RESULTS: All but two patients could be classified into one of the 16 subgroups, with 94% in the Persistent fluctuating or Episodic patterns. In the largest subgroup, "Mild Persistent fluctuating" (25%), mean (SD) pain intensity was 3.4 (0.6) and mean days with pain 130. Patients grouped as "Moderate Episodic" (24%) reported a mean pain intensity of 2.7 (0.6) and 39 days with pain. Eight of the 16 subgroups each contained less than 1% of the cohort. Patients in the Persistent fluctuating pattern scored higher than the other patterns in terms of reduced function and psychosocial factors. CONCLUSIONS: The same subgroups seem to fit neck and low back pain patients, with pain that typically persists and varies in intensity or is episodic. Patients in a Persistent fluctuating pattern are more bothered by their pain than those in other patterns. The low back pain definitions can be used on patients with neck pain, but with the majority of patients classified into 8 subgroups, there seems to be a redundancy in the original model.

Long-term repeatability of force, endurance time and muscle activity during isometric contractions.

We determined the repeatability and correlations between force, endurance and muscle activity during isometric contractions over three years. Twenty-six subjects, with and without complaints of the shoulder and neck, performed standardized maximal and submaximal shoulder-abduction contractions and wrist extension-contractions at yearly intervals from 1997 to 1999. Peak forces developed during maximal contraction and the endurance times of submaximal contractions during shoulder abduction and wrist extension were measured. Electromyography (EMG) of muscle activity was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles. Root mean square EMG amplitudes were calculated. We found statistically significant associations between peak forces developed during wrist extension and shoulder abduction, and between endurance times of submaximal wrist extension and shoulder abduction. No statistically significant changes in peak force and EMG(peak) were found over the measurement years. The responses were not statistically significantly influenced by gender, or neck and shoulder pain. However, we observed considerable intra-individual variation in the inter-year measurements particularly for the responses to submaximal contraction. Such large variations represent a challenge when attempting to use the responses to interpret the effects of therapies.

Cross-cultural adaptation of the Norwegian versions of the Roland-Morris Disability Questionnaire and the Oswestry Disability Index.

OBJECTIVE: To evaluate reliability and construct validity of the Norwegian versions of the Roland Morris Disability Questionnaire and the modified Oswestry Disability Index. DESIGN: Translation of two functional status questionnaires and a cross-sectional study of measurement properties. METHODS: The questionnaires were translated and back-translated following the Guillemin criteria. The Norwegian versions were tested for 55 patients with acute low back pain and 50 patients with chronic low back pain. Test-retest with a 2-day interval was performed in a subsample of 28 patients from the chronic sample. Reliability was assessed by repeatability according to Bland and Altman, intraclass coefficient and coefficient of variation. Internal consistency was assessed by Cronbach's alpha. Concurrent construct validity was assessed with correlations between the questionnaires and the SF-36, Disability Rating Index and pain intensity. RESULTS: Repeatability of the Roland Morris Disability Questionnaire was 4 points, coefficient of variation 15% and intraclass correlation coefficient 0.89, and of the modified Oswestry Disability Index 11, 12% and 0.88, respectively. Internal consistency was 0.94 for both questionnaires. The questionnaires correlated highly with the physical functioning scale of SF-36, moderately with pain, and low with mental scales of the SF-36. CONCLUSION: The reliability and construct validity of the Norwegian versions of the Roland Morris Disability Questionnaire and the modified Oswestry Disability Index are acceptable for assessing functional status of Norwegian-speaking patients with low back pain.

Muscle contractile properties during intermittent nontetanic stimulation in rat skeletal muscle.

To examine changes in contractile properties and mechanisms of fatigue during submaximal nontetanic skeletal muscle activity, in situ perfused soleus (60-min protocol) and extensor digitorum longus (EDL; 10-min protocol) muscles of the rat were electrically stimulated intermittently at low frequency. The partly fused trains of contractions showed a two-phase change in appearance. During the first phase, relaxation slowed, one-half relaxation time increased, and maximal relaxation first derivative of force (dF/dt) decreased. Developed force during the trains was reduced and was closely related to the rate of relaxation in this first phase. During the second phase, relaxation became faster again, one-half relaxation time decreased, and force returned to resting levels between contractions in a train. In contrast, developed force remained reduced, so that peak force of the contractions was 51% (soleus) and 30% (EDL) of control. In the soleus muscle, the changes in contractile properties were not related to ATP, creatine phosphate, or lactate content. The changes in contractile properties fit best with a mechanism of fatigue involving changes in Ca(2+) handling by the sarcoplasmic reticulum.

Classification of problems, clinical findings and treatment goals in patients with low back pain using the ICIDH-2 beta-2.

PURPOSE: This study aimed to explore problems, clinical findings and treatment goals presented by patients with low back pain and their physiotherapists, and to which extent this information could be classified according to the ICIDH-2 Beta-2. METHOD: An explorative field trial of 12 physiotherapists and 28 patients with low back pain in outpatient practices. RESULTS: Pain and impairments related to muscles, joints and structures of the lower back, pelvis and hips were most often presented. Functioning related to daily activities and participation in society was often described in the treatment goals, but vaguely formulated. Most of the problems and clinical findings could be classified according to the ICIDH-2 Beta-2. However, several treatment goals could not be classified. CONCLUSIONS: There was a lack of specification in several of the detailed codes of the Body, Function and Structure Dimension, particularly regarding classification of pain and muscle functions. The main limitation concerned the detailed version of the Body Function and Structure Dimension, which provided no additional precision compared with the short version when classifying pain and other impairments. However, the ICIDH-2 offers a tool to enhance the precision of clinical findings and the possibilities for performing a multidimensional evaluation of functioning related to patients with low back pain.

Cognitive performance and muscle activation in workers with chronic shoulder myalgia.

The present study determined whether workers with chronic shoulder myalgia exhibit impaired cognitive performance and elevated muscle activation. Ten workers with and 10 without shoulder myalgia performed three sessions, each consisting of three different cognitive tasks. Subjective pain was reported on visual analogue scales (VAS). Surface electromyography was recorded bilaterally from the trapezius and forearm extensor muscles. Speed and error rates of the cognitive work were almost the same in the two groups, but the myalgia subjects reported more stress during the work. In two of the cognitive tasks, both groups increased their speed of performance to the same extent from the first to the third session, despite pain increase in the myalgia subjects. In the biomechanically demanding task, the speed fell from the first to the third session in both groups. Muscle activation was equal in the two groups and constant throughout the protocol despite changes in speed of performance and pain. It is concluded that localized chronic muscle pain does not significantly alter cognitive performance, or muscle activation level during cognitive work. However, the perceived burden of work is larger, as indicated by the higher stress reported during cognitive work in the myalgia subjects compared with their pain-free controls.

Muscle activation in the contralateral passive shoulder during isometric shoulder abduction in patients with unilateral shoulder pain.

Studies have shown an increased muscle activation at the opposite passive side during unilateral contractions. The purpose of the present study was to examine the influence of pain on muscle activation in the passive shoulder during unilateral shoulder abduction. Ten patients with unilateral rotator tendinosis of the shoulder and nine healthy controls performed unilateral maximal voluntary contractions (MVC) and sustained submaximal contractions with and without subacromial injections of local anaesthetics of the afflicted shoulder. Muscle activation was recorded by electromyography (EMG) from the trapezius, deltoid, infraspinatus and supraspinatus muscles in both shoulders. During MVCs, the EMG amplitude from muscles of the passive afflicted side was not different in patients and controls, and was not influenced by pain alterations. In contrast, the EMG amplitude from the muscles of the passive unafflicted side was lower in the patients and increased after pain reduction. During the sustained submaximal contraction the EMG amplitude increased gradually in the passive shoulder to 15-30% of the EMG amplitude observed during MVC. This response was not influenced by differences in pain. We conclude that muscle activation of the passive shoulder was closely related to the activation of the contracting muscles and thus related to central motor drive, and not directly influenced by changes in pain.

Muscle activation after supervised exercises in patients with rotator tendinosis.

OBJECTIVE: To determine whether pain reduction induced by supervised exercises over several months results in increased maximal force and muscle activation. DESIGN: Before-after trial. PARTICIPANTS: Ten patients with unilateral rotator tendinosis and more than 3 months' duration of pain. INTERVENTION: Supervised exercises for 3 to 6 months. OUTCOME MEASURES: Maximal abduction force, muscle activation by surface electromyogram (EMG), and pain were assessed during brief maximal voluntary isometric contractions (MVC) before and after fatigue. EMG and pain were assessed during sustained submaximal contraction, performed with the shoulder 45 degrees abducted. The testing protocol was performed before and after supervised exercises. RESULTS: In the afflicted shoulder, resting pain was reduced after supervised exercises and no longer differed from the unafflicted side. The increase in pain during contraction was almost the same before and after treatment. MVC force increased, but significant side differences remained. EMG increased for trapezius and deltoid muscles in both afflicted and unafflicted arms. Fatigue development and recovery was unaltered by the exercise regimen. CONCLUSIONS: Pain reduction after supervised exercises was associated with an improved MVC force, but the side difference in maximal force generation was maintained. Muscle activity during maximal contraction increased in both the afflicted and unafflicted sides.

Loss of potassium from muscle during moderate exercise in humans: a result of insufficient activation of the Na+-K+-pump?

In this study, we have investigated whether the muscle net potassium (K+) loss, observed during two-legged intermittent static knee-extensions at 30% MVC (n = 9), is caused by an insufficient activation of the Na+-K+-pumps. Furthermore, we have investigated whether the changes in the K+ homeostasis can be causally related to fatigue. K+ loss was calculated from the arterio-venous concentration difference and plasma flow. In three subjects, femoral venous K+ concentration was measured continuously with a K+ selective electrode. Na+-K+-pump activity was estimated from the rate of removal of K+ from the blood during 30-s pauses inserted into the exercise protocol. A large net K+ loss took place during the first minutes of exercise, but diminished quickly and disappeared after 20 min. An increasing net K+ loss reappeared after 30 min. Only 10% of the lost K+ had been regained after the 20-min recovery. A lag in the activation of the Na+-K+-pumps may explain the K+ loss at the beginning of exercise, but gradual pump activation prevented a net K+ loss after 20 min of exercise. The reappearance of the net K+ loss in the later stage of exercise and the subsequent slow recovery of intracellular K+ seemed to be caused by an insufficient further activation of the pumps, rather than by the capacity of the pumps being surpassed. Fatigue was not related to the accumulation of K+ in the interstitium. However, during exercise, the decrease in intracellular K+ content was linearly related to the fall of maximal force. We conclude that during repeated isometric contractions, insufficient activation of the Na+-K+-pumps causes a continuous muscle K+ loss which was associated with fatigue.

Skeletal muscle fatigue in normal subjects and heart failure patients. Is there a common mechanism?

Skeletal muscle fatigue develops gradually during all forms of exercise, and develops more rapidly in heart failure patients. The fatigue mechanism is still not known, but is most likely localized to the muscle cells themselves. During high intensity exercise the perturbations of the Na+ and K+ balance in the exercising muscle favour depolarization, smaller action potentials and inexcitability. The Na+, K+ pump becomes strongly activated and limits, but does not prevent the rise in extracellular Na+, K+ pump concentration and intracellular Na+ concentration. However, by virtue of its electrogenic property the pump may contribute in maintaining excitability and contractility by keeping the cells more polarized than the ion gradients predict. With prolonged exercise perturbations of Na+ and K+ are smaller and fatigue may be associated with altered cellular handling of Ca2+ and Mg2+. Release of Ca2+ from the sarcoplasmic reticulum (SR) is reduced in the absence of changes of the cellular content of Ca2+ and Mg2+. In heart failure several clinical reports indicate severe electrolyte perturbations in skeletal muscle. However, in well controlled studies small or insignificant changes are found. We conclude that with high intensity exercise perturbations of Na+ and K+ in muscle cells may contribute to fatigue, whereas with endurance type of exercise and in heart failure patients the skeletal muscle fatigue is more likely to reside in the intracellular control of Ca2+ release and reuptake.

Isometric abduction muscle activation in patients with rotator tendinosis of the shoulder.

OBJECTIVE: To examine the influence of pain on activation in brief maximal and sustained submaximal isometric abduction in patients with rotator tendinosis of the shoulder. DESIGN: Randomized, controlled experimental trial. PARTICIPANTS: Ten patients with complaints of at least 3 months' duration (median range, 1 to 2 years) and nine healthy controls. INTERVENTION: Patients and controls were randomized into subacromial local anesthetic injection on 2 different days. METHODS: The uninvolved shoulder was tested first, elbow flexed 90 degrees, shoulder abducted 45 degrees. The protocol consisted of three brief maximal voluntary contractions (MVCs), followed by a sustained submaximal contraction until exhaustion and three MVCs during a 20-minute recovery period. Electromyography (EMG) was obtained bilaterally from the supraspinatus, infraspinatus, upper trapezius, and middle deltoid muscles. Pain was scored on a visual analogue scale (0 to 100). RESULTS: Mean pain rating on MVC of the involved side of patients was reduced from 28 to 10 by subacromial injection. Mean MVC force improved from 163N to 184N (95% confidence interval for the difference, 14 to 29N). The accompanying EMG amplitude during MVC increased significantly in three of the four muscles examined. Pain, force, and EMG of the uninvolved side and in controls were unaltered. Endurance time and EMG (given as microV) during the submaximal contraction were not influenced by pain. MVC did not fully recover during the postexhaustive period, while the corresponding EMG amplitudes were comparable to values in unfatigued muscle. CONCLUSION: Pain reduced central motor drive during maximal efforts in the unfatigued state, but no additional reduction was seen after a sustained submaximal contraction.

Measurement of human muscle fatigue.

Human muscle fatigue has been studied using a wide variety of exercise models, protocols and assessment methods. Based on the definition of fatigue as 'any reduction in the maximal capacity to generate force or power output', the different methods to measure fatigue are discussed. It is argued that reliable and valid measures must include either assessment of maximal voluntary contraction force or power, or the force generated by electrical stimulation. By comparing tetanic stimulation and maximal voluntary contraction force one may reveal whether fatigue is of central origin, or whether peripheral mechanisms are involved. Adequate use of twitch interpolation provides an even more sensitive measure for central fatigue. Indirect methods as endurance times and electromyography show variable responses during exercise and no close relationship to fatigue. Hence these methods are of limited value in measurement of human muscle fatigue.

Dissociation between metabolic and contractile responses during intermittent isometric exercise in man.

This study examines the temporal changes in high-energy phosphate and metabolic levels, and in force-generating capacity, during and after voluntary submaximal repetitive isometric exercise (RIE). Eight male subjects performed one-legged RIE with the knee extensors at 40% maximum voluntary contraction (MVC) target force (duty cycle: 6 s contraction, 4 s rest) in a 48 cm bore whole body 1.5 T superconducting magnet. Phosphocreatine (PCr), inorganic phosphate (P(i)), ATP and pH were measured every 9 s. Force-generating capacity was repeatedly measured using MVC force and electrically stimulated contractions (sequential train of impulses of 1-100 Hz). During RIE, MVC declined gradually by 56 +/- 5% (mean +/- S.E.M.). Electrically stimulated force also declined, with a disproportionally large drop in low-frequency force, seen as a decline from 0.76 +/- 0.02 to 0.33 +/- 0.02 in 20:50 Hz force ratio. The PCr decline during RIE was 65 +/- 9%, in most subjects seen as a rapid initial drop followed by less or no further decline to exhaustion. pH declined in parallel by 0.18 +/- 0.04 units, whilst ATP levels remained unchanged throughout the exercise. PCr, P(i) and pH recovered to near control values within 5 min of exhaustion. Force, however, was not fully restored after 30 min recovery. The results support the hypothesis that fatigue from submaximal RIE is unrelated to changes in P(i) and H+ levels. The decline in 20:50 Hz force ratio implies that fatigue may be associated with excitation-contraction coupling impairment. No sudden changes were observed in mechanical or metabolic factors at exhaustion. Exhaustion was probably not caused by lack of substrates for ATP resynthesis, since pH had decreased only marginally.

Mechanical behavior of skeletal muscle during intermittent voluntary isometric contractions in humans.

Changes in contractile speed and force-fusion properties were examined during repetitive isometric contractions with the knee extensors at three different target force levels. Seven healthy subjects were studied at target force levels of 30, 45, and 60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s rest were continued until exhaustion. Contractile speed was determined for contractions elicited by electrical stimulation at 1-50 Hz given during exercise and a subsequent 27-min recovery period. Contraction time remained unchanged during exercise and recovery, except for an initial rapid shift in the twitch properties. Half relaxation time (RT1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT1/2 values were not fully restored to preexercise levels. During exercise at 60% MVC, the RT1/2 decreased for twitches and increased for the 50-Hz stimulation. In the recovery period after 60% MVC, RT1/2 values declined toward those seen after the 30 and 45% MVC exercise. The force oscillation amplitude in unfused tetani relative to the mean force increased during exercise at 30 and 45% MVC but remained unaltered during the 60% MVC exercise. This altered force-fusion was closely associated with the changes in RT1/2. The faster relaxation may at least partly explain the increased energy cost of contraction reported previously for the same type of exercise.

Metabolic heat production during fatigue from voluntary repetitive isometric contractions in humans.

The effect of repetitive isometric knee extensions on the energy cost of contraction was examined. The rate of temperature rise (dT/dt) was determined in test contractions at 30 and 50% of maximal voluntary contraction (MVC) force before and during 30% MVC repetitive isometric exercise (RIE) to exhaustion and regularly in a 30-min postexercise recovery period (n = 9). Pulmonary O2 uptake and muscle temperature (Tmus) were determined at regular intervals. During the 30% MVC test contractions, dT/dt was 5.6 +/- 0.6 mK/s in unfatigued muscle, increasing linearly by 68% during exercise. In the 50% MVC test contractions, dT/dt rose by 84% from 9.8 +/- 1.1 mK/s. dT/dt determined during test contractions at both force levels did not decrease significantly throughout the 30-min postexercise recovery period. The rise in dT/dt was paralleled by 76% increased in O2 uptake. In contrast, Tmus rose initially and then leveled off. The present data indicate that RIE induced a gradual rise in the rate of energy turnover associated with isometric force production. Neither increased Tmus nor recruitment of less economic type II fibers can fully explain the increased energy cost. We suggest that energetic changes may occur at the cellular level and argue that this may be associated with the changes in muscle mechanics occurring during fatigue from submaximal voluntary RIE.

Nonlinear relationship between heat production and force during voluntary contractions in humans.

The rate of temperature rise (dT/dt) in the vastus lateralis muscle of seven subjects was measured at four to five locations in each muscle during voluntary isometric contractions ranging from 10 to 90% of maximal voluntary contraction (MVC) force. dT/dt increased from 3.1 +/- 1.1 mK/s at 10% MVC to 14.5 +/- 1.3 mK/s at 90% MVC. In the typical subject, the increase in dT/dt with force was markedly higher between 30 and 70% MVC than in the upper and lower force ranges. The estimated ratio between heat rate in active muscle and force was six times higher at 10% MVC than at 90% MVC, indicating a markedly increasing economy of contraction with increasing force. The lower contraction economy at low forces may be explained by an increased rate of energy turnover associated with force generation and relaxation when motor units are contracting at low firing rates. Hence, we argue that recruitment and rate coding may have a profound effect on the economy of contraction at different force levels.

Pain and fatigue induced by exercise in fibromyalgia patients and sedentary healthy subjects.

OBJECTIVE: To examine whether general feelings of fatigue, exercise-induced pain in the extremities, and exertion were different in female patients with fibromyalgia syndrome (FS) compared with sedentary healthy women. METHODS: Thirty-seven FS patients and 20 healthy subjects were studied. Cardiovascular fitness was assessed by Aastrand's indirect, submaximal method. The period of repetitive dynamic muscle contractions and sustained static muscle contraction were measured. General feelings of fatigue before exercise and exercise-induced extremity pain were assessed by visual analogue scales. Exercise-induced exertion was recorded by Borg's Rating Scale of Perceived Exertion. RESULTS: No significant group difference in cardiovascular fitness was found (p = 0.8). In the FS patients general fatigue was (median 95% confidence interval) 69 (59 - 75) versus 32 (22 - 47) for the healthy controls (p < 0.0001). At the moment of interrupting the bicycle test, the perceived exertion score was 17 (16 - 18) among patients versus 13 (13 - 15) among controls (p < 0.0001). Compared with the controls, high exercise-induced extremity pain was found after sustained static and repetitive dynamic muscle contractions in the FS patients (p < 0.004), and 24 hours later the patients' pain intensities had not returned to pre-exercise values (p < 0.01). CONCLUSION: High general fatigue, exercise-induced extremity pain, exertion and 24 hours post-exercise extremity pain in FS patients compared with healthy controls could not be explained by any group difference in cardiovascular fitness.

Muscle fatigue in early fibromyalgia.

OBJECTIVE: To compare the processes connected with muscle fatigue during prolonged exercise in patients with fibromyalgia (FM) with studies of healthy individuals. METHODS: Nine female patients with FM, all employed in full or part time, performed repetitive isometric contractions of the quadriceps muscles at 30% of maximal voluntary contraction (MVC) force in 6 s with 4 s rest between until exhaustion. RESULTS: The MVC and electrically stimulated tetanic forces fell continually and almost in parallel, indicating no central fatigue. The relaxation rate of the muscle became gradually faster as previously seen in healthy subjects. Plasma catecholamine concentrations remained unchanged during the exercise period, whereas a modest rise in blood lactate and plasma K+ was seen. The electromyographic (EMG) amplitude recorded during contractions increased gradually, while no changes were seen in EMG recorded in resting periods between contractions. Heart rate and VO2 rose gradually and a small exercise induced pain was recorded. CONCLUSION: Our study showed normal physiological muscle fatigue responses, except for the lack of increase in plasma catecholamine concentrations during exercise.

Metabolic correlates of fatigue from different types of exercise in man.

It is well established that muscle fatigue, defined as a decline in maximal force generating capacity, is a common response to muscular activity. To what extent metabolic factors contribute to the reduced muscle function is still debated. Metabolic effects can affect muscle through different processes, either through a reduced ATP supply or by effects on EC-coupling or crossbridge dynamics. Observations from in vitro experiments are often extrapolated to interpret fatigue mechanisms from measurements performed in vivo, without recognizing that the biochemical reactions involved can be quite different depending on such factors as activation pattern, mode and duration of exercise. During repeated submaximal contractions, there is a negligible accumulation of H+ and inorganic phosphate, and hence fatigue must be ascribed to other factors. Substrate depletion might contribute to exhaustion, but cannot explain the gradual loss of maximal force. Curiously, the energetic cost of contraction increases progressively during repeated isometric but not during concentric contractions. With contractions involving high-force or high power output, fatigue is better related to H2PO4- than to pH, but still other factors seem to play a role.