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.

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.

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.

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.

[Can processes connected to muscular activity explain development of chronic pain?]. / Kan prosesser knyttet til muskeltretthet forklare utvikling av kronisk smerte?

The pathogenetic mechanisms of chronic muscle pain are not known, but may be related to incomplete recovery between bouts of work. During monotonous repetitive static muscle contraction, fatigue develops gradually, parallel to an increase in oxidative metabolism combined with a change in Ca(2+)-homeostasis in a few muscle fibres. Ischemia or high lactate probably do not contribute to fatigue in this state. We review some recent results on muscle fatigue during a low force static work situation and also discuss the significance of K+ released from muscle as an important extracellular messenger.