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PubMed1979

Influence of temperature on isometric contraction and passive muscular tension in paramyotonia congenita (Eulenburg).

Four patients without symptoms of episodic hyperkalemic weakness from two families with paramyotonia congenita (Eulenburg) are described. 1. Maximum voluntary muscle contraction of the upper and lower arm was studied under isometric conditions at different temperatures. If the temperature was lowered stepwise, distinct paresis occured at 32--31 degrees C which increased with the amount of muscular effort. The upper arm muscles, however, developed weakness gradually after cooling. 2. During cooling of the resting muscle, the EMG showed dense spontaneous activity of the fibrillary type, which decreased again at about 30 degrees C. It can be assumed that in paramyotonia congenita cooling produces muscle cell membrane depolarization which at a critical level causes the firing of action potentials and finally muscular paresis. 3. Increasing muscular stiffness can be interpreted as abnormally slow muscular relaxation after isometric contraction. In the forearm muscles the time to 3/4 relaxation after cooling was about six times normal, in the upper arm muscles only two times normal. As an additional parameter the mechanical resistance to passive stretching of a muscle has been studied. This passive muscular tension increased simultaneously with the onset of weakness. 4. The close relation between weakness and stiffness suggest that both symptoms are caused by the same basic defect which is probably located in the sarcolemma. It is suggested that a defect of the sodium channel causes a cooling-dependent increase in sodium conductance. Raised intracellular sodium causes in the first place membrane depolarization, and in the second place depression of calcium reuptake through competition by sodium for calcium binding sites. This would explain muscle stiffness and delayed relaxation as well.

Arm↗

PubMed2003

Voluntary drive-dependent changes in vastus lateralis motor unit firing rates during a sustained isometric contraction at 50% of maximum knee extension force.

The purpose of the present study was to relate the expected inter-subject variability in voluntary drive of the knee extensor muscles during a sustained isometric contraction to the changes in firing rates of single motor units. Voluntary activation, as established with superimposed electrical stimulation was high (range: 91-99%, n=8) during a short maximal contraction, but was lower (range: 69-100%) in most subjects at the point of force failure during a sustained (49.1+/-10.1 s) fatiguing contraction at 50% of maximum force. On a different experimental day the firing behaviour of 27 single motor units was recorded with wire electrodes in the vastus lateralis muscle, 24 of which could be monitored from the time of recruitment to the point of force failure (53.6+/-9.8 s). Motor unit firing behaviour differed considerably among subjects. During the second half of the sustained, fatiguing contraction the changes in firing rate firing rate variability of early recruited units ranged from -10% to +100% and from -50% to +160% respectively among subjects. There were significant positive linear relations between voluntary activation, on the one hand, and rectified surface electromyogram (rsEMG, r=0.82), the changes in motor unit firing rate ( r=0.49) and firing rate variability ( r=0.50) towards the point of force failure on the other. The present data suggest that differences in voluntary drive that appear among subjects during fatigue may be an important determinant of motor unit firing behaviour.

Action Potentials↗

PubMed1994

Electromyographic median frequency changes during isometric contraction of the back extensors to fatigue.

STUDY DESIGN: This was a cross-sectional study involving 229 healthy, back pain-free individuals. OBJECTIVES: To examine the relationship between electromyographic manifestations of fatigue and endurance time during isometric contraction of the back extensors to fatigue. SUMMARY OF BACKGROUND DATA: Despite the wide-spread use of electromyography to monitor muscle fatigue, its relationship with endurance time has not been well investigated. METHODS: Using skin-surface electrodes, electromyographic signals were recorded from thoracic (T10) and lumbar (L3) regions of erector spinae during an isometric endurance test, and the rate of change in median frequency of the electromyographic power spectrum (MFGRAD) was calculated. RESULTS: MFGRAD was significantly higher at L3 than at T10. The best predictor of endurance time was given by the greater MFGRAD observed at either region. MFGRAD calculated over a submaximal time period (50% total time or 60 sec) also correlated significantly with endurance time. Women displayed a significantly longer endurance time and lower MFGRAD than men. CONCLUSIONS: Endurance appears to be limited by the most fatigable region of the muscle group. MFGRAD is a suitable technique for monitoring back muscle fatigue, even when it is determined over a submaximal time period. The back extensors of women are less fatigable than those of men when the same task is performed.

Adult↗

PubMed2001

Influence of joint position on electromyographic and torque generation during maximal voluntary isometric contractions of the hamstrings and gluteus maximus muscles.

STUDY DESIGN: Repeated measures analysis of joint angle effects on hip and knee muscle electromyographic (EMG) activity. OBJECTIVES: To simultaneously determine angle-dependent changes in maximal voluntary isometric contraction (MVIC) torque and EMG activity during hip extension and knee flexion. BACKGROUND: Procedures for normalizing EMG data and for determining torque-angle relationships for various joint motions both entail asking subjects to exert an MVIC. The implicit assumption in these paradigms is that magnitude of the EMG response is at a constant, maximum level so that observed angle-dependent variations in torque are due to mechanical factors, such as muscle length and muscle moment arm. METHODS AND MEASURES: Fifty subjects (25 men and 25 women) participated in this study (age, 23.5 +/- 4.6 y; range, 18-38 y). Subjects performed maximal isometric knee flexion at 4 knee angles and maximal isometric hip extension at 4 hip angles. The dependent variables were normalized root-mean-square EMG and torque. The process for normalizing EMG and torque data consisted of determining the largest mean value for each subject across testing positions for the muscle of interest. That value was designated as corresponding to 100% MVIC, and all other data for that muscle were expressed as a percentage of the MVIC value. Repeated measures was used to determine angle-dependent changes in normalized MVIC-torque and MVIC-EMG values for each muscle group. RESULTS: Mean torque-angle relationships were generally consistent with previous reports, though considerable intersubject variability was observed. There were significant angle-dependent differences in maximal EMG for both the hamstring and gluteus maximus muscles. Mean percentages of hamstring MVIC-EMG at knee angles of 30 degrees (81 +/- 19) and 60 degrees (82 +/- 22) were greater than at 0 degrees (68 +/- 20) or 90 degrees (74 +/- 20). The mean percentage of gluteus maximus MVIC-EMG at a hip angle of 0 degrees (94 +/- 10) was greater than at 30 degrees (84 +/- 13), 60 degrees (80 +/- 14), or 90 degrees (64 +/- 20), and gluteus maximus maximal voluntary isometric EMG at 90 degrees was less than at all other angles. These differences could not be explained solely by muscle length-dependent effects on EMG amplitude, suggesting that despite instructions for maximal effort, motor unit activation was not maintained at a constant, maximal level throughout the range of motion. The form of the EMG/angle relationships differed markedly from the torque-angle relationships. CONCLUSIONS: These findings have implications for the use of MVIC-EMG for reference values in EMG normalization procedures and for the interpretation of mechanisms underlying the torque-angle relationships observed in vivo.

Adult↗

PubMed2004

Reliability of EMG measurements for trunk muscles during maximal and sub-maximal voluntary isometric contractions in healthy controls and CLBP patients.

The purpose of this study was to compare the reliability of trunk muscle activity measured by means of surface electromyography (EMG) during maximal and sub-maximal voluntary isometric contractions (MVC/sub-MVC) over repeated trials within-day and between-days in healthy controls and patients with chronic low back pain (CLBP). Eleven volunteers (six controls and five CLBP patients) were assessed twice with a 1-week interval. Surface EMG signals were recorded bilaterally from six trunk muscles. Intra-class correlation coefficients (ICC) and standard error of measurement as a percentage of the grand mean (%SEM) were calculated. MVC and sub-MVC showed excellent within-day reliability in both healthy controls and CLBP patients (ICC mean 0.91; range 0.75-0.98; %SEM mean 4%; range 1-12%). Sub-MVC for both groups between-days showed excellent reliability (ICC mean 0.88; range 0.78-0.97; %SEM mean 7%; range 3-11%). The between-days MVC for both groups showed trends towards lower levels of reliability (ICC mean 0.70; range 0.19-0.99; %SEM mean 17%; range 4-36%) when compared to sub-MVC. Findings of the study provide evidence that sub-MVC are preferable for amplitude normalisation when assessing EMG signals of trunk muscles between-days.

Abdomen↗

PubMed1994

Recovery of conduction velocity of muscle fiber action potential after strenuous isometric contraction.

The change of conduction velocity of muscle fiber action potentials of individual motor units after loading of 50% maximal voluntary contraction was examined in biceps brachii muscles of twenty healthy male subjects. A surface electrode array, consisting of fifteen electrodes spaced at 5 mm intervals, was used to detect EMG signals during minimal isometric contraction which is adjusted so as to generate action potentials in a single motor unit. Conduction velocity was measured from the recorded signals of 14 channels. Mean conduction velocity of all subjects was 3.33 +/- 0.28 m/s (n = 20). A significant decrease of the conduction velocity was observed after contraction maintaining 50% of maximal voluntary contraction torque as long as possible. From the results of conduction velocity decrease during 6 min minimal contraction following the contraction at 50% of maximal strength, subjects were divided into two groups, having endurance times exceeding 50 s and less than 50 s, respectively. The former showed significant recovery during minimal contraction after loading, while the latter showed no apparent recovery. It was demonstrated that the conduction velocity of muscle action potentials obtained with minimal contraction decreased after strenuous contraction and the extent of its recovery was related to the endurance time of contraction which the subjects could maintain 50% of maximal strength.

Action Potentials↗

PubMed1991

Motor units of the fourth deep lumbrical muscle of the adult rat: isometric contractions and fibre type compositions.

1. Isometric twitch and tetanic tensions were recorded from whole muscles and single motor units in fourth deep lumbrical muscles isolated from young adult (60 days) rats. Muscles were superfused with oxygenated Ringer solution at 25 degrees C except where stated otherwise. 2. It was confirmed that the muscle is supplied most commonly by eleven motor axons, nine via the lateral plantar nerve (LPN), and two via the sural nerve (SN). Motor units whose axons were isolated from either LPN or SN were studied. There was no difference in mean motor unit size. 3. In their unfused tetani most units showed 'sag' and some 'no sag', with no segregation between LPN and SN. 'No sag' units were always small (unit tetanic tension less than 8% whole-muscle tetanic tension), tended to be relatively slowly contracting and relaxing during an isometric twitch, and tended to have relatively low twitch:tetanus ratios. Units showing sag ranged from large to small. 4. In some motor units muscle fibres were depleted of their glycogen by repetitive stimulation at 30 degrees C in glucose-free Ringer solution, and the muscle and its unstimulated control frozen and sectioned. Neighbouring sections were stained for glycogen and for binding of two myosin-specific antibodies, one specific for slow myosin and the other for type IIA myosin. Myosin ATPase and succinic dehydrogenase histochemistry were also carried out in some muscles. 5. Serial reconstructions showed that all or virtually all extrafusal fibres in the muscle were present in a midbelly section, and that the myosin type of individual fibres did not change significantly along their length. Spindle profiles were seen frequently and in two muscles eight and twelve spindles were identified. 6. Of twenty-six motor units examined twenty contained almost exclusively muscle fibres of the recently described type IIX. All these units showed sag in their isometric tetani. 7. Six units each contained 50% or more of slow myosin-containing fibres (IIC and a few type I). The remaining fibres in these units were IIA. All these units were therefore of mixed fibre composition, and are discussed as IIC/IIA units. In whole muscles slow-myosin-containing fibres were generally distributed evenly (non-randomly) throughout the muscle cross-section. 8. Whole muscles contained on average 970 fibres (S.D. +/- 70) of which 82 (+/- 9) were slow-myosin-containing. A few muscles from older rats (3-24 months) contained very few such fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed2001

Sex differences in the fatigability of arm muscles depends on absolute force during isometric contractions.

Women are capable of longer endurance times compared with men for contractions performed at low to moderate intensities. The purpose of the study was 1) to determine the relation between the absolute target force and endurance time for a submaximal isometric contraction and 2) to compare the pressor response and muscle activation patterns of men [26.3 +/- 1.1 (SE) yr] and women (27.5 +/- 2.3 yr) during a fatiguing contraction performed with the elbow flexor muscles. Maximal voluntary contraction (MVC) force was greater for men (393 +/- 23 vs. 177 +/- 7 N), which meant that the average target force (20% of MVC) was greater for men (79.7 +/- 6.5 vs. 36.7 +/- 2.0 N). The endurance time for the fatiguing contractions was 118% longer for women (1,806 +/- 239 vs. 829 +/- 94 s). The average of the rectified electromyogram (%MVC) for the elbow flexor muscles at exhaustion was similar for men (31 +/- 2%) and women (30 +/- 2%). In contrast, the heart rate and mean arterial pressure (MAP) were less at exhaustion for women (94 +/- 6 vs. 111 +/- 7 beats/min and 121 +/- 5 vs. 150 +/- 6 mmHg, respectively). The target force and change in MAP during the fatiguing contraction were exponentially related to endurance time (r(2) = 0.68 and r(2) = 0.64, respectively), whereas the change in MAP was linearly related to target force (r(2) = 0.51). The difference in fatigability of men and women when performing a submaximal contraction was related to the absolute contraction intensity and was limited by mechanisms that were distal to the activation of muscle.

Adult↗

PubMed1988

Estimation of blood flow by 133Xe clearance in human masseter muscle during rest, endurance of isometric contraction, and recovery.

Sixty-four measurements of 133Xe clearance from the masseters of 9 individuals, who had no facial or oral pain, were used to evaluate changes in intramuscular blood flow (IMBF) caused by endurance of isometric contraction at approx. 50 per cent of maximum voluntary EMG activity. On average, the IMBF during endurance increased by 4.2 times in 6 individuals and decreased 62 per cent of the rest flow in 3 individuals. Overall, the group average of endurance flow was 2.5 times higher than the initial rest flow. The group average of post-endurance hyperaemia was 27 times higher than the initial rest flow and 11 times higher than the endurance flow. IMBF during the final rest was 1.7 times higher than the initial rest flow. The relative IMBF during endurance was negatively correlated to muscle effort, and the post-endurance flow in relation to the endurance flow presented a compensatory positive correlation to the muscle effort. Although the average IMBF tended to increase during endurance, it was insufficient to meet the demand of the much higher relative increase in muscle activity, as indicated by the pronounced post-endurance hyperaemia, especially after long endurance.

Adult↗

PubMed2003

Chronic flexibility gains: effect of isometric contraction duration during proprioceptive neuromuscular facilitation stretching techniques.

The aim of this study was to assess the effect of to isometic contraction durations during propiceptive neuromuscular facilitation stretching on gains inflexion at the hip. Forty-three women (M age = 20.0 years, SD = 1.3) were assigned to one of three groups: 5-s isometric contraction (5-IC), 10-IC, and control. Flexibility was assessed at baseline and Weeks 3 and 6. Analysis of covariance, controlling for pretest differences, showed a significant interaction, F(2, 33) = 44.1, p < .001. Flexibility was significant lower in the control group relative to the 5-IC and 10-IC groups and in the 5-IC group relative to the 10-IC group at 3 and 6 weeks (3 weeks = 101.2 +/- 1.4 degrees, 114.3 +/- 1.5 degrees, 120.5 +/- 1.3 degrees; 6 weeks = 103.0 +/- 1.4 degrees, 126.1 +/- 1.6 degrees, 133.3 +/- 1.4 degrees for control, 5-IC and 10-IC groups, respectively). A longer contraction time led to greater increases in flexibility.

Adult↗

PubMed1993

Bilateral deficit is larger for step than for ramp isometric contractions.

The force produced by a muscle group during a maximum effort bilateral task has been reported to be less than that produced during a maximum effort unilateral task. It was hypothesized that if decreased activation of primarily fast motor units is responsible for this bilateral deficit, 1) the bilateral deficit would be larger in step than in ramp isometric contractions and 2) the rate of torque generation in step contractions would be slower in bilateral than in unilateral tasks. Twelve healthy male subjects performed unilateral and bilateral isometric knee joint extension tasks in which maximum torque was generated under ramp and step conditions. The maximum torques produced in the bilateral tasks were 17.0 and 24.6% less than those produced in the unilateral tasks during the ramp and step conditions, respectively (both P < 0.001). The bilateral torque deficit for the step condition was significantly greater than that for the ramp condition (P = 0.012). The rate of torque generation in the step condition was 19.7% slower in the bilateral than in the unilateral task (P = 0.010). Thus the results from these torque measurements support the notion that decreased activation of primarily fast motor units may be responsible for the bilateral deficit.

Adult↗

PubMed1991

Changes produced by increased hydrostatic pressure in isometric contractions of rat fast muscle.

1. Muscle fibre bundles isolated from the extensor digitorum longus (a fast muscle) of the rat were exposed to different hydrostatic pressures (range 0.1-10 MPa), in order to determine the pressure dependence of their isometric contractions. 2. The pressure dependent changes in the contractions were reversible and linearly related to pressure. 3. The peak tension, the time to peak and the time to half-relaxation of a twitch contraction increased with pressure; the mean (+/- S.E.M.) percentage increases were 5.9 +/- 0.5% MPa-1, 2.7 +/- 0.2% MPa-1 and 2.7 +/- 0.4% MPa-1, respectively. 4. In a fused tetanus, the tension was typically depressed at high pressure (0.9 +/- 0.16% MPa-1); the half-time of tension rise was decreased (2.1 +/- 0.2% MPa-1) and the half-time of exponential relaxation was increased (2.4 +/- 0.3% MPa-1).

Animals↗

PubMed2004

Long-lasting body leanings following neck muscle isometric contractions.

Our objective was to investigate the neural mechanisms and the functional relevance of motor effects that develop involuntarily following the release of a sustained isometric muscle contraction. The few data available in the literature deal only with post-contractions occurring in a body segment. Although these data emphasise the role of proprioceptive input, the question as to whether this phenomenon is of central or peripheral origin remains unclear. Given the leading role of neck muscle proprioceptive input in body orientation and posture regulation, we designed two experiments to test for postural posteffects after voluntary and involuntary neck muscle contraction. The spatiotemporal characteristics of the posteffects were analysed by means of stabilometric recordings following 30-s isometric contraction of splenius, trapezius and levator muscle groups, and 30-s electrically-induced contraction of the levator muscle group. Results show that a postural response occurred after voluntary contraction of each muscle group tested, which was oriented in the plane of action of the muscle, and lasted 14 min at least. In contrast, no clearly oriented body leanings were found after electrical stimulation of the levator muscle, except for a slight increase in natural postural instability. Data are interpreted as a change in the postural reference resulting from an increase in proprioceptive inflow accompanying mainly the voluntary muscle contraction.

Adult↗

PubMed1995

Effects of temperature on motor unit action potentials during isometric contraction.

The effect of temperature on motor unit action potential (MUAP) configuration and recruitment was studied using automatic decomposition electromyography (ADEMG) recordings from a concentric needle electrode placed in the first dorsal interosseous (FDI) muscle of 10 normal adult subjects during isometric contraction. Focally cooling the FDI resulted in prolonged MUAP duration (P < 0.001, ANOVA), a finding congruent with those of Buchthal. Focal ulnar cooling at the elbow resulted in the increased MUAP frequency. In contrast to previous studies, there were no significant differences in amplitude or turns. Greater understanding of normal motor unit electrophysiology is necessary to improve diagnostic accuracy of EMG testing.

Adult↗

PubMed1995

Mechanical properties of aponeurosis and tendon of the cat soleus muscle during whole-muscle isometric contractions.

Recent studies have suggested that the mechanical properties of aponeurosis are not similar to the properties of external tendon. In the present study, the lengths of aponeurosis, tendon, and muscle fascicles were recorded individually, using piezoelectric crystals attached to the surface of each structure during isometric contractions in the cat soleus muscle. We used a surgical microscope to observe the surface of the aponeurosis, which revealed a confounding effect on measures of aponeurosis length due to sliding of a thin layer of epimysium over the proximal aponeurosis. After correcting for this artifact, the stiffness computed for aponeurosis was similar to tendon, with both increasing from around 8 F0/Lc (F0 is maximum isometric force and Lc is tissue length) at 0.1 F0 to 30 F0/Lc at forces greater than 0.4 F0. At low force levels only (0.1 F0), aponeurotic stiffness increased somewhat as fascicle length increased. There was a gradient in the thickness of the aponeurosis along its length: its thickness was minimal at the proximal end and maximal at the distal end, where it converged to form the external tendon. This gradient in thickness appeared to match the gradient in tension transmitted along this structure. We conclude that the specific mechanical properties of aponeurosis are similar to those of tendon.

Animals↗

PubMed1998

Metabolic and myoelectrical effects of acute hypoxaemia during isometric contraction of forearm muscles in humans: a combined 31P-magnetic resonance spectroscopy-surface electromyogram (MRS-SEMG) study.

1. Failure of muscle force during sustained fatiguing contraction is associated with myoelectrical and metabolic alterations. However, the inter-relationships between these two types of events remain unclear. The purpose of this study was to examine the effects of decreased oxygen availability during sustained contraction on myoelectrical and metabolic changes, thereby addressing the issue of fatigue. 2. 31P-Magnetic resonance spectra and surface electromyograms were simultaneously recorded in six subjects (three women and three men) performing isometric contraction of forearm flexor muscles sustained at 60% maximum value of force under aerobic or acute hypoxaemic conditions (inhalation of a gas mixture containing 12% O2). 3. The 5 min hypoxaemic rest preceding contraction did not affect the phosphocreatine level and pH value. Under both conditions of oxygen availability, the magnitude of metabolic changes remained similar and the duration of contraction was unaffected (similar workload). However, hypoxaemia significantly reduced the rate of changes in integrated surface electromyogram activity measured in the high-frequency band. Correlative analysis of magnetic resonance spectroscopy and surface electromyogram data shows that for a given surface electromyogram change, metabolic variations were always larger under hypoxaemic conditions. 4. These results suggest that hypoxaemia does not alter metabolic changes, i.e. decrease in pH and phosphocreatine during static contraction. The downward shift of the relationships between myoelectrical and metabolic changes under hypoxaemia points to the existence of a better excitation-contraction coupling in acute hypoxaemia compared with normoxia and this is indicative of an adaptative mechanism.

Acute Disease↗

PubMed2000

Effects of aging on EMG variables during fatiguing isometric contractions.

The purpose of this study was to evaluate the neuromuscular adaptation that occurred with aging, by comparing young and aged subjects with respect to changes in surface EMG from the tibialis anterior muscle during fatiguing contractions. EMG variables such as the averaged rectified value (ARV), median frequency (MDF), and muscle fiber conduction velocity (MFCV) were calculated during maximal (MVC, 3 sec) and submaximal (60% MVC, 60 sec) isometric contractions. Muscular force, ARV, MDF, and MFCV during MVC were significantly greater in the young than in the elderly (p < 0.05). EMG amplitude increased and the waveform slowed in all subjects during submaximal contractions, indicating the development of local muscle fatigue. As fatigue progressed, the ARV increased and the MDF and MFCV decreased significantly (p < 0.01). The fatigue-induced changes in the MDF and MFCV were significantly smaller in aged than in young subjects (p < 0.05), a trend also seen in the ARV change, which means that the elderly cannot be fatigued as much as the young with contractions of the same relative intensity. These results as a whole suggest that the aged subjects hold an adaptive motor strategy to cope with age-related neuromuscular deteriorations, due to the decline of motor unit activation and selective atrophy of fast twitch muscle fibers.

Adult↗

PubMedDate not supplied

Repeatability study of mechanomyography in submaximal isometric contractions using coefficient of variation and intraclass correlation coefficient.

The within-day and between-day repeatability of the mechanomyogram (MMG) was assessed using the coefficient of variation (CV) and the intraclass correlation coefficient (ICC) and was compared with that of the electromyogram (EMG). The MMG and EMG were recorded simultaneously during isometric elbow flexion trials at different submaximal levels of 10% to 90% MVC. The testing session consisting of 9 submaximal trials was repeated 8 times on the same day for estimation of the within-day variation. In order to examine the between-day variation, the same testing session was also performed 8 times over 3 weeks with a 2-day rest interval between each session. The CVs within-day and between-day in both the MMG and EMG did not demonstrate any significant differences relating to the magnitude of force exerted. The CVs combined over all the force levels were approximately 10% within the same day and 25% between days for both the MMG and EMG. These corresponded to the within-day ICC of approximately 0.95 and the between-day ICC of 0.80. The repeatability of the MMG during submaximal isometric contractions of biceps brachii muscles is considered to be similar to that of the more established EMG.

Adult↗