Skeletal muscle fatigue and endurance in young and old men and women

The effects of increasing age on skeletal muscle fatigue and endurance were assessed in 22 healthy young (14 men and 8 women; mean age, 28 +/- 6 years) and 16 healthy old (8 men and 8 women; mean age, 73 +/- 3 years) individuals. All subjects performed 100 repeated maximum dynamic knee extensions at 90 degrees.s-1 (1.57 rad.s-1) using an isokinetic dynamometer (Cybex II). Peak torque was recorded during every contraction, and for each individual the maximal voluntary contraction (MVC), the fatigue rate, the endurance level, and the relative reduction in muscle force were determined. MVC and endurance level were significantly lower in old men and women, but there was no discernible difference in relative muscle force reduction and fatigue rate between young and old individuals. We conclude that thigh muscles of older individuals are weaker than those of younger individuals, but relative to their strength, older individuals have similar properties as younger individuals with respect to muscle fatigue and endurance.     

EMG power spectrum of respiratory and skeletal muscles during static contraction in healthy man

Changes in EMG power spectrum during isometric voluntary contraction maintained until exhaustion in the range of 20-80% MVC were studied in three skeletal muscles (adductor pollicis or AP, vastus lateralis, and medialis) and two respiratory muscles (diaphragm and rectus abdominis). Quantitative EMG analysis consisted of computation of the median frequency (MF) of power spectra and also the continuous measurement of EMG power in two bands of high (EH) and low (EL) frequencies using bandpass filters. This allowed the calculation of the H/L ratio and its time constant of decay rate (TC delta H/L) throughout the sustained static contraction. The main results were: (1) highly significant, positive correlations between TC delta H/L and the maximal MF changes and also the endurance time to fatigue; (2) EMG changes were determined early, within the first 10-20 s of contraction; and (3) EL always increased throughout the fatiguing isometric contraction, but EH changes markedly varied within the five muscle groups studied. These observations are discussed in terms of the differences in muscle fiber composition and also the variations in motor unit recruitment.     

Electromyographic and neuromuscular variables in post-polio subjects

OBJECTIVE: Post-polio subjects experience functional deterioration many years after developing acute poliomyelitis and have been shown previously to have a deficit in strength recovery after isometric activity. This study characterized the size and stability of the motor units in a group of post-polio subjects with macro and single fiber electromyography (EMG) and correlated these variables with isometric strength, endurance, "work capacity," and strength recovery after fatiguing isometric exercise. DESIGN: A cohort of 12 post-polio subjects was tested for neuromuscular function. Electromyographic variables were determined on a separate day. SETTING: Volunteers were recruited from the community and tested in our neuromuscular research laboratory. SUBJECTS: A volunteer sample was obtained from advertisements. All subjects acknowledged post-polio syndrome symptoms. MAIN OUTCOME MEASURES: Neuromuscular variables were isometric knee extension peak torque, endurance (time to exhaustion) at 40% of maximal torque, tension time index, and recovery of torque at 10 minutes. Electromyographic variables were macro EMG and single fiber EMG (percent blocking and jitter). RESULTS: Macro EMG amplitude was ninefold the control value, and both jitter and blocking were greatly increased in comparison to control values. Isometric strength significantly (p < .05) correlated negatively with macro EMG amplitude. CONCLUSIONS: The weakest subjects had the greatest number of muscle fibers within the motor unit (as measured by macro EMG amplitude). Jitter and blocking did not correlate with neuromuscular function.     

Quadriceps weakness in knee osteoarthritis: the effect on pain and disability

OBJECTIVES: (1) To determine the importance of quadriceps strength, structural change, and psychological status in terms of knee pain in the community. (2) To determine the relative importance of quadriceps function, structural change, and psychological status with respect to disability in subjects with knee pain. METHODS: 300 men and women with pain and 300 controls without pain (aged 40-79) were seen. Isometric quadriceps strength (MVC) was measured and muscle activation was assessed by twitch superimposition. Disability (Western Ontario McMaster Osteoarthritis Index (WOMAC)) and anxiety and depression were assessed (Hospital Anxiety and Depression Index (HAD)). Radiographs were obtained of the tibiofemoral and patellofemoral joints and total score for osteophyte, narrowing, and sclerosis calculated for each knee. RESULTS: Subjects with knee pain had lower voluntary quadriceps strength than those without pain (p < 0.005). Quadriceps activation was also lower (p < 0.005), but did not fully explain the reduction in strength. When analysed by multiple logistic regression: quadriceps strength (odds ratio 18.8, CI 4.8, 74.1 for MVC < or = 10 kgF); depression (odds ratio 2.4, CI 1.0, 5.5 for HAD score > or = 8); and radiographic change (odds ratio 4.1, CI 1.9, 8.6 for radiographic score > or = 4) were independently associated with pain. In those with knee pain, disability was independently associated with quadriceps strength (odds ratio 8.2, CI 1.5, 44.4 for MVC < or = 10 kgF) and depression (odds ratio 6.2, CI 2.1, 18.0 for HAD score > or = 8); but not with radiographic score. CONCLUSIONS: Quadriceps strength is strongly associated with knee pain and disability in the community, even when activation and psychological factors are taken into account. This has important therapeutic implications.     

Gender-related differences in left ventricular filling dynamics in older subjects after endurance exercise training

The present study was designed to determine if gender affects the adaptive response to endurance exercise training of left ventricular filling dynamics in older individuals. Recently, it was shown that gender influences the cardiovascular responses to endurance exercise training in older subjects. Older men improve left ventricular systolic performance and increase maximal cardiac output in response to endurance exercise training, whereas older women do not. Twelve men (65 +/- 1 years old; mean +/- SE) and 10 women (64 +/- 1) were studied before and after 9 months of endurance exercise training. Maximal O2 uptake was determined during treadmill exercise. Left ventricular filling dynamics and ejection fraction (EF) at rest and during supine exercise were assessed by Tc-99m radionuclide ventriculography. When expressed relative to body weight, maximal O2 uptake (VO2 max) was increased by 24% (27.3 +/- 1.5 to 34.0 +/- 1.5 ml/kg/min; p < .01) in men and 27% (21.9 +/- 1.0 to 27.8 +/- 1.0 ml/kg/min; p < .01) in women in response to endurance exercise training. In men, the time-to-peak filling rate (TPFR) decreased (-19.8 +/- 6.7 ms; p < .05) during exercise at a comparable heart rate in response to training. In contrast, the change in TPFR in women (+2.7 +/- 6.0 ms) was small and insignificant. Peak filling rate (PFR) at rest and during exercise was similar before and after training in men and women. The change in left ventricular systolic reserve at a comparable heart rate from pre-to posttraining improved in men (delta EF 4 +/- 3%; p < .05), but not in women (-2 +/- 3%). The results indicate that the adaptive response of left ventricular filling dynamics to endurance exercise training is influenced by gender in older subjects. Older men show improvement in left ventricular filling dynamics, whereas older women do not.     

Electrical stimulation of human tibialis anterior: (A) contractile properties are stable over a range of submaximal voltages; (B) high- and low-frequency fatigue are inducible and reliably assessable at submaximal voltages

OBJECTIVES: To investigate the validity and reliability of submaximal voltage stimulation for assessing the 'fresh' contractile properties of human tibialis anterior muscle (TA) and the efficacy of such stimulation in inducing and assessing high- and low-frequency fatigue. INTERVENTIONS: (A) Contractile properties of fresh TA were assessed in six normal volunteers using multifrequency stimulation trains (comprising 2 seconds at each of 10, 20 and 50 Hz, arranged contiguously) over a range of submaximal voltages. (B) On three separate occasions, fatigue was induced in the TA of 10 normal volunteers by means of a 3-minute unbroken sequence of the described multifrequency stimulation trains, delivered at a 'standardized' submaximal voltage. This fatiguing protocol was preceded by discrete multifrequency stimulation trains, at the same standardized voltage, but followed by discrete multifrequency trains delivered over a range of submaximal voltages (which included the standardized voltage). OUTCOME MEASURES: In experiment A the 10:50 Hz and 20:50 Hz force ratios were analysed for between-voltages variability using coefficients of variation (CVs), and for trends using Friedman tests and post-hoc Wilcoxon tests. In experiment B low-frequency fatigue was detected using 10:50 Hz and 20:50 Hz force ratios derived from the discrete multifrequency trains. High-frequency fatigue was calculated from the decline in high-frequency force which occurred during the fatiguing protocol itself. Each parameter was assessed for between-days repeatability using CVs. RESULTS: In experiment A the 'fresh' 10:50 Hz force ratio was clearly unreliable at voltages which generated <10% of maximal voluntary contractile force (MVC) (CV< or =29.7%), but was reasonably reliable at voltages which generated 20-30% of MVC (CV < or = 11.5%; p = 0.847). The 'fresh' 20:50 Hz force ratio was,in contrast, extremely reliable throughout the tested voltage range (CV< or =5.8%; p = 0.636) in fresh muscle. In experiment B paired t-tests indicated that the fatiguing protocol induced significant high-frequency fatigue (p <0.0037) and low-frequency fatigue (p <0.0008 for 'fresh' versus 'fatigued' 10:50 Hz force ratio; p <0.0001 for 'fresh' versus 'fatigued' 20:50 Hz force ratio). In muscle thus fatigued, the 20:50 Hz force ratio was extremely reliable in the 20-33% of MVC range (CV < or =7.3%; p = 0.847). Between-days repeatability was poor for the 10:50 Hz force ratio in both fresh and fatigued muscle (CV < or =23.8 and 44.4% respectively), but was highly acceptable for both voluntary and stimulated fatigue indices and for the 20:50 Hz force ratio, the latter in both fresh and fatigued muscle. CONCLUSIONS: These results confirm the validity and reliability of submaximal voltages in assessing contractile properties (including low-frequency fatiguability) and inducing fatigue of human TA.     

Quadriceps strength and fatigue assessed by magnetic stimulation of the femoral nerve in man

There is no nonvolitional method of assessing quadriceps strength which both supramaximally activates the muscle and is acceptable to subjects. In 10 normal subjects and 10 patients with suspected muscle weakness we used magnetic stimulation of the femoral nerve to elicit an isometric twitch and measured twitch tension (TwQ), surface electromyogram in addition to the maximum voluntary contraction force (MVC). Supramaximality was achieved in all subjects at a mean of 83% of maximum stimulator output. When supramaximal, TwQ was reproducible (mean coefficient of variation 3.6%, range 0.7-10.9) and correlated well with MVC (r2 = 0.83, P<0.001). In 7 normal subjects we measured TwQ before and after a fatiguing protocol; after 20 min TwQ was a mean of 55% (range 29-77%) of baseline and remained substantially reduced at 90 min. Magnetic femoral nerve stimulation is a painless, supramaximal method of assessing quadriceps strength and fatigue which is likely to be of value in clinical and physiological studies.     

Effects of fatigue and chewing training on maximal bite force and endurance

The purpose of the study was to evaluate the effects of chewing training on the strength and resistance to fatigue of the masticatory muscles. Twenty-five healthy adults were divided into an experimental group (7 men, 10 women) and a control group (4 men, 4 women). The experimental group chewed a special hard chewing gum one hour daily for 28 days. Maximal bite force and endurance were measured. The maximal bite force already showed a significant increase in the experimental group by the middle of the experimental period (p < 0.05), reached the highest values by the end of the training period (p < 0.001), and also remained at high levels 2 weeks after (p < 0.001). Subjects with weak initial maximal bite force values showed the highest increase in their strength after training (r = -0.66, p < 0.01). This type of training did not influence the endurance time during maximal clenching or the reduction in this endurance time after a dynamic fatigue test. No significant differences were found between the maximal bite force before and that after the fatigue test under either untrained or trained conditions. In the control group no significant differences were found between the recording sessions. In conclusion, 4 weeks training with a hard chewing gum seems to influence the functional capacity of the masticatory muscles and increase their strength.     

Motor unit recruitment during prolonged isometric contractions

Motor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P < 0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed "motor unit rotation" and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.     

The effect of strength and endurance training on gait, balance, fall risk, and health services use in community-living older adults

BACKGROUND: The study tested the effect of strength and endurance training on gait, balance, physical health status, fall risk, and health services use in older adults. METHODS: The study was a single-blinded, randomized controlled trial with intention-to-treat analysis. Adults (n = 105) age 68-85 with at least mild deficits in strength and balance were selected from a random sample of enrollees in a health maintenance organization. The intervention was supervised exercise (1-h sessions, three per week, for 24-26 weeks), followed by self-supervised exercise. Exercise groups included strength training using weight machines (n = 25), endurance training using bicycles (n = 25), and strength and endurance training (n = 25). Study outcomes included gait tests, balance tests, physical health status measures, self-reported falls (up to 25 months of follow-up), and inpatient and outpatient use and costs. RESULTS: There were no effects of exercise on gait, balance, or physical health status. Exercise had a protective effect on risk of falling (relative hazard = .53, 95% CI = .30-.91). Between 7 and 18 months after randomization, control subjects had more outpatient clinic visits (p < .06) and were more likely to sustain hospital costs over $5000 (p < .05). CONCLUSIONS: Exercise may have beneficial effects on fall rates and health care use in some subgroups of older adults. In community-living adults with mainly mild impairments in gait, balance, and physical health status, short-term exercise may not have a restorative effect on these impairments.     

Correlation between contractile strength and myosin heavy chain isoform composition in human skeletal muscle

PURPOSE: The purpose of this study was to investigate the relationship between myosin heavy chain (MHC) composition and maximal contraction strength of the human quadriceps femoris muscle. METHODS: Muscle biopsies were obtained from m. vastus lateralis in your highly physical active males (N = 7). The MHC composition of muscle homogenates was determined by electrophoresis techniques (SDS-PAGE). Isokinetic peak torque and constant-angle torque (50 degrees knee flexion) were obtained during slow (30 degrees.s-1), medium (120 degrees.s-1), and fast (240 degrees.s-1) maximal concentric and eccentric quadriceps contractions and expressed relative to muscle volume. RESULTS: The percentage of MHC II in the quadriceps muscle was positively correlated (rs = 0.61-0.93; P < 0.05-0.01) to maximal concentric quadriceps strength obtained at medium to high knee angular velocity. In contrast, no consistent pattern of correlation was observed for maximal eccentric quadriceps strength. CONCLUSIONS: The relationship observed between muscular MHC composition and maximal contractile strength is suggested to appear as a consequence of MHC -related differences in contractile force-velocity characteristics and/or contractile Rate of Force Development (RFD).     

Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function

The relation between muscle pain, muscle activity, and muscle co-ordination is still controversial. The present human study investigates the influence of experimental muscle pain on resting, static, and dynamic muscle activity. In the resting and static experiments, the electromyography (EMG) activity and the contraction force of m. tibialis anterior were assessed before and after injection of 0.5 ml hypertonic saline (5%) into the same muscle. In the dynamic experiment, injections of 0.5 ml hypertonic saline (5%) were performed into either m. tibialis anterior (TA) or m. gastrocnemius (GA) and the muscle activity and co-ordination were investigated during gait on a treadmill by EMG recordings from m. TA and m. GA. At rest no evidence of EMG hyperactivity was found during muscle pain. The maximal voluntary contraction (MVC) during muscle pain was significantly lower than the control condition (P < 0.05). During a static contraction at 80% of the pre-pain MVC muscle pain caused a significant reduction in endurance time (P < 0.043). During dynamic contractions, muscle pain resulted in a significant decrease of the EMG activity in the muscle, agonistic to the painful muscle (P < 0.05), and a significant increase of the EMG activity of the muscle, antagonistic to the painful muscle (P < 0.05). Muscle pain seems to cause a general protection of painful muscles during both static and dynamic contractions. The increased EMG activity of the muscle antagonistic to the painful muscle is probably a functional adaptation of muscle co-ordination in order to limit movements. Modulation of muscle activity by muscle pain could be controlled via inhibition of muscles agonistic to the movement and/or excitation of muscles antagonistic to the movement. The present results are in accordance with the pain-adaptation model (Lund, J.P., Stohler, C.S. and Widmer, C.G. In: H. Vaer?y and H. Merskey (Eds.), Progress in Fibromyalgia and Myofascial Pain. Elsevier, Amsterdam, 1993, pp. 311-327.) which predicts increased activity of antagonistic muscle and decreased activity of agonistic muscle during experimental and clinical muscle pain.     

Changes in torque and electromyographic activity of the quadriceps femoris muscles following isometric training

BACKGROUND AND PURPOSE: The purpose of this study was to examine the effect of isometric training of the quadriceps femoris muscles, at different joint angles, on torque production and electromyographic (EMG) activity. SUBJECTS: One hundred seven women were randomly assigned to one of four groups. Three groups trained with isometric contractions three times per week at a knee flexion angle of 30, 60, or 90 degrees. The fourth group, which served as a control, did not exercise. METHODS: Isometric torque was measured using a dynamometer, and EMG activity was measured using a multichannel EMG system. Measurements were obtained during maximal isometric contraction of the quadriceps femoris muscles at 15-degree increments from 15 to 105 degrees of knee flexion. Measurements were taken before and after 8 weeks of training. RESULTS: Following isometric exercise, increased torque and EMG activity occurred not only at the angle at which subjects exercised, but also at angles in the range of motion at which exercise did occur. Further analyses indicated that exercising in the lengthened position for the quadriceps femoris muscles (90 degrees of knee flexion) produced increased torque across all angles measured and appeared to be the more effective position for transferring strength and EMG activity to adjacent angles following isometric training as compared with the shorter positions of the muscle (30 degrees and 60 degrees of knee flexion). CONCLUSION AND DISCUSSION: These findings suggest that an efficient method for increasing isometric knee extension torque and EMG activity throughout the entire range of motion is to exercise with the quadriceps femoris muscles in the lengthened position.     

The relationship between physical restraint removal and falls and injuries among nursing home residents

BACKGROUND: A major reason cited for continued restraint use in American nursing homes is the widely held belief that restraint reduction will lead to fall-related incidents and injuries. METHODS: This study represents an analysis of data collected in a clinical trial of interventions aimed at reducing the use of restraints in nursing homes. Two different designs were employed to test the relationship between restraint reduction and falls/injuries. First, multiple logistic regression was used to compare fall/injury rates in subjects who had restraints removed (n = 38) to those who continued to be restrained (n = 88); second, survival analysis was employed to test the relationship between physical restraint removal and falls/injuries at the institutional level by comparing fall/injury rates among three nursing homes (n = 633) with varying rates of restraint reduction. RESULTS: Based on the multiple logistic regression analysis, there was no indication of increased risk of falls or injuries with restraint removal. Moreover, restraint removal significantly decreased the chance of minor injuries due to falls (adjusted odds ratio: 0.3, 95% CI: 0.1, 0.9; p < .05). The survival analysis demonstrated that the nursing home that had the least restraint reduction (11%) had a 50% higher rate of falls (p < .01) and more than twice the rate of fall-related minor injuries (p < .001) when compared to the homes with 23% and 56% restraint reduction, respectively. CONCLUSIONS: Physical restraint removal does not lead to increases in falls or subsequent fall-related injury in older nursing home residents.     

Target agreements between insurance carriers and hospitals as instrument for modifying hospital length of stay]

Serotonin has been implicated to play an important role in regulating emotions and behavior, and it is well accepted that the platelet serotonergic system mirrors the presynaptic central serotonergic system. Since prevalence of psychiatric problems increases with age and women are known to be more vulnerable than men, the present investigation was carried out to study the relationship between serotonin activity and age in women. Levels of serotonin (5-hydroxytryptamine, 5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in platelets and plasma in women (n = 49) aged 40-84 years (30 women aged 40-60 years and 19 women aged 61-84 years). There was a significant age difference between the two groups (mean: 47.6 +/- 5.91 years in the younger and 73.0 +/- 6.83 years in the older women, respectively, p < 0.00001). Platelet 5-HT as well as 5-HIAA levels were significantly higher in older women as compared to those in the younger women (89.41 +/- 21.95 ng/10(8) platelets in younger vs. 112.9 +/- 36.07 in older women, p < 0.02, and 1.20 +/- 1.10 in younger vs. 2.19 +/- 1.88 ng/10(8) platelets in older women, p < 0.05, respectively). Pearson correlation coefficients determined in the combined group (n = 49) showed a significant positive correlation between platelet 5-HT and age (r = 0.31, p < 0.03). Plasma 5-HT levels on the other hand were lower in older women compared to those in the younger women (4.50 +/- 3.20 in younger vs. 1.04 +/- 1.28 ng/ml plasma in older women, p < 0.0001) and a significant negative correlation was observed between plasma 5-HT and age (r = -0.44, p < 0.002). Plasma 5-HIAA concentration did not differ between the two groups. Platelet 5-HT levels in the younger group were independent of ethnicity. Since high serotonin activity has also been associated with psychiatric problems, our results of increased concentration of platelet 5-HT as well as 5-HIAA with age may have implications in predisposing aging women to behavioral/psychiatric problems.     

Anthropometric determination of thigh volumes and thigh forces following acute training of increasing intensity in adult men

The purpose of the present study was to investigate the influence of submaximal training of increasing intensity on the rate of fatigue and on the anthropometry of the quadriceps muscle. A group of 24 middle-aged male subjects trained three times a week for 12 weeks at incremental exercise intensities which elicited 66%-83% of the maximal heart rate reserve; 11 male subjects acted as controls. A purpose-built, isometric chair with an adjustable force transducer ensured a standard posture for each subject during a 20 s maximal voluntary contraction (MVC). The muscle plus bone mass of the thigh was calculated from a previously validated anthropometric method reinforced by measurements derived from water displacement and ultrasonics. Training-induced submaximal relationships were assessed by means of two ergometer tests of progressive intensity. Following training, the subjects' blood lactate concentrations had decreased from 5.5 (SD 1.7) to 3.9 (SD 1.4) mmol.1(-1) (P < 0.01) at the final exercise intensities. They increased thigh volumes from 10.7 (SD 1.7) to 11.0 (SD 1.7) 1 (P < 0.05) and thigh forces in 47.5% (P < 0.05) of the measurements. Heart rates and rates of perceived exertion were reduced (P < 0.01) following training; for the controls, all the above parameters remained constant. These results would seem to indicate that a submaximal type of training could enable a group of middle-aged men to increase the isometric forces of the thigh muscles by almost 50% when completing a MVC test following training. This suggests that this is a more suitable, safer form of maintaining and increasing maximal thigh forces for this, and older, age groups than specific isometric training alone.     

Interrater reliability of six tests of trunk muscle function and endurance

Some studies have shown a relationship between trunk muscle strength and low back pain. Measures of trunk muscle strength and endurance, which are feasible in the clinical setting, are needed. The purpose of this study was to determine interrater reliability of six tests of abdominal and trunk extensor muscle strength and endurance. The tests included abdominal and extensor dynamic endurance, hand-held dynamometry of isometric flexion and extension, and abdominal and extensor static endurance. Thirty-nine healthy workers were recruited as subjects. Each was tested by three raters on 3 days within 1 week. Intraclass correlation coefficients (ICC) and the standard error of measurement (SEM) were calculated: abdominal dynamic endurance ICC = .89, SEM = 8 repetitions; extensor dynamic endurance ICC = .78, SEM = 9 repetitions; abdominal isometric force ICC = .25, SEM = 60 N; extensor isometric force ICC = .24, SEM = 68 N; abdominal static endurance ICC = .51, SEM = 35 seconds; extensor static endurance ICC = .59, SEM = 20 seconds. The dynamic endurance tests had acceptable interrater reliability. For the others, reliability was poor and the SEMs were large.     

Skeletal muscle function and its relation to exercise tolerance in chronic heart failure

OBJECTIVES: This study sought to define the relation between muscle function and bulk in chronic heart failure (HF) and to explore the association between muscle function and bulk and exercise capacity. BACKGROUND: Skeletal muscle abnormalities have been postulated as determinants of exercise capacity in chronic HF. Previously, muscle function in chronic HF has been evaluated in relatively small numbers of patients and with variable results, with little account being taken of the effects of muscle wasting. METHODS: One hundred male patients with chronic HF and 31 healthy male control subjects were studied. They were matched for age (59.0 +/- 1.0 vs. 58.7 +/- 1.7 years [mean +/- SEM]) and body mass index (26.6 +/- 0.4 vs. 26.3 +/- 0.7 kg/m2). We assessed maximal treadmill oxygen consumption (VO2), quadriceps maximal isometric strength, fatigue (20-min protocol, expressed in baseline maximal strength) and computed tomographic cross-sectional area (CSA) at midthigh. RESULTS: Peak VO2 was lower in patients (18.0 +/- 0.6 vs. 33.3 +/- 1.4 ml/min per kg, p < 0.0001), although both groups achieved a similar respiratory exchange ratio at peak exercise (1.15 +/- 0.01 vs. 1.19 +/- 0.03, p = 0.13). Quadriceps (582 vs. 652 cm2, p < 0.05) and total leg muscle CSA (1,153 vs. 1,304 cm2, p < 0.005) were lower in patients with chronic HF. Patients were weaker than control subjects (357 +/- 12 vs. 434 +/- 18 N, p < 0.005) and also exhibited greater fatigue at 20 min (79.1% vs. 92.1% of baseline value, p < 0.0001). After correcting strength for quadriceps CSA, significant differences persisted (5.9 +/- 0.2 vs. 7.0 +/- 0.3 N/cm2, p < 0.005), indicating reduced strength per unit muscle. In patients, but not control subjects, muscle CSA significantly correlated with peak absolute VO2 (R = 0.66, p < 0.0001) and is an independent predictor of peak absolute VO2. CONCLUSIONS: Patients with chronic HF have reduced quadriceps maximal isometric strength. This weakness occurs as a result of both quantitative and qualitative abnormalities of the muscle. With increasing exercise limitation there is increasing muscle weakness. This progressive weakness occurs predominantly as a result of loss of quadriceps bulk. In patients, this muscular atrophy becomes a major determinant of exercise capacity.     

Muscle fiber conduction velocity (MFCV) after fatigue in elderly subjects

The changes in muscle fiber conduction velocity (MFCV) and median frequency (MDF) during and after muscle fatigue were investigated in 9 younger (mean age: 29.3) and 7 elderly (mean age: 72.0) healthy subjects to determine if age has any effect on them. The surface EMG of the abductor digiti minimi muscle was examined at 50% of maximal voluntary contraction (MVC) during a prefatigue session. The subjects were instructed to continue applying maximal force until the force declined to 50% MVC. EMG signals were measured during brief subsequent contractions at 50% MVC until 60 minutes after fatigue as a postfatigue session. The decrement in MFCV and MDF at 50% MVC before and after fatigue was 73.4%, 67.3% in the younger and 71.2%, 66.7% in the elderly subjects, respectively. MFCV and MDF recovered to initial value after fatigue more slowly in the elderly subjects (recovery time: elderly: MFCV 6.0 min., MDF 6.0 min.; younger: MFCV 2.77 min., MDF 3.00 min.) (P < 0.05, Mann-Whitney test). The over-shooting recovery phase was recognized in both age groups, but the elderly subjects had a smaller and shorter one. The slower recovery in the elderly suggested that they possibly had less potential to recovery the membrane potential propagation, metabolic capacity and more type 1 fiber composition. The smaller and shorter overshooting in the elderly might be due to less increase of muscle swelling and/or less membrane hyperpolarization.     

Electrophysiologic evaluation of denervated muscles in incomplete paraplegia using macro electromyography

OBJECTIVE: To evaluate denervated muscles in persons with incomplete paraplegia due to thoracolumbar spinal injury (TLSI) using macro electromyography in determining indications for functional electrical stimulation (FES). DESIGN: A randomized clinical trial and a criterion standard. SETTING: A department of orthopedic surgery in a university hospital. PATIENTS AND OTHER PARTICIPANTS: Eighteen patients with incomplete paraplegia, including 11 with TSLI, and 50 healthy adults. INTERVENTION: Area and amplitude of macro motor unit potential (macro MUP) were measured at the tibialis anterior, the vastus lateralis, and the vastus medialis. The normal limits of macro MUP parameters were defined based on values from healthy subjects. Abnormal denervated muscles were detected by macro EMG and conventional EMG in paralytic patients. The correlation between macro MUP parameter values and muscle forces of the tibialis anterior and quadriceps femoris induced by electrical stimulation was analyzed. MAIN OUTCOME MEASURES: The number of abnormal muscles, parameter values, and muscle force induced by electrical stimulation. RESULTS: Abnormal muscles were found only in the TLSI patients and 13 abnormal muscles were detected by macro EMG only. The abnormal muscles defined by macro EMG showed insufficient contraction induced by electrical stimulation. The increase of parameter value negatively correlated with the muscle force (tibialis anterior area r=-.797, amplitude r=-.866; quadriceps area r=-.866, amplitude r=-.893; p < .001). CONCLUSIONS: These results suggest that macro EMG is useful in detecting denervated muscles, in determining indications for FES, and in predicting FES effects before implantation of electrodes.