Force Steadiness Research Articles

Abnormalities in Motor Physiology in Bipolar Disorder

The primary purpose of this study was to examine motor physiology disturbances in a group of patients with bipolar disorder by using sensitive instrumental procedures. The secondary aim of the study was to examine the effects of the affective state on motor functions. The authors studied 67 individuals meeting DSM-IV criteria for bipolar disorder and 47 healthy comparison subjects. Electromechanical measures of force steadiness and scaling of movement velocity to distance were performed. The authors found that performance on the force steadiness and velocity scaling measures was significantly poorer in the subjects with bipolar disorder, with 58% and 63% of the patients scoring outside the normal range on the force steadiness and velocity scaling measures, respectively. Eighty-four percent of subjects showed impairment on either measure, and 37% showed impairment on both measures. The force steadiness and velocity scaling abnormalities appeared to occur independently of one another and also were not associated with affective state or current medication status. This study demonstrates that motor abnormalities suggestive of basal ganglia dysfunction occur in many patients with bipolar disorder.

Power output, isometric strength and steadiness in the leg muscles of pre- and postmenopausal women; the effects of hormone replacement therapy

There are conflicting reports of the effects of hormone replacement therapy (HRT) on strength preservation in postmenopausal women, while any effect on power output has received little attention. Decreased steadiness of force generation has been reported in older muscles and may be related to the hormonal changes associated with the menopause, but the effect of HRT has not been investigated. We have studied the effect of HRT on strength, power output and isometric force steadiness in healthy women. Sixteen young (aged 27.4 +/- 1.4 years, mean +/- SEM) and 29 postmenopausal women were studied. Fifteen of the latter were taking HRT (68.1 +/- 1.4 years, HRT+) and 14 (70.5 +/- 1.5 years, HRT-) had never done so. During isometric quadriceps contractions the force steadiness (coefficient of variation of force) was measured at 10, 25, 50 and 100% maximum voluntary contraction (MVC). The average power generated by an explosive leg extension was recorded. The HRT- group generated less power (110.2 +/- 7.2 W) than both the HRT+ (136.5 +/- 10.9 W, P = 0.027) and young (136.2 +/- 5.8 W, P = 0.027) subjects. Power output was similar in the HRT+ and younger subjects. The HRT- subjects were weaker than the younger ones (241.3 +/- 14.0 N vs. 297.6 +/- 13 N, P = 0.006). The strength of the HRT+ group (255.5 +/- 14 N) was not significantly different to the other two groups. There was no difference in steadiness between the three groups at any of the force levels. HRT appears to maintain power output to a greater extent than isometric strength in postmenopausal women. There was no evidence for an effect of either age or HRT on isometric steadiness in the quadriceps.

Force Steadiness in the Lower Extremities as an Independent Predictor of Functional Performance in Older Women

The relationship between isometric force control and functional performance is unknown. Submaximal steadiness and accuracy were measured during a constant force-matching task at 50% of maximal isometric voluntary contraction (MVC) of the knee extensors in 19 older women (70-89 years). Other variables included MVC, rate of torque development, and EMG activity. Functional performance was assessed during maximal performance of walking endurance, chair rising, and stair climbing. Isometric steadiness (but not accuracy) was found to independently predict chair-rise time and stair-climbing power and explained more variance in these tasks than any other variable. Walking endurance was related to muscle strength but not steadiness. These results suggest that steadiness is an independent predictor of brief, stressful functional-performance tasks in older women with mild functional impairment. Thus, improving steadiness might help reduce functional limitations or disability in older adults.

The relation between force magnitude, force steadiness, and muscle co-contraction in the thumb during precision grip

The aim of this study was to investigate the problem of agonist–antagonist co-contracti during a precision force task performed at different force levels. Using a precision grip, seven young adults performed a constant force matching task (10, 22.5, 35, 47.5, and 60% maximum) as accurately as possible (10 trials per force level). Muscle co-contraction in the thumb was monitored by the surface EMG activity of the extensor pollicis longus (EPL) and the flexor pollicis brevis (FPB), and the ratio between those EMG activities (EPL/FPB). Results showed that both EMG activities increased as grip force increased, but the EPL/FPB ratio decreased over the range of force investigated. Force steadiness (as expressed by the coefficient of variation, CV) appeared as a U-shape function of the force level (with maximal steadiness at 22.5%). Separate analyses at each force level showed no correlation between CV and EMG indices. In addition, the contrast between trials with high and low CV revealed no significant difference in terms of our EMG indices. We conclude that muscle co-contraction and grip force steadiness depend on grip force magnitude, but grip force steadiness does not depend on muscle co-contraction.

Isotonic Force Steadiness of the Leg Extensors is Dependent on Intensity and Contraction Type in Pre-Clinically Disabled Older Adults

Isotonic Force Steadiness of the Leg Extensors is Dependent on Intensity and Contraction Type in Pre-Clinically Disabled Older Adults

Isotonic Force Steadiness of the Leg Extensors is Dependent on Intensity and Contraction Type in Pre-Clinically Disabled Older Adults

0835 Various studies have indicated that apparently healthy older adults have reduced steadiness during eccentric contractions and at low intensities in the finger and elbow flexor muscles, with no difference in the knee extensors. Because of their difficulty performing tasks of everyday living, pre-clinically disabled older adults may show altered steadiness patterns compared to previous research in older adults. Purpose: The purpose of this study was to determine the effect of contraction type and intensity on isotonic and isometric steadiness in pre-clinically disabled older adults. Methods: Thirty older subjects (aged 74.7 ± 7.64 years) with difficulty performing either a chair rise or stair ascent/descent tasks were enrolled into the study. Following assessment of maximal voluntary contraction (MVC), subjects performed two concentric, eccentric and isometric contractions at three different intensities. During isotonic tasks subjects lifted and lowered a load at 0.23 rad/sec with 5%, 30% and 65% of MVC. During isometric tasks, subjects were asked to hold 10, 25 & 30% of MVC. Fluctuations around the average force level during isotonic tasks and coefficient of variation for force during isometric tasks were used for statistical analysis. Data was analyzed using a two-way (Factors: contraction type*intensity for isotonic contractions) and one-way (Factor: intensity for isometric contractions) repeated measures analysis of variance. Results: For isotonic contractions, there was no intensity*contraction type interaction (p = 0.88). However, subjects were significantly less steady during high intensity compared to low intensity contractions (5% = 0.08 ± 0.04, 30% = 0.15 ± 0.09, 65% = 0.22 ± 0.14; p 0.00). Additionally, the main effect for intensity showed that older adults were less steady during concentric (0.17 ± 0.12) compared to eccentric (0.13 ± 0.10) contractions. Subjects showed similar force fluctuations between each intensity during isometric contractions (10% = 1.5 ± 1.3, 25% = 1.3 ± 1.1, 50% = 1.2 ± 0.58; p = 0.23). Conclusion: These results suggest that pre-clinically disabled older adults show reduced steadiness at higher intensities and during concentric contractions.

Low- or high-intensity strength training partially restores impaired quadriceps force accuracy and steadiness in aged adults.

Because many daily tasks are executed at only a fraction of maximal strength, an understanding of submaximal force control may be important for improving function in aged adults. We compared the effects of low- and high-intensity (LI and HI, respectively) strength training on maximal and explosive strength and on the accuracy (force error) and steadiness (variability) of submaximal quadriceps force in elderly humans. Older subjects (age, 72 years; n = 27) had 57% lower maximal strength in comparison with young subjects (age, 21 years; n = 10). Older subjects had 190% (19 N), 50% (1 N), and 80% (4 N) more force error in matching 25 N of quadriceps force during eccentric, isometric, and concentric contractions, and had 157%, 0%, and 60% more variability in these forces compared with young subjects. Force error and force variability were correlated with each other but not with maximal strength. Thirty sessions of LI (n = 9 participants) or HI (n = 9 participants) training of equal total work increased maximal strength in the older subjects by 29%. Training also significantly reduced force error and variability--by 31% and 30%, respectively--of eccentric and concentric contractions. A control group of older subjects (n = 9) showed no significant changes in any variables. LI or HI strength training was equally effective in partially restoring elderly adults' maximal strength and control of submaximal force.

Motor-unit synchronization increases EMG amplitude and decreases force steadiness of simulated contractions.

The purpose of the study was to determine the effect of motor-unit synchronization on the surface electromyogram (EMG) and isometric force using a computer model of muscle contraction. The EMG and force were simulated by generating muscle fiber action potentials, defining motor-unit mechanical characteristics and territories, estimating motor-unit action potentials, specifying motor-unit discharge times, and imposing various levels of motor-unit synchronization. The output (EMG and force) was simulated at 11 levels of excitation, ranging from 5 to 100% of maximum. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; however, the number of motor units recruited and the average discharge rate of each unit was constant across synchronization conditions for a given level of excitation. Two levels of synchronization were imposed on the discharge times: a moderate and a high level, which approximated the experimentally observed range of motor-unit synchronization. The moderate level of synchrony caused the average EMG to increase by approximately 65%, whereas the high level caused a 130% increase in the EMG with respect to the no-synchrony condition. Neither synchrony condition influenced the magnitude of the average force. However, motor-unit synchronization did increase the amplitude of the fluctuations in the simulated force, especially at intermediate levels of excitation. In conclusion, motor-unit synchronization increased the amplitude of the average rectified EMG and decreased the steadiness of the force exerted by the muscle in simulated contractions.

Motor Unit Synchronization Increases EMG Amplitude and Force Fluctuations in Simulated Isometric Contractions.

1455 Experimental measurements have found significant temporal associations between the action potentials discharged by most active motor units. Because of technical limitations, however, these studies of short-term synchronization have generally been limited to low contraction intensities. The purpose of this study was to quantify the effects of motor unit synchronization on whole muscle EMG and force over the entire range of contraction intensities in simulated isometric contractions. The EMG and force were simulated by generating muscle fiber action potentials, defining motor unit mechanical characterisitcs and territories, estimating motor unit action potentials, specifying motor unit discharge times, and imposing various levels of motor unit synchronization. These parameters were based on experimental measurements made on the first dorsal interosseus muscle. The output (EMG and force) was simulated at 11 levels of excitation, ranging from 5% to 100% of maximum. To synchronize the motor unit activity, the timing of selected motor unit discharge times was adjusted; however, neither the number of motor units recruited nor the average discharge rate of the active motor units was altered for the two conditions (syncrhony and no synchrony) at each level of excitation. Two synchronization schemes were compared: random and weighted. The weighted scheme imposed greater synchronization among motor units with similar recruitment thresholds. The two schemes had similar effects on the simulated EMG and force. At each level of excitation, the imposed synchronization had marked effects on the EMG amplitude but not on the avearge force. However, the fluctuations in the force record were substantially greater in the presence of motor unit synchronization. These findings indicate that an increase in motor unit synchronization can produce an increase in EMG amplitude and a decline in the steadiness of force.

Isometric force production by motor units of extensor digitorum communis muscle in man

Isometric force production by motor units of extensor digitorum communis muscle in man

Portable clinical tracking-task instrument

It has been known that tracking tasks can be useful for detecting small but significant changes in neurological function. However the size, cost and complexity of the equipment have, to date, precluded their widespread use for evaluation of clinical trials. In development for nine years, a battery of tremour and tracking tasks has been designed and evaluated. It is portable, and easy to administer and score. It evaluates neurological functions associated with steadiness, reaction time, speed, and co-ordination of the upper extremities. Instrumentation includes a central electronics package with power supply and test timers, a digital data readout, a television display, a chart recorder, and position, force, and accelerometer transducers. Available tests include force steadiness, resting and sustention tremour, and random, sinusoid, step and critical tracking in pursuit and compensatory modes. Data analysis can be online or offline.