A unique form of light-load training improves steadiness and performance on some functional tasks in older adults.

Beginning Movement Load (BML) training is a unique form of light-load training that comprises a lengthening-shortening sequence of muscle actions about multiple degrees of freedom. The purpose of the study was to determine the effectiveness of BML training at improving the performance of old adults on four functional tasks and to identify some of the neuromuscular adaptations that contributed to these gains. Healthy old adults (67.5 ± 5.23 years) were randomly assigned to either a BML training group (n = 17) or a control group (n = 7). The training group exercised with a 30% of the one repetition-maximum (1-RM) load and performed five to seven sets of 15 repetitions, three times per week for 8 weeks. BML training increased maximal voluntary contraction (MVC) force significantly for the knee extensors (31.6%), but not the elbow flexors (9.8%), and improved the steadiness of isometric contractions (10%, 30%, and 65% MVC forces). Training-associated changes in times for ascending and descending stairs and one-legged balance, but not the chair rise, were predicted by changes in selected combinations of MVC force and steadiness. The attributes of BML training that enabled it to elicit functionally meaningful adaptations in the neuromuscular system of older adults should be explored with more mechanistic studies.

Contralateral Effects of Unilateral Strength and Skill Training: Modified Delphi Consensus to Establish Key Aspects of Cross-Education.

BACKGROUND: Cross-education refers to increased motor output (i.e., force generation, skill) of the opposite, untrained limb following a period of unilateral exercise training. Despite extensive research, several aspects of the transfer phenomenon remain controversial. METHODS: A modified two-round Delphi online survey was conducted among international experts to reach consensus on terminology, methodology, mechanisms of action, and translational potential of cross-education, and to provide a framework for future research. RESULTS: Through purposive sampling of the literature, we identified 56 noted experts in the field, of whom 32 completed the survey, and reached consensus (75% threshold) on 17 out of 27 items. CONCLUSION: Our consensus-based recommendations for future studies are that (1) the term 'cross-education' should be adopted to refer to the transfer phenomenon, also specifying if transfer of strength or skill is meant; (2) functional magnetic resonance imaging, short-interval intracortical inhibition and interhemispheric inhibition appear to be promising tools to study the mechanisms of transfer; (3) strategies which maximize cross-education, such as high-intensity training, eccentric contractions, and mirror illusion, seem worth being included in the intervention plan; (4) study protocols should be designed to include at least 13-18 sessions or 4-6 weeks to produce functionally meaningful transfer of strength, and (5) cross-education could be considered as an adjuvant treatment particularly for unilateral orthopedic conditions and sports injuries. Additionally, a clear gap in views emerged between the research field and the purely clinical field. The present consensus statement clarifies relevant aspects of cross-education including neurophysiological, neuroanatomical, and methodological characteristics of the transfer phenomenon, and provides guidance on how to improve the quality and usability of future cross-education studies.

Tutorial: Analysis of motor unit discharge characteristics from high-density surface EMG signals.

Recent work demonstrated that it is possible to identify motor unit discharge times from high-density surface EMG (HDEMG) decomposition. Since then, the number of studies that use HDEMG decomposition for motor unit investigations has increased considerably. Although HDEMG decomposition is a semi-automatic process, the analysis and interpretation of the motor unit pulse trains requires a thorough inspection of the output of the decomposition result. Here, we report guidelines to perform an accurate extraction of motor unit discharge times and interpretation of the signals. This tutorial includes a discussion of the differences between the extraction of global EMG signal features versus the identification of motor unit activity for physiological investigations followed by a comprehensive guide on how to acquire, inspect, and decompose HDEMG signals, and robust extraction of motor unit discharge characteristics.

Eccentric contractions require unique activation strategies by the nervous system.

Eccentric contractions occur when activated muscles are forcibly lengthened. This mode of muscle function occurs frequently in the activities of daily living and in athletic competition. This review examines the experimental evidence that provides the foundation for our current understanding of the benefits, consequences, and control of eccentric contractions. Over the past several decades, numerous studies have established that eccentric contractions can maximize the force exerted and the work performed by muscle; that they are associated with a greater mechanical efficiency; that they can attenuate the mechanical effects of impact forces; and that they enhance the tissue damage associated with exercise. More recent evidence adds a new feature to this repertoire by suggesting a new hypothesis: that the neural commands controlling eccentric contractions are unique. Examination of this hypothesis is critical because the existence of such a control scheme would increase substantially the complexity of the strategies that the nervous system must use to control movement.

Maximum bilateral contractions are modified by neurally mediated interlimb effects.

The force exerted by a single limb during a maximal bilateral contraction has been found to be less than the force associated with a maximal unilateral contraction. The purpose of this study was to determine whether this bilateral deficit is due to neural mechanisms. For one experiment, three groups of subjects (untrained, cyclists, and weight lifters) performed maximal one- or two-limb isometric tasks for which the two-limb combinations were either both legs or the left arm and the right leg. The untrained subjects exhibited a bilateral deficit, the cyclists did not, and the weight lifters produced a bilateral facilitation. Although the changes in electromyogram did not completely parallel the changes in force, variability in the filtered electromyogram associated with the maximal contraction was too great for reliable interpretation. The arm-leg task demonstrated that the bilateral deficit affects only homologous contralateral muscles. For the second experiment, two groups of subjects (bilateral deficit and bilateral facilitation) performed maximal left leg contractions while the right leg either rested or was activated with electrical stimulation. All subjects produced an increase in the maximal voluntary left leg force during right leg electromyostimulation. The magnitude of the increase was greatest for the bilateral facilitation subjects. These results suggest that interlimb interactions during maximal bilateral contractions are mediated by neural mechanisms.

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.

Sympathetic activation is associated with increases in EMG during fatiguing exercise.

The purpose of this study was to test the hypothesis that efferent sympathetic neural discharge is coupled with the development of muscle fatigue during voluntary exercise in humans. In 12 healthy subjects (aged 20-34 yr) we measured heart rate (HR), arterial blood pressure (AP), and noncontracting, skeletal muscle sympathetic nerve activity (MSNA) in the leg (peroneal nerve) before (control) and during each of three trials of submaximal (30% of maximum) isometric handgrip exercise performed to exhaustion. In six of the subjects of eletromyographic (EMG) activity of the exercising forearm was also measured. HR and AP increased significantly (P less than 0.05) in the 1st min of exercise in all trials. In contrast, neither MSNA nor EMG activity increased significantly above control during the 1st min of exercise, but both parameters subsequently increased in a progressive and parallel manner (P less than 0.05). The overall correlation coefficient between MSNA and EMG activity (144 observations) was 0.85 (P less than 0.001). With successive trials the magnitudes of the increases in HR, AP, MSNA, and EMG activity were greater at any absolute point in time during exercise. These results indicate that sympathetic activation to noncontracting skeletal muscle is directly related to the development of muscle fatigue (as assessed by the change in EMG) during prolonged isometric exercise in humans. Furthermore, our findings demonstrate that previous fatiguing contractions alter the time course of the sympathetic neural adjustments to exercise.

Neurobiology of muscle fatigue.

Muscle fatigue encompasses a class of acute effects that impair motor performance. The mechanisms that can produce fatigue involve all elements of the motor system, from a failure of the formulation of the descending drive provided by suprasegmental centers to a reduction in the activity of the contractile proteins. We propose four themes that provide a basis for the systematic evaluation of the neural and neuromuscular fatigue mechanisms: 1) task dependency to identify the conditions that activate the various mechanisms; 2) force-fatigability relationship to explore the interaction between the mechanisms that results in a hyperbolic relationship between force and endurance time; 3) muscle wisdom to examine the association among a concurrent decline in force, relaxation rate, and motor neuron discharge that results in an optimization of force; and 4) sense of effort to determine the role of effort in the impairment of performance. On the basis of this perspective with an emphasis on neural mechanisms, we suggest a number of experiments to advance our understanding of the neurobiology of muscle fatigue.

Short-term immobilization has a minimal effect on the strength and fatigability of a human hand muscle.

The purpose of this study was to determine the association between reduced fatigability typically observed in disused muscle and an improved resistance to the impairment of neuromuscular propagation. Endurance time of an isometric contraction sustained at 35% of maximum voluntary contraction (MVC) force and the fatigue-induced change in the evoked compound muscle action potential (M wave) were measured in the first dorsal interosseus muscle of human subjects before, during, and after 3 (n = 9) or 5 wk (n = 2) of immobilization. The immobilization procedure caused a substantial decline in the chronic electromyographic (EMG) activity (to 4% of control value) of the first dorsal interosseus muscle. Endurance time was found to be significantly correlated to the maintenance of M-wave amplitude during the fatigue task. However, neither of these variables was significantly affected by immobilization. Also, immobilization had no significant effect on the prefatigue values of MVC force and EMG or twitch contraction time or on the postfatigue changes in MVC force and EMG, M wave duration, twitch amplitude, and contraction time. In the unfatigued muscle, immobilization did cause an increase in twitch force (153%) and a decrease in M-wave amplitude (67%). It appears, therefore, that a healthy first dorsal interosseus muscle is generally resistant to adaptation when its use has been reduced for 3-5 wk by immobilization.

Activation of the arousal response can impair performance on a simple motor task.

The purpose of this study was to determine the effect of arousal in men and women on the moment-to-moment performance of a simple motor task. We examined the control of a precision task in the presence and absence of imposed stressors. Twenty-nine subjects (14 men, 15 women; 18--44 yr) were randomly assigned to either a control group or one of two stressor groups, Mental Math or Electric Shock. Subjects presented with Math and Shock stressors, which lasted 10 min, experienced significant increases in cognitive and physiological arousal compared with baseline and control subjects. Heart rate, systolic blood pressure, and electrodermal activity were elevated 5--80% with presentation of the stressors, whereas diastolic blood pressure and salivary cortisol were unchanged. The greater levels of cognitive and physiological arousal were associated with reductions in steadiness of a pinch grip for the Shock subjects (approximately 130% reduction from baseline) but not for the subjects in the Math group, who experienced heightened arousal but no change in steadiness (10% reduction from baseline). Although women exhibited more of a reduction in steadiness than men, the effect was largely unrelated to the magnitude of the change in arousal.

Older adults can maximally activate the biceps brachii muscle by voluntary command.

Because some of the decline in strength with age may be explained by an impairment of muscle activation, the purpose of this study was to determine the activation level achieved in biceps brachii by older adults during a maximum voluntary contraction (MVC). This capability was assessed with two superimposition techniques: one calculated the activation level that was achieved during an MVC, and the other provided an estimate of the expected MVC force based on extrapolation with submaximal forces. The activation level in biceps brachii was incomplete (< 100%) for the young (n = 16) and elderly (n = 16) subjects, with the elderly subjects exhibiting the greater deficit. In contrast, there was no difference between the measured and expected MVC forces for either group of subjects, whether the extrapolation involved a third-order polynomial or linearization of the data. Because of the lower signal-to-noise ratio associated with the measurement of activation level and the greater number of measurements that contributed to the estimate of the expected MVC force, we conclude that the older adults were able to achieve complete activation of the biceps brachii muscle during an MVC.

Training-related enhancement in the control of motor output in elderly humans.

The increase in motor unit force that occurs with aging has been hypothesized to cause a decline in the ability to maintain a constant submaximal force. To test this hypothesis, young and elderly subjects performed a 12-wk strength-training program that was intended to increase motor unit force. The training program caused similar increases (%initial) in the training load (137.4 +/- 17.2%), twitch force (23.1 +/- 7.4%), and maximum voluntary contraction force (39.2 +/- 6.8%) of the first dorsal interosseus muscle for the young and elderly subjects. The increase in strength was associated with a modest increase in muscle volume (7% of initial value) and a nonmonotonic increase in the surface-recorded electromyogram that was significant at week 8 but not at week 12. The elderly subjects reduced the variability in force at the lower target forces (2.5, 5.0, and 20.0% maximum voluntary contraction force). This improvement, however, was unrelated to changes in the distribution of motor unit forces, which was not consistent with the hypothesis that the greater coefficient of variation for the force fluctuations is due to increased motor unit forces.

Activation of the arousal response and impairment of performance increase with anxiety and stressor intensity.

The purpose of this study was to determine the effect of trait anxiety and stressor intensity on arousal and motor performance during a pinch task. We examined the steadiness of a precision task in the presence and absence of an imposed stressor on subjects with moderate and low trait anxiety. Subjects with the 26 highest and 14 lowest anxiety scores were assigned to one of three groups: a control group (5 women, 5 men), a moderate-anxiety group (8 women, 8 men), or a low-anxiety group (7 women, 7 men). Subjects in the anxiety groups received electric shocks and experienced significant increases in cognitive and physiological arousal compared with baseline and control subjects, especially subjects in the moderate-anxiety group. Heart rate, systolic blood pressure, and electrodermal activity were elevated during the stressor, whereas diastolic blood pressure was unchanged. Cognitive and physiological arousal tended to increase with stressor intensity and was accompanied by changes in steadiness. Although steadiness was markedly reduced with the highest intensity of shock, the average electromyogram activity was unaffected by the stressor. These findings indicate that the increase in arousal and the impairment of steadiness increased with trait anxiety and with the intensity of the noxious stimulus.

Long-term activity in upper- and lower-limb muscles of humans.

Despite limited data on humans, previous studies suggest that there is an association between the duration of daily muscle activity and the proportion of type I muscle fibers. We quantified the activity of limb muscles in healthy men and women during normal use and compared these measurements with published reports on fiber-type proportions. Seven men (age range = 21-28 yr) and seven women (age range = 18-26 yr) participated in two 10-h recording sessions. Electromyogram (EMG) activity of four muscles in nondominant upper (first dorsal interosseus and biceps brachii) and lower limbs (vastus medialis and vastus lateralis) was recorded with surface electrodes. Hand and arm muscles were active for 18% of the recording time, whereas leg muscles were active for only 10% of the recording time. On average, upper-limb muscles were activated 67% more often than lower-limb muscles. When lower-limb muscles were activated, however, the mean amplitude of each burst was greater in leg muscles [18 and 17% maximum voluntary contraction (MVC)] compared with hand (8% MVC) and arm (6% MVC) muscles. Temporal association in activity between pairs of muscles was high for the two lower-limb muscles (r2 = 0.7) and relatively weak for the two upper-limb muscles (r2 = 0.09). Long-term muscle activity was only different between men and women for the biceps brachii muscle. We found no relation between duration of muscle activity in 10-h recordings and the reported values of type I fibers in men and women.

Coactivation of the antagonist muscle does not covary with steadiness in old adults.

The purpose of the study was to determine the association between steadiness and activation of the agonist and antagonist muscles during isometric and anisometric contractions. Young (n = 14) and old (n = 15) adults used the first dorsal interosseus muscle to perform constant-force and constant-load tasks (2.5, 5, 20, 50, and 75% maximum) with the left index finger. Steadiness was quantified as the coefficient of variation of force and the SD of acceleration normalized to the load lifted. The old adults were less steady at most target forces with isometric contractions (2.5, 5, and 50%) and with most loads during the anisometric contractions (2.5, 5, and 20%). Furthermore, the old adults were less steady when performing lengthening contractions (up to 50%) compared with shortening contractions, whereas there was no difference for young adults. The reduced steadiness exhibited by the old adults during these tasks was not associated with differences in the average level of agonist muscle electromyogram or with coactivation of the antagonist muscle.

Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction.

The activity of 50 single motor units was recorded in the biceps brachii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on motor unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval on the discharge rates of single motor units during a fatiguing contraction. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contraction, whereas the discharge rates of most newly recruited units were either constant or increased slightly. The absolute threshold forces of recruitment and derecruitment decreased, and the variability of interimpulse intervals increased after the fatigue task. The change in motor unit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be predicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units declined despite an increase in the excitatory drive to the motoneuron pool during the fatigue task.

Reduced motor unit activation of muscle spindles and tendon organs in the immobilized cat hindlimb.

Six weeks of limb immobilization of a healthy muscle (cat tibialis posterior) at a short length resulted in a significant reduction of mean fiber area for all fiber types (I, 71% of control; IIa, 77% of control; IIb, 79% of control), whereas fiber type proportions were unchanged. For motor units, there was a reduction in peak tetanic force (type slow > fast fatigue resistant > fast fatigable); an increase in the twitch-to-tetanus ratio for fast fatigue-resistant and slow units; and no effect on the twitch force, twitch time course, or fatigability. The reduction in peak force was greater than expected because of fiber atrophy in slow units. Immobilization had a minimal effect on muscle spindle afferent (Ia and spindle group II) responses to a ramp-and-hold stretch of the passive muscle. Tendon organ (Ib) afferents had an increased responsiveness to stretch after immobilization but only when the muscle was stretched from a short resting length. However, immobilization reduced the modulation of muscle afferent discharge in response to tetanic contractions of single motor units. The decline in responsiveness of spindles was a result of the reduced tetanic force of motor units. In contrast, tendon organs in immobilized muscle were twice as likely to convey no information on the contraction of a single motor unit and were more likely to be unloaded, suggesting that immobilization caused the functional denervation of some muscle fibers. Thus the responses of muscle spindles and tendon organs in immobilized muscle reflected atrophic changes in extrafusal fibers but did not provide evidence for substantial disturbance of receptor function.

Strength training improves the steadiness of slow lengthening contractions performed by old adults.

When old adults participate in a strength-training program with heavy loads, they experience an increase in muscle strength and an improvement in the steadiness of submaximal isometric contractions. The purpose of this study was to determine the effect of light- and heavy-load strength training on the ability of old adults to perform steady submaximal isometric and anisometric contractions. Thirty-two old adults (60-91 yr) participated in a 4-wk training program of a hand muscle. Both the light- and heavy-load groups increased one-repetition maximum and maximal voluntary contraction (MVC) strength and experienced similar improvements in the steadiness of the isometric and shortening and lengthening contractions. The increase in MVC strength was greater for the heavy-load group and could not be explained by changes in muscle activation. Before training, the lengthening contractions were less steady than the shortening contractions with the lightest loads (10% MVC). After training, there was no difference in steadiness between the shortening and lengthening contractions, except with the lightest load. These improvements were associated with a reduced level of muscle activation, especially during the lengthening contractions.

Coexistence of twitch potentiation and tetanic force decline in rat hindlimb muscle.

An experimental protocol designed to assess fatigability in motor units has been applied to two hindlimb muscles of anesthetized adult rats to study the effects of whole-muscle fatigue on the isometric twitch. Both soleus and extensor digitorum longus exhibited a linear relationship between fatigability (i.e., force decline after a 360-s fatigue test) and the magnitude of the twitch force following the fatigue test. Twitch force after the fatigue test was potentiated (i.e., greater than the value before the fatigue test) in many muscles, despite the development of considerable fatigue. This coexistence of fatigue and twitch potentiation was observed in 7% (5/70) of soleus and 48% (31/64) of extensor digitorum longus muscles. The coexistence was exhibited only by the least fatigable muscles of the fast-contracting extensor digitorum longus. The extensor digitorum longus muscles that did not exhibit twitch potentiation probably experienced a higher proportion of muscle-fiber inactivation, such as due to failure of neuromuscular propagation, that was induced by the fatigue regimen.

Sensitivity of muscle proton spin-spin relaxation time as an index of muscle activation.

The purpose of this study was to determine the minimum number of contractions that are needed to detect an increase in the muscle proton spin-spin relaxation time (T2) at a given exercise intensity. Five healthy human subjects performed five sets of an exercise that included concentric and eccentric contractions of the elbow-flexor muscles with loads that were 25 or 80% of maximum. With the 80% load, the five sets involved 1, 2, 5, 10, or 20 repetitions of the exercise; with the 25% load the five sets were 2, 5, 10, 20, or 40 repetitions. The upper arm of each subject was imaged before and immediately after each set of the exercise. Spin-echo images (repetition time/echo time = 2,000 ms/30, 60, 90, and 120 ms) were collected using an extremity coil, and T2 values were calculated. The signal intensity was measured from the elbow-flexor and -extensor muscles and from the bone marrow of the humerus. With the 80% load, T2 increased in the short head of the biceps brachii after two repetitions of the elbow exercise and after five repetitions in the brachialis and the long head of the biceps brachii. With the 25% load, T2 became longer after five repetitions of the exercise for the short head of the biceps brachii and after 10 repetitions for the brachialis and the long head of the biceps brachii. T2 varied linearly with the number of contraction repetitions for each of the elbow-flexor muscles at either load (r2 > or = 0.97, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)