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.

Task-dependent effect of limb immobilization on the fatigability of the elbow flexor muscles in humans.

Because short-term limb immobilization produces selective adaptations in the neuromuscular system that probably interact with the task-dependent expression of muscle fatigue, the purpose of this study was to determine the effects of limb immobilization on the ability of human subjects to sustain isometric contractions at low and moderate submaximal forces. Four weeks of elbow joint immobilization caused a substantial decrease in the daily activity of biceps brachii during immobilization, a significant reduction in the cross-sectional area and volume of the elbow flexor muscles as measured by magnetic resonance imaging, and a decline in the maximum voluntary contraction (MVC) activation and force of the elbow flexor muscles. Immobilization had a task-dependent effect on muscle fatigue with a substantially increased endurance time (reduced fatigability) at a low force (20% MVC) and no statistical effect at a moderate force (65% MVC).

Neural adaptations with chronic physical activity.

Chronic activity patterns, such as strength training, limb immobilization, and aging, produce marked adaptations in both the muscular and nervous systems. In this brief review, some of the involved mechanisms are examined as they are revealed through studies on the maximality, specificity, and pattern of the neural drive to muscle. The studies on maximality indicate that it is difficult to activate maximally a muscle by voluntary command, the capacity varies across muscles, tasks, and training, and the maximum discharge rates of motor neurons decreases with immobilization and increases with strength training.

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.

The influence of age on the assessment of motor unit activation in a human hand muscle.

Two indices of motor unit recruitment, the ramp-force and repetitive-discharge thresholds, were compared in the first dorsal interosseus muscle of ten young and twelve elderly subjects. The purpose was to determine the effect of age on the relationship between the two recruitment thresholds and the spike-triggered average force of motor units. Each subject performed three tasks requiring isometric abduction of the left index finger: a maximum voluntary contraction (MVC), a ramp-and-hold contraction, and a repetitive-discharge task.

Morphological features and activation patterns of motor units.

The force that a muscle exerts depends on which motor units have been recruited and at what rate they are discharging action potentials. Because of differences in motor-unit morphology (innervation ratio, average cross-sectional area, specific force, and geometric distribution of muscle fibers), the maximum motor-unit force within a motor-neuron pool is not constant but rather can vary by approximately 50 times. Consequently, muscle force is affected by which motor units have been activated.

Mechanisms of muscle fatigue: Central factors and task dependency.

Muscle fatigue can be caused by a number of different mechanisms that result in an acute reduction in the ability to perform a motor task. These mechanisms include the physiological processes that range from the motivation associated with performing the task through to the force exerted by the contractile proteins once they are activated. Two issues are examined in this brief review.

Limitations of the surface electromyography technique for estimating motor unit synchronization.

Motor unit synchronization was estimated from the surface electromyograms (EMG) of the first dorsal interosseus muscle of human volunteers by a simplified surface-EMG technique (Milner-Brown et al. 1973, 1975). Single motor units were identified from intramuscular recordings and were used to obtain a spike-triggered average of the surface-EMG.

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.

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.

Neurobiology of muscle fatigue. Advances and issues.

Throughout this epilogue, we have emphasized that rapid advances in understanding of neural and muscular aspects of fatigue have occurred since the 1980 London Symposium. However, in each instance of progress, from the single muscle fiber to the forebrain, the application of more precise techniques have raised important new questions. Neuroscientists and muscle physiologists have expanded opportunities for rigorous study of a topic of major scientific and social importance.

The scientific contributions of Brenda Bigland-Ritchie.

Brenda Bigland-Ritchie has made seminal contributions to our understanding of skeletal muscle physiology--the energy cost of muscle when it shortens or is forcibly stretched, the relationship between EMG and force, the behavior of single motor units, and above all, the processes underlying neuromuscular fatigue. More than this, she has stimulated inquiry into the search for reflex mechanisms which may serve to balance the activity of the spinal cord with that of the fatiguing muscles. Her use of human volunteers for much of this work is extraordinary, and represents a major strength.

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.

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.

Association between muscle architecture and quadriceps femoris H-reflex.

The purpose of the present study was to establish the relationship between muscle architecture and H-reflex recordings in quadriceps femoris muscle. H-reflexes were elicited in human quadriceps femoris muscle over a broad area of skin to document the shape and amplitude of the H-potentials. This, in combination with recording monopolar and bipolar H-potentials, was performed to determine the location and method for measuring maximum-amplitude H-reflexes.

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.

Motor unit behavior in Parkinson's disease.

This article reviews the literature related to motor unit behavior in Parkinson's disease (PD). The focus is on bradykinesia, or slowed movement. There is sparse literature on muscular performance in PD, as PD is regarded as a disease of higher motor centers.

Reduced control of motor output in a human hand muscle of elderly subjects during submaximal contractions.

1. The effect of age on the motor output of the first dorsal interosseous muscle of 22 (6 female, 16 male) human subjects was investigated. The purpose of the study was to determine the effect of age on the control of muscle force and the associated changes in the discharge behavior and mechanical properties of single motor units.

Impairment of neuromuscular propagation during human fatiguing contractions at submaximal forces.

1. The purpose of the study was to examine the dependence of neuromuscular propagation impairment on the level of isometric force sustained to the endurance limit. The task involved human volunteers sustaining a submaximal abduction force with the index finger by activating the first dorsal interosseous muscle as long as possible.

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.

Fatigue-related changes in motor unit action potentials of adult cats.

The purpose of this study was to quantify the changes in motor-unit action potentials (MUAP) and force during a standard motor-unit fatigue test. MUAP waveforms were characterized by the measurement of amplitude, duration, area, and shape (as reflected in a coefficient of proportionality). Fatigue-resistant motor units exhibited small, but statistically significant, changes in MUAP amplitude and area during the fatigue test, whereas fatigable motor units displayed variable changes in MUAP amplitude, duration, and area.

Estimating the strength of common input to human motoneurons from the cross-correlogram.

1. The relationship between the motor unit discharge pattern (rate and variability) and synchronization of motor unit pairs was studied in the first dorsal interosseus muscle of human subjects. In separate trials of up to 4 min duration, subjects voluntarily controlled the mean discharge rate of an identified motor unit at one of several prescribed rates (range 7.