Gary Kamen

Neural adaptations to resistive exercise: mechanisms and recommendations for training practices.

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation.An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated.There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation.Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force.Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the integrity of the joint. Thus far, it is not clear what the CNS will optimise: force production or joint integrity.The following recommendations are made by the authors based on the existing literature. Motor learning theory and imagined contractions should be incorporated into strength-training practice. Static contractions at greater muscle lengths will transfer across more joint angles. Submaximal eccentric contractions should be used when there are issues of muscle pain, detraining or limb immobilisation. The reversal of antagonists (antagonist-to-agonist) proprioceptive neuromuscular facilitation contraction pattern would be useful to increase the rate of tension development in older adults, thus serving as an important prophylactic in preventing falls. When evaluating the neural changes induced by strength training using EMG recording, antagonist EMG activity should always be measured and evaluated.

PHYSIOLOGY AND INTERPRETATION OF THE ELECTROMYOGRAM

The purpose of this review is to consider some issues in the interpretation of the electromyogram (EMG) and to discuss current areas of controversy regarding use of the EMG. We consider the underlying physiology and origin of the EMG signal and offer an abbreviated discussion of measurement issues and selected factors that affect the characteristics of the EMG signal. We discuss many of the problems affecting interpretation, including normalization, crosstalk, and issues specific to contraction. In the final section, we consider topics of current interest in electromyography, such as muscle fatigue, task specificity, multichannel representations, and muscle fiber conduction velocity. We present, in addition, alternative analysis techniques. This review should interest researchers and clinicians who seek to obtain the valuable information inherent in the EMG while respecting the potential sources of variance and misinterpretation.

Adaptations in maximal motor unit discharge rate to strength training in young and older adults

Six young (mean = 23 years) and 6 older (mean = 76 years) adults participated in isometric resistance training 5 days/week for 6 weeks. The task involved isometric fifth finger abduction. Maximal motor unit discharge rates (MUDRs) were obtained from the abductor digiti minimi of each hand at 0, 2, 14, and 42 days of training using a quadrifilar needle electrode and automatic spike recognition software. In agreement with previous findings, maximal MUDR at baseline was significantly lower in older adults (P < 0.001), averaging 51.5 (±17.13) HZ in young and 43.3 (±14.88) HZ in older adults. In response to resistance training, maximal voluntary force increased 25% in young and 33% in older subjects (P < 0.001). Maximal MUDR increased significantly (11% young, 23% older) on day 2 [F(3,36) = 2.58, P < 0.05], but in older subjects returned to baseline levels thereafter. These adaptations in abductor digiti minimi MUDR suggest a two‐part response to strengthening fifth finger abduction: early disinhibition followed by altered MU activation.© 2001 John Wiley & Sons, Inc. Muscle Nerve 24:542–550, 2001

Adaptations in motor unit discharge activity with force control training in young and older human adults

Abstract Six young (aged 18–22 years) and six older (aged 66–76 years) healthy humans participated in a visually guided isometric force modulation training program designed to improve accurate control of force during ankle dorsiflexion. Isometric force and the discharge activity of motor units (MU) supplying the tibialis anterior muscle were sampled concurrently at the beginning of the study, following 2 weeks of force modulation training and again after a 4 week retention period which followed immediately. The initial maximal voluntary force (MVC) and MU discharge rates were similar between young and older adults at 10–60% MVC while MU discharge rates during maximal effort were significantly reduced in older adults. Following the 2 weeks of force modulation training, both young and older adults demonstrated significant improvements in force accuracy (44% young, 48% older) and significantly reduced MU discharge rates at 30%, 40%, and 60% MVC. Young adults also demonstrated increased MVC force (11%), while older adults demonstrated significantly increased (30%) maximal MU discharge rate. Thus, following 2 weeks of force modulation training, young and older individuals demonstrated similar MU discharge rates at all force levels. The MU discharge rate adaptations were retained after the 4 week retention period. In young adults, improved force accuracy and increased MVC force were accompanied by significantly reduced MU recruitment thresholds. In the older subjects, improved force accuracy was accompanied by an increase in the difference between the recruitment-derecruitment force threshold and significantly reduced antagonist co-contraction. Age-related alterations in force regulation and MU discharge activity cannot be explained solely on the basis of contractile changes in senescent muscle. Rather, reliance on compensatory neuromuscular changes including antagonist muscle co-contraction is suggested.

Adaptations in muscular activation of the knee extensor muscles with strength training in young and older adults

The purpose of this study was to compare the extent of muscular activation during maximal voluntary knee extension contractions in old and young individuals and to examine the effects of resistance training on muscular activation in each group. The interpolated twitch technique was used to estimate muscular activation during two pre-training baseline tests, and after two and six weeks of resistance training. Throughout the study, the older group was 30% less strong than the young group (p=0.02). The training protocol was effective in both groups with overall isometric strength gains of 30 and 36% in the older (p=0.01) and young (p<0.01) groups, respectively. 10-RM training loads increased by 66% in the old group (p<0.01) and by 77% in the young group (p<0.01) throughout training. At the first baseline test, a 2% difference in muscular activation between groups (p=0.3) did not explain the large disparity in strength. Muscular activation increased by 2% in both groups throughout training (p<0.01). Despite considerably less muscular strength in the older group, muscular activation was greater than 95% of maximum and appears to be equal in both young and older individuals. Both groups demonstrated similar but small increases in muscular activation throughout training.

Detecting Balance Deficits in Frequent Fallers Using Clinical and Quantitative Evaluation Tools

OBJECTIVE: This investigation sought to determine whether older idiopathic frequent fallers could be distinguished from healthy older adults on the basis of balance and movement coordination tests. A secondary objective was to determine the relationships among clinical balance test scores, balance performance data obtained by accelerometry, and quantitative motor coordination tests.

An Accelerometry-Based System for the Assessment of Balance and Postural Sway

The aim of this investigation was to develop an inexpensive, efficient system for the clinical assessment of static and dynamic balance and postural sway using accelerometry-based measurements. Subjects consisted of 10 young (range 18–32 years) and 10 older (range 69–86 years) individuals screened for polypharmacy and history of cardiovascular, neurological or orthopedic health conditions. A lightweight uniaxial accelerometer and general-purpose microcomputer were used to obtain measurements of postural sway. Customized software was written to acquire the data and provide a real-time display consisting of amplitude and frequency characteristics of the sway profile. Intraclass reliability coefficients greater than R = 0.75 were obtained in both eyes-open and challenging-standing balance tasks. Preliminary results demonstrate that the instrumentation can be used to discriminate among balance tasks and to differentiate healthy older adults from those with a tendency toward frequent falls. Moreover, the technique described yields a simple-to-administer, inexpensive procedure that can be conducted in the home or another natural environment. Accelerometry also allows for balance training and re-learning, using tasks that might ordinarily pose a balance challenge for the older or frail adult.

Adverse events associated with eccentric exercise protocols: six case studies

PURPOSE Rhabdomyolysis is a condition characterized by muscle damage and degeneration of muscle cells after strenuous, overexertion exercise. Although the incidence of severe rhabdomyolysis is rare, this condition can be dangerous and even fatal. Eccentric exercise protocols are currently being used to induce and study mild forms of muscle damage. However, serious adverse events can occur in these laboratory investigations. The purpose of this report was to expose some of the adverse events resulting from performance of eccentric exercise protocols to study muscle damage in humans. METHODS The following case studies involved an eccentric exercise protocol where two sets of 25 maximal eccentric actions of the elbow flexors were performed, separated by a 5-min rest period. RESULTS Case reports are presented that reveal prolonged losses in the ability of the muscle to generate force lasting 43-47 d, extreme swelling of the exercised arm lasting several weeks, and greatly elevated serum creatine kinase levels. CONCLUSIONS Although adverse events resulting from eccentric exercise are rare, our laboratory has observed a 3% incidence rate during the past year. Investigators should be knowledgeable of the sequelae of events that are associated with muscle damage after high-force eccentric exercise and take appropriate precautions.

Age-dependent effects of muscle vibration and the Jendrassik maneuver on the patellar tendon reflex response☆

OBJECTIVE To explore possible effects of aging on the excitability of spinal reflexes. DESIGN Using a cross-sectional design, the influences of muscle vibration and the Jendrassik maneuver on patellar tendon reflex function were compared between 30 young adults and 15 older adults. SETTING Motor control research laboratory. SUBJECTS The young adults were volunteers of college age. The older adults (74.5 +/- 4.14 yr) were volunteers from the local community. All subjects were free of medications and neurological conditions that would affect normal neuromuscular responses. MAIN OUTCOME MEASURES A force-time curve analysis of the patellar tendon reflex response was used to assess the inhibition and facilitation of spinal reflexes. In the experimental protocol to assess spinal reflex inhibition, 100 Hz vibration was applied to the right quadriceps muscle. In another experimental protocol, spinal reflex facilitation was assessed using the Jendrassik maneuver. To perform the Jendrassik maneuver, subjects were instructed to grasp their hands together and to pull as hard as possible while breathing normally. After a 2-second count, the tendon tap was delivered to the right leg and the subject was instructed to relax. In both experimental protocols, control patellar tendon reflexes were collected. RESULTS Analysis of variance for reflex peak force revealed a significant 30% reduction in the amount of vibration-induced reflex inhibition with increasing age, and a similar 33% reduction in the amount of Jendrassik maneuver facilitation observed for the older adults as compared with the younger adults. CONCLUSION These results support the hypothesis that inhibitory and excitatory influences acting on the alpha motoneuron pool are different in young and older adults.

The effect of handgrip span on isometric exercise performance.

Fourteen male and eight female volunteers served as subjects in these experiments lo determine the effect of hand tool dimensions on isometric strength, endurance, the surface EMG above the active muscle, and the cardiovascular responses to isometric exercise. As reported by others, we found that for each individual, there existed one handgrip size at which he or she could exert the greatest isometric strength. Endurance was the same at any work load relative to the maximum strength for a given grip dimension. The EMG and blood pressure responses to isometric exercise were the same at any given grip span: however, the heart rate response was lowest when subjects worked with their muscles at the optimal grip span.