Adaptation of mechanical properties of muscle to high force training in man

Combined action observation and mental imagery versus neuromuscular electrical stimulation as novel therapeutics during short-term knee immobilization

Limb immobilization causes rapid declines in muscle strength and mass. Given the role of the nervous system in immobilization-induced weakness, targeted interventions may be able to preserve muscle strength, but not mass, and vice versa. The purpose of this study was to assess the effects of two distinct interventions during 1 week of knee joint immobilization on muscle strength (isometric and concentric isokinetic peak torque), mass (bioimpedance spectroscopy and ultrasonography), and neuromuscular function (transcranial magnetic stimulation and interpolated twitch technique). Thirty-nine healthy, college-aged adults (21 males, 18 females) were randomized into one of four groups: immobilization only (n = 9), immobilization + action observation/mental imagery (AOMI) (n = 10), immobilization + neuromuscular electrical stimulation (NMES) (n = 12), or control group (n = 8). The AOMI group performed daily video observation and mental imagery of knee extensions. The NMES group performed twice daily stimulation of the quadriceps femoris. Based on observed effect sizes, it appears that AOMI shows promise as a means of preserving voluntary strength, which may be modulated by neural adaptations. Strength increased from PRE to POST in the AOMI group, with +7.2% (Cohen's d = 1.018) increase in concentric isokinetic peak torque at 30°/s. However, NMES did not preserve muscle mass. Though preliminary, our findings highlight the specific nature of clinical interventions and suggest that muscle strength can be independently targeted during rehabilitation. This study was prospectively registered: ClinicalTrials.gov NCT05072652.

Effects of high resistance muscle training on corticospinal output during motor fatigue assessed by transcranial magnetic stimulation

Introduction: Central fatigue refers to a reduced drive of motor cortical output during exercise, and performance can be enhanced after training. However, the effects of training on central fatigue remain unclear. Changes in cortical output can be addressed non-invasively using transcranial magnetic stimulation (TMS). The aim of the study was to compare responses to TMS during a fatiguing exercise before and after a 3 weeks lasting resistance training, in healthy subjects.

Methods: The triple stimulation technique (TST) was used to quantify a central conduction index (CCI = amplitude ratio of central conduction response and peripheral nerve response) to the abductor digiti minimi muscle (ADM) in 15 subjects. The training consisted of repetitive isometric maximal voluntary contractions (MVC) of ADM for 2 min twice a day. Before and after this training, TST recordings were obtained every 15 s during an 2 min exercise of MVC of the ADM, where subjects performed repetitive contractions of the ADM, and repeatedly during a recovery period of 7 min.

Results: There was a consistent decrease of force to approximately 40% of MVC in all experiments and in all subjects, both before and after training. In all subjects, CCI decreased during exercise. While before training, theCCI decreased to 49% (SD 23.7%) after 2 min of exercise, it decreased after training onlyto 79% (SD 26.4%) after exercise (p < 0.01).

Discussion: The training regimen increased the proportion of target motor units that could be activated by TMS during a fatiguing exercise. The results point to a reduced intracortical inhibition, which may be a transient physiological response to facilitate the motor task. Possible underlying mechanisms at spinal and supraspinal sites are discussed.

Measuring subject specific muscle model parameters of the first dorsal interosseous in vivo

Subject specific musculoskeletal models typically base some or all of their parameters on a source other than the subject being modeled. Evidence demonstrates that cadaveric measurements do not always scale appropriately to every subject, yet many musculoskeletal models still rely heavily on cadaveric based data. This study focused on the First Dorsal interosseous (FDI) given its unique function as the sole abductor of the second metacarpophalangeal joint. There were two purposes to this study: (1) to describe the procedures that can be used in vivo to determine the properties of a model of the FDI. (2). To determine the model parameters required to characterize the FDI for a group of four subjects. Parameters were determined using ultrasound imaging and a custom-built finger dynamometer. Some parameters were measured directly while other parameters had to be estimated using a least-squares criterion. For example, the parameters for the force-length properties were determined by fitting a model to experimentally determined data, with maximum isometric force values ranging from 86 to 102 N, and optimum lengths from 41 to 53 mm. It was shown that full characterization is possible for the FDI with parameters that are physiologically reasonable, but which showed variability between subjects. This model and approach for parameter identification will allow for more detailed analysis of the function of the FDI.

The arrangement of fascicles in whole muscle

The architecture of the muscle fascicles, here meaning their lengths and their arrangement relative to one another, has important implications for the force a muscle can produce. Therefore, quantifying this architectural arrangement and understanding the implications of the architecture are important for understanding muscle function in vivo. There were two purposes of this study: (1) to assess, via blunt dissection, the number and the length of all the fascicles comprising the First Dorsal Interosseous (FDI) muscle and (2) to visually identify, via magnetic resonance imaging (MRI), the arrangement of the fascicles comprising the FDI. Simple blunt dissection of all the fascicles comprising four FDI muscles and their subsequent measurement demonstrated that the fascicles comprising the whole muscle were not as long as the muscle belly from which they were extracted. Muscle fascicles are surrounded by connective tissue hence the paths of the fascicles in two whole FDI muscles were identified via MRI by tracking the connective tissue surrounding the fascicles. The fascicles had a spiral pattern along the length of each muscle, within both muscles many of the fascicles were arranged in series with other fascicles. These architectural features of the fascicles of the FDI have important implications for the force-length and force-velocity properties of the whole muscle.

Transcutaneous neuromuscular electrical stimulation: influence of electrode positioning and stimulus amplitude settings on muscle response

The aim of the study was to investigate the influence of two different transcutaneous neuromuscular electrical stimulation procedures on evoked muscle torque and local tissue oxygenation. In the first one (MP mode), the cathode was facing the muscle main motor point and stimulus amplitude was set to the level eliciting the maximal myoelectrical activation according to the amplitude of the evoked electromyogram (EMG); in the second one (RC mode), the electrodes were positioned following common reference charts for electrode placement while stimulus amplitude was set according to subject tolerance. Tibialis Anterior (TA) and Vastus Lateralis (VL) muscles of 10 subjects (28.4 ± 8.2 years) were tested in specific dynamometers to measure the evoked isometric torque. The EMG and near-infrared spectroscopy probes were placed on muscle belly to detect the electrical activity and local metabolic modifications of the stimulated muscle, respectively. The stimulation protocol consisted of a gradually increasing frequency ramp from 2 to 50 Hz in 7.5 s. Compared to RC mode, in MP mode the contractile parameters (peak twitch, tetanic torque, area under the torque build-up) and the metabolic solicitation (oxygen consumption and hyperemia due to metabolites accumulation) resulted significantly higher for both TA and VL muscles. MP mode resulted also to be more comfortable for the subjects. Based on the assumption that proper mechanical and metabolic stimuli are necessary to induce muscle strengthening, our results witness the importance of an optimized, i.e., comfortable and effective, stimulation to promote the aforementioned muscle adaptive modifications.

Architectural properties of the first dorsal interosseous muscle

Muscle architecture is considered to reflect the function of muscle in vivo, and is important for example to clinicians in designing tendon-transfer and tendon-lengthening surgeries. The purpose of this study was to quantify the architectural properties of the FDI muscle. It is hypothesized that there will be consistency, that is low variability, in the architectural parameters used to describe the first dorsal interosseous muscle because of its clear functional role in index finger motion. The important architectural parameters identified were those required to characterize a muscle adequately by modeling. Specifically the mass, cross-sectional area, and length of the tendon and muscle were measured in cadavers along with the muscle fiber optimum length and pennation angle, and the moment arm of the first dorsal interosseous at the metacarpophalangeal joint. These parameters provide a characterization of the architecture of the first dorsal interosseous, and were used to indicate the inherent variability between samples. The results demonstrated a large amount of variability for all architectural parameters measured; leading to a rejection of the hypothesis. Ratios designed to describe the functioning of the muscles in vivo, for example the ratio of tendon to fiber optimum lengths, also demonstrated a large variability. The results suggest that function cannot be deduced from form for the first dorsal interosseous, and that subject-specific architectural parameters may be necessary for the formulation of accurate musculoskeletal models or making clinical decisions.

Evoked tetanic torque and activation level explain strength differences by side

Previous studies have demonstrated that healthy young people typically have side-to-side differences in knee strength of about 10% when the peak torque generated by the stronger leg is contrasted with that of the weaker leg. However, the mechanisms responsible for side-to-side differences in knee strength have not been clearly defined. The current study tested the hypothesis that side-to-side knee extensor strength differences are explained by inter-limb variations in voluntary activation, antagonistic hamstrings activity, and electrically evoked torque at rest. Twenty-two volunteers served as subjects. Side-to-side differences in quadriceps activation and electrically evoked knee extensor torque explained 69% of the strength differences by side. Antagonistic hamstrings activity did not contribute significantly. The results suggest both central and peripheral mechanisms contribute to inter-limb variations in strength.

The Adaptations to Strength Training

High-resistance strength training (HRST) is one of the most widely practiced forms of physical activity, which is used to enhance athletic performance, augment musculo-skeletal health and alter body aesthetics. Chronic exposure to this type of activity produces marked increases in muscular strength, which are attributed to a range of neurological and morphological adaptations. This review assesses the evidence for these adaptations, their interplay and contribution to enhanced strength and the methodologies employed. The primary morphological adaptations involve an increase in the cross-sectional area of the whole muscle and individual muscle fibres, which is due to an increase in myofibrillar size and number. Satellite cells are activated in the very early stages of training; their proliferation and later fusion with existing fibres appears to be intimately involved in the hypertrophy response. Other possible morphological adaptations include hyperplasia, changes in fibre type, muscle architecture, myofilament density and the structure of connective tissue and tendons. Indirect evidence for neurological adaptations, which encompasses learning and coordination, comes from the specificity of the training adaptation, transfer of unilateral training to the contralateral limb and imagined contractions. The apparent rise in whole-muscle specific tension has been primarily used as evidence for neurological adaptations; however, morphological factors (e.g. preferential hypertrophy of type 2 fibres, increased angle of fibre pennation, increase in radiological density) are also likely to contribute to this phenomenon. Changes in inter-muscular coordination appear critical. Adaptations in agonist muscle activation, as assessed by electromyography, tetanic stimulation and the twitch interpolation technique, suggest small, but significant increases. Enhanced firing frequency and spinal reflexes most likely explain this improvement, although there is contrary evidence suggesting no change in cortical or corticospinal excitability. The gains in strength with HRST are undoubtedly due to a wide combination of neurological and morphological factors. Whilst the neurological factors may make their greatest contribution during the early stages of a training programme, hypertrophic processes also commence at the onset of training.

Cross Education

Resistance training can be defined as the act of repeated voluntary muscle contractions against a resistance greater than those normally encountered in activities of daily living. Training of this kind is known to increase strength via adaptations in both the muscular and nervous systems. While the physiology of muscular adaptations following resistance training is well understood, the nature of neural adaptations is less clear. One piece of indirect evidence to indicate that neural adaptations accompany resistance training comes from the phenomenon of 'cross education', which describes the strength gain in the opposite, untrained limb following unilateral resistance training. Since its discovery in 1894, subsequent studies have confirmed the existence of cross education in contexts involving voluntary, imagined and electrically stimulated contractions. The cross-education effect is specific to the contralateral homologous muscle but not restricted to particular muscle groups, ages or genders. A recent meta-analysis determined that the magnitude of cross education is approximately equal to 7.8% of the initial strength of the untrained limb. While many features of cross education have been established, the underlying mechanisms are unknown. This article provides an overview of cross education and presents plausible hypotheses for its mechanisms. Two hypotheses are outlined that represent the most viable explanations for cross education. These hypotheses are distinct but not necessarily mutually exclusive. They are derived from evidence that high-force, unilateral, voluntary contractions can have an acute and potent effect on the efficacy of neural elements controlling the opposite limb. It is possible that with training, long-lasting adaptations may be induced in neural circuits mediating these crossed effects. The first hypothesis suggests that unilateral resistance training may activate neural circuits that chronically modify the efficacy of motor pathways that project to the opposite untrained limb. This may subsequently lead to an increased capacity to drive the untrained muscles and thus result in increased strength. A number of spinal and cortical circuits that exhibit the potential for this type of adaptation are considered. The second hypothesis suggests that unilateral resistance training induces adaptations in motor areas that are primarily involved in the control of movements of the trained limb. The opposite untrained limb may access these modified neural circuits during maximal voluntary contractions in ways that are analogous to motor learning. A better understanding of the mechanisms underlying cross education may potentially contribute to more effective use of resistance training protocols that exploit these cross-limb effects to improve the recovery of patients with movement disorders that predominantly affect one side of the body.

Training adaptations in the behavior of human motor units

The purpose of this brief review is to examine the neural adaptations associated with training, by focusing on the behavior of single motor units. The review synthesizes current understanding on motor unit recruitment and rate coding during voluntary contractions, briefly describes the techniques used to record motor unit activity, and then evaluates the adaptations that have been observed in motor unit activity during maximal and submaximal contractions. Relatively few studies have directly compared motor unit behavior before and after training. Although some studies suggest that the voluntary activation of muscle can increase slightly with strength training, it is not known how the discharge of motor units changes to produce this increase in activation. The evidence indicates that the increase is not attributable to changes in motor unit synchronization. It has been demonstrated, however, that training can increase both the rate of torque development and the discharge rate of motor units. Furthermore, both strength training and practice of a force-matching task can evoke adaptations in the discharge characteristics of motor units. Because the variability in discharge rate has a significant influence on the fluctuations in force during submaximal contractions, the changes produced with training can influence motor performance during activities of daily living. Little is known, however, about the relative contributions of the descending drive, afferent feedback, spinal circuitry, and motor neuron properties to the observed adaptations in motor unit activity.

Oestrogen status in relation to the early training responses in human thumb adductor muscles

AIMS

The aims of this study were to identify the mechanisms for the early response to training in women of different oestrogen status and to determine whether any oestrogen and exercise effects on these would be additive.

METHODS

We monitored training (ten 5-s contractions per day for 12 weeks)-induced changes in the size, strength, voluntary activation capacity and index of crossbridge force state (i.e. rapid stretch to isometric torque ratio), in the thumb adductor muscles of postmenopausal [eight who had never used, and 14 who were using, hormone replacement therapy (HRT)] and seven premenopausal eumenorrhoeic women. The contralateral untrained muscle was used as a control.

RESULTS

There was a significant effect of oestrogen status on the magnitude of training-induced strength increment, with the non-HRT postmenopausal group exhibiting the greatest benefits (28 +/- 6%, P = 0.024) from training. There were no significant or commensurate changes in either cross-sectional area or voluntary activation capacity. The index of crossbridge force state improved most in the no-HRT group (19 +/- 7%, P < 0.05).

CONCLUSIONS

Presence, rather than absence of oestrogen, is associated with relatively higher muscle function which limits the potential for any further training-induced increments in muscle performance, as would be expected if the muscle strengthening actions of training and oestrogen share a common, partially saturable physiological pathway. The mechanism that is involved in the early training-induced strength increment in the three differing oestrogen groups cannot be due to increased size or recruitment. It would appear instead that increased motor unit firing frequency is involved.

Maximal motor unit firing rates during isometric resistance training in men

This study measured changes in maximal voluntary contraction (MVC) force, percentage maximal activation, maximal surface EMG, M-wave amplitude and average motor unit firing rates during the initial 3 weeks of isometric resistance training of the quadriceps muscle. Ten men participated in a resistance training programme three times a week for 3 weeks and 10 men participated as a control group. In the training group, MVC increased by 35% (from 761 +/- 77 to 1031 +/- 78 N) by the end of the 3 weeks. There were no changes in mean motor unit firing rates during submaximal or maximal voluntary contractions of 50 (15.51 +/- 1.48 Hz), 75 (20.23 +/- 1.85 Hz) or 100% MVC (42.25 +/- 2.72 Hz) with isometric resistance training. There was also no change in maximal surface EMG relative to the M-wave amplitude. However, there was a small increase in maximal activation (from 95.7 +/- 1.83 to 98.44 +/- 0.66%) as measured by the twitch interpolation technique. There were no changes in any of the parameters measured in the control group. It is suggested that mechanisms other than increases in average motor unit firing rates contributed to the increase in maximal force output with resistance training. Such mechanisms may include a combination of increased motor unit recruitment, enhanced protein synthesis, and changes in motor unit synchronization and muscle activation patterns across the quadriceps synergy.

Cross-Education of Arm Muscular Strength Is Unidirectional in Right-Handed Individuals

PURPOSE

Cross-education of strength is a neural adaptation defined as the increase in strength of the untrained contralateral limb after unilateral training. The purpose was to determine the effect of the direction of transfer on cross-education in right-handed individuals.

METHODS

Thirty-nine strongly right-handed females were randomized into a left-hand training (LEFT), right-hand training (RIGHT), or nontraining control (CON) group. Strength training was 6 wk of maximal isometric ulnar deviation, 4x wk(-1). Peak torque, muscle thickness (ultrasound), and EMG activity were assessed before and after training in both limbs.

RESULTS

The change in strength in the untrained limb was greatest in the RIGHT group (39.2%; P < 0.01), whereas no significant changes in strength were observed for the untrained limb of the LEFT group (9.3%) or for either of the CON group limbs (10.4 and 12.2%). Strength training also increased trained limb strength in the LEFT (41.9%, P < 0.01) and the RIGHT (25.9%; P < 0.01) groups. Training groups increased trained limb muscle thickness (RIGHT and LEFT combined: 4.1%) compared to CON (-4.0%) (P < 0.01). There were no changes in muscle thickness of untrained limbs compared to CON. Trained limb agonist EMG activation increased with training (P < 0.05) with no change for the antagonist. Changes in untrained limb EMG were not different compared to CON.

CONCLUSIONS

Cross-education with hand strength training occurs only in the right-to-left direction of transfer in right-handed individuals. We conclude that cross-education of arm muscular strength is most pronounced to the nondominant arm.

Randomized controlled trial of strength training in post-polio patients

Many post-polio patients develop new muscle weakness decades after the initial illness. However, its mechanism and treatment are controversial. The purpose of this study was to test the hypotheses that: (1) after strength training, post-polio patients show strength improvement comparable to that seen in the healthy elderly; (2) such training does not have a deleterious effect on motor unit (MU) survival; and (3) part of the strength improvement is due to an increase in voluntary motor drive. After baseline measures including maximum voluntary contraction force, voluntary activation index, motor unit number estimate, and the tetanic tension of the thumb muscles had been determined, 10 post-polio patients with hand involvement were randomized to either the training or control group. The progressive resistance training program consisted of three sets of eight isometric contractions, three times weekly for 12 weeks. Seven healthy elderly were also randomized and trained in a similar manner. Changes in the baseline parameters were monitored once every 4 weeks throughout the training period. The trained post-polio patients showed a significant improvement in their strength (P < 0.05). The magnitude of gain was greater than that seen in the healthy elderly (mean +/- SE, 41 +/- 16% vs. 29 +/- 8%). The training did not adversely affect MU survival and the improvement was largely attributable to an increase in voluntary motor drive. We therefore conclude that moderate intensity strength training is safe and effective in post-polio patients.

Variable frequency trains enhance torque independent of stimulation amplitude

AIM

Variable frequency trains have been reported to enhance force of fatigued human skeletal muscle. More rapid calcium turnover and/or enhanced stiffness may be responsible for the augmented torque-time integral during surface stimulation at moderate amplitude. In contrast, it has recently been suggested that variable frequency train enhancement occurs only at low forces as a result of preferential stimulation of fast fibres and/or altered motor unit recruitment. If correct, this would limit the practical benefit of variable frequency trains. Accordingly, we tested the hypothesis that torque augmentation by variable frequency trains in fatigued skeletal muscle was independent of stimulation amplitude.

METHODS

The m. quadriceps femoris of six males was stimulated with constant frequency trains (six 200-micros square waves separated by 70 ms) or variable frequency trains (first interpulse interval 5 ms) at an amplitude that initially evoked approximately 25 or approximately 50% of maximal voluntary isometric torque.

RESULTS

After 180 constant frequency trains (50% duty cycle), isometric peak torque decreased approximately 63%. In fatigued muscle, variable frequency trains enhanced the torque-time integral by approximately 23% over that for constant frequency trains and this effect was independent of stimulation amplitude. This was due to greater peak torque and less slowing of rise time.

CONCLUSION

These responses show that the torque-time integral can be enhanced at both moderate and high stimulation amplitudes. As such, it is suggested that neither recruitment nor preferential activation of fast muscle is responsible for the "catch-like" property that can be demonstrated in fatigued human skeletal muscle.

Neural adaptations with chronic activity patterns in able-bodied humans

The purpose of this article is to review the neural adaptations that occur in able-bodied humans with alterations in chronic patterns of physical activity. The adaptations are categorized as those related to cortical maps, motor command, descending drive, muscle activation, motor units, and sensory feedback. We focused on the adaptations that occur with such activities as strength training, limb immobilization, and limb unloading. For these types of interventions, the adaptations are widely distributed throughout the nervous system, but those changes that are observed with strength training are often not the converse of those found with reduced-use protocols.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Force-length characteristics of in vivo human skeletal muscle

In the present study, the in vivo force-length relations of the human soleus (SOL) and tibialis anterior (TA) muscles were estimated. Measurements were taken in six men at ankle angles from 30 degrees of dorsiflexion to 45 degrees of plantarflexion in steps of 15 degrees, and involved dynamometry, electrical stimulation, ultrasonography and magnetic resonance imaging (MRI). For each muscle and ankle angle studied the following three measurements were carried out: (1) dynamometry-based measurement of maximal voltage tetanic moment, (2) ultrasound-based measurement of pennation angle and fibre length and (3) MRI-based measurement of tendon moment arm length. Tendon forces were calculated dividing moments by moment arm lengths, and muscle forces were calculated dividing tendon forces by the cosine of pennation angles. In the transition from 30 degrees of dorsiflexion to 45 degrees of plantarflexion the SOL muscle fibre length decreased from 3.8 to 2.4 cm and its force decreased from 3330 to 290 N. Over the same range of ankle angles the TA muscle fibre length increased from 3.7 to 6 cm and its force increased from 157 to 644 N. Over the longest muscle fibre lengths reached the force of both muscles remained approximately constant. These results indicate that the intact human SOL and TA muscles operate in the ascending limb and plateau region of the force-length relationship. Similar conclusions were reached when calculating the theoretical operating range of the two muscle sarcomeres in the study.

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.

In vivo human tendon mechanical properties

1. The aim of the present study was to measure the mechanical properties of human tibialis anterior (TA) tendon in vivo. 2. Measurements were taken in five males at the neutral ankle position and involved: (a) isometric dynamometry upon increasing the voltage of percutaneous electrical stimulation of the TA muscle, (b) real-time ultrasonography for measurements of the TA tendon origin displacement during contraction and tendon cross-sectional area, and (c) magnetic resonance imaging for estimation of the TA tendon length and moment arm. 3. From the measured joint moments and estimated moment arms, the values of tendon force were calculated and divided by cross-sectional area to obtain stress values. The displacements of the TA tendon origin from rest to all contraction intensities were normalized to tendon length to obtain strain values. From the data obtained, the tendon force-displacement and stress-strain relationships were determined and the tendon stiffness and Young's modulus were calculated. 4. Tendon force and stress increased curvilinearly as a function of displacement and strain, respectively. The tendon force and displacement at maximum isometric load were 530 N and 4.1 mm, and the corresponding stress and strain values were 25 MPa and 2.5 %, respectively. The tendon stiffness and Young's modulus at maximum isometric load were 161 N mm-1 and 1.2 GPa, respectively. These results are in agreement with previous reports on in vitro testing of isolated tendons and suggest that under physiological loading the TA tendon operates within the elastic 'toe' region.

Maximal motor unit discharge rates in the quadriceps muscles of older weight lifters

Although the existence of "neural factors" is regularly cited as an important contributor to muscular strength, we have little specific knowledge regarding the existence of such neural factors or how they contribute to the expression of muscular force.

PURPOSE

The present investigation sought to assess maximal motor unit discharge rates in older, highly resistance-trained adults to determine whether maximal motor unit discharge rates might be one such neural contributor to maximal strength production.

METHODS

Subjects consisted of seven well-trained older weight lifters (ages 67-79 yr) and five untrained age-matched older adults. While subjects performed 50 and 100% maximal voluntary knee extensor contractions (MVC), recordings from groups of motor units were obtained from the rectus femoris muscle by using an indwelling electrode. Off-line analysis was performed to identify individual motor unit firing occurrences and to compute maximal motor unit discharge rates.

RESULTS

As expected, knee extension strength in the trained weight lifters (367.0 +/- 72.0 N) was significantly greater than that in the control subjects (299.9 +/- 35.9 N; P < 0.05). Motor unit discharge rates were similar in the two subject groups at the 50% MVC force level (P > 0.05), but maximal (100% MVC) motor unit discharge rate in the weight lifters (23.8 +/- 7.71 pps) was significantly greater than that in the age-matched controls (19.1 +/- 6.29 pps; P < 0.05).

CONCLUSION

Motor unit discharge rates may comprise an important neural factor contributing to maximal strength in older adults.

Training-induced adaptations in the central command and peripheral reflex components of the pressor response to isometric exercise of the human triceps surae

1. The effect of calf raise training of the dominant limb on the pressor response to isometric exercise of the triceps surae was examined in the trained dominant limb and the contralateral untrained limb. Blood pressure and heart rate responses to electrically evoked and voluntary exercise at 30 % maximum voluntary contraction (MVC), followed by post-exercise circulatory occlusion (PECO), were compared before and after a 6 week training period. 2. In the trained limb the diastolic blood pressure rise seen during electrically evoked exercise was reduced by 27 % after training. However, the response during PECO was not significantly affected. 3. During voluntary exercise of the trained limb, diastolic blood pressure rise was reduced by 28 %, and heart rate rise was significantly attenuated after training. During PECO no significant effects of training were observed. 4. Voluntary exercise of the untrained limb resulted in a 24 % reduction in diastolic blood pressure rise after the training period, and a significant attenuation of the heart rate increase during exercise. Responses to electrically evoked exercise and PECO of the untrained limb remained unaltered after training. 5. Attenuation of blood pressure and heart rate responses, in the contralateral untrained limb, during voluntary but not electrically evoked exercise, indicates a training-induced alteration in central command.

The effects of training through high-frequency electrical stimulation on the physiological properties of single human thenar motor units

The relative impact of training on motor units (MUs) with differing physiological characteristics remains controversial. To examine this issue, we longitudinally tracked the contractile and electrical characteristics of six human thenar MUs in 2 young healthy subjects before, during, and following an intermittent, high-frequency electrical stimulation program. Responses of MUs with differing baseline physiological characteristics varied widely. While the twitch and maximal tetanic tensions of the slower and fatigue-resistant MUs increased, tensions of the faster and more fatigable MUs declined. The fatigue resistance of the faster and more fatigable MUs, on the other hand, increased while that of the slower MUs remained unchanged. Although electrical stimulation of individual MUs allowed their training to be precisely controlled, it will be of practical importance to determine whether similar divergent MU contractile changes also occur with voluntary training.

Changes in single motor unit behaviour contribute to the increase in contraction speed after dynamic training in humans

1. The adaptations of the ankle dorsiflexor muscles and the behaviour of single motor units in the tibialis anterior in response to 12 weeks of dynamic training were studied in five human subjects. In each training session ten series of ten fast dorsiflexions were performed 5 days a week, against a load of 30-40% of the maximal muscle strength. 2. Training led to an enhancement of maximal voluntary muscle contraction (MVC) and the speed of voluntary ballistic contraction. This last enhancement was mainly related to neural adaptations since the time course of the muscle twitch induced by electrical stimulation remained unaffected. 3. The motor unit torque, recorded by the spike-triggered averaging method, increased without any change in its time to peak. The orderly motor unit recruitment (size principle) was preserved during slow ramp contraction after training but the units were activated earlier and had a greater maximal firing frequency during voluntary ballistic contractions. In addition, the high frequency firing rate observed at the onset of the contractions was maintained during the subsequent spikes after training. 4. Dynamic training induced brief (2-5 ms) motor unit interspike intervals, or 'doublets'. These doublets appeared to be different from the closely spaced (+/-10 ms) discharges usually observed at the onset of the ballistic contractions. Motor units with different recruitment thresholds showed doublet discharges and the percentage of the sample of units firing doublets was increased by training from 5.2 to 32.7%. The presence of these discharges was observed not only at the onset of the series of spikes but also later in the electromyographic (EMG) burst. 5. It is likely that earlier motor unit activation, extra doublets and enhanced maximal firing rate contribute to the increase in the speed of voluntary muscle contraction after dynamic training.

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. The data on specificity demonstrate that: strength can be increased by training with imagined contractions; the velocity specificity of isokinetic training is evident with intended contractions; the strength training influences the untrained homologous muscle in the contralateral limb; the bilatral deficit can become a bilateral facilitation with appropriate training; and that eccentric contractions appear to involve a different activation scheme compared to isometric and concentric contractions. Finally, the literature on the pattern of the neural drive suggests that: coactivation varies with training and often decreases as skill level increases; measures of motor-unit synchronization reveal changes in neuronal connectivity with physical training; the reflex potentiation varies across muscles, individuals, and activity patterns; the modulation of the H-reflex amplitude with training involves changes in the motor neuron; and the motor neurons exhibit a bistable, excitability property that may be influenced by exercise. Despite the breadth of this evidence, there remain substantial gaps in our knowledge, particularly regarding the symmetry of adaptations with increased and decreased chronic physical activity.

Greater cross education following training with muscle lengthening than shortening

The hypothesis was tested that the magnitude of cross education is greater following training with muscle lengthening than shortening. Changes in contralateral concentric, eccentric, and isometric strength and vastus lateralis and biceps femoris surface electromyographic (EMG) activity were analyzed in groups of young men who exercised the ipsilateral quadriceps with either eccentric (N = 7) or concentric (N = 8) contractions for 36 sessions over 12 wk. Control subjects (N = 6) did not train. Concentric training increased concentric strength 30% and isometric strength 22%, and eccentric training increased eccentric strength 77% and isometric strength 39% (all P < 0.05). Eccentric training improved eccentric strength three times more than the concentric training improved concentric strength (P < 0.05), and eccentric compared with concentric training improved isometric strength about 2 times more (P < 0.05). The eccentric group improved significantly from pre- to mid-training in eccentric and isometric strength (P < 0.05). The control group showed no significant changes (P < 0.05). Surface EMG activity of the vastus lateralis increased 2.2 times (pre- to mid-training), 2.8 (mid- to post-training) and 2.6 more (pre- to post-training) (P < 0.05) in the eccentric than concentric group. No significant changes in EMG activity occurred in the control group (P > 0.05). It was concluded that the greater cross education following training with muscle lengthening is most likely being mediated by both afferent and efferent mechanisms that allow previously sedentary subjects to achieve a greater activation of the untrained limb musculature.

Effect of electrical stimulation training on the contractile characteristics of the triceps surae muscle

This study aimed to assess the effects of training using electrical stimulation (ES) on the contractile characteristics of the triceps surae muscle. A selection of 12 subjects was divided into two groups (6 control, 6 experimental). The ES sessions were carried out using a stimulator. Flexible elastomer electrodes were used. The current used discharged pulses lasting 200 microseconds at 70 Hz. Contraction time was 5 s and rest time 15 s. The session lasted 10 min for each muscle. Training sessions were three times a week for 4 weeks. Biomechanical tests were performed using an isokinetic ergometer. Subjects performed plantar flexions of the ankle over a concentric range of movement at different angular velocities (60, 120, 180, 240, 300, 360 degrees.s-1) and held isometric contractions for 5 s at several ankle flexion angles (-30/-15/0/15 degrees-0 corresponded to foot flexion of 90 degrees relative to the leg axis). The force-velocity relationship was seen to shift evenly upwards under the influence of ES (P < 0.05). The increased force during the "after" test was greater (P < 0.05) for ankle angle positions of 15 degrees and -30 degrees, which demonstrated a link between the training angle and the gain in strength. No change was noted in the cross-sectional area of the muscle. The results showed that ES allowed the contractile qualities of muscle to be developed in isometric and dynamic conditions. Nervous mechanisms can account for most of these adaptations.

Strength training alters contractile properties of the triceps brachii in men aged 65-78 years

Voluntary and electrically evoked contractile properties were studied in the triceps brachii following a 24-week dynamic strength training program in ten men aged 65-78 years. Eight men of a similar age were control subjects. A resistance overload program was undertaken three times per week with subjects performing four sets of six to eight repetitions at 80% of their one repetition on maximum (1RM). Maximum voluntary contraction (MVC) and contractile properties were measured at 0, 12, and 24 weeks in the exercise group and at 0 and 24 weeks in the controls. The 1RM was used to assess dynamic strength at 0 and 24 weeks in the exercise group. Contractile measures consisted of supramaximal isometric twitch and post-activation twitch parameters. Muscle size was estimated from anthropometric measurements. Compared with the control group, the exercise group MVC increased by about 20% and time to peak tension was slowed by about 11%. Also in the exercise group the peak rate of torque development of the potentiated twitch was reduced by about 10%. Twitch potentiation was substantial in both groups (about 140%) and unaffected by training. The 1 RM increased by about 30%, and there was a non-significant positive change of 8.6% in the muscle plus bone cross-sectional area in the exercise group. The results show that the force generating capacity of the triceps brachii in these men can be significantly improved for up to 24 weeks using concentric overload training. Furthermore, the finding of slowed twitch properties and no change in peak twitch amplitude substantiate and extend the limited data currently available on intrinsic contractile changes in the elderly.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuromuscular adaptations during the acquisition of muscle strength, power and motor tasks

Neuromuscular performance is determined not only by the size of the involved muscles, but also by the ability of the nervous system to appropriately activate the muscles. Adaptive changes in the nervous system in response to training are referred to as neural adaptation. This article briefly reviews current evidence regarding the neural adaptations during the acquisition of muscle strength power and motor tasks and will be organized under four main topics, namely: (i) muscle strength gain: neural factors versus hypertrophy, (ii) neural adaptations during power training, (iii) neuromuscular adaptations during the acquisition of a motor task, and (iv) neuromuscular adaptations during a ballistic movement.

Neuromuscular electrical stimulation and voluntary exercise

Neuromuscular electrical stimulation (NMES) has been in practice since the eighteenth century for the treatment of paralysed patients and the prevention and/or restoration of muscle function after injuries, before patients are capable of voluntary exercise training. More recently NMES has been used as a modality of strengthening in healthy subjects and highly trained athletes, but it is not clear whether NMES is a substitute for, or a complement to, voluntary exercise training. Moreover the discussion of the mechanisms which underly the specific effects of NMES appears rather complex at least in part because of the disparity in training protocols, electrical stimulation regimens and testing procedures that are used in the various studies. It appears from this review of the literature that in physical therapy, NMES effectively retards muscle wasting during denervation or immobilisation and optimises recovery of muscle strength during rehabilitation. It is also effective in athletes with injured, painful limbs, since NMES contributes to a shortened rehabilitation time and aids a safe return to competition. In healthy muscles, NMES appears to be a complement to voluntary training because it specifically induces the activity of large motor units which are more difficult to activate during voluntary contraction. However, there is a consensus that the force increases induced by NMES are similar to, but not greater than, those induced by voluntary training. The rationale for the complementarity between NMES and voluntary exercise is that in voluntary contractions motor units are recruited in order, from smaller fatigue resistant (type I) units to larger quickly fatiguable (type II) units, whereas in NMES the sequence appears to be reversed. As a training modality NMES is, in nonextreme situations such as muscle denervation, not a substitute for, but a complement of, voluntary exercise of disused and healthy muscles.

Changes in force, cross-sectional area and neural activation during strength training and detraining of the human quadriceps

Four male subjects aged 23-34 years were studied during 60 days of unilateral strength training and 40 days of detraining. Training was carried out four times a week and consisted of six series of ten maximal isokinetic knee extensions at an angular velocity of 2.09 rad.s-1. At the start and at every 20th day of training and detraining, isometric maximal voluntary contraction (MVC), integrated electromyographic activity (iEMG) and quadriceps muscle cross-sectional area (CSA) assessed at seven fractions of femur length (Lf), by nuclear magnetic resonance imaging, were measured on both trained (T) and untrained (UT) legs. Isokinetic torques at 30 degrees before full knee extension were measured before and at the end of training at: 0, 1.05, 2.09, 3.14, 4.19, 5.24 rad.s-1. After 60 days T leg CSA had increased by 8.5% +/- 1.4% (mean +/- SEM, n = 4, p less than 0.001), iEMG by 42.4% +/- 16.5% (p less than 0.01) and MVC by 20.8% +/- 5.4% (p less than 0.01). Changes during detraining had a similar time course to those of training. No changes in UT leg CSA were observed while iEMG and MVC increased by 24.8% +/- 10% (N.S.) and 8.7% +/- 4.3% (N.S.), respectively. The increase in quadriceps muscle CSA was maximal at 2/10 Lf (12.0% +/- 1.5%, p less than 0.01) and minimal, proximally to the knee, at 8/10 Lf (3.5% +/- 1.2%, N.S.). Preferential hypertrophy of the vastus medialis and intermedius muscles compared to those of the rectus femoris and lateralis muscles was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle fatigue, effects of training and disuse

The study of muscle fatigue started about a century ago, when it was proposed that the observed decrease in force during prolonged voluntary contractions resulted from changes in central processes which reduced the motor drive. In the middle of this century it was noticed that this loss of force could not be restored by maximal electrical stimulation of the motor nerve, and thus the importance of peripheral mechanisms, located beyond the motoneuron, was emphasized. However, it was not clear which peripheral site was most important in decreasing the muscle mechanical capacity during fatigue. More recently, the comparison between peripheral failures during sustained and intermittent contractions indicated that recorded mechanical changes underwent deterioration which was not closely related to the recorded electrical changes. It was thus proposed that muscle intracellular processes dominate the force decrease during muscle fatigue. This concept has been substantiated by the study of standard fatigue tests performed in control, trained, and disused human muscles, as reviewed in this paper.

Specificity of joint angle in isometric training

Six healthy women (21.8 +/- 0.4 y) did isometric strength training of the left plantarflexors at an ankle joint angle of 90 degrees. Training sessions, done 3 times per week for 6 weeks, consisted of 2 sets of ten 5 s maximal voluntary contractions. Prior to and following the training, and in random order, voluntary and evoked isometric contraction strength was measured at the training angle and at additional angles: 5 degrees, 10 degrees, 15 degrees, and 20 degrees intervals in the plantarflexion and dorsiflexion directions. Evoked contraction strength was measured as the peak torque of maximal twitch contractions of triceps surae. Training increased voluntary strength at the training angle and the two adjacent angles only (p less than 0.05). Time to peak twitch torque was not affected by training. Twitch half relaxation time increased after training (p = 0.013), but the increase was not specific to the training angle. There was a small (1.1%, p less than 0.05) increase in calf circumference after training. Evoked twitch torque did not increase significantly at any joint angle. It was therefore concluded that a neural mechanism is responsible for the specificity of joint angle observed in isometric training.

Activation of human muscles at short muscle lengths during maximal static efforts

1. Human muscle endurance is apparently enhanced during maximal voluntary contractions at short muscle lengths (McKenzie & Gandevia, 1987) but the ability of subjects to activate muscles fully at short lengths has not been established. Therefore this study examined the voluntary capacity to activate muscles fully at control (near resting) lengths and at decreased muscle lengths. Changes in mechanical properties of twitch responses to electrical stimulation of relaxed muscles at short muscle lengths were also documented. The abductor digiti minimi, elbow flexors and tibialis anterior were studied in five subjects. 2. For the three muscle groups, the mean reduction in twitch force between the control and short muscle lengths ranged from 46-51%. AT the short length there was a 9-13% reduction in the contraction time and a 21-27% reduction in the half-relaxation time. Maximal voluntary force declined by 21-49% at the short muscle length. A reduction in muscle length produced a shift to the right of the force-frequency curve as determined by brief trains of electrical stimuli. 3. During maximal efforts single or brief trains of two to four supramaximal stimuli, delivered to the parent nerve or motor point, failed to increase the force at a latency appropriate for onset of a muscle twitch in some but not all attempts. Each subject achieved 'maximal activation' of the muscle in a similar proportion of attempts at the control and short muscle lengths. 4. These results suggest that maximal voluntary activation of motoneurone pools is possible at short muscle lengths and that the central nervous system is able to maintain the discharge of motoneurones close to 'fusion' frequency despite a decrease in the temporal characteristics of the isometric twitch.

Muscle strength and its development. New perspectives

Skeletal muscle undergoes substantial adaptation when it is subjected to a strength training regimen. At one extreme, these effects are manifested as profound morphological changes, such as those exemplified by bodybuilders. However, it is possible to increase strength without any change in muscle size. This dissociation underscores the notion that strength is not solely a property of muscle but rather it is a property of the motor system. The nervous system seems to be of paramount importance for the expression and development of strength. Indeed, it is probable that increases in strength can be achieved without morphological changes in muscle but not without neural adaptations. This review focuses on the role of the nervous system in the development of strength. In the strength literature, 3 topics exemplify the importance of the nervous system in strength development. These 3 topics are considered in detail in the review: electromyostimulation, cross-training effects, and EMG-force relationships. Evidence is presented from several different paradigms emphasising the significant contribution of neural mechanisms to the gains in strength with short term training. Although little is known about the specific neural mechanisms associated with strength training adaptations, the literature emphasises that the measure of human performance known as strength can be influenced by a variety of neurophysiological processes.