The effect of strength training on the force of twitches evoked by corticospinal stimulation in humans

Relationship between intended force and actual force: comparison between athletes and non-athletes

This cross-sectional study aimed to investigate whether athletes (ATHL) and non-athletes (NON-ATHL) individuals had similar accuracy in matching intended to actual force during ballistic (BAL) and tonic (TON) isometric contractions. In this cross-sectional study, the subjects were divided into ATHL (n = 20; 22.4 ± 2.3 yrs; 73.2 ± 15.7 kg; 1.76 ± 0.08 m) and NON-ATHL (n = 20; 24.6 ± 2.4 yrs; 68.2 ± 15.0 kg; 1.73 ± 0.1 m) groups. The isometric quadriceps strength was measured with a load cell applied to a custom-built chair. For each condition, subjects performed at first three maximal voluntary isometric contractions (MVIC) as reference. Then, subjects had to match three intended force intensities expressed in percentage of the MVIC (i.e., 25%, 50%, and 75%) without any external feedback. Subjects performed three trials for each force intensity. The accuracy (AC) was calculated as the absolute difference in percentage between the intended and the actual force. A Likert scale was administered for each trial to assess the subjective matching between the intended and the actual force. Statistical analysis showed that the ATHL group was more accurate (p < 0.001) than the NON-ATHL group. In contrast, the AC (p < 0.001) was lower when the force intensities increased independently from the group. Moreover, significantly higher AC (p < 0.001) and lower aggregate Likert scores (p < 0.001) were found in BAL than TON conditions. These results suggest that (i) sports practice could enhance muscle recruitment strategies by increasing the AC in the isometric task; (ii) differences between intended and actual force appeared to be intensity-dependent with lower AC at high force intensities; (iii) different control systems act in modulating BAL and TON contractions.

Corticospinal adaptations following resistance training and its relationship with strength: A systematic review and multivariate meta-analysis

Neural adaptations to resistance training (RT) and their correlation with muscle strength remain partially understood. We conducted a systematic review and multivariate meta-analysis to examine the effects of metronome-paced (MP), self-paced (SP), and isometric (IM) training on M1 and corticospinal pathway activity. Following MP RT, a significant increase in corticospinal excitability was observed, correlating with increased strength. Conversely, no significant relationship was found after SP or IM training. RT also reduced the duration of the cortical silent period, but this change did not predict strength changes and was not specific to any training modality. No significant effects were found for short-interval intracortical inhibition. Our findings suggest that changes in corticospinal excitability may contribute to strength gains after RT. Furthermore, the relationship between these adaptations and strength appears dependent on the type of training performed.

Does the reticulospinal tract mediate adaptation to resistance training in humans?

Resistance training increases volitional force-producing capacity, and it is widely accepted that such an increase is partly underpinned by adaptations in the central nervous system, particularly in the early phases of training. Despite this, the neural substrate(s) responsible for mediating adaptation remains largely unknown. Most studies have focused on the corticospinal tract, the main descending pathway controlling movement in humans, with equivocal findings. It is possible that neural adaptation to resistance training is mediated by other structures; one such candidate is the reticulospinal tract. The aim of this narrative mini-review is to articulate the potential of the reticulospinal tract to underpin adaptations in muscle strength. Specifically, we 1) discuss why the structure and function of the reticulospinal tract implicate it as a potential site for adaptation; 2) review the animal and human literature that supports the idea of the reticulospinal tract as an important neural substrate underpinning adaptation to resistance training; and 3) examine the potential methodological options to assess the reticulospinal tract in humans.

Behavior of motor units during submaximal isometric contractions in chronically strength-trained individuals

Neural and morphological adaptations combine to underpin the enhanced muscle strength following prolonged exposure to strength training, although their relative importance remains unclear. We investigated the contribution of motor unit (MU) behavior and muscle size to submaximal force production in chronically strength-trained athletes (ST) versus untrained controls (UT). Sixteen ST (age: 22.9 ± 3.5 yr; training experience: 5.9 ± 3.5 yr) and 14 UT (age: 20.4 ± 2.3 yr) performed maximal voluntary isometric force (MViF) and ramp contractions (at 15%, 35%, 50%, and 70% MViF) with elbow flexors, whilst high-density surface electromyography (HDsEMG) was recorded from the biceps brachii (BB). Recruitment thresholds (RTs) and discharge rates (DRs) of MUs identified from the submaximal contractions were assessed. The neural drive-to-muscle gain was estimated from the relation between changes in force (ΔFORCE, i.e. muscle output) relative to changes in MU DR (ΔDR, i.e. neural input). BB maximum anatomical cross-sectional area (ACSA_MAX) was also assessed by MRI. MViF (+64.8% vs. UT, P < 0.001) and BB ACSA_MAX (+71.9%, P < 0.001) were higher in ST. Absolute MU RT was higher in ST (+62.6%, P < 0.001), but occurred at similar normalized forces. MU DR did not differ between groups at the same normalized forces. The absolute slope of the ΔFORCE - ΔDR relationship was higher in ST (+66.9%, P = 0.002), whereas it did not differ for normalized values. We observed similar MU behavior between ST athletes and UT controls. The greater absolute force-generating capacity of ST for the same neural input demonstrates that morphological, rather than neural, factors are the predominant mechanism for their enhanced force generation during submaximal efforts.NEW & NOTEWORTHY In this study, we observed that recruitment strategies and discharge characteristics of large populations of motor units identified from biceps brachii of strength-trained athletes were similar to those observed in untrained individuals during submaximal force tasks. We also found that for the same neural input, strength-trained athletes are able to produce greater absolute muscle forces (i.e., neural drive-to-muscle gain). This demonstrates that morphological factors are the predominant mechanism for the enhanced force generation during submaximal efforts.

Fatigability of the knee extensors following high- and low-load resistance exercise sessions in trained men

PURPOSE

Fatigability after gym-based resistance exercises with high and low loads has not been well described, thus limiting the translation of exhaustive low-weight prescription into athletic practice. We compared the fatigability and recovery of the knee extensor muscles for up to 1H after sessions that involved either high- or low-load resistance exercises.

METHODS

16 trained men performed two resistance exercise sessions between 5 and 7 days apart. The LIGHT session involved five sets to task failure at 50% of maximal knee-extension strength, whereas the HEAVY session accrued repetitions across seven sets at intensities ≥ 80% maximal knee-extension strength. Measures of quadriceps maximal torque and rate of torque development were measured before, after, and 1H after each exercise session. Muscle activation (electromyography and voluntary activation) and contractility were measured from doublet stimulation of the femoral nerve during and after maximal contractions, respectively.

RESULTS

Greater declines in maximal rate of torque development were observed after the LIGHT compared with the HEAVY session (p < 0.001), with full recovery after 1H. Voluntary activation (100-Hz doublet stimulation) and surface electromyograms were reduced immediately after the HEAVY session only (p < 0.05), with greater declines in quadriceps twitch amplitudes after the LIGHT session (p < 0.01). Voluntary activation (100-Hz doublet stimulation) was reduced at 1H after both the HEAVY and LIGHT sessions (p < 0.05).

CONCLUSIONS

Despite differences in the decreases in muscle activation and contractility after high- and low-load resistance-exercise sessions, recovery of neuromuscular function was essentially complete after 1H of rest for both sessions.

The Central Mechanisms of Resistance Training and Its Effects on Cognitive Function

Resistance exercise is used extensively in athletic and general populations to induce neuromuscular adaptations to increase muscle size and performance. Exercise parameters such as exercise frequency, intensity, duration and modality are carefully manipulated to induce specific adaptations to the neuromuscular system. While the benefits of resistance exercise on the neuromuscular system are well documented, there is growing evidence to suggest that resistance exercise, even when performed acutely, can lead to neuroplastic changes within the central nervous system (CNS) and improve cognitive functioning. As such, resistance exercise has been proposed as a novel adjuvant rehabilitation strategy in populations that suffer from neurological or neurocognitive impairments (i.e. Parkinson's and Alzheimer's dementia) or even to attenuate age-related declines in cognitive health. In this review, we present evidence for the neuroplastic effects and cognitive benefits of resistance exercise and propose some of the underlying mechanisms that drive neuroplasticity following resistance training. We will further discuss the effects of exercise parameters, in particular exercise frequency, intensity, duration and modality to improve cognitive health. Lastly, we will highlight some of the existing limitations in the literature surrounding the use of resistance exercise to improve cognitive function and propose considerations to improve future studies in this field. In summary, the current evidence supports the role of resistance exercise, as a stand alone or in combination with aerobic exercise, for benefiting cognitive health and that it should be considered as an adjuvant therapy to treat age- or disease-related cognitive declines.

Declines in muscle contractility and activation during isometric contractions of the knee extensors vary with contraction intensity and exercise volume

NEW FINDINGS

What is the central question of this study? Is there a critical threshold beyond which the loss of muscle contractility is regulated by the level of muscle activation during single-limb exercise of differing intensities and volumes? What is the main finding and its importance? Plateaus in the decline in muscle contractility during single-limb knee extension depended on both exercise volume and contraction intensity.  A plateau was only evident with an increase in exercise volume.  Muscle activation increased and did not decline despite substantial reductions in contractility. The findings indicate that the decrease in muscle contractility exhibited by resistance-trained men during the performance of submaximal isometric contractions with the knee extensors was not regulated by the level of muscle activation.

ABSTRACT

Our study examined the influence of contraction intensity and exercise volume on changes in muscle contractility and activation of the knee extensor muscles. Maximal voluntary torque (MVT) and rate of change in torque, surface electromyograms, voluntary activation, V-waves and quadriceps resting twitch measures were assessed in 10 resistance-trained men during two experimental sessions. Each session began with an initial baseline series of contractions at a fixed intensity of 40% or 80% MVT. The 40%-only session continued with five contractions to task failure at 40% MVT. The 80% session continued with five contractions to failure each at 80%, 60% and 40% MVT. Greater reductions in MVT were observed during the baseline contractions of the 40%-only session compared with the 80% session at each matched-volume time point (P < 0.05), with similar changes in twitch values (P < 0.001). MVT and twitch values plateaued at each intensity during the 80% session and were significantly different across intensities: 80% > 60% > 40% (P < 0.001). There were no differences for measures during the five contractions at 40% MVT performed on the different days, despite a greater volume of exercise performed prior to the 40% MVT during the 80% session. At each contraction intensity, a plateau in contractility loss was observed as more contractions were performed. We found that initial increases in muscle activation were maintained in the presence of increases in exercise volume and, in contrast to the critical-threshold hypothesis, did not decline in parallel with reductions in muscle contractility.

Corticospinal excitability and motor representation after long-term resistance training

It is poorly understood how the central nervous system adapts to resistance training, especially after years of exposure. We compared corticospinal excitability and motor representation assessed with transcranial magnetic stimulation (TMS) between long-term resistance trained (LRT, ≥3 years) versus untrained (UNT) males (n = 15/group). Motor-evoked potentials (MEPs) were obtained from the biceps brachii during isometric elbow flexion. Stimulus-response curves were created at the hotspot during 10% maximum voluntary torque (MVT) contractions. Maximum peak-to-peak MEP amplitude (MEPmax) was acquired with 100% stimulator output intensity, whilst 25%-100% MVT was produced. Maps were created during 10% MVT contractions, with an individualised TMS intensity eliciting 20% MEPmax at the hotspot. LRT had a 48% lower stimulus-response curve slope than UNT (p < .05). LRT also had a 66% larger absolute map size, although TMS intensity used for mapping was greater in LRT versus UNT (48% vs. 26% above active motor threshold) to achieve a target 20% MEPmax at the hotspot, due to the lower slope of LRT. Map size was strongly correlated with the TMS intensity used for mapping (r = 0.776, p < .001). Once map size was normalised to TMS intensity, there was no difference between the groups (p = .683). We conclude that LRT had a lower stimulus-response curve slope/excitability, suggesting higher neural efficiency. TMS map size was overwhelmingly determined by TMS intensity, even when the MEP response at the hotspot was matched among individuals, likely due to larger current spread with higher intensities. Motor representation appears similar between LRT and UNT given no difference in the normalised map size.

The knowns and unknowns of neural adaptations to resistance training

The initial increases in force production with resistance training are thought to be primarily underpinned by neural adaptations. This notion is firmly supported by evidence displaying motor unit adaptations following resistance training; however, the precise locus of neural adaptation remains elusive. The purpose of this review is to clarify and critically discuss the literature concerning the site(s) of putative neural adaptations to short-term resistance training. The proliferation of studies employing non-invasive stimulation techniques to investigate evoked responses have yielded variable results, but generally support the notion that resistance training alters intracortical inhibition. Nevertheless, methodological inconsistencies and the limitations of techniques, e.g. limited relation to behavioural outcomes and the inability to measure volitional muscle activity, preclude firm conclusions. Much of the literature has focused on the corticospinal tract; however, preliminary research in non-human primates suggests reticulospinal tract is a potential substrate for neural adaptations to resistance training, though human data is lacking due to methodological constraints. Recent advances in technology have provided substantial evidence of adaptations within a large motor unit population following resistance training. However, their activity represents the transformation of afferent and efferent inputs, making it challenging to establish the source of adaptation. Whilst much has been learned about the nature of neural adaptations to resistance training, the puzzle remains to be solved. Additional analyses of motoneuron firing during different training regimes or coupling with other methodologies (e.g., electroencephalography) may facilitate the estimation of the site(s) of neural adaptations to resistance training in the future.

The increase in muscle force after 4 weeks of strength training is mediated by adaptations in motor unit recruitment and rate coding

KEY POINTS

Previous studies have indicated that several weeks of strength training is sufficient to elicit significant adaptations in the neural drive sent to the muscles. There are few data, however, on the changes elicited by strength training in the recruitment and rate coding of motor units during voluntary contractions. We show for the first time that the discharge characteristics of motor units in the tibialis anterior muscle tracked across the intervention are changed by 4 weeks of strength training with isometric voluntary contractions. The specific adaptations included significant increases in motor unit discharge rate, decreases in the recruitment-threshold force of motor units and a similar input-output gain of the motor neurons. The findings suggest that the adaptations in motor unit function may be attributable to changes in synaptic input to the motor neuron pool or to adaptations in intrinsic motor neuron properties.

ABSTRACT

The strength of a muscle typically begins to increase after only a few sessions of strength training. This increase is usually attributed to changes in the neural drive to muscle as a result of adaptations at the cortical or spinal level. We investigated the change in the discharge characteristics of large populations of longitudinally tracked motor units in tibialis anterior before and after 4 weeks of strength training the ankle-dorsiflexor muscles with isometric contractions. The adaptations exhibited by 14 individuals were compared with 14 control subjects. High-density electromyogram grids with 128 electrodes recorded the myoelectric activity during isometric ramp contractions to the target forces of 35%, 50% and 70% of maximal voluntary force. The motor unit recruitment and derecruitment thresholds, discharge rate, interspike intervals and estimates of synaptic inputs to motor neurons were assessed. The normalized recruitment-threshold forces of the motor units were decreased after strength training (P < 0.05). Moreover, discharge rate increased by 3.3 ± 2.5 pps (average across subjects and motor units) during the plateau phase of the submaximal isometric contractions (P < 0.001). Discharge rates at recruitment and derecruitment were not modified by training (P < 0.05). The association between force and motor unit discharge rate during the ramp-phase of the contractions was also not altered by training (P < 0.05). These results demonstrate for the first time that the increase in muscle force after 4 weeks of strength training is the result of an increase in motor neuron output from the spinal cord to the muscle.

Neuroplasticity following short-term strength training occurs at supraspinal level and is specific for the trained task

AIMS

Different modalities of strength training cause performance enhancements, which are specific for the trained task. However, the involved mechanisms are still largely unknown. It has been demonstrated that strength training could induce neuroplasticity, which might underlie the performance improvements during the first training sessions. Thus, we hypothesized to find task-specific neuroplasticity after a short-term strength training of two distinct strength tasks.

METHODS

Young healthy male subjects were exposed to 4 sessions of either maximal isometric explosive (EXPL group, N = 9) or slow sustained (SUS group, N = 10) knee extensions. Pre- and post-training, we measured H-reflexes and motor evoked potentials (MEPs) in the vastus lateralis (VL) at the onset of both strength tasks.

RESULTS

Pre- and post-training, H-reflexes remained unchanged in both groups. MEP areas were lower in the trained task in both groups and remained unchanged in the untrained task.

CONCLUSION

This study demonstrated that short-term strength training induces specific neuroplasticity for the trained task only. The fact that MEPs were lower but H-reflex amplitudes remained unchanged at the onset of the trained tasks suggests that strength training elicited neuroplasticity at supraspinal level that most likely reflect an improved task-specific corticospinal efficiency.

The effects of d-aspartic acid supplementation in resistance-trained men over a three month training period: A randomised controlled trial

Context

Research on d-aspartic acid (DAA) has demonstrated increases in total testosterone levels in untrained men, however research in resistance-trained men demonstrated no changes, and reductions in testosterone levels. The long-term consequences of DAA in a resistance trained population are currently unknown.

Objective

To evaluate the effectiveness of DAA to alter basal testosterone levels over 3 months of resistance training in resistance-trained men.

Design

Randomised, double-blind, placebo controlled trial in healthy resistance-trained men, aged 18–36, had been performing regular resistance training exercise for at least 3 d.w^-1 for the previous 2 years. Randomised participants were 22 men (d-aspartic acid n = 11; placebo n = 11) (age, 23.8±4.9 y, training age, 3.2±1.5 y).

Intervention

D-aspartic acid (6 g.d^-1, DAA) versus equal-weight, visually-matched placebo (PLA). All participants performed 12 weeks of supervised, periodised resistance training (4 d.w^-1), with a program focusing on all muscle groups.

Measures

Basal hormones, total testosterone (TT), free testosterone (FT), estradiol (E₂), sex-hormone-binding globulin (SHBG) and albumin (ALB); isometric strength; calf muscle cross-sectional area (CSA); calf muscle thickness; quadriceps muscle CSA; quadriceps muscle thickness; evoked V-wave and H-reflexes, were assessed at weeks zero (T1), after six weeks (T2) and after 12 weeks (T3).

Results

No change in basal TT or FT were observed after the intervention. DAA supplementation (n = 10) led to a 16%, 95% CI [-27%, -5%] reduction in E₂ from T1-T3 (p<0.01). The placebo group (n = 9) demonstrated improvements in spinal responsiveness (gastrocnemius) at the level of the alpha motoneuron. Both groups exhibited increases in isometric strength of the plantar flexors by 17%, 95% CI [7%, 28%] (p<0.05) as well as similar increases in hypertrophy in the quadriceps and calf muscles.

Conclusions

The results of this paper indicate that DAA supplementation is ineffective at changing testosterone levels, or positively affecting training outcomes. Reductions in estradiol and the blunting of peripheral excitability appear unrelated to improvements from resistance training.

Trial registration

Australian New Zealand Clinical Trials Registry ACTRN12617000041358

Wrist Resistance Training Improves Motor Control and Strength

Chu, E, Kim, Y-S, Hill, G, Kim, YH, Kim, CK, and Shim, JK. Wrist resistance training improves motor control and strength. J Strength Cond Res 32(4): 962-969, 2018-The aim of this study was to investigate the effects of a 6-week direction-specific resistance training program on isometric torque control and isokinetic torque strength of the wrist joint. Nineteen subjects were randomly assigned to either the wrist training group (n = 9) or the control group (n = 10). The training group performed wrist exercises in 6 directions (flexion, extension, pronation, supination, radial deviation, and ulnar deviation), whereas the control group did not. Data were collected on the isometric torque control, 1-repetition maximum (1RM) strength, and isokinetic maximum torque (angular velocity of 60° per second wrist movements) before and after 6 weeks of resistance training and at 2-week intervals during training. The training group showed significant decreases in isometric torque control error in all 6 directions after 2 weeks of resistance training, whereas the control group did not show significant increase or decrease. After 4 weeks of training, the training group showed significant increases in maximum strength in all 6 directions as assessed by 1RM strength and isokinetic strength tests, whereas the control group did not show any statistically significant changes. This study shows that motor control significantly improves within the first 2 weeks of resistance training, whereas the wrist strength significantly improves within the first 4 weeks of resistance training. Based on the findings of this study, coaches and trainers should consider wrist resistance training to improve athletes' muscular strength and control of the wrist muscles.

The Effects of Mirror Feedback during Target Directed Movements on Ipsilateral Corticospinal Excitability

Mirror visual feedback (MVF) training is a promising technique to promote activation in the lesioned hemisphere following stroke, and aid recovery. However, current outcomes of MVF training are mixed, in part, due to variability in the task undertaken during MVF. The present study investigated the hypothesis that movements directed toward visual targets may enhance MVF modulation of motor cortex (M1) excitability ipsilateral to the trained hand compared to movements without visual targets. Ten healthy subjects participated in a 2 × 2 factorial design in which feedback (veridical, mirror) and presence of a visual target (target present, target absent) for a right index-finger flexion task were systematically manipulated in a virtual environment. To measure M1 excitability, transcranial magnetic stimulation (TMS) was applied to the hemisphere ipsilateral to the trained hand to elicit motor evoked potentials (MEPs) in the untrained first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles at rest prior to and following each of four 2-min blocks of 30 movements (B1–B4). Targeted movement kinematics without visual feedback was measured before and after training to assess learning and transfer. FDI MEPs were decreased in B1 and B2 when movements were made with veridical feedback and visual targets were absent. FDI MEPs were decreased in B2 and B3 when movements were made with mirror feedback and visual targets were absent. FDI MEPs were increased in B3 when movements were made with mirror feedback and visual targets were present. Significant MEP changes were not present for the uninvolved ADM, suggesting a task-specific effect. Analysis of kinematics revealed learning occurred in visual target-directed conditions, but transfer was not sensitive to mirror feedback. Results are discussed with respect to current theoretical mechanisms underlying MVF-induced changes in ipsilateral excitability.

Enhanced Corticospinal Excitability and Volitional Drive in Response to Shortening and Lengthening Strength Training and Changes Following Detraining

There is a limited understanding of the neurological adaptations responsible for changes in strength following shortening and lengthening resistance training and subsequent detraining. The aim of the study was to investigate differences in corticospinal and spinal responses to resistance training of the tibialis anterior muscle between shortening or lengthening muscle contractions for 4 weeks and after 2 weeks of detraining. Thirty-one untrained individuals were assigned to either shortening or lengthening isokinetic resistance training (4 weeks, 3 days/weeks) or a non-training control group. Transcranial magnetic stimulation and peripheral nerve stimulation (PNS) were used to assess corticospinal and spinal changes, respectively, at pre-, mid-, post-resistance training and post detraining. Greater increases changes (P < 0.01) in MVC were found from the respective muscle contraction training. Motor evoked potentials (expressed relative to background EMG) significantly increased in lengthening resistance training group under contraction intensities ranging from 25 to 80% of the shortening and lengthening contraction intensity (P < 0.01). In the shortening resistance training group increases were only seen at 50 and 80% of both contraction type. Volitional drive (V-wave) showed a greater increase following lengthening resistance training (57%) during maximal lengthening contractions compared to maximal shortening contractions following shortening resistance training (23%; P < 0.001). During the detraining period MVC and V-wave did not change (P > 0.05), although MEP amplitude decreased during the detraining period (P < 0.01). No changes in H-reflex were found pre to post resistance training or post detraining. Modulation in V-wave appeared to be contraction specific, whereby greatest increases occurred following lengthening resistance training. Strength and volitional drive is maintained following 2 weeks detraining, however corticospinal excitability appears to decrease when the training stimulus is withdrawn.

Acute Strength Training Increases Responses to Stimulation of Corticospinal Axons

PURPOSE

Acute strength training of forearm muscles increases resting twitch forces from motor cortex stimulation. It is unclear if such effects are spinal in origin and if they also occur with training of larger muscles. With the use of subcortical stimulation of corticospinal axons, the current study examined if one session of strength training of the elbow flexor muscles leads to spinal cord changes and if the type of training is important.

METHODS

In experiment 1, 10 subjects completed ballistic isometric training, ballistic concentric training, and no training (control) on separate days. In experiment 2, 13 subjects completed ballistic isometric training and slow-ramp isometric training. Before and after training, transcranial magnetic stimulation over the contralateral motor cortex elicited motor-evoked potentials (MEPs) in the resting biceps brachii, and electrical stimulation of corticospinal tract axons at the cervicomedullary junction elicited cervicomedullary motor-evoked potentials (CMEPs). Motor-evoked potential and CMEP twitch forces were also measured.

RESULTS

In experiment 1, CMEPs and CMEP twitch forces were significantly facilitated after ballistic isometric training compared to control. In experiment 2, MEPs, MEP twitch forces, CMEPs, and CMEP twitch forces increased for 15 to 25 min after ballistic and slow-ramp isometric training.

CONCLUSION

Via processes within the spinal cord, one session of strength training of the elbow flexors increases net output from motoneurons projecting to the trained muscles. Likely mechanisms include increased efficacy of corticospinal-motoneuronal synapses or increased motoneuron excitability. However, the rate of force generation during training is not important for inducing these changes. A concomitant increase in motor cortical excitability is likely. These short-term changes may represent initial neural adaptations to strength training.

The Magnitude of Peripheral Muscle Fatigue Induced by High and Low Intensity Single-Joint Exercise Does Not Lead to Central Motor Output Reductions in Resistance Trained Men

Purpose

To examine quadriceps muscle fatigue and central motor output during fatiguing single joint exercise at 40% and 80% maximal torque output in resistance trained men.

Method

Ten resistance trained men performed fatiguing isometric knee extensor exercise at 40% and 80% of maximal torque output. Maximal torque, rate of torque development, and measures of central motor output and peripheral muscle fatigue were recorded at two matched volumes of exercise, and after a final contraction performed to exhaustion. Central motor output was quantified from changes in voluntary activation, normalized surface electromyograms (EMG), and V-waves. Quadriceps muscle fatigue was assessed from changes in the size and shape of the resting potentiated twitch (Q._pot.tw). Central motor output during the exercise protocols was estimated from EMG and interpolated twitches applied during the task (VA_sub).

Results

Greater reductions in maximal torque and rate of torque development were observed during the 40% protocol (p<0.05). Maximal central motor output did not change for either protocol. For the 40% protocol reductions from pre-exercise in rate and amplitude variables calculated from the Q._pot.tw between 66.2 to 70.8% (p<0.001) exceeded those observed during the 80% protocol (p<0.01). V-waves only declined during the 80% protocol between 56.8 ± 35.8% to 53.6 ± 37.4% (p<0.05). At the end of the final 80% contraction VA_sub had increased from 91.2 ± 6.2% to 94.9 ± 4.7% (p = 0.005), but a greater increase was observed during the 40% contraction where VA_sub had increased from 67.1 ± 6.1% to 88.9 ± 9.6% (p<0.001).

Conclusion

Maximal central motor output in resistance trained men is well preserved despite varying levels of peripheral muscle fatigue. Upregulated central motor output during the 40% contraction protocol appeared to elicit greater peripheral fatigue. V-waves declines during the 80% protocol suggest intensity dependent modulation of the Ia afferent pathway.

The effects of a protein enriched diet with lean red meat combined with a multi-modal exercise program on muscle and cognitive health and function in older adults: study protocol for a randomised controlled trial

Background

Age-related muscle wasting has been strongly implicated with falls and fractures in the elderly, but it has also been associated with cognitive decline and dementia. Progressive resistance training (PRT) and adequate dietary protein are recognised as important contributors to the maintenance of muscle health and function in older adults. However, both factors also have the potential to improve brain function and prevent cognitive decline via several pathways, including the regulation of various growth and neurotrophic factors [insulin-like growth factor-1 (IGF-1)]; brain-derived growth factor (BDNF)] and/or the modulation of systemic inflammation. The primary aim of this study is to investigate whether a modest increase in dietary protein achieved through the consumption of lean red meat three days per week, when combined with PRT, can enhance muscle mass, size and strength and cognitive function in community-dwelling older people.

Methods/Design

The study design is a 48-week randomised controlled trial consisting of a 24-week intervention with a 24-week follow-up. Men and women (n=152) aged 65 years and over residing in the community will be randomly allocated to: 1) PRT and provided with 220 g (raw weight) of lean red meat to be cooked and divided into two 80 g servings on each of the three days that they complete their exercise session, or 2) control PRT in which participants will be provided with and advised to consume ≥1 serving (~1/2 cup) of rice and/or pasta or 1 medium potato on each of the three training days. The primary outcome measures will be muscle mass, size and strength and cognitive function. Secondary outcomes will include changes in: muscle function, neural health (corticospinal excitability and inhibition and voluntary activation), serum IGF-1 and BDNF, adipokines and inflammatory markers, fat mass and inter-/intra-muscular fat, blood pressure, lipids and health-related quality of life. All outcome measures will be assessed at baseline and 24 weeks, with the exception of cognitive function and the various neurobiological and inflammatory markers which will also be assessed at week 12.

Discussion

The findings from this study will provide important new information on whether a modest increase in dietary protein achieved through the ingestion of lean red meat can enhance the effects of PRT on muscle mass, size and strength as well as cognitive function in community-dwelling older adults. If successful, the findings will form the basis for more precise exercise and nutrition guidelines for the management and prevention of age-related changes in muscle and neural health and cognitive function in the elderly.

Trial registration

Australian New Zealand Clinical Trials Registry: ACTRN12613001153707. Date registered 16^th October, 2013.

Endocrine alterations from concentric vs. eccentric muscle actions: a brief review

Resistance exercise has a positive effect on many tissues, including heart, bone, skeletal muscle, and nervous tissue. Eccentric muscle actions offer a unique and a potentially beneficial form of exercise for maintaining and improving health. During resistance exercise, the effects of gravity, and mechanical properties of the sarcomere and connective tissue in skeletal muscle allow a greater muscle load during an eccentric (lengthening) muscle contraction than a concentric (shortening) muscle contraction. Consequently, older patients, patients with muscle or limb movement limitations or injuries, as well as cancer patients may be able to benefit from isolated eccentric muscle actions. There are specific physiological responses to eccentric muscle contractions. This review will describe the effects of different eccentric muscle contraction protocols on endocrine responses that could have positive effects on different tissues and recommend direction for future research.

Differences in supraspinal and spinal excitability during various force outputs of the biceps brachii in chronic- and non-resistance trained individuals

Motor evoked potentials (MEP) and cervicomedullary evoked potentials (CMEP) may help determine the corticospinal adaptations underlying chronic resistance training-induced increases in voluntary force production. The purpose of the study was to determine the effect of chronic resistance training on corticospinal excitability (CE) of the biceps brachii during elbow flexion contractions at various intensities and the CNS site (i.e. supraspinal or spinal) predominantly responsible for any training-induced differences in CE. Fifteen male subjects were divided into two groups: 1) chronic resistance-trained (RT), (n = 8) and 2) non-RT, (n = 7). Each group performed four sets of ∼5 s elbow flexion contractions of the dominant arm at 10 target forces (from 10%-100% MVC). During each contraction, subjects received 1) transcranial magnetic stimulation, 2) transmastoid electrical stimulation and 3) brachial plexus electrical stimulation, to determine MEP, CMEP and compound muscle action potential (Mmax) amplitudes, respectively, of the biceps brachii. All MEP and CMEP amplitudes were normalized to Mmax. MEP amplitudes were similar in both groups up to 50% MVC, however, beyond 50% MVC, MEP amplitudes were lower in the chronic RT group (p<0.05). CMEP amplitudes recorded from 10-100% MVC were similar for both groups. The ratio of MEP amplitude/absolute force and CMEP amplitude/absolute force were reduced (p<0.012) at all contraction intensities from 10-100% MVC in the chronic-RT compared to the non-RT group. In conclusion, chronic resistance training alters supraspinal and spinal excitability. However, adaptations in the spinal cord (i.e. motoneurone) seem to have a greater influence on the altered CE.

Neural pathways mediating cross education of motor function

Cross education is the process whereby training of one limb gives rise to enhancements in the performance of the opposite, untrained limb. Despite interest in this phenomenon having been sustained for more than a century, a comprehensive explanation of the mediating neural mechanisms remains elusive. With new evidence emerging that cross education may have therapeutic utility, the need to provide a principled evidential basis upon which to design interventions becomes ever more pressing. Generally, mechanistic accounts of cross education align with one of two explanatory frameworks. Models of the “cross activation” variety encapsulate the observation that unilateral execution of a movement task gives rise to bilateral increases in corticospinal excitability. The related conjecture is that such distributed activity, when present during unilateral practice, leads to simultaneous adaptations in neural circuits that project to the muscles of the untrained limb, thus facilitating subsequent performance of the task. Alternatively, “bilateral access” models entail that motor engrams formed during unilateral practice, may subsequently be utilized bilaterally—that is, by the neural circuitry that constitutes the control centers for movements of both limbs. At present there is a paucity of direct evidence that allows the corresponding neural processes to be delineated, or their relative contributions in different task contexts to be ascertained. In the current review we seek to synthesize and assimilate the fragmentary information that is available, including consideration of knowledge that has emerged as a result of technological advances in structural and functional brain imaging. An emphasis upon task dependency is maintained throughout, the conviction being that the neural mechanisms that mediate cross education may only be understood in this context.

Corticospinal responses of resistance-trained and un-trained males during dynamic muscle contractions

Little is known regarding the modulation and the plasticity of the neural pathway interconnecting elements of the central nervous system and skeletal muscle in resistant-trained individuals. The aim of the study was to compare corticospinal and spinal responses measured during dynamic muscle contractions of the tibialis anterior in resistance trained (RT) and un-trained (UT) males. Nine UT and 10 RT male volunteers reported to the laboratory 24h following a familiarisation session. Motor evoked potentials (MEPs) and the cortical silent period were evoked using transcranial magnetic stimulation at a range of contraction intensities and was delivered as the ankle passed 90° during shortening and lengthening contractions. The Hoffmann reflex (H-reflex) and V-waves were evoked with peripheral nerve stimulation. Despite the RT group being significantly stronger during shortening (28%; P=0.023: CI=1.27-15.1Nm), lengthening (25%; P=0.041: CI=0.27-17.0Nm) and isometric muscle actions (20%; P=0.041; CI=0.77-14.9Nm), no differences between the groups existed for corticospinal or spinal variables. Lack of detectable differences between RT and UT individuals may be linked to minimal exposure to task specific, isolated high intensity resistance training of the TA muscle.

Active robotic training improves locomotor function in a stroke survivor

Background

Clinical outcomes after robotic training are often not superior to conventional therapy. One key factor responsible for this is the use of control strategies that provide substantial guidance. This strategy not only leads to a reduction in volitional physical effort, but also interferes with motor relearning.

Methods

We tested the feasibility of a novel training approach (active robotic training) using a powered gait orthosis (Lokomat) in mitigating post-stroke gait impairments of a 52-year-old male stroke survivor. This gait training paradigm combined patient-cooperative robot-aided walking with a target-tracking task. The training lasted for 4-weeks (12 visits, 3 × per week). The subject’s neuromotor performance and recovery were evaluated using biomechanical, neuromuscular and clinical measures recorded at various time-points (pre-training, post-training, and 6-weeks after training).

Results

Active robotic training resulted in considerable increase in target-tracking accuracy and reduction in the kinematic variability of ankle trajectory during robot-aided treadmill walking. These improvements also transferred to overground walking as characterized by larger propulsive forces and more symmetric ground reaction forces (GRFs). Training also resulted in improvements in muscle coordination, which resembled patterns observed in healthy controls. These changes were accompanied by a reduction in motor cortical excitability (MCE) of the vastus medialis, medial hamstrings, and gluteus medius muscles during treadmill walking. Importantly, active robotic training resulted in substantial improvements in several standard clinical and functional parameters. These improvements persisted during the follow-up evaluation at 6 weeks.

Conclusions

The results indicate that active robotic training appears to be a promising way of facilitating gait and physical function in moderately impaired stroke survivors.

Muscle coordination is habitual rather than optimal

When sharing load among multiple muscles, humans appear to select an optimal pattern of activation that minimizes costs such as the effort or variability of movement. How the nervous system achieves this behavior, however, is unknown. Here we show that contrary to predictions from optimal control theory, habitual muscle activation patterns are surprisingly robust to changes in limb biomechanics. We first developed a method to simulate joint forces in real time from electromyographic recordings of the wrist muscles. When the model was altered to simulate the effects of paralyzing a muscle, the subjects simply increased the recruitment of all muscles to accomplish the task, rather than recruiting only the useful muscles. When the model was altered to make the force output of one muscle unusually noisy, the subjects again persisted in recruiting all muscles rather than eliminating the noisy one. Such habitual coordination patterns were also unaffected by real modifications of biomechanics produced by selectively damaging a muscle without affecting sensory feedback. Subjects naturally use different patterns of muscle contraction to produce the same forces in different pronation-supination postures, but when the simulation was based on a posture different from the actual posture, the recruitment patterns tended to agree with the actual rather than the simulated posture. The results appear inconsistent with computation of motor programs by an optimal controller in the brain. Rather, the brain may learn and recall command programs that result in muscle coordination patterns generated by lower sensorimotor circuitry that are functionally "good-enough."

Assessing the neurophysiological effects of botulinum toxin treatment for adults with focal limb spasticity: a systematic review

To examine effectiveness of botulinum toxin therapy (BoNT), accurate quantification of the neurological component of underlying spasticity is needed.

PURPOSE

Systematically identify and summarize literature on neurophysiological methods to test the effectiveness of BoNT, discuss methodological aspects, and recommend best techniques for clinical and research settings.

METHODS

Electronic databases were searched using specific keywords. Reviews and reference lists were hand-checked to find relevant studies. Studies in adults with focal limb spasticity were included.

RESULTS

Over 300 articles were scanned for relevance, and a list of 90 relevant articles was made. After excluding reviews and studies in healthy controls or non-spasticity related BoNT, 22 articles were reviewed. Neurophysiological techniques included electromyography (EMG; n = 11), electrical nerve stimulation (n = 13), transcranial magnetic stimulation (TMS; n = 1), functional magnetic resonance imaging (fMRI; n = 1), reflex threshold (n = 4), Achilles tendon reflex (n = 1), reflex latency (n = 1).

DISCUSSION

Maximum M-wave amplitude (M-max) and stretch reflex threshold techniques were successfully used to measure the neurophysiological effects of BoNT and have high test-retest reliability. EMG techniques require effective magnitude normalization techniques and establishing test-retest reliability.

CONCLUSION

Serial electrical nerve stimulation can be incorporated into clinical practice to assist in follow-up. We recommend using M-max and stretch reflex threshold techniques to assess the neurophysiological effects.

Early neural responses to strength training

The neural adaptations that accompany strength training have yet to be fully determined. Here we sought to address this topic by testing the idea that strength training might share similar mechanisms with some forms of motor learning. Since ballistic motor learning is accompanied by a shift in muscle twitches induced by transcranial magnetic stimulation (TMS) toward the training direction, we sought to investigate if these changes also occur after single isometric strength training sessions with various contraction duration and rate of force development characteristics (i.e., brief or sustained ballistic contractions or slow, sustained contractions). Twitch force resultant vectors and motor-evoked potentials (MEPs) induced by TMS were measured before and after single sessions of strength training involving the forearm muscles. Participants ( n = 12) each performed three training protocols (each consisting of 4 sets of 10 repetitions) and served as their own control in a counterbalanced order. All three training protocols caused a significant ( P < 0.05) shift in TMS-induced twitch force resultant vectors toward the training direction, followed by a gradual shift back toward the pretraining direction. The strongest effect was found when training involved both ballistic and sustained force components. There were no large or consistent changes in the direction of twitches evoked by motor nerve stimulation for any of the three training protocols. We suggest that these early neural responses to strength training, which share similar corticospinal changes to motor learning, might reflect an important process that precedes more long-term neural adaptation that ultimately enhance strength.

Neural adaptations to strength training: moving beyond transcranial magnetic stimulation and reflex studies

It has long been believed that training for increased strength not only affects muscle tissue, but also results in adaptive changes in the central nervous system. However, only in the last 10 years has the use of methods to study the neurophysiological details of putative neural adaptations to training become widespread. There are now many published reports that have used single motor unit recordings, electrical stimulation of peripheral nerves, and non-invasive stimulation of the human brain [i.e. transcranial magnetic stimulation (TMS)] to study neural responses to strength training. In this review, we aim to summarize what has been learned from single motor unit, reflex and TMS studies, and identify the most promising avenues to advance our conceptual understanding with these methods. We also consider the few strength training studies that have employed alternative neurophysiological techniques such as functional magnetic resonance imaging and electroencephalography. The nature of the information that these techniques can provide, as well as their major technical and conceptual pitfalls, are briefly described. The overall conclusion of the review is that the current evidence regarding neural adaptations to strength training is inconsistent and incomplete. In order to move forward in our understanding, it will be necessary to design studies that are based on a rigorous consideration of the limitations of the available techniques, and that are specifically targeted to address important conceptual questions.