The Effect of Task Complexity Influencing Bilateral Transfer.

International Journal of Exercise Science 10(8): 1174-1183, 2017. Bilateral transfer is a well-known phenomenon whereby training one limb results in improvement in the untrained homologous limb. However, despite evidence across a range of motor skill paradigms, the influence of motor skill complexity on the magnitude of bilateral transfer has not yet been fully explored. The aim of this preliminary study was to compare bilateral transfer effects between three dexterity tasks with the hypothesis that the complexity of the task, the volume of time training, and the amount of improvement in the trained hand would positively influence bilateral transfer. Using a randomized cross-over design, 14 young healthy participants (mean age of 22.6 ± 6.6 years; eight female) completed three finger dexterity tasks (O’Connor dexterity, Purdue pegboard, and Mirror Purdue pegboard tasks) with one week rest between each task. Each task required training with the participant’s dominant hand with pre and post testing in both the dominant and non-dominant hands. The Mirrored Purdue pegboard task showed the greatest rate of improvement in the dominant hand. Similarly, the greatest bilateral transfer effect was found in the Mirrored Purdue task. Interestingly, the amount of time training was not a factor associated with bilateral transfer. In conclusion, this study has demonstrated that the value of task complexity, but not the volume of practice, correlated with the magnitude of bilateral transfer to the non-dominant hand

Priming the Motor Cortex With Anodal Transcranial Direct Current Stimulation Affects the Acute Inhibitory Corticospinal Responses to Strength Training

Synaptic plasticity in the motor cortex (M1) is associated with strength training and can be modified by transcranial direct current stimulation (tDCS). The M1 responses to strength training increase when anodal-tDCS is applied during training due to gating. An additional approach to improve the M1 responses to strength training, which has not been explored, is to use anodal-tDCS to prime the M1 before a bout of strength training. We examined the priming effects of anodal-tDCS of M1 on the acute corticospinal responses to strength training. In a randomized double-blinded cross-over design, changes in isometric strength, corticospinal-excitability and inhibition (assessed as area under the recruitment curve [AURC] using transcranial magnetic stimulation [TMS]) were analysed in 13 adults exposed to 20-min of anodal and sham-tDCS followed by a strength training session of the right elbow-flexors. We observed a significant decrease in isometric elbow-flexor strength immediately following training (11-12%; P < 0.05) which was not different between anodal-tDCS and sham-tDCS. TMS revealed a 24% increase in AURC for corticospinal-excitability following anodal-tDCS and strength training; this increase was not different between conditions. However, there was a 14% reduction in AURC for corticospinal inhibition when anodal-tDCS was applied prior to strength training when compared to sham-tDCS and strength training (all P < 0.05). Priming anodal-tDCS had a limited effect in facilitating corticospinal-excitability following an acute bout of strength training. nterestingly, the interaction of anodal-tDCS and strength training appears to affect the excitability of intracortical inhibitory circuits of the M1 via non-homeostatic mechanisms

Cross-education of wrist extensor strength is not influenced by non-dominant training in right-handers

Purpose Cross-education of strength has been proposed to be greater when completed by the dominant limb in right handed humans. We investigated whether the direction of cross-education of strength and corticospinal plasticity are different following right or left limb strength training in right-handed participants. Methods Changes in strength, muscle thickness and indices of corticospinal plasticity were analyzed in 23 adults who were exposed to 3-weeks of either right-hand strength training (RHT) or left-hand strength training (LHT). Results Maximum voluntary wrist extensor strength in both the trained and untrained limb increased, irrespective of which limb was trained, with TMS revealing reduced corticospinal inhibition. Conclusions Cross-education of strength was not limited by which limb was trained and reduced corticospinal inhibition was not just confined to the trained limb. Critically, from a behavioral perspective, the magnitude of cross-education was not limited by which limb was trained

Task‐specific strength increases after lower‐limb compound resistance training occurred in the absence of corticospinal changes in vastus lateralis

Neural adaptations subserving strength increases have been shown to be task‐specific, but responses and adaptation to lower‐limb compound exercises such as the squat are commonly assessed in a single‐limb isometric task. This two‐part study assessed neuromuscular responses to an acute bout (Study A) and 4 weeks (Study B) of squat resistance training at 80% of one‐repetition‐maximum, with measures taken during a task‐specific isometric squat (IS) and non‐specific isometric knee extension (KE). Eighteen healthy volunteers (25 ± 5 years) were randomised into either a training (n = 10) or a control (n = 8) group. Neural responses were evoked at the intracortical, corticospinal and spinal levels, and muscle thickness was assessed using ultrasound. The results of Study A showed that the acute bout of squat resistance training decreased maximum voluntary contraction (MVC) for up to 45 min post‐exercise (−23%, P < 0.001). From 15–45 min post‐exercise, spinally evoked responses were increased in both tasks (P = 0.008); however, no other evoked responses were affected (P ≥ 0.240). Study B demonstrated that following short‐term resistance training, participants improved their one repetition maximum squat (+35%, P < 0.001), which was reflected by a task‐specific increase in IS MVC (+49%, P = 0.001), but not KE (+1%, P = 0.882). However, no training‐induced changes were observed in muscle thickness (P = 0.468) or any evoked responses (P = 0.141). Adjustments in spinal motoneuronal excitability are evident after acute resistance training. After a period of short‐term training, there were no changes in the responses to central nervous system stimulation, which suggests that alterations in corticospinal properties of the vastus lateralis might not contribute to increases in strength

Adaptations in corticospinal excitability and inhibition are not spatially confined to the agonist muscle following strength training

Purpose: We used transcranial magnetic stimulation (TMS) to determine the corticospinal responses from an agonist and synergist muscle following strength training of the right elbow flexors. Methods: Motor evoked potentials were recorded from the Biceps Brachii and Flexor Carpi Radialis during a submaximal contraction from 20 individuals (10 women, 10 men, aged 18-35 years; training group; n = 10 and control group; n = 10) before and after three weeks of strength training at 80% of 1-repetition maximum (1-RM). To characterise the input-output properties of the corticospinal tract, stimulus-response curves for corticospinal excitability and inhibition of the right Biceps Brachii and Flexor Carpi Radialis were constructed and assessed by examining the area under the recruitment curve (AURC). Results: Strength training resulted in a 29% (P < 0.001) increase in 1-RM Biceps Brachii strength and this was accompanied by a 19% increase in isometric strength of the wrist flexors (P = 0.001). TMS revealed an increase in corticospinal excitability AURC and a decrease in silent period duration AURC for the Biceps Brachii and Flexor Carpi Radialis following strength training (all P < 0.05). However, the changes in corticospinal function were not associated with increased muscle strength. Conclusion: These findings show that the corticospinal responses to strength training of a proximal upper limb muscle is not spatially restricted, but rather, results in a change in connectivity, among an agonist and a synergistic muscle relevant to force production

A single session of submaximal grip strength training with or without high-definition anodal-TDCS produces no cross-education of maximal force

BACKGROUND: Previous studies suggest that cross-education of strength may be modulated by increased corticospinal excitability of the ipsilateral primary motor cortex (M1) due to cross-activation. However, no study has examined the influence of bilateral TDCS of both M1 and how it affects corticospinal excitability, cross-activation and cross-education of muscle strength.METHOD: Twelve participants underwent three conditions in a randomized crossover design: (1) submaximal grip training and single-site unilateral-high definition-TDCS (2) submaximal grip training and bilateral anodal-high definition-TDCS, and (3) submaximal grip training and sham-high definition-TDCS. Submaximal gripping task involved a single-session of unilateral training which was squeezing the transducer at 70% of maximum voluntary isometric contraction (MVIC) grip force and performing four sets of 10 isometric contractions. Anodal-high definition-TDCS was applied for 15 min at 1.5 mA over right M1 or left and right M1s, and in a sham condition. Participants were pseudorandomized to receive either single-site or bilateral M1 stimulation with each session separated by one-week. Before and after each session, MVIC force of ipsilateral and contralateral gripping, ipsilateral stimulus-response curve, short-interval intracortical inhibition, cortical silent period, intracortical facilitation, long-interval intracortical inhibition, and cross-activation were measured.RESULTS: MVIC of the trained arm decreased by 43% (P=0.04) after training. We observed no changes in MVIC of the untrained hand and in any of the TMS measures (all P&gt;0.05).CONCLUSION: A single session of submaximal grip training with or without anodal-high definition-TDCS produces no cross-education of maximal grip force nor does it affect the excitability of the ipsilateral M1

Evidence of Altered Corticomotor Inhibition in older Adults with a History of Repetitive Neurotrauma. A Transcranial Magnetic Stimulation study

International concern continues regarding the association between the long-term neurophysiologic changes from repetitive neurotrauma associated with contact and collision sports. This study describes corticomotor changes in retired contact/collision sport athletes and controls, between the ages of 30 and 70 years. Retired athletes (n=152; 49.1±8.5 years) and controls (n=72; 47.8±9.5 years) were assessed using single and paired-pulse transcranial magnetic stimulation (TMS) for active motor threshold (aMT), motor evoked potential and cortical silent period duration (expressed as MEP:cSP ratio), and short- and long-interval intracortical inhibition (SICI and LICI). Motor threshold, MEP:cSP, SICI and LICI for both groups were correlated across age. Controls showed significant moderate correlations for MEP:cSP ratios at 130% (rho=0.48, p50 years), this study is the first to characterize corticomotor differences between retired athletes and controls across the lifespan. These results, demonstrating pathophysiological differences in retired athletes across the lifespan, provide a foundation to utilise evoked potentials as a prodromal marker in supplementing neurological assessment for traumatic encephalopathy syndrome associated with contact/collision sport athletes that is currently lacking physiological biomarkers

Unilateral Strength Training Imparts a Cross-Education Effect in Unilateral Knee Osteoarthritis Patients

Background: Worldwide, 86 million individuals over the age of 20 were diagnosed with knee osteoarthritis (KOA) in 2020. Hallmark features of KOA are the loss in knee extensor strength, increasing knee pain severity, and deficits in functional performance. There is a critical need for the investigation into potential cost-effective therapeutic interventions in the treatment of KOA. A potential therapeutic option is the cross-education phenomenon. Methods: This was a non-blinded randomized control trial, with a 4-week intervention, with a pre, post and follow-up assessment (3 months post intervention). Outcome measures of isometric knee extensor strength, rectus femoris muscle thickness and neuromuscular activation were assessed at all-time points. Results: Compared to age-matched KOA controls, 4 weeks of unilateral strength training in end-stage KOA patients increased strength of the untrained affected KOA limb by 20% (p &lt; 0.05) and reduced bilateral hamstring co-activation in the KOA intervention group compared to the KOA control group (p &lt; 0.05). Conclusions: A 4-week-long knee extensor strength training intervention of the contralateral limb in a cohort with diagnosed unilateral KOA resulted in significant improvements to knee extensor strength and improved neuromuscular function of the KOA limb. Importantly, these results were maintained for 3 months following the intervention

Tracking the corticospinal responses to strength training

Purpose The motor cortex (M1) appears to be a primary site of adaptation following both a single session, and repeated strength-training sessions across multiple weeks. Given that a single session of strength-training is sufficient to induce modification at the level of the M1 and corticospinal tract, this study sought to determine how these acute changes in M1 and corticospinal tract might accumulate across the course of a 2-week heavy-load strength-training program. Methods Transcranial magnetic stimulation (TMS) was used to infer corticospinal excitability (CSE), intracortical facilitation (ICF), short and long-interval intracortical inhibition (SICI and LICI) and silent period duration prior to and following each training session during a 2-week heavy-load strength-training period. Results Following 2-weeks of strength-training, increases in strength (15.5%, P = 0.01) were accompanied by an increase in CSE (44%, P = 0.006) and reductions in both silent period duration (14%, P < 0.0001) and SICI (35%, P = 0.0004). Early training sessions acutely increased CSE and ICF, and acutely reduced silent period duration and SICI. However, later training sessions failed to modulate SICI and ICF, with substantial adaptations occurring offline between training sessions. No acute or retained changes in LICI were observed. Co-contraction of antagonists reduced by 36% following 2-weeks of strength-training. Conclusions Collectively, these results indicate that corticospinal plasticity occurs within and between training sessions throughout a training period in distinct early and later stages that are modulated by separate mechanisms of plasticity. The development of strength is akin to the previously reported changes that occur following motor skill training

Corticospinal and spinal adaptations to motor skill and resistance training: Potential mechanisms and implications for motor rehabilitation and athletic development.

Optimal strategies for enhancing strength and improving motor skills are vital in athletic performance and clinical rehabilitation. Initial increases in strength and the acquisition of new motor skills have long been attributed to neurological adaptations. However, early increases in strength may be predominantly due to improvements in inter-muscular coordination rather than the force-generating capacity of the muscle. Despite the plethora of research investigating neurological adaptations from motor skill or resistance training in isolation, little effort has been made in consolidating this research to compare motor skill and resistance training adaptations. The findings of this review demonstrated that motor skill and resistance training adaptations show similar short-term mechanisms of adaptations, particularly at a cortical level. Increases in corticospinal excitability and a release in short-interval cortical inhibition occur as a result of the commencement of both resistance and motor skill training. Spinal changes show evidence of task-specific adaptations from the acquired motor skill, with an increase or decrease in spinal reflex excitability, dependant on the motor task. An increase in synaptic efficacy of the reticulospinal projections is likely to be a prominent mechanism for driving strength adaptations at the subcortical level, though more research is needed. Transcranial electric stimulation has been shown to increase corticospinal excitability and augment motor skill adaptations, but limited evidence exists for further enhancing strength adaptations from resistance training. Despite the logistical challenges, future work should compare the longitudinal adaptations between motor skill and resistance training to further optimise exercise programming