Neuromuscular Mechanisms Underlying Changes in Force Production during an Attentional Focus Task

Combining an Internal Attentional Focus With Mirror Motor Observation Enhances Mechanical Output During Isokinetic Leg-Extension Exercise

When carrying out a motor task, the direction of the performer's attentional focus can affect mechanical output and muscle activation. Cortical excitability increases with the observation of motor skills. However, it is unknown if this effect can be additive to that resulting from an internal attentional focus during resistance exercise. A crossover-study design was employed to examine the acute effects of combining mirror self-motor observation (MO) and internally focused verbal instructions on mechanical output and muscle activation during isokinetic concentric knee-extension exercise. Ten participants were tested in 2 different conditions: verbal alone and verbal + MO-mirror. The combination of verbal + MO-mirror attenuated the decrease in torque output in response to 6 sets of 10 isokinetic contractions (P = .043). Interestingly, this effect was paired by a lower-level antagonist/agonist coactivation with the verbal + MO-mirror condition (P = .031). No other differences between conditions were noted. Taken together, these results suggest that the combination of both cueing modalities elicits a more effective contraction strategy during knee-extension exercise. Ultimately, this provides preliminary evidence of better motor performance and heightened fatigue resistance in response to isokinetic exercise.

Effects of Diabetes and Attentional Focus on Learning of a Novel Balance Task

BACKGROUND AND PURPOSE

Chronic diabetes is a prevalent systemic disease that impairs neuromotor functioning and often leads to increased risk of falls. Adopting an external focus of attention during motor skill practice has been shown to improve learning outcomes; however, it has not been examined in this population. We examined how attentional focus instructions (internal vs external) affect balance performance and learning in older adults with and without diabetes.

METHODS

Fifty-three older adults (27 with diabetes, 63.7 ± 7.0 years) participated in the randomized, pre-post intervention study. The balance training involved 50 practice trials of a stabilometer task that was novel to all participants. Participants were randomized to receive either internal or external focus task instruction. Task performance was assessed at baseline, during training, and during a retention test. Primary outcomes were changes in balance task performance before and after training.

RESULTS

Participants who received external focus instruction showed a significantly greater increase in balance performance than individuals who received internal focus instruction (95% confidence interval, 0.02-4.05; P = 0.048). While participants with diabetes exhibited poorer baseline task performance (P = 0.02), both groups improved their relative task performance after training (95% confidence interval, 5.25-18.14; P < 0.0001).

DISCUSSION AND CONCLUSIONS

Adopting an external focus of attention benefits performance during short-term training of a novel balance task in older adults with and without diabetes. Participants with diabetes were capable of learning the challenging balance task with practice, at a relative rate similar to those without diabetes. This information may be useful for designing interventional strategies to improve physical function and mitigate fall risks in older adults with diabetes.

Attentional focus differentially modulates the corticospinal and intracortical excitability during dynamic and static exercise

Although attentional focus affects motor performance, whether corticospinal excitability and intracortical modulations differ between focus strategies depending on the exercise patterns remains unclear. In the present study, using single- and paired-pulse transcranial magnetic stimulation and peripheral nerve stimulation, we demonstrated changes in the cortical and spinal excitability under external focus (EF) and internal focus (IF) conditions with dynamic or static exercise. Participants performed the ramp-and-hold contraction task of right index finger abduction against an object (sponge or wood) with both exercises. They were asked to concentrate on the pressure on the sponge/wood induced by finger abduction under the EF condition, and on the index finger itself under the IF condition. Motor-evoked potential (MEP) and F-wave in the premotor, phasic, or tonic phase, and short- and long-interval intracortical inhibition (SICI and LICI, respectively), and intracortical facilitation (ICF) in the premotor phase were examined by recording surface electromyographic activity in the right first dorsal interosseous muscle. Increments in the MEP amplitude were larger under the EF condition than under the IF condition in the dynamic, but not static, exercise. The F-wave, SICI, and LICI did not differ between focus conditions in both exercises. In the dynamic exercise, interestingly, ICF was greater under the EF condition than under the IF condition and positively correlated with the MEP amplitude. These results indicate that corticospinal excitability and intracortical modulations to attentional focus differ depending on exercise patterns, suggesting that attentional focus differentially affects the central nervous system responsible for diverse motor behaviors.NEW & NOTEWORTHY We investigated attentional focus-dependent corticospinal and intracortical modulations in dynamic or static exercise. The corticospinal excitability was modulated differentially depending on the focus of attention during dynamic, but not static exercise. Although the reduction of intracortical GABAergic inhibition was comparable between focus conditions in both exercises, intracortical facilitation was smaller when focusing on the internal environments in the dynamic exercise, resulting in lower activation of the corticospinal tract.

Within- and between-day reliability and repeatability of neuromuscular function assessment in females and males

The study evaluated the reliability and repeatability of the force and surface electromyography activity (EMG) outcomes obtained through voluntary and electrically evoked contractions of knee extensors in females (n = 18) and males (n = 20) and compared these data between sexes. Maximal isometric voluntary contractions (iMVCs) of knee extensors associated with electrical stimulation of the femoral nerve were performed over 4 days (48-h interval), with the first day involving familiarization procedures, the second involving three trials (1-h interval), and the third and fourth involving just one trial. The intraclass correlation coefficient (ICC), coefficient of variation (CV), and repeatability of outcomes from within- and between-day trials were determined for each sex. Females presented lower maximal voluntary force during iMVC (iMVCForce) and associated vastus lateralis EMG activity (root mean square, RMSVL), force evoked by potentiated doublet high-frequency (Db100Force) and single stimuli (Qtw), and M-wave amplitude than males (P ≤ 0.01, partial eta squared ≥0.94). Voluntary activation (VA) and RMSVL/M-wave amplitude did not differ between sexes. iMVCForce, VA, Db100Force, Qtw, and M-wave amplitude were the most reliable outcomes in within-day trials, with similar results between sexes (ICC > 0.62; CV < 6.4%; repeatability: 12.2%-22.6%). When investigating between-day trials, the iMVCForce, VA, Db100Force, and Qtw were the most reliable (ICC > 0.66; CV < 7.5%; repeatability: 13.2%-33.45%) with similar results between sexes. In conclusion, females presented lower iMVCForce and evoked response than males. Although reliability and repeatability statistics vary between trials, data (e.g., from EMG or force signal), and sexes, most of the outcomes obtained through this technique are reliable in females and males.NEW & NOTEWORTHY Although reliability and repeatability of knee extensors vary according to the type of neuromuscular function outcome (e.g., from force or EMG responses), the trial intervals (i.e., hours or days), and the sex of the participant, most force and EMG outcomes obtained through these neuromuscular assessment protocols present ICC > 0.75, very good CV (<10%), and repeatability <25% in within- and between-day trials in both sexes.

The Influence of Resistance Training Experience on the Efficacy of Motor Imagery for Acutely Increasing Corticospinal Excitability

Both motor imagery and resistance-training enhance motor function and corticospinal excitability. We tested the hypothesis that young participants with significant resistance-training experience would show heightened corticospinal excitability during a single session of motor imagery training. Fifty-six participants (mean ± SD age = 22 ± 2 years) were divided into resistance-trained and untrained groups. Forty-one upper-body resistance trained (21 males, 20 females; mean ± SD relative one repetition maximum bench press = 0.922 ± 0.317 kg/kg) and 15 untrained (4 males, 11 females; mean ± SD relative one repetition maximum bench press = 0.566 ± 0.175 kg/kg) participants visited the laboratory on three separate occasions. The first visit served as the familiarization session. During visits 2 and 3, participants engaged in a hand/wrist motor imagery protocol or rested quietly (control condition) in a randomized order. Before and after the interventions, single-pulse transcranial magnetic stimulation (TMS) over the motor cortex was used to measure resting motor-evoked potential amplitude of the first dorsal interosseous muscle. Our main finding was that motor imagery acutely increased corticospinal excitability by ~64% (marginal means pre = 784.1 µV, post = 1246.6 µV; p < 0.001, d = 0.487). However, there was no evidence that the increase in corticospinal excitability was influenced by resistance-training experience. We suspect that our results may have been influenced by the specific nature of the motor imagery task. Our findings have important implications for motor imagery prescription and suggest that motor imagery training may be equally beneficial for both resistance-trained and untrained populations. This study was prospectively registered at ClinicalTrials.gov (Identifier: NCT03889548).

External and internal focus of attention differentially modulate corticospinal excitability in anticipatory postural adjustments

Whether attentional focus modulates the corticospinal excitability of the lower limb muscles in anticipatory postural adjustments (APAs) when performing a ballistic movement of the upper limb remains unclear. The present study used transcranial magnetic stimulation (TMS) to examine the corticospinal excitability of the lower limb muscles along with the kinematic profiles during dart throwing with different attentional foci, external focus (EF) and internal focus (IF). In 13 healthy participants, TMS was applied immediately before electromyographic onset of the tibialis anterior (TA) muscle, and the motor evoked potential (MEP) was recorded in the TA and soleus (SOL) muscles. The performance accuracy was significantly higher in the EF condition than in the IF condition. In both EF and IF conditions, MEP amplitude in the TA muscle, but not the SOL muscle, was significantly higher immediately before TA muscle onset (- 100, - 50, and 0 ms) compared to the control. In particular, the MEP increment in the TA muscle before TA muscle onset (- 50 and 0 ms) was significantly larger in the EF condition than in the IF condition. Our findings provide the first evidence for the modulation of corticospinal excitability in APA by changing attentional focus.

Effects of Attentional Focus on Muscular Endurance: A Meta-Analysis

Several studies explored the effects of attentional focus on resistance exercise, but their analysed outcomes most commonly involved surface electromyography variables. Therefore, the effects of attentional focus on resistance exercise performance remain unclear. The aim of this review was to perform a meta-analysis examining the acute effects of external focus vs. internal focus vs. control on muscular endurance. Five databases were searched to find relevant studies. The data were pooled in a random-effects meta-analysis. In the analysis for external vs. internal focus of attention, there were seven comparisons with 14 study groups. In the analyses for external focus vs. control and internal focus vs. control, there were six comparisons with 12 study groups. An external focus of attention enhanced muscular endurance when compared with an internal focus (Cohen's d: 0.58; 95% confidence interval (CI): 0.34 and 0.82) and control (Cohen's d: 0.42; 95% CI: 0.08 and 0.76). In the analysis for internal focus vs. control, there was no significant difference between the conditions (Cohen's d: -0.19; 95% CI: -0.45 and 0.07). Generally, these results remained consistent in the subgroup analyses for upper-body vs. lower-body exercises. From a practical perspective, the results presented in this review suggest that individuals should use an external focus of attention for acute enhancement of muscular endurance.

Arthrogenic Muscle Inhibition: Best Evidence, Mechanisms, and Theory for Treating the Unseen in Clinical Rehabilitation

CONTEXT

Arthrogenic muscle inhibition (AMI) impedes the recovery of muscle function following joint injury, and in a broader sense, acts as a limiting factor in rehabilitation if left untreated. Despite a call to treat the underlying pathophysiology of muscle dysfunction more than three decades ago, the continued widespread observations of post-traumatic muscular impairments are concerning, and suggest that interventions for AMI are not being successfully integrated into clinical practice.

OBJECTIVES

To highlight the clinical relevance of AMI, provide updated evidence for the use of clinically accessible therapeutic adjuncts to treat AMI, and discuss the known or theoretical mechanisms for these interventions.

EVIDENCE ACQUISITION

PubMed and Web of Science electronic databases were searched for articles that investigated the effectiveness or efficacy of interventions to treat outcomes relevant to AMI.

EVIDENCE SYNTHESIS

122 articles that investigated an intervention used to treat AMI among individuals with pathology or simulated pathology were retrieved from 1986 to 2021. Additional articles among uninjured individuals were considered when discussing mechanisms of effect.

CONCLUSION

AMI contributes to the characteristic muscular impairments observed in patients recovering from joint injuries. If left unresolved, AMI impedes short-term recovery and threatens patients' long-term joint health and well-being. Growing evidence supports the use of neuromodulatory strategies to facilitate muscle recovery over the course of rehabilitation. Interventions should be individualized to meet the needs of the patient through shared clinician-patient decision-making. At a minimum, we propose to keep the treatment approach simple by attempting to resolve inflammation, pain, and effusion early following injury.

Acute and Long-Term Effects of Attentional Focus Strategies on Muscular Strength: A Meta-Analysis

This review aimed to perform a meta-analysis examining the following: (a) acute effects of adopting an internal focus vs. external focus of attention on muscular strength; and (b) long-term effects of adopting an internal focus vs. external focus of attention during resistance training on gains in muscular strength. We searched through five databases to find eligible studies. Random-effects meta-analyses of standardized mean differences were conducted to analyze the data. Ten studies were included. In the meta-analysis for the acute effects, there was a significant positive effect of external focus on muscular strength (standardized mean difference: 0.34; 95% confidence interval: 0.22, 0.46). In the meta-analysis for the long-term effects, there was no significant difference between training with an internal focus and external focus on muscular strength gains (standardized mean difference: 0.32; 95% confidence interval: –0.08, 0.73). In the subgroup analysis for lower-body exercises, we found a significant positive effect of training with an external focus on muscular strength gains (standardized mean difference: 0.47; 95% confidence interval: 0.07, 0.87). In summary, our findings indicate an acute increase in muscular strength when utilizing an external focus of attention. When applied over the long-term, using an external focus of attention may also enhance resistance training-induced gains in lower-body muscular strength.