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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. What is the central question of this study?Action observation and mental imagery (AOMI) may be a possible and potentially promising rehabilitation strategy via facilitation of neural adaptations, whereas neuromuscular electrical stimulation (NMES) may augment muscle strength and mass via peripheral mechanisms. We compared the ability of these interventions to distinctly preserve strength versus mass during one week of knee joint immobilization.What is the main finding and its importance?Effect sizes suggested that AOMI preserved voluntary strength, and may be used therapeutically to target neural adaptations during immobilization. [ABSTRACT FROM AUTHOR]

This manuscript represents the second phase of a clinical trial designed to examine the effects of knee joint immobilization and retraining on muscle strength and mass. In Phase 2, we examined sex differences in the recovery of multiple indices of muscle quality after a resistance training-based rehabilitation program. Following 1 week of immobilization, 27 participants (16 males, 11 females) exhibiting weakness underwent twice weekly resistance training sessions designed to re-strengthen their left knee. Unilateral retraining sessions utilizing leg press, extension, and curl exercises were conducted until participants could reproduce their pre-immobilization knee extension isometric maximal voluntary contraction (MVC) peak torque. Post-immobilization, both sexes demonstrated impaired MVC peak torque (males = −10.8%, females = −15.2%), specific torque (−9.8% vs. −13.1%), echo intensity of the vastus lateralis (+6.9% vs. +5.9%) and rectus femoris (+5.9% vs. +2.1), and extracellular water/intracellular water ratio (+7.8% vs. +9.0%). The number of retraining sessions for peak torque to return to baseline for males (median = 1, mean = 2.13) versus females (median = 2, mean = 2.91) was not significantly different, though the disparity in recovery times may be clinically relevant. Following retraining, specific torque was the only muscle-quality indicator that improved along with MVC peak torque (males = 20.1%, females = 22.4%). Our findings indicate that measures of muscle quality demonstrate divergent recovery rates following immobilization, with muscle mass lagging behind improvements in strength. Greater immobilization-induced strength loss among females suggests that sex-specific rehabilitation efforts may be justified. [ABSTRACT FROM AUTHOR]

Transcranial magnetic stimulation (TMS) is a research tool that has potential to provide new insights into strength training-induced adaptations. However, using TMS to study the lower limbs is challenging, and sex-specific reliability has yet to be reported. We examined the reliability of corticospinal excitability and silent periods for the rectus femoris, vastus lateralis, and biceps femoris in both sexes. Thirteen males and 14 females reported to the laboratory twice. During both trials, a double cone coil was used to deliver 20 pulses to the rectus femoris hotspot with a stimulator output of 130% of active motor threshold. Motor-evoked potential peak-to-peak amplitude, which reflects corticospinal excitability, and silent period duration were quantified. Our results offer 4 novel findings. First, corticospinal excitability and silent period demonstrated higher reliability for the females. Second, regardless of sex and muscle, the silent period was more reliable than corticospinal excitability. Third, reliability was highest for our target muscle (rectus femoris), with lower reliability for the vastus lateralis and biceps femoris, suggesting that these methods cannot be used to study coactivation. Fourth, active motor threshold showed less variability than corticospinal excitability and silent period but increased at trial 2 in females. Many of the intraclass correlation coefficients were excellent ($0.90), although we attribute this finding to variability between subjects. Reliability of lower-limb TMS measures may be sex, muscle, and variable dependent. Our findings suggest that both males and females should be included in lower-limb TMS research, although combining data between sexes should be approached cautiously. [ABSTRACT FROM AUTHOR]

This review examines the experimental evidence regarding unilateral resistance training frequency, intensity, the type of training, training volume, and adjuvant therapies on the cross-education of strength. CINAHL, MEDLINE, APA PsycInfo, SPORTDiscus, and Web of Science were systematically searched with gray literature searches and pearling of references thereafter. Experiments were included in the review if they performed a unilateral resistance training intervention that directly compared the dose of a training variable on the cross-education response in healthy or clinical populations following a minimum of two weeks of training. Experiments must have reported maximal strength outcomes for the untrained limb. For each experiment, the study population, intervention methods, the dosage of the training variable being studied, and the outcomes for the untrained, contralateral limb were identified and collectively synthesized. The search returned a total of 912 articles, 57 of which qualified for inclusion. The results show that experimental trials have been conducted on resistance training frequency (n = 4), intensity (n = 7), the type of training (n = 26), training volume (n = 3), and adjuvant therapies (n = 17) on the cross-education of strength. This review maps the available evidence regarding exercise design and prescription strategies to promote the cross-education of strength. It appears that traditional resistance training frequencies (i.e., 2–3×/week) at high intensities are effective at promoting cross-education with eccentric muscle actions showing additive benefits. There is experimental evidence that neuromodulatory techniques can augment cross-education when layered with unilateral resistance training versus training alone. Registration identifier (osf.io/9sh5b). The cross-education of strength is moderated by exercise design and prescription in clinical and non-clinical populations. This review synthesizes the available evidence regarding exercise design strategies for unilateral resistance training and provides evidence-based recommendations for the prescription of unilateral training to maximize the cross-education of strength. Greater insights regarding the timing and effectiveness of cross-education interventions in clinical scenarios will strengthen the use of unilateral resistance training for individuals who may benefit from its use. [ABSTRACT FROM AUTHOR]

Osseointegration implant (OI) surgery is the latest rehabilitation technology for amputees, where a bone-anchored implant obviates the limitations of traditional socket prostheses. The bone mineral density (BMD) in the periprosthetic and other anatomical regions can be used to assess bone remodelling following OI surgery. Currently, limited studies have used BMD measurements in reporting post-operative OI outcomes and the association between the maintenance of BMD and implant efficacy has remained elusive. This review captured and analysed all studies that have reported the BMD as an objective outcome measure in patients with trans-femoral or trans-tibial OI. The PubMed, Medline, Scopus and Web of Science databases were searched using the terms 'amputation', 'osseointegration' and 'bone mineral density'. A total of 6 studies involving human participants were included for analysis. All studies used dual X-ray absorptiometry and/or X-rays for measuring BMD. Rehabilitation of trans-femoral or trans-tibial amputation using OI may help restore healthy BMD by enabling physiological bone loading. However, there is a low correlation between the BMD around the OI and the success of OI surgery or the risk of periprosthetic fractures. This review summarises the current evidence on BMD assessment in OI for lower limb amputee rehabilitation. Despite the great variability in the results, the available evidence suggests that OI may help restore BMD following surgery. The limited evidence calls for further investigation, as well as the development of a standard BMD measurement protocol. [ABSTRACT FROM AUTHOR]

Carr, JC and Ye, X. Strength and electromyographic responses of upper and lower limbs during maximal intermittent contractions in males and females. J Strength Cond Res 36(9): 2403–2409, 2022—This study examined the strength and electromyographic (EMG) responses of upper vs. lower limb muscles during intermittent maximal contractions in both sexes. Twenty subjects (n = 7 women) performed a fatiguing protocol (6, 30-second intermittent maximal isometric contractions with a 50% duty cycle) with either the elbow flexors or the knee extensors on separate visits. Bipolar surface EMG signals were detected from the biceps brachii and vastus lateralis muscles (n = 5 women retained). Women maintained more of their maximal force than men (Δforce: men vs. women = −55.0 ± 12.8% vs. −43.3 ± 9.9%, p = 0.042). Although force loss was similar between the elbow flexors and knee extensors, the EMG responses showed greater reductions for the biceps brachii than those for the vastus lateralis (Δamplitude: biceps brachii vs. vastus lateralis: −32.0 ± 22.3% vs. −18.9 ± 28.9%; Δmedian frequency: biceps brachii vs. vastus lateralis: −31.1 ± 14.5% vs. −10.3 ± 17.0%). During a series of maximal intermittent isometric contractions with 30 seconds of recovery between work bouts, women are more fatigue resistant than men. In addition, the greater electrophysiological fatigue exhibited by the biceps brachii than that by the vastus lateralis suggests that high-intensity contractions involving elbow flexion will have a greater rate of fatigue progression than those involving knee extension. [ABSTRACT FROM AUTHOR]

The purpose of this experiment was to evaluate the bilateral index in force and electromyographic (EMG) responses for the dominant and non-dominant hands during maximal handgrip contractions in males and females. Thirty-two right-handed participants (16 females) performed maximal unilateral and bilateral handgrip contractions on two separate visits. Bilateral indices were computed for maximal force, rate of force development (RFD100), EMG amplitude, and the rate of EMG rise (RER). There was a bilateral deficit for maximal force in the dominant (−4.98 ± 7.39%, p < 0.001; d = 0.674) but not the non-dominant hand (−1.57 ± 9.10%, p = 0.334; d = 0.173). No deficits were observed for rapid force. The non-dominant flexor carpi radialis showed a bilateral facilitation in EMG amplitude (+12.32 ± 19.29%, p < 0.001; d = 0.638), yet a bilateral deficit for RER (−22.10 ± 27.80%, p < 0.001; d = 0.795). No sex differences were observed for any of the bilateral indices. These data suggest that maximal but not rapid force is susceptible to a bilateral deficit during contractions of the hands. The EMG responses did not parallel the force data. We show sex does not influence the magnitude or direction of the bilateral index in this muscle group. [ABSTRACT FROM AUTHOR]

Fifteen healthy subjects (mean ± SD age = 27.6 ± 4.0 yrs) performed a submaximal isometric muscle action of the dominant leg extensors at 50% of the maximum voluntary contraction (MVC). Bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis (VL) and vastus medialis (VM) during the muscle action and processed with a wavelet analysis designed specifically for EMG signals. The resulting EMG signals in each wavelet band were cross-correlated between the VL and VM. The results indicated significant coherence between the two muscles, with cross-correlation coefficient values that were generally above 0.5 and occasionally as high as 0.8-0.9. In addition, the strongest coherence was located primarily in wavelet bands 1-5 (center frequencies ranging from 6.90-92.36 Hz). Collectively, these findings demonstrated frequency-specific coherence between the VL and VM, thereby suggesting that the two muscles are likely not controlled independently by the central nervous system. The importance of the frequency-specific nature of this coherence is yet unknown, but may be related to inter-muscular coordination of a shared fiber type. [ABSTRACT FROM AUTHOR]