Voluntary co-contraction of ankle muscles alters motor unit discharge characteristics and reduces estimates of persistent inward currents

Motoneuronal persistent inward currents (PICs) are facilitated by neuromodulatory inputs but are highly sensitive to local inhibitory circuits. Estimates of PICs are reduced by group Ia reciprocal inhibition, and increased with the diffuse actions of neuromodulators released during remote muscle contraction. However, it remains unknown how motoneurons function in the presence of simultaneous excitatory and inhibitory commands. To probe this topic, we investigated motor unit discharge patterns and estimated PICs during voluntary co-contraction of ankle muscles, which simultaneously demands the contraction of agonist-antagonist pairs. Twenty participants performed triangular ramps of both co-contraction (simultaneous dorsiflexion and plantar flexion) and isometric dorsiflexion to a peak of 30% of their maximum muscle activity from a maximal voluntary contraction. Motor unit spike trains were decomposed from high-density surface EMG activity recorded from tibialis anterior using blind source separation algorithms. Voluntary co-contraction altered motor unit discharge rate characteristics. Discharge rate at recruitment and peak discharge rate were modestly reduced (∼6% change; P < 0.001; d = 0.22) and increased (∼2% change; P = 0.001, d = -0.19), respectively, in the entire dataset but no changes were observed when motor units were tracked across conditions. The largest effects during co-contraction were that estimates of PICs (ΔF) were reduced by ∼20% (4.47 vs. 5.57 pulses per second during isometric dorsiflexion; P < 0.001, d = 0.641). These findings suggest that, during voluntary co-contraction, the inhibitory input from the antagonist muscle overcomes the additional excitatory and neuromodulatory drive that may occur due to the co-contraction of the antagonist muscle, which constrains PIC behaviour. KEY POINTS: Voluntary co-contraction is a unique motor behaviour that concurrently provides excitatory and inhibitory synaptic input to motoneurons. Co-contraction of agonist-antagonist pairs alters agonist motor unit discharge characteristics, consistent with reductions in persistent inward current magnitude.

Voluntary co-contraction of ankle muscles alters motor unit discharge characteristics and reduces estimates of persistent inward currents

Motoneuronal persistent inward currents (PICs) are both facilitated by neuromodulatory inputs and highly sensitive to local inhibitory circuits (e.g., Ia reciprocal inhibition). Methods aimed to increase group Ia reciprocal inhibition from the antagonistic muscle have been successful in decreasing PICs, and the diffuse actions of neuromodulators released during activation of remote muscles have increased PICs. However, it remains unknown how motoneurons function in the presence of simultaneous excitatory and inhibitory commands. To probe this topic, we investigated motor unit (MU) discharge patterns and estimated PICs during voluntary co-contraction of ankle muscles, which simultaneously demands the contraction of agonist-antagonist pairs. Twenty young adults randomly performed triangular ramps (10s up and down) of both co-contraction (simultaneous dorsiflexion and plantarflexion) and isometric dorsiflexion to a peak of 30% of their maximum muscle activity from a maximal voluntary contraction. Motor unit spike trains were decomposed from high-density surface electromyography recorded over the tibialis anterior (TA) using blind source separation algorithms. Voluntary co-contraction altered motor unit discharge rate characteristics, decreasing estimates of PICs by 20% (4.47 pulses per second (pps) vs 5.57 pps during isometric dorsiflexion). These findings suggest that, during voluntary co-contraction, the inhibitory input from the antagonist muscle overcomes the additional excitatory and neuromodulatory drive that may occur due to the co-contraction of the antagonist muscle, which constrains PIC behavior.

The effect of rotator cuff physical exercise combined with electrically stimulated antagonist on shoulder rotator cuff strength

BACKGROUND

An elastic band (EB) is generally used with a low load for rotator cuff physical exercise, but the resulting increase in muscle strength is insufficient. We assessed the efficacy on external rotator muscle strength of the shoulder joint; of a hybrid training system (HTS) that resists the motion of a volitionally contracting agonist muscle using the force generated by its electrically stimulated antagonist vs. general rotator cuff exercise with EB.

METHODS

Twenty healthy men with no shoulder joint disorders were randomized to 6 weeks of triweekly 10-min rotator cuff exercise with HTS or EB in a clinical research laboratory. Isokinetic concentric external rotator muscle strength at angular velocities of 60°/s and 180°/s (CON60, CON180, respectively) and isokinetic eccentric external rotator muscle strength at an angular velocity of 60°/s (ECC60) were measured as rotator cuff function before and after 6 weeks of intervention.

RESULTS

There were no significant intergroup differences in baseline characteristics. There were statistically significant differences (p = 0.0358, p = 0.0213, respectively) in the increase in CON180 (mean ± SD) and ECC60 between the HTS group (Δ6.0 ± 6.0Nm, p = 0.015; Δ7.5 ± 4.7Nm p = 0.0007, respectively) and the EB group (Δ0.3 ± 5.2Nm, p = 0.8589; Δ1.8 ± 5.3 Nm p = 0.3133, respectively). There was a trend toward CON60 increasing in the HTS group (Δ4.7 ± 6.5Nm, p = 0.0494) which was greater than in the control group (Δ-0.9 ± 6.3Nm, p = 0.6637) (inter-group, p = 0.0677).

CONCLUSIONS

The results of this study support the conclusion that HTS is more effective for increasing external rotator muscle strength more effectively than EB. HTS would be useful for rotator cuff physical exercise.

Mind-Muscle Connection: Verbal Instructions Alter Electromyographic Activity for Elbow Flexors and Extensors During Co-Contraction Training

Co-contraction training has demonstrated similar electromyographic (EMG) activity levels compared to conventional strength training. Since verbal instructions can increase EMG activity on target muscles during conventional exercises, the same should occur during co-contraction. In this study we analyzed whether different verbal instructions would alter the EMG activity of target muscles - biceps brachii (BB) and triceps brachii lateral head (TB) - during co-contraction training for the elbow joint. Seventeen males with experience in strength training performed a co-contraction set in two verbal instruction conditions to emphasize either elbow flexion or elbow extension. Surface electrodes were fixed over biceps brachii and triceps brachii lateral head muscles. We measured EMG mean amplitude and analyzed data with 2-way ANOVA. We found a significant interaction between muscle and verbal instruction (p = 0.002). Post hoc tests indicated that verbal instructions (p = 0.001) influenced the BB EMG activity (elbow flexion: M = 68.74, SD = 17.96%; elbow extension: M = 53.47, SD = 16.13%); and also showed difference (p = 0.006) in the EMG activity between BB and TB with verbal instruction emphasizing the elbow extension (BB: M = 53.47, SD = 16.13%; TB: M = 69.18, SD = 21.79%). There was a difference in the EMG ratio of BB/TB (p = 0.001) when focusing on elbow flexion (M = 1.09, SD = 0.30) versus elbow extension (M = 0.81, SD = 0.25). As verbal instruction modified the magnitude of muscle recruitment during co-contractions for elbow joint muscles, there is a clear mind-muscle connection of importance to this method of training. Also, of importance to trainers, verbal instructions seemed to affect individuals differentially.

Physical strategies to prevent disuse-induced functional decline in the elderly

Disuse situations can have serious adverse health consequences in the elderly, including mainly functional impairment with subsequent increase in the risk of falls or morbimortality. The present review provides clinicians and care givers with detailed and practical information on the feasibility and effectiveness of physical strategies that are currently available to prevent or attenuate the functional decline that occurs secondarily to disuse situations in the elderly, notably in the hospital setting. In this context, active approaches such as resistance exercises and maximal voluntary contractions, which can be performed both isometrically and dynamically, are feasible during most immobilization situations including in hospitalized old people and represent powerful tools for the prevention of muscle atrophy. Aerobic exercise should also be prescribed whenever possible to reduce the loss of cardiovascular capacity associated with disuse periods. Other feasible strategies for patients who are unwilling or unable to perform volitional exercise comprise neuromuscular electrical stimulation, vibration, and blood flow restriction. However, they should ideally be applied synchronously with voluntary exercise to obtain synergistic benefits.

El efecto de los programas de fuerza muscular sobre la capacidad funcional. Revisión sistemática

Introducción. El número de estudios relacionados con la fuerza muscular y la funcionalidad invitan al análisis en profundidad de sus resultados antes de su aplicación profesional.Objetivo. Desarrollar una revisión sistemática para la construcción de programas de actividad física centrados en el entrenamiento de fuerza muscular y la capacidad funcional de sedentarios entre los 19 y 79 años.Materiales y métodos. Se emplearon los parámetros PRISMA, Chocrane y de la Universidad de York para el diseño y ejecución de revisiones sistemáticas. Además, se garantizaron criterios de calidad y especificidad estrictos que permitieron identificar 14 categorías de análisis, de las cuales emergieron las pautas de programación que se informan en la revisión sistemática.Resultados. 49 estudios con nivel de evidencia 1+ (24%), 1- (33%), 2++ (4%), 2+ (29%) y 2- (10%) cumplieron con los criterios de selección establecidos y permitieron alimentar las 14 categorías propuestas y hacer una síntesis de contenido.Conclusión. Es posible elevar el efecto de los programas de actividad física sobre la fuerza muscular y la funcionalidad a partir de la identificación y consideración de unas variables de programación (categoría) básicas que se sustentan en la calidad de evidencia científica circulante.

Maximal repetition performance, rating of perceived exertion, and muscle fatigue during paired set training performed with different rest intervals

BACKGROUND/OBJECTIVE

The purpose of this study was to examine rest interval length between agonist-antagonist paired set training (PS) on maximal repetition performance, rating of perceived exertion, and neuromuscular fatigue.

METHODS

Fourteen trained men (age, 24.2 ± 1.1 years; height, 175 ± 5.5 cm; body mass, 76.6 ± 7.0 kg) performed two experimental protocols in random order with 2 minutes (P2) or 4 minutes (P4) between agonist-antagonist PS, which consisted of a bench press set followed immediately by a seated row set with 8-repetition maximum loads, respectively. A total of three PS were performed for each rest interval protocol. The total repetitions performed and the rating of perceived exertion were recorded for each exercise set within each rest interval protocol. Electromyography signals were recorded for the posterior deltoid, biceps brachii, pectoralis major, and triceps brachii muscles during the SR exercise. The electromyography signals were then used to calculate a fatigue index for each rest interval protocol.

RESULTS

No significant differences were identified in the total repetitions completed between rest interval protocols for the bench press (P2 = 22.9 ± 1.3 and P4 = 22.6 ± 0.8) and seated row (P2 = 25.4 ± 1.7 and P4 = 25.1 ± 1.3). However, a significantly higher fatigue index was found for all muscles under the P2 versus the P4 protocol.

CONCLUSION

When performing agonist-antagonist PS, prescribing a shorter rest interval between PS may induce higher levels of fatigue, albeit with similar total repetitions versus a longer rest interval.

Trainability of muscular activity level during maximal voluntary co-contraction: comparison between bodybuilders and nonathletes

Antagonistic muscle pairs cannot be fully activated simultaneously, even with maximal effort, under conditions of voluntary co-contraction, and their muscular activity levels are always below those during agonist contraction with maximal voluntary effort (MVE). Whether the muscular activity level during the task has trainability remains unclear. The present study examined this issue by comparing the muscular activity level during maximal voluntary co-contraction for highly experienced bodybuilders, who frequently perform voluntary co-contraction in their training programs, with that for untrained individuals (nonathletes). The electromyograms (EMGs) of biceps brachii and triceps brachii muscles during maximal voluntary co-contraction of elbow flexors and extensors were recorded in 11 male bodybuilders and 10 nonathletes, and normalized to the values obtained during the MVE of agonist contraction for each of the corresponding muscles (% EMGMVE). The involuntary coactivation level in antagonist muscle during the MVE of agonist contraction was also calculated. In both muscles, % EMGMVE values during the co-contraction task for bodybuilders were significantly higher (P<0.01) than those for nonathletes (biceps brachii: 66±14% in bodybuilders vs. 46±13% in nonathletes, triceps brachii: 74±16% vs. 57±9%). There was a significant positive correlation between a length of bodybuilding experience and muscular activity level during the co-contraction task (r = 0.653, P = 0.03). Involuntary antagonist coactivation level during MVE of agonist contraction was not different between the two groups. The current result indicates that long-term participation in voluntary co-contraction training progressively enhances muscular activity during maximal voluntary co-contraction.