Effect of stretching on agonist-antagonist muscle activity and muscle force output during single and multiple joint isometric contractions

Eight moderately active male subjects where tested for peak force in an isometric knee extension test and peak force and rate of force development in an isometric squat test. Both tests where performed at a 100 degrees knee angle and average integrated electromyography (IEMG) was measured from the vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) muscles. Subjects performed the two conditions, stretching (S) or control (C) in a randomized order. Subjects where tested for baseline strength measures in both the isometric knee extension and isometric squat and then either stretched or sat quietly for 10 min. Following S or C subjects where then tested at six time points. Following S peak force in the isometric knee extension was significantly (P < or = 0.05) less than C at 1, 2, 8 and 16 min post. No significant difference in peak force was found between S and C in the isometric squat. However, following S the rate of force development in the isometric squat was significantly less than C at immediately post. No significant differences where observed in IEMG of the VM or VL between S and C in either the isometric knee extension or isometric squat. However, IEMG significantly decreased in the BF at 1 min post after S in comparison with C in both the isometric knee extension and isometric squat. Stretching appears to decrease muscle force output in a single joint isometric contraction and rate of force development in a multiple joint isometric contraction. Possible changes in agonist-antagonist muscle activity patterns need to be further examined.     

外加弹性紧身装置对田径运动员下肢肌力、疲劳与肌肉活动的影响

目的:利用下肢等动肌力测试,配合生物信号分析工具——肌电图与肌动图,探讨不同紧度的外加紧身装置对力量输出、疲劳表现以及肌肉活动的影响。方法:利用力量传感器和砝码测得紧身装置的张力-长度曲线,并据此选取紧度张力分别为66.4N和85.8N的中等和高等紧度负荷。由12名田径专项男运动员分别完成无紧身装置、中等紧度负荷和高等紧度负荷三种紧度条件下的测试,包括5秒最大等长收缩和两种速度(60°.s-1和300°.s-1)下的连续25次最大伸膝等动向心运动,并同步采集力量输出参数(力矩、功率、做功)、股直肌肌电和肌动信号(均方根振幅)。结果:三种紧度条件下峰值力矩、峰值功率、前5次平均功率和总做功量无显著性差异;在两种速度下的最大等动向心运动中,相比无紧度条件,高等紧度条件下最大力矩衰减更为缓慢,同时肌电振幅随着外加紧度增加(从无到高)呈显著递减趋势(P<0.05),但肌动振幅的变化却不明显。结论:紧身装置未改变下肢短时间收缩时的肌力和爆发力,然而通过增加一定范围的紧度能够帮助肌肉在募集更少运动单位的情况下维持相似的力量输出,进而对肌肉疲劳和长时间的运动表现产生积极的影响。     

Mechanomyography and electromyography during and after fatiguing shoulder eccentric contractions in males and females

The aim of this study was to investigate changes in mechanomyographic (MMG) and the surface electromyographic (EMG) signals during and after fatiguing shoulder eccentric contractions in a group consisting of 12 males and 12 females. Exerted force, MMG, EMG, pain and rate of perceived exertion were assessed before, during and after repeated high-intensity eccentric exercises. Bouts of eccentric contractions caused a decrease in the exerted force for males (P<0.05) and an increase in the rate of perceived exertion and pain for both genders (P<0.05). During eccentric exercise, the root mean square (RMS) values of the MMG signal increased (P<0.05). The mean power frequency (MPF) values of the EMG signal decreased at the end of each eccentric bout for both genders (P<0.05); the decrease was higher for females compared with males (P<0.05). Immediately after eccentric exercise in static abduction of the upper limbs, the MMG RMS and MPF values increased (P<0.05). The present study showed that (1) neuromuscular changes associated with pain and changes in muscle stiffness and (2) changes in motor units strategy during fatigue development in shoulder muscle are reflected in the MMG and EMG signals.     

Shoulder and arm muscle activity during elastic band exercises performed in a hospital bed

Objective: Muscle atrophy is a common side-effect of bed rest during hospitalization. However, resistance training may minimize or even prevent these complications. Therefore, we evaluated the efficiency of four upper-body elastic resistance exercises that could be performed while lying or sitting in a hospital bed.

Methods: Twenty-two healthy subjects performed three repetitions of each exercise in a supine and seated position with a perceived intensity of 3 (low) and 8 (high) on the Borg CR10 Scale. Surface electromyography was collected from 12 shoulder and arm muscles (e.g. trapezius, deltoideus, and biceps brachii), and normalized to a maximal voluntary isometric contraction (nEMG).

Results: During all exercises performed at high intensity, moderate (>40%) to high (>60%) levels of nEMG were found for the majority of the analysed muscles, e.g. deltoideus (from 37% to 69%, median 57.5%), trapezius (from 43% to 66%, median 51%), and infraspinatus (from 54% to 66%, median 59%), with the exception of pectoralis major (from 29% to 47%, median 39.5%) and latissimus dorsi (from 15% to 22%, median 18.5%). No significant differences were found between the supine and seated positions for any of the exercises.

Conclusion: This study showed that high levels of shoulder and arm muscle activity can be achieved while lying or sitting in a hospital bed using appropriate exercises with elastic bands. The data presented here can be used by physiotherapists as a guideline for selecting suitable and effective strengthening exercises during in-hospital rehabilitation to counteract bed-rest related muscle atrophy in the upper body.     

Effects of velocity loss during resistance training on athletic performance,strength gains and muscle adaptations

We compared the effects of two resistance training (RT) programs only differing in the repetition velocity loss allowed in each set: 20% (VL20) vs 40% (VL40) on muscle structural and functional adaptations. Twenty‐two young males were randomly assigned to a VL20 (n = 12) or VL40 (n = 10) group. Subjects followed an 8‐week velocity‐based RT program using the squat exercise while monitoring repetition velocity. Pre‐ and post‐training assessments included: magnetic resonance imaging, vastus lateralis biopsies for muscle cross‐sectional area (CSA) and fiber type analyses, one‐repetition maximum strength and full load‐velocity squat profile, countermovement jump (CMJ), and 20‐m sprint running. VL20 resulted in similar squat strength gains than VL40 and greater improvements in CMJ (9.5% vs 3.5%, P < 0.05), despite VL20 performing 40% fewer repetitions. Although both groups increased mean fiber CSA and whole quadriceps muscle volume, VL40 training elicited a greater hypertrophy of vastus lateralis and intermedius than VL20. Training resulted in a reduction of myosin heavy chain IIX percentage in VL40, whereas it was preserved in VL20. In conclusion, the progressive accumulation of muscle fatigue as indicated by a more pronounced repetition velocity loss appears as an important variable in the configuration of the resistance exercise stimulus as it influences functional and structural neuromuscular adaptations.     

Knee isokinetic muscle strength and balance ratio in female soccer players of different age groups: a cross-sectional study

ABSTRACT

Background: As a consequence of years of soccer training and sexual maturation, there is an increase in lower body muscle mass and strength especially in the knee extensors and flexors muscles. In this context, the lack of knee joint stability, resulting from strength imbalance between knee extensor and flexors muscles, has been associated with knee injuries. The aim of this study was to compare the knee flexor and extensor muscle peak torque, average power, contralateral deficit, conventional and functional balance ratios of female soccer players from different age groups.

Methods: Sixty-six female soccer players were divided into four groups: under 13 (U13), under 15 (U15), under 17 (U17) years old and professional (PRO). Flexor and extensor knee muscle strength in concentric and eccentric actions of both limbs were assessed using isokinetic dynamometer.

Results: For the dominant limb, the knee concentric extensor muscles peak torques, assessed at 60 and at 240 deg/sec, and the average power of U15 group were significantly higher than U13 group. Extensor muscle average power of the PRO group was higher than U17. Dominant knee flexor average power of U15 was significantly higher than the U13 group. Peak torque at 60 deg/sec and 240 deg/sec and average power of the PRO group were higher than the U17 group. No differences were found regarding the eccentric action for flexor and extensor muscles. Conventional and functional balance ratios were similar among all age group, except for U13, which presented higher values than the U15 group for the dominant limb.

Conclusions: The greatest improvement in muscular performance occur when the athlete starts practicing soccer (after U13) and when they become professional (after U17) and the balance ratios, and muscle balance ratios remain stable in all age groups, although they are below the recommended level in the literature, which may increase the risk for lower limb injury.     

Pre-heel-strike peroneal muscle activity during walking and running with and without an external ankle support

The electromyographic activity of the peroneus longus and brevis muscles was recorded during 100 strides of walking and running on a treadmill in 9 experienced runners with and without an inversion restricting external ankle support. The duration of peroneal activity in the stance phase increased from median 61% of this stage when jogging to 88% when running fast. Pre-heel-strike activity was present in median 10% of all strides when walking, in 28% when jogging, in 40% when running at a medium pace and in 45% when running fast. Application of an external ankle support reduced pre-heel-strike activity by 50% for jogging and medium-pace running. Our results suggest that an unacceptable degree of inversion of the ankle/foot complex in late swing phase produces a stimulus for peroneal muscle contraction before the foot is loaded at heel-strike.     

Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure

Aging is characterized by loss of spinal motor neurons (MNs) due to apoptosis, reduced insulin-like growth factor I signaling, elevated amounts of circulating cytokines, and increased cell oxidative stress. The age-related loss of spinal MNs is paralleled by a reduction in muscle fiber number and size (sarcopenia), resulting in impaired mechanical muscle performance that in turn leads to a reduced functional capacity during everyday tasks. Concurrently, maximum muscle strength, power, and rate of force development are decreased with aging, even in highly trained master athletes. The impairment in muscle mechanical function is accompanied and partly caused by an age-related loss in neuromuscular function that comprise changes in maximal MN firing frequency, agonist muscle activation, antagonist muscle coactivation, force steadiness, and spinal inhibitory circuitry. Strength training appears to elicit effective countermeasures in elderly individuals even at a very old age (>80 years) by evoking muscle hypertrophy along with substantial changes in neuromuscular function, respectively. Notably, the training-induced changes in muscle mass and nervous system function leads to an improved functional capacity during activities of daily living.     

The benefit of combining clinical and radiological assessments in diagnosis of inherited muscle diseases

Purpose

To study the benefit of combining clinical and radiological assessments in the diagnosis of inherited muscle diseases.

Subjects and methods

This cross sectional study included 45 patients presenting with manifestations of muscle disease. They were subjected to thorough clinical assessment and MRI examination of thigh and leg muscles. Independent clinical and radiological assessments were performed followed by combining the clinical categorization and radiological signs together to assess the strength of collaborating the clinico-radiological assessments in diagnosis of rare muscle diseases.

Results

Clinical assessment suggested the diagnosis in 42% of the cases and radiological assessment blind to clinical data suggested the diagnosis in 20% of the cases. Combining both was able to suggest the diagnosis in 56% of the cases. Also the radiological assessment supported the clinical diagnosis of some rare muscle diseases uncommonly reported in Egypt.

Conclusion

Combining clinical and radiological assessments of muscle diseases is helpful to suggest a reliable diagnosis, especially in circumstances where genetic diagnosis is expensive or not accessible.     

Effects of acute and 2‐week administration of oral salbutamol on exercise performance and muscle strength in athletes

Our objective was to investigate effects of acute and 2‐week administration of oral salbutamol on repeated sprint ability, exercise performance, and muscle strength in elite endurance athletes. Twenty male elite athletes [VO_2max: 69.4 ± 1.8 (Mean ± SE) mL/min/kg], aged 25.9 ± 1.4 years, were included in a randomized, double‐blinded and placebo‐controlled parallel study. At baseline, after acute administration, and again after 2‐week administration of the study drugs (8 mg salbutamol or placebo), subjects' maximal voluntary contraction (MVC) of m. quadriceps and isometric endurance of m. deltoideus were measured, followed by three repeated Wingate tests. Exercise performance at 110% of VO_2max was determined on a bike ergometer. Acute administration of salbutamol increased peak power during first Wingate test by 4.1 ± 1.7% (P < 0.05). Two‐week administration of salbutamol increased (P < 0.05) peak power during first and second Wingate test by 6.4 ± 2.0 and 4.2 ± 1.0%. Neither acute nor 2‐week administration of salbutamol had any effect on MVC, exercise performance at 110% of VO_2max or on isometric endurance. No differences were observed in the placebo group. In conclusion, salbutamol benefits athletes' sprint ability. Thus, the present study supports the restriction of oral salbutamol in competitive sports.     

Effect of loading on unintentional lifting velocity declines during single sets of repetitions to failure during upper and lower extremity muscle actions

The purpose of this study was to examine the effect of different loads on repetition speed during single sets of repetitions to failure in bench press and parallel squat. Thirty-six physical active men performed 1-repetition maximum in a bench press (1 RM (BP)) and half squat position (1 RM (HS)), and performed maximal power-output continuous repetition sets randomly every 10 days until failure with a submaximal load (60 %, 65 %, 70 %, and 75 % of 1RM, respectively) during bench press and parallel squat. Average velocity of each repetition was recorded by linking a rotary encoder to the end part of the bar. The values of 1 RM (BP) and 1 RM (HS) were 91 +/- 17 and 200 +/- 20 kg, respectively. The number of repetitions performed for a given percentage of 1RM was significantly higher (p < 0.001) in half squat than in bench press performance. Average repetition velocity decreased at a greater rate in bench press than in parallel squat. The significant reductions observed in the average repetition velocity (expressed as a percentage of the average velocity achieved during the initial repetition) were observed at higher percentage of the total number of repetitions performed in parallel squat (48 - 69 %) than in bench press (34 - 40 %) actions. The major finding in this study was that, for a given muscle action (bench press or parallel squat), the pattern of reduction in the relative average velocity achieved during each repetition and the relative number of repetitions performed was the same for all percentages of 1RM tested. However, relative average velocity decreased at a greater rate in bench press than in parallel squat performance. This would indicate that in bench press the significant reductions observed in the average repetition velocity occurred when the number of repetitions was over one third (34 %) of the total number of repetitions performed, whereas in parallel squat it was nearly one half (48 %). Conceptually, this would indicate that for a given exercise (bench press or squat) and percentage of maximal dynamic strength (1RM), the pattern of velocity decrease can be predicted over a set of repetitions, so that a minimum repetition threshold to ensure maximal speed performance is determined.     

Effect of aging on regional neuromuscular regulation within human rectus femoris muscle during stair ascent and descent

Our recent studies showed the rectus femoris (RF) muscle is regionally regulated during the level walking and this unique neuromuscular activation pattern is influenced by aging (Watanabe et al., 2014, 2016 J Biomech). We aimed to investigate and compare regional neuromuscular activation patterns along the RF muscle between the young and elderly during the stair walking. Fourteen young men (age: 20.4 ± 1.0 years) and 14 elderly men (age: 73.8 ± 5.9 years) performed the stair ascent and descent. Fifteen trials of three steps were performed for both stair ascent and descent. The spatial distribution of surface electromyography (EMG) within the RF muscle was assessed by central locus activation (CLA), which is calculated from 18 surface electrodes along the longitudinal line of the muscle. CLA was significantly moved along the muscle during the stair ascent and descent in both young and elderly (p < 0.05). Significant differences in CLA were showed at the stance phase of the ascent (12.5 ± 0.7 and 11.4 ± 1.7 cm from most proximal electrodes for the young and elderly, p < 0.05) and at the swing phase of the descent (11.4 ± 1.5 and 10.3 ± 1.5 cm from most proximal electrodes for the young and elderly, p < 0.05). These results suggest that the regional neuromuscular activation within the RF muscle is affected by aging during the stair walking.     

Isometric training and long‐term adaptations: Effects of muscle length,intensity, and intent: A systematic review

Isometric training is used in the rehabilitation and physical preparation of athletes, special populations, and the general public. However, little consensus exists regarding training guidelines for a variety of desired outcomes. Understanding the adaptive response to specific loading parameters would be of benefit to practitioners. The objective of this systematic review, therefore, was to detail the medium‐ to long‐term adaptations of different types of isometric training on morphological, neurological, and performance variables. Exploration of the relevant subject matter was performed through MEDLINE, PubMed, SPORTDiscus, and CINAHL databases. English, full‐text, peer‐reviewed journal articles and unpublished doctoral dissertations investigating medium‐ to long‐term (≥3 weeks) adaptations to isometric training in humans were identified. These studies were evaluated further for methodological quality. Twenty‐six research outputs were reviewed. Isometric training at longer muscle lengths (0.86%‐1.69%/week, ES = 0.03‐0.09/week) produced greater muscular hypertrophy when compared to equal volumes of shorter muscle length training (0.08%‐0.83%/week, ES = ?0.003 to 0.07/week). Ballistic intent resulted in greater neuromuscular activation (1.04%‐10.5%/week, ES = 0.02‐0.31/week vs 1.64%‐5.53%/week, ES = 0.03‐0.20/week) and rapid force production (1.2%‐13.4%/week, ES = 0.05‐0.61/week vs 1.01%‐8.13%/week, ES = 0.06‐0.22/week). Substantial improvements in muscular hypertrophy and maximal force production were reported regardless of training intensity. High‐intensity (≥70%) contractions are required for improving tendon structure and function. Additionally, long muscle length training results in greater transference to dynamic performance. Despite relatively few studies meeting the inclusion criteria, this review provides practitioners with insight into which isometric training variables (eg, joint angle, intensity, intent) to manipulate to achieve desired morphological and neuromuscular adaptations.     

Effects of macronutrient intake on thigh muscle mass during home‐based walking training in middle‐aged and older women

We examined whether post‐exercise macronutrient supplementation during a 5‐month home‐based interval walking training (IWT) accelerated exercise‐induced increases in skeletal muscle mass and strength in healthy middle‐aged and older women. Thirty‐five women (41–78 years) were randomly divided into two groups: IWT alone (CNT, n = 18) or IWT plus post‐exercise macronutrient (7.6 g protein, 32.5 g carbohydrate, and 4.4 g fat) supplementation (NUT, n = 17). For IWT, all subjects were instructed to repeat five or more sets of 3‐min low‐intensity walking at 40% peak aerobic capacity (Vo_{2 peak}), followed by a 3‐min high‐intensity walking above 70% Vo_{2 peak} per day for 4 or more days per week. We determined Vo_{2 peak}, thigh muscle tissue area by computer tomography, and thigh muscle strength in all subjects before and after IWT. We found that an increase in hamstring muscle tissue area was 2.8 ± 1.2% in NUT vs ?1.0 ± 0.7% in CNT and that in isometric knee flexion force was 16.3 ± 3.7% in NUT vs 6.5 ± 3.0% in CNT; both were significantly higher in NUT than in CNT (both, P < 0.001). Thus, post‐exercise macronutrient supplementation enhanced the increases in thigh muscle mass and strength, although partially, in home‐based IWT in middle‐aged and older women.     

Effect of cooling during inter‐exercise periods on subsequent intramuscular water movement and muscle performance

To evaluate the effects of cooling between exercise sessions on intramuscular water movement and muscle performance, the lower extremities of nine untrained men were assigned to either a cooling protocol (20‐min water immersion, 15 °C) or a noncooling protocol. Each subject performed two exercise sessions involving maximal concentric knee extension and flexion (three repetitions, 60°/s; followed by 50 repetitions, 180°/s). The peak torque at 60°/s and total work, mean power, and decrease rate of torque value at 180°/s were evaluated. Axial magnetic resonance diffusion‐weighted images of the mid‐thigh were obtained before and after each exercise session. Apparent diffusion coefficient (ADC) values for the quadriceps and hamstrings were calculated for evaluating intramuscular water movement. Both groups exhibited significantly increased ADC values for the quadriceps and hamstrings after each exercise session. These ADC values returned to the pre‐exercise level after water immersion. No significant difference was observed in muscle performance from first exercise session to the next in either group, except for increased total work and mean power in knee flexion in the cooled group. Cooling intervention between exercise sessions decreased exercise‐induced elevation of intramuscular water movement and had some beneficial effects on muscle endurance of knee flexors, but not knee extensors.     

Heritability estimates of muscle strength‐related phenotypes: A systematic review and meta‐analysis

The purpose of this study was to clarify the heritability estimates of human muscle strength‐related phenotypes (H²‐msp). A systematic literature search was conducted using PubMed (through August 22, 2016). Studies reporting the H²‐msp for healthy subjects in a sedentary state were included. Random‐effects models were used to calculate the weighted mean heritability estimates. Moreover, subgroup analyses were performed based on phenotypic categories (eg, grip strength, isotonic strength, jumping ability). Sensitivity analyses were also conducted to investigate potential sources of heterogeneity of H²‐msp, which included age and sex. Twenty‐four articles including 58 measurements were included in the meta‐analysis. The weighted mean H²‐msp for all 58 measurements was 0.52 (95% confidence intervals [CI]: 0.48–0.56), with high heterogeneity (I²=91.0%, P<.001). Subgroup analysis showed that the heritability of isometric grip strength, other isometric strength, isotonic strength, isokinetic strength, jumping ability, and other power measurements was 0.56 (95% CI: 0.46–0.67), 0.49 (0.47–0.52), 0.49 (0.32–0.67), 0.49 (0.37–0.61), 0.55 (0.45–0.65), and 0.51 (0.31–0.70), respectively. The H²‐msp decreased with age (P<.05). In conclusion, our results indicate that the influence of genetic and environmental factors on muscle strength‐related phenotypes is comparable. Moreover, the role of environmental factors increased with age. These findings may contribute toward an understanding of muscle strength‐related phenotypes.