Neuromuscular adaptation during skill acquisition on a two degree-of-freedom target-acquisition task: Isometric torque production

In this study we attempted to identify the principles that govern the changes in neural control that occur during repeated performance of a multiarticular coordination task. Eight participants produced isometric flexion/extension and pronation/supination torques at the radiohumeral joint, either in isolation (e.g., flexion) or in combination (e.g., flexion - supination), to acquire targets presented by a visual display. A cursor superimposed on the display provided feedback of the applied torques. During pre- and postpractice tests, the participants acquired targets in eight directions located either 3.6 cm (20% maximal voluntary contraction [MVC]) or 7.2 cm (40% MVC) from a neutral cursor position. On each of five consecutive days of practice the participants acquired targets located 5.4 cm (30% MVC) from the neutral position. EMG was recorded from eight muscles contributing to torque production about the radiohumeral joint during the pre- and posttests. Target-acquisition time decreased significantly with practice in most target directions and at both target torque levels. These performance improvements were primarily associated with increases in the peak rate of torque development after practice. At a muscular level, these changes were brought about by increases in the rates of recruitment of all agonist muscles. The spatiotemporal organization of muscle synergies was not significantly altered after practice. The observed adaptations appear to lead to performances that are generalizable to actions that require both greater and smaller joint torques than that practiced, and may be successfully recalled after a substantial period without practice. These results suggest that tasks in which performance is improved by increasing the rate of muscle activation, and thus the rate of joint torque development, may benefit in terms of the extent to which acquired levels of performance are maintained over time.

Muscle synergies at the elbow in static and oscillating isometric torque tasks with dual degrees of freedom

This study's aim was to identify the effect of oscillation of torques in isometric tasks under identical mechanical conditions on the muscle synergies used. It was hypothesized that bi-functional muscles would play a lesser role in torque oscillation, because they would also generate an undesired oscillation. Thus, changes in muscle synergies were expected as a consequence of oscillation in torque generation. The effect of the trajectory of torque generation was investigated in dual-degrees-of-freedom submaximal isometric oscillation torque tasks at the elbow. The torques were flexion-extension and supination-pronation. Oscillation torques were compared with static torque generations at four torque positions during oscillation. Muscle activity was determined with surface electromyography. Compared with the static torque tasks, the oscillation tasks showed an overall increased muscle activity. The oscillation tasks, however, showed similar activity patterns and muscle synergies compared to the static composite tasks. It was found that the motor system is well able to control different orthogonal combinations of slow torque oscillations and constant torques by employing a single oscillating muscle synergy.

THE SUMMATION OF VOLUNTARY AND ELECTRICAL ELICITED STIMULUS INCREASES JOINT TORQUE?

The aim of this study is to analyze the effect of neuromuscular electrical stimulation (NMES) on myoelectrical activity and on joint torque during isometric plantar flexion contraction. Ten healthy young adult subjects participate in this study. The electrodes for NMES are placed along posterior thigh along ciatic nerve trajectory. It is measured the myoelectrical activity and the isometric torque generated by ankle plantar flexion with an isokinetic dynamometer. The conditions of isometric contractions are maximum isometric voluntary contraction (MIVC), NMES, and association of both (MIVC+NMES). The results show lower torque during NMES and larger SOL activity compare to the others. Besides, in order to keep the same objective task (to produce the same level of torque), neuromuscular adaptations are necessary on the common drive.

The force-velocity relationship of the human soleus muscle during submaximal voluntary lengthening actions

In experiments on isolated animal muscle, the force produced during active lengthening contractions can be up to twice the isometric force, whereas in human experiments lengthening force shows only modest, if any, increase in force. The presence of synergist and antagonist muscle activation associated with human experiments in situ may partly account for the difference between animal and human studies. Therefore, this study aimed to quantify the force-velocity relationship of the human soleus muscle and assess the likelihood that co-activation of antagonist muscles was responsible for the inhibition of torque during submaximal voluntary plantar flexor efforts. Seven subjects performed submaximal voluntary lengthening, shortening(at angular, velocities of +5, -5, +15, -15 and +30, and -30degrees s(-1)) and isometric plantar flexor efforts against an ankle torque motor. Angle-specific (90degrees) measures of plantar flexor torque plus surface and intramuscular electromyography from soleus, medial gastrocnemius and tibialis anterior were made. The level of activation (30% of maximal voluntary isometric effort) was maintained by providing direct visual feedback of the soleus electromyogram to the subject. In an attempt to isolate the contribution of soleus to the resultant plantar flexion torque, activation of the synergist and antagonist muscles were minimised by: (1) flexing the knee of the test limb, thereby minimising the activation of gastrocnemius, and (2) applying an anaesthetic block to the common peroneal nerve to eliminate activation of the primary antagonist muscle, tibialis anterior and the synergist muscles, peroneus longus and peroneus brevis. Plantar flexion torque decreased significantly (P<0.05) after blocking the common peroneal nerve which was likely due to abolishing activation of the peroneal muscles which are synergists for plantar flexion. When normalised to the corresponding isometric value, the force-velocity relationship between pre- and post-block conditions was not different. In both conditions, plantar flexion torques during shortening actions were significantly less than the isometric torque and decreased at faster velocities. During lengthening actions, however, plantar flexion torques were not significantly different from isometric regardless of angular velocity. It was concluded that the apparent inhibition of lengthening torques during voluntary activation is not due to co-activation of antagonist muscles. Results are presented as mean (SEM).

Adaptation to chronic eccentric exercise in humans: the influence of contraction velocity

We compared changes in muscle fibre composition and muscle strength indices following a 10 week isokinetic resistance training programme consisting of fast (3.14 rad(.)s(-1)) or slow (0.52 rad(.)s(-1)) velocity eccentric muscle contractions. A group of 20 non-resistance trained subjects were assigned to a FAST (n = 7), SLOW (n = 6) or non-training CONTROL (n = 7) group. A unilateral training protocol targeted the elbow flexor muscle group and consisted of 24 maximal eccentric isokinetic contractions (four sets of six repetitions) performed three times a week for 10 weeks. Muscle biopsy samples were obtained from the belly of the biceps brachii. Isometric torque and concentric and eccentric torque at 0.52 and 3.14 rad(.)s(-1) were examined at 0, 5 and 10 weeks. After 10 weeks, the FAST group demonstrated significant [mean (SEM)] increases in eccentric [29.6 (6.4)%] and concentric torque [27.4 (7.3) %] at 3.14 rad(.)s(-1), isometric torque [21.3 (4.3)%] and eccentric torque [25.2 (7.2) %] at 0.52 rad(.)s(-1). The percentage of type I fibres in the FAST group decreased from [53.8 (6.6)% to 39.1 (4.4)%] while type lib fibre percentage increased from [5.8 (1.9)% to 12.9 (3.3)%; P < 0.05]. In contrast. the SLOW group did not experience significant changes in muscle fibre type or muscle torque. We conclude that neuromuscular adaptations to eccentric training stimuli may be influenced by differences in the ability to cope with chronic exposure to relatively fast and slow eccentric contraction velocities. Possible mechanisms include greater cumulative damage to contractile tissues or stress induced by slow eccentric muscle contractions.

Functional changes of human quadriceps muscle injured by eccentric exercise

The present study evaluated functional changes of quadriceps muscle after injury induced by eccentric exercise. Maximal isometric torque of quadriceps and the surface electromyography (root mean square, RMS, and median frequency, MDF) of the vastus medialis oblique (VMO) and vastus lateralis (VL) muscles were examined before, immediately after and during the first 7 days after injury. Serum creatine kinase (CK) levels and magnetic resonance imaging (MRI) were used to identify muscle injury. The subject was used as her own control and percent refers to pre-injury data. Experiments were carried out with a sedentary 23-year-old female. Injury was induced by 4 bouts of 15 maximal isokinetic eccentric contractions (angular velocity of 5º/s; range of motion from 40º to 110º of knee flexion). The isometric torque of the quadriceps (knee at 90º flexion) decreased 52% immediately after eccentric exercise and recovered on the 5th day. The highest reduction of RMS occurred on the 2nd day after injury in both VL (63%) and VMO (66%) and only VL recovered to the pre-injury level on the 7th day. Immediately after injury, the MDF decreased by 5 and 3% (VMO and VL, respectively) and recovered one day later. Serum CK levels increased by 109% on the 2nd day and were still increased by 32% on the 7th day. MRI showed large areas of injury especially in the deep region of quadriceps. In conclusion, eccentric exercise decreased the isometric torque and electromyographic signals of quadriceps muscle, which were recovered in one week, despite the muscle regeneration signals.

Low-frequency fatigue at maximal and submaximal muscle contractions

Skeletal muscle force production following repetitive contractions is preferentially reduced when muscle is evaluated with low-frequency stimulation. This selective impairment in force generation is called low-frequency fatigue (LFF) and could be dependent on the contraction type. The purpose of this study was to compare LFF after concentric and eccentric maximal and submaximal contractions of knee extensor muscles. Ten healthy male subjects (age: 23.6 ± 4.2 years; weight: 73.8 ± 7.7 kg; height: 1.79 ± 0.05 m) executed maximal voluntary contractions that were measured before a fatigue test (pre-exercise), immediately after (after-exercise) and after 1 h of recovery (after-recovery). The fatigue test consisted of 60 maximal (100%) or submaximal (40%) dynamic concentric or eccentric knee extensions at an angular velocity of 60°/s. The isometric torque produced by low- (20 Hz) and high- (100 Hz) frequency stimulation was also measured at these times and the 20:100 Hz ratio was calculated to assess LFF. One-way ANOVA for repeated measures followed by the Newman-Keuls post hoc test was used to determine significant (P < 0.05) differences. LFF was evident after-recovery in all trials except following submaximal eccentric contractions. LFF was not evident after-exercise, regardless of exercise intensity or contraction type. Our results suggest that low-frequency fatigue was evident after submaximal concentric but not submaximal eccentric contractions and was more pronounced after 1-h of recovery.

Effect of L-carnitine on exercise performance in patients with mitochondrial myopathy

Exercise intolerance due to impaired oxidative metabolism is a prominent symptom in patients with mitochondrial myopathy (MM), but it is still uncertain whether L-carnitine supplementation is beneficial for patients with MM. The aim of our study was to investigate the effects of L-carnitine on exercise performance in MM. Twelve MM subjects (mean age±SD=35.4±10.8 years) with chronic progressive external ophthalmoplegia (CPEO) were first compared to 10 healthy controls (mean age±SD=29±7.8 years) before they were randomly assigned to receive L-carnitine supplementation (3 g/daily) or placebo in a double-blind crossover design. Clinical status, body composition, respiratory function tests, peripheral muscle strength (isokinetic and isometric torque) and cardiopulmonary exercise tests (incremental to peak exercise and at 70% of maximal), constant work rate (CWR) exercise test, to the limit of tolerance [Tlim]) were assessed after 2 months of L-carnitine/placebo administration. Patients with MM presented with lower mean height, total body weight, fat-free mass, and peripheral muscle strength compared to controls in the pre-test evaluation. After L-carnitine supplementation, the patients with MM significantly improved their Tlim (14±1.9 vs 11±1.4 min) and oxygen consumption ( V ˙ O 2 ) at CWR exercise, both at isotime (1151±115 vs 1049±104 mL/min) and at Tlim (1223±114 vs 1060±108 mL/min). These results indicate that L-carnitine supplementation may improve aerobic capacity and exercise tolerance during high-intensity CWRs in MM patients with CPEO.

Neuromuscular adaptations in endurance-trained boys and men

Competitive sports participation in youth is becoming increasingly more common in the Western world. It is widely accepted that sports participation, specifically endurance training, is beneficial for physical, psychomotor, and social development of children. The research on the effect of endurance training in children has focused mainly on healthrelated benefits and physiological adaptations, particularly on maximal oxygen uptake. However, corresponding research on neuromuscular adaptations to endurance training and the latter's possible effects on muscle strength in youth is lacking. In children and adults, resistance training can enhance strength and mcrease muscle activation. However, data on the effect of endurance training on strength and neuromuscular adaptations are limited. While some evidence exists demonstrating increased muscle activation and possibly increased strength in endurance athletes compared with untrained adults, the neuromuscular adaptations to endurance training in children have not been examined. Thus, the purpose of this study was to examine maximal isometric torque and rate of torque development (RID), along with the pattern of muscle activation during elbow and knee flexion and extension in muscle-endurancetrained and untrained men and boys. Subjects included 65 males: untrained boys (n=18), endurance-trained boys (n=12), untrained men (n=20) and endurance-trained men (n=15). Maximal isometric torque and rate of torque development were measured using an isokinetic dynamometer (Biodex III), and neuromuscular activation was assessed using surface electromyography (SEMG). Muscle strength and activation were assessed in the dominant arm and leg, in a cross-balanced fashion during elbow and knee flexion and extension. The main variables included peak torque (T), RTD, rate of muscle activation (Q30), Electro-mechanical delay (EMD), time to peak RTD and co-activation index. Age differences in T, RTD, electro-mechanical delay (EMD) and rate of muscle activation (Q30) were consistently observed in the four contractions tested. Additionally, Q30, nonnalized for peak EMG amplitude, was consistently higher in the endurancetrained men compared with untrained men. Co-activation index was generally low in all contractions. For example, during maximal voluntary isometric knee extension, men were stronger, had higher RTD and Q30, whether absolute or nonnalized values were used. Moreover, boys exhibited longer EMD (64.8 ± 18.5 ms vs. 56.6 ± 15.3 ms, for boys and men respectively) and time to peak RTD (112.4 ± 33.4 ms vs. 100.8 ± 39.1 ms for boys and men, respectively). In addition, endurance-trained men had lower T compared with untrained men, yet they also exhibited significantly higher nonnalized Q30 (1.9 ± 1.2 vs. 1.1 ± 0.7 for endurance-trained men and untrained men, respectively). No training effect was apparent in the boys. In conclusion, the findings demonstrate muscle strength and activation to be lower in children compared with adults, regardless of training status. The higher Q30 of the endurance-trained men suggests neural adaptations, similar to those expected in response to resistance training. The lower peak torque may su9gest a higher relative involvement oftype I muscle fibres in the endurance-trained athletes. Future research is required to better understand the effect of growth and development on muscle strength and activation patterns during dynamic and sub-maximal isometric contractions. Furthennore, training intervention studies could reveal the effects of endurance training during different developmental stages, as well as in different muscle groups.

Do Neuro-Muscular Adaptations Occur in Endurance-Trained Boys and Men?

Most research on the effects of endurance training has focused on endurance training's health-related benefits and metabolic effects in both children and adults. The purpose of this study was to examine the neuromuscular effects of endurance training and to investigate whether they differ in children (9.0-12.9 years) and adults (18.4-35.6 years). Maximal isometric torque, rate of torque development (RTD), rate of muscle activation (Q30), electromechanical delay (EMD), and time to peak torque and peak RTD were determined by isokinetic dynamometry and surface electromyography (EMG) in elbow and knee flexion and extension. The subjects were 12 endurance-trained and 16 untrained boys, and 15 endurance-trained and 20 untrained men. The adults displayed consistently higher peak torque, RTD, and Q30, in both absolute and normalized values, whereas the boys had longer EMD (64.7+/-17.1 vs. 56.6+/-15.4 ms) and time to peak RTD (98.5+/-32.1 vs. 80.4+/-15.0 ms for boys and men, respectively). Q30, normalized for peak EMG amplitude, was the only observed training effect (1.95+/-1.16 vs. 1.10+/-0.67 ms for trained and untrained men, respectively). This effect could not be shown in the boys. The findings show normalized muscle strength and rate of activation to be lower in children compared with adults, regardless of training status. Because the observed higher Q30 values were not matched by corresponding higher performance measures in the trained men, the functional and discriminatory significance of Q30 remains unclear. Endurance training does not appear to affect muscle strength or rate of force development in either men or boys.

Taxa de desenvolvimento de força em diferentes velocidades de contrações musculares

O objetivo deste estudo foi comparar a taxa de desenvolvimento de força (TDF) nas contrações isométrica e isocinética concêntrica a 60°.s-1 e 180°.s-1. Quatorze indivíduos do gênero masculino (idade = 23,1 ± 2,8 anos; estatura = 174 ± 31,3cm; massa corporal = 81 ± 12kg) realizaram inicialmente uma familiarização ao equipamento isocinético. Posteriormente, os indivíduos realizaram em ordem randômica cinco contrações isocinéticas máximas para os extensores do joelho a 60°.s-1 e 180°.s-1 para determinar o torque máximo concêntrico (TMC) e duas contrações isométricas máximas de 3s para determinar o torque máximo isométrico (TMI). O TMI (301,4 ± 56,0N.m) foi maior do que o TMC a 60°.s-1 (239,8 ± 42,2N.m) e 180°.s-1 (175,0 ± 32,5 N.m). O TMC a 60°.s-1 foi maior do que o TMC a 180°.s-1. Para os intervalos de 0-30ms e 0-50ms, a TDF na condição isométrica (1.196,6 ± 464,6 e 1.326,5 ± 514,2N.m.s-1, respectivamente) foi similar à TDF a 60°.s-1 (1.035,4 ± 446,2 e 1.134,3 ± 448,4N.m.s-1) e maior do que a 180°.s-1 (656,7 ± 246,6 e 475,2 ± 197,9N.m.s-1), sendo ainda que a TDF na contração concêntrica a 180°.s-1 foi menor do que a 60°.s-1. No intervalo de 0-100ms, a TDF da contração isométrica (1.248,8 ± 417,4N.m.s-1) foi maior que a obtida na contração isocinética rápida (909,2 ± 283,4N.m.s-1). A TDF obtida na contração isocinética lenta (1.005,4 ± 247,7N.m.s-1) foi similar à obtida na contração isométrica e na concêntrica isocinética rápida. No intervalo 0-150ms, a TDF isométrica (1.084,2 ± 332,1N.m.s-1) foi maior do que as concêntricas (60°.s-1 e 180°.s-1) (834,8 ± 184,2 e 767,6 ± 201,8N.m.s-1, respectivamente), não existindo diferenças entre estas duas últimas. Conclui-se que a TDF é dependente do tipo e da velocidade de contração, suportando a hipótese de que maiores velocidades de contração acarretam maior inibição do drive neural no início do movimento.

Moderate intensity and volume downhill run does not impair knee joint stability at early and late phases of quadriceps/hamstrings contraction

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Efeitos do treinamento isocinético e isométrico sobre a taxa de desenvolvimento de força máxima

The objective of the present study was to compare the effects of a high speed isokinetic training (180°.s-1) and an isometric training (75°) on the maximum rate of force development (RFDmax) measured in the isokinetic and isometric modes. Twenty seven male non active subjects participated of this study (Mean + SD = body mass 78.6 + 14.1 kg; stature 175.1 + 8.9 cm; age 22.6 + 3.8 years). They were randomly divided into three groups: Control (GC); Isokinetic training (GISOC) and; Isometric training (GISOM). The subjects were submitted in different days to the following pre training protocols: 1) Familiarization to the isokinetic dynamometer tests; 2) Five maximum concentric isokinetic contractions of the knee extensors (180°.s-1) to access the maximum concentric torque (TMC) and the concentric RFDmax; 3) Two maximum isometric contractions of the knee extensors (75°) to access the maximum isometric torque (TMI) and the isometric RFDmax. The same tests were repeated after the training period, but without the familiarization session. Eighteen training sessions were performed (3 times per week). The GISOC performed the entire training whit concentric isokinetic contractions whit the speed of 180°.s-1. The GISOM performed the entire training whit isometric contractions whit the angle between the thigh end the leg being 75° (0° = full knee extension). TMI, TMC, concentric RFDmax, isometric RFDmax values of the GC was not different between pre and post training. GISOM increased only the TMI and the GSIOC increased the TMC, concentric RFDmax and isometric RFDmax. Furthermore, the GISOC had a higher percentage increase of the isometric RFDmax than the isokinetic RFDmax. Based on these results, it is possible to conclude that the increase in maximum strength corresponded to the training specificity theory, unlike to the RFDmax. Thus the use of isometric contraction ...(Complete abstract click electronic access below)

Treinamento de força unilateral: efeitos sobre a taxa de desenvolvimento de força do membro contralateral

The aim of this study was to analyze the effects of unilateral isokinetic strength training at high speed (180°.s-1) of the dominant leg on the rate of force development (RFD) of the contralateral limb. The study included 18 male individuals, apparently healthy, with an average of 23 years, and without regular practice of physical activities. The subjects were divided randomly into two groups: control group (GC) and isokinetic group (GISOC). Assessments were made of the values of peak isometric torque (PT isom) and RFD of both lower limbs in the pre-training. So, the GISOC underwent an isokinetic strength training with high speed (180°.s-1) for 6 weeks, with the dominant leg only and then reassessed. It was found that there was no significant improvement in value of PT isom (p> 0.05), and the RFD for the dominant limb had a significant improvement (p< 0.05) compared to the pre-training and that there was no transfer of values to the contralateral side (p> 0.05). It can be concluded that the isokinetic training at high speed was not enough for significant cross education