Neuromuscular fatigue development during maximal concentric and isometric knee extensions.

This study aimed to investigate mechanisms of neuromuscular fatigue during maximal concentric and isometric leg extensions inducing similar torque decrements. Nine physically active men performed two separate fatiguing sessions maintained until similar torque decreases were obtained. The first session, only conducted under isokinetic concentric conditions (CON), consisted of three series of 30 maximal voluntary concentric knee extensions (60 degrees/s). The second session, exclusively isometric (ISO), mimicked the torque decreases registered during the CON session while performing three long-lasting ISO contractions. Maximal voluntary torque, activation level (twitch interpolation technique), electromyographic activity (root mean square and median frequency) of the vastus lateralis muscle, and electrically evoked doublet-twitch mechanical properties were measured before and at the end of each of the three series. After the three series, similar torque decrements were obtained for both fatiguing procedures. The total fatiguing contraction durations were not different among procedures. With equivalent voluntary torque decrements, the doublet-twitch amplitude reduction was significantly greater (P<0.01) during the two first series of the CON procedure compared with ISO. No difference was observed for the third series. Although no difference was recorded with fatigue for median frequency changes between CON and ISO, activation levels and root mean square values demonstrated greater reductions (P<0.05) for all three series during the ISO procedure compared with CON. Performing CON or ISO fatiguing exercises demonstrated different fatigue origins. With CON exercises, peripheral fatigue developed first, followed by central fatigue, whereas with ISO exercises the fatigue pattern was inverted.     

Characteristics and lateral dominance of hand grip and elbow flexion powers in young male adults

This study aimed to clarify the characteristics and the lateral dominance of hand grip power and elbow flexion power. The subjects were 15 healthy young males (mean age 22.1+/-0.7 yr, mean height 171.3+/-3.4 cm, mean mass 64.5+/-4.1 kg). All subjects were right-handed. Peak power was measured by both hands with 6 different loads of 20%-70% of maximum voluntary contraction. The maximum voluntary contraction of hand grip movement and elbow flexion movement was significantly larger in the dominant hand. Peak power of the dominant hand was larger in all loads in hand grip movement and in loads of 20% and 30% of maximum voluntary contraction in elbow flexion movement. In short, lateral dominance was confirmed. Peak power was significantly larger in hand grip movement than in elbow flexion movement in both hands. Peak velocity decreased with increasing loads in both movements, but peak power increased until about 50% of maximum voluntary contraction and then decreased. The peak power ratio of the dominant hand to the nondominant hand was significantly larger in hand grip movement than in elbow flexion movement in all loads and the peak power ratio in elbow flexion movement was more marked in light loads. In conclusion, both powers showed lateral dominance. Lateral dominance is more marked in hand grip power.     

Neuromuscular fatigue during dynamic maximal strength and hypertrophic resistance loadings

The purpose of this study was to compare the acute neuromuscular fatigue during dynamic maximal strength and hypertrophic loadings, known to cause different adaptations underlying strength gain during training. Thirteen healthy, untrained males performed two leg press loadings, one week apart, consisting of 15 sets of 1 repetition maximum (MAX) and 5 sets of 10 repetition maximums (HYP). Concentric load and muscle activity, electromyography (EMG) amplitude and median frequency, was assessed throughout each set. Additionally, maximal bilateral isometric force and muscle activity was assessed pre-, mid-, and up to 30 min post-loading. Concentric load during MAX was decreased after set 10 (P<0.05), while the load was maintained throughout HYP. Both loadings caused large reductions in maximal isometric force (MAX=-30±6.4% vs. HYP=-48±9.7%, P<0.001). The decreased concentric and isometric strength during MAX loading was accompanied by reduced EMG amplitude (P<0.05). Conversely, hypertrophic loading caused decreased median frequency only during isometric contractions (P<0.01). During concentric contractions, EMG amplitude increased and median frequency decreased in HYP (P<0.01). Our results indicate reduced neural drive during MAX loading and more complex changes in muscle activity during HYP loading.     

Power produced by maximal velocity elbow flexion

An analysis of horizontal elbow flexion at maximal velocity was made to determine how different loads affected power output. Twenty male subjects operated a specially constructed dynamometer initially performing a maximal effort isometric trial with the elbow fully extended and then three dynamic trials at each of three loads equal to 75, 50, and 25 per cent of the maximal isometric strength. Angular acceleration was used to calculate forearm torque, and power was obtained by taking the product of torque and angular velocity. Power was found to be a cubic function of time and a fourth-order polynomial function of angular displacement reaching a peak early in the movement. The 50 per cent load resulted in a higher peak level of power than either the 25 or 75 per cent loads.     

Supraspinal fatigue during intermittent maximal voluntary contractions of the human elbow flexors.

Responses to transcranial magnetic stimulation in human subjects (n = 9) were studied during series of intermittent isometric maximal voluntary contractions (MVCs) of the elbow. Stimuli were given during MVCs in four fatigue protocols with different duty cycles. As maximal voluntary torque fell during each protocol, the torque increment evoked by cortical stimulation increased from approximately 1.5 to 7% of ongoing torque. Thus "supraspinal" fatigue developed in each protocol. The motor evoked potential (MEP) and silent period in the elbow flexor muscles also changed. The silent period lengthened by 20-75 ms (lowest to highest duty cycle protocol) and recovered significantly with a 5-s rest. The MEP increased in area by >50% in all protocols and recovered significantly with 10 s, but not 5 s, of rest. These changes are similar to those during sustained MVC. The central fatigue demonstrated by the torque increments evoked by the stimuli did not parallel the changes in the electromyogram responses. This suggests that part of the fatigue developed during intermittent exercise is "upstream" of the motor cortex.     

Neuromuscular fatigue after maximal stretch-shortening cycle exercise

Strojnik, V., and P. V. Komi. Neuromuscular fatigueafter maximal stretch-shortening cycle exercise. J. Appl. Physiol. 84(1): 344-350, 1998.To examinesome possible sites of fatigue during short-lasting maximally intensivestretch-shortening cycle exercise, drop jumps on an inclined sledgeapparatus were analyzed. Twelve healthy volunteers performed jumpsuntil they were unable to maintain jumping height >90% of theirmaximum. After the workout, the increases in the blood lactateconcentration and serum creatine kinase activation were statisticallysignificant (P < 0.001 and P < 0.05, respectively) but rathersmall in physiological terms. The major changes after the workout wereas follows: the single twitch was characterized by smaller peak torque(P < 0.05) and shorter time to peak(P < 0.05) and half-relaxation time(P < 0.01). The double-twitch torqueremained at the same level (P > 0.05), but with a steeper maximal slope of torque rise(P < 0.05); during 20- and 100-Hzstimulation the torque declined (both P < 0.01) and the maximal voluntarytorque changed nonsignificantly but with a smaller maximal slope oftorque rise (P < 0.01) and a higheractivation level (P < 0.05),accompanied by an increased electromyogram amplitude. These findingsindicate that the muscle response after the short-lasting consecutivemaximum jumps on the sledge apparatus may involve two distinctmechanisms acting in opposite directions:1) The contractile mechanism seemsto be potentiated through a shorterCa²⁺ transient and fastercross-bridge cycling, as implied by twitch changes.2) High-frequency action potentialpropagation shows an impairment, which is suggested as the possibledominant reason for fatigue in exercise of this type.

     

Scapular kinematics during unloaded and maximal loaded isokinetic concentric and eccentric shoulder flexion and extension movements

Characterization of scapular kinematics under demanding load conditions might aid to distinguish between physiological and clinically relevant alterations. Previous investigations focused only on submaximal external load situations. How scapular movement changes with maximal load remains unclear. Therefore, the present study aimed to evaluate 3D scapular kinematics during unloaded and maximal loaded shoulder flexion and extension. Twelve asymptomatic individuals performed shoulder flexion and extension movements under unloaded and maximal concentric and eccentric loaded isokinetic conditions. 3D scapular kinematics assessed with a motion capture system was analyzed for 20° intervals of humeral positions from 20° to 120° flexion. Repeated measures ANOVAs were used to evaluate kinematic differences between load conditions for scapular position angles, scapulohumeral rhythm and scapular motion extent. Increased scapular upward rotation was seen during shoulder flexion and extension as well as decreased posterior tilt and external rotation during eccentric and concentric arm descents of maximal loaded compared to unloaded conditions. Load effects were further seen for the scapulohumeral rhythm with greater scapular involvement at lower humeral positions and increased scapular motion extent under maximal loaded shoulder movements. With maximal load applied to the arm physiological scapular movement pattern are induced that may imply both impingement sparing and causing mechanisms.     

Non-linear dynamics in muscle fatigue and strength model during maximal self-perceived elbow extensors training

Our aim was to determine the dynamics in muscle strength increase and fatigue development during repetitive maximal contraction in specific maximal self-perceived elbow extensors training program. We will derive our functional model for m. triceps brachii in spirit of traditional Hill’s two-component muscular model and after fitting our data, develop a prediction tool for this specific training system. Thirty-six healthy young men (21±1.0 y, BMI 25.4±7.2 kg/m²), who did not take part in any formal resistance exercise regime, volunteered for this study. The training protocol was performed on the isoacceleration dynamometer, lasted for 12 weeks, with a frequency of five sessions per week. Each training session included five sets of 10 maximal contractions (elbow extensions) with a 1 min resting period between each set. The non-linear dynamic system model was used for fitting our data in conjunction with the Levenberg–Marquardt regression algorithm. As a proper dynamical system, our functional model of m. triceps brachii can be used for prediction and control. The model can be used for the predictions of muscular fatigue in a single series, the cumulative daily muscular fatigue and the muscular growth throughout the training process. In conclusion, the application of non-linear dynamics in this particular training model allows us to mathematically explain some functional changes in the skeletal muscle as a result of its adaptation to programmed physical activity—training.     

Neuromuscular fatigue during a long-duration cycling exercise.

The effects of prolonged cycling on neuromuscular parameters were studied in nine endurance-trained subjects during a 5-h exercise sustained at 55% of the maximal aerobic power. Torque during maximal voluntary contraction (MVC) of the quadriceps muscle decreased progressively throughout the exercise (P < 0.01) and was 18% less at the end of exercise compared with the preexercise value. Peak twitch torque, contraction time, and total area of mechanical response decreased significantly (P < 0.05) after the first hour of exercise. In contrast, changes in M-wave characteristics were significant only after the fourth hour of the exercise. Significant reductions (P < 0.05) in electromyographic activity normalized to the M wave occurred after the first hour for the vastus lateralis muscle but only at the end of the exercise for the vastus medialis muscle. Muscle activation level, assessed by the twitch interpolation technique, decreased by 8% (P < 0.05) at the end of the exercise. The results suggest that the time course is such that the contractile properties are significantly altered after the first hour, whereas excitability and central drive are more impaired toward the latter stages of the 5-h cycling exercise.     

Nonuniform shortening in the biceps brachii during elbow flexion.

This study tested the common assumption that skeletal muscle shortens uniformly in the direction of its fascicles during low-load contraction. Cine phase contrast magnetic resonance imaging was used to characterize shortening of the biceps brachii muscle in 12 subjects during repeated elbow flexion against 5 and 15% maximum voluntary contraction (MVC) loads. Mean shortening was relatively constant along the anterior boundary of the muscle and averaged 21% for both loading conditions. In contrast, mean shortening was nonuniform along the centerline of the muscle during active elbow flexion. Centerline shortening in the distal region of the biceps brachii (7.3% for 5% MVC and 3.7% for 15% MVC) was significantly less (P < 0.001) than shortening in the muscle midportion (26.3% for 5% MVC and 28.2% for 15% MVC). Nonuniform shortening along the centerline was likely due to the presence of an internal aponeurosis that spanned the distal third of the longitudinal axis of the biceps brachii. However, muscle shortening was also nonuniform proximal to the centerline aponeurosis. Because muscle fascicles follow the anterior contour and centerline of the biceps brachii, our results suggest that shortening is uniform along anterior muscle fascicles and nonuniform along centerline fascicles.     

Hierarchical genetic algorithm versus static optimization-investigation of elbow flexion and extension movements

The applicability of static optimization (and, respectively, frequently used objective functions) for prediction of individual muscle forces for dynamic conditions has often been discussed. Some of the problems are whether time-independent objective functions are suitable, and how to incorporate muscle physiology in models. The present paper deals with a twofold task: (1) implementation of hierarchical genetic algorithm (HGA) based on the properties of the motor units (MUs) twitches, and using multi-objective, time-dependent optimization functions; and (2) comparison of the results of the HGA application with those obtained through static optimization with a criterion "minimum of a weighted sum of the muscle forces raised to the power of n". HGA and its software implementation are presented. The moments of neural stimulation of all MUs are design variables coding the problem in the terms of HGA. The main idea is in using genetic operations to find these moments, so that the sum of MUs twitches satisfies the imposed goals (required joint moments, minimal sum of muscle forces, etc.). Elbow flexion and extension movements with different velocities are considered as proper illustration. It is supposed that they are performed by two extensor muscles and three flexor muscles. The results show that HGA is a suitable means for precise investigation of motor control. Many experimentally observed phenomena (such as antagonistic co-contraction, three-phasic behavior of the muscles during fast movements) can find their explanation by the properties of the MUs twitches. Static optimization is also able to predict three-phasic behavior and could be used as practicable and computationally inexpensive method for total estimation of the muscle forces.     

Neuromuscular fatigue of elbow flexor muscles of dominant and non-dominant arms in healthy humans.

The purpose of this study was to assess differences in fatigue-related changes in variables related to structures within the neuromuscular system, between the dominant and non-dominant elbow flexor muscles of right-handed individuals. Two experimental sessions were performed on the right arm and one on the left arm. For each session, maximum voluntary torque, level of voluntary activation, M-wave amplitude, twitch/train or twitch/doublet torque ratio and EMG median frequency were obtained before and up to 20 min after a sustained maximum isometric fatigue task. Our main results were: 1) reproducible fatigue-induced changes in all variables of interest between the two sessions performed with the right arm, 2) significantly greater failure in voluntary activation and neuromuscular propagation with sustained activity for the non-dominant compared with dominant side, and 3) no effect of dominance on MVC torque, endurance time, and fatigue-induced changes in EMG median frequency and elicited torques. These results suggest that the preferential use of elbow flexor muscles with the dominant arm leads to more fatigue resistance in certain structures/mechanisms of the neuromuscular system, but not in others.     

Reduced elbow extension torque during vibrations

Impact sports and vibration platforms trigger vibrations within soft tissues and the skeleton. Although the long-term effects of vibrations on the body have been studied extensively, the acute effects of vibrations are little understood. This study determined the influence of acute vibrations at different frequencies and elbow angles on maximal isometric elbow extension torque and muscle activity. Vibrations were generated by a pneumatic vibrator attached to the lever of a dynamometer, and were applied on the forearm of 15 healthy female subjects. The subjects were instructed to push maximally against the lever at three different elbow angles, while extension torque and muscle activity were quantified and compared between vibration and non-vibration (control) conditions. A change in vibration frequency had no significant effects on torque and muscle activity although vibrations in general decreased the maximal extension torque relative to the control by 1.8% (±5.7%, p>0.05), 7.4% (±7.9%, p<0.01), and 5.0% (±8.2%, p<0.01) at elbow angles of 60°, 90°, and 120°, respectively. Electromyographic activity increased significantly between ～30% and 40% in both triceps and biceps with vibrations. It is speculated that a similar increase in muscle activity between agonist and antagonist, in combination with an unequal increase in muscle moment arms about the elbow joint, limit the maximal extension torque during exposure to vibrations. This study showed that maximal extension torque decreased during vibration exposure while muscle activity increased and suggests that vibrations may be counterproductive during activities requiring maximal strength but potentially beneficial for strength training.     

The use of correlation integrals in the study of localized muscle fatigue of elbow flexors during maximal efforts.

Innovative applications of non-linear time series analysis have recently been used to investigate physiological phenomena. In this study, we investigated the feasibility of using the correlation integral to monitor the localized muscle fatigue process in the biceps brachii during sustained maximal efforts. The subjects performed isometric maximum contractions until failure in elbow flexion (90 degrees from neutral). The median and the 70th percentile frequency of the Surface electromyography (SEMG) power spectrum, the integrated SEMG, and the Correlation Integral (CI) were evaluated during the trials. The linear correlation between these variables and the elbow torque production was used to quantify the ability of a parameter to follow the fatiguing process. The CI had the highest linear correlation with torque (0.77 (0.12SD)), while the spectral indices correlations with torque were much lower. The decreasing trend of the torque production was followed by the spectral indices for only the beginning part of the contraction, while the CI increased sharply after the torque production fell to about 0.60 of the MVC. This suggests that the CI is sensitive to different changes of the SEMG signal during fatigue than the spectral variables.     

Neuromuscular fatigue during repetitive stimulation in elderly and young adults.

The purpose of this investigation was to examine the integrity of neuromuscular transmission and impulse propagation during fatigue by examining the muscle compound action potential (M wave) in elderly and young adults. The tibialis anterior muscle of nine elderly [mean = 67.7 (SE 1.7) years] and nine young [mean = 26.7 (SE 1.2) years] adults was maximally stimulated repetitively at frequencies of 20, 30 or 40 Hz for 60 s on separate occasions. There was a significantly smaller resting M wave amplitude [7.9 (SE 0.4) mV versus 9.9 (SE 0.6) mV] and M wave area [0.038 (SE 0.005) mV s versus 0.06 (SE 0.004) mV.s] in the elderly versus the young adults respectively. Measurement of the evoked muscle contractile properties revealed significantly (P < 0.05) longer twitch durations and a significantly (P < 0.05) greater peak twitch torque [4.6 (SE 0.4) Nm versus 3.2 (SE 0.5) Nm] in the elderly versus the young adults, respectively. The elderly adults had a significantly greater torque decline during the 20-Hz trial; however, the decline in torque during the 30-Hz and 40-Hz trials was similar in the elderly and the young adults (30 Hz: 40%; 40 Hz: 56%). Throughout each of the stimulation trials, the decline in torque was accompanied by a significant reduction in M wave amplitude (20 Hz: 14%; 30 Hz: 53%; 40 Hz: 67%); M wave area also declined significantly during the 30-Hz (31%) and 40-Hz (53%) trials. There was no significant difference between the elderly and the young adults in the reduction in the M wave amplitude or area during each trial.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationships between force-time parameters and muscle oxygenation kinetics during maximal sustained isometric grip and maximal repeated rhythmic grip with different contraction frequencies

The purposes of this study were to examine the relationships between various force-time parameters and muscle oxygenation kinetics during maximal sustained isometric grip (SIG) and maximal repeated rhythmic grips (RRG) with different grip intervals (interval times: 5, 4, 3, and 2 s). Subjects were 10 healthy young males, aged 20-26 years (height 173.9+/-7.3 cm, body mass 71.5+/-11.2 kg). After measuring maximal grip force, each subject performed the SIG and RRG tests with a target frequency of 12, 15, 20, and 30 grips.min(-1) (interval times: 5, 4, 3, and 2 s, respectively) for 6 min. The decreasing time until 80% MVC showed significant and high correlations with final force values in RRGs with over 3 s intervals (r=0.866-0.941), but not in the SIG and RRG with a 2 s interval. The time at the lowest Oxy-Hb/Mb value showed a significant and high correlation with the time at the highest Deoxy-Hb/Mb value only in the SIG and RRG with a 2 s interval (r=0.825-0.916). Oxy-Hb/Mb decreases markedly and deoxy-Hb/Mb increases after the onset of SIG due to the obstruction of blood flow caused by the increase in intramuscular pressure. A similar physiological response to that of SIG occurs also in RRG with a 2 s interval, but RRGs with intervals over 3 s achieve more resumption of blood flow in the muscular relaxation phase. Hence, in spite of the same RRGs, it was determined that RRGs with intervals over 3 s differ significantly in a changing pattern of grip force and muscle oxygen kinetics from RRGs with a 2 s interval.     

Isokinetic elbow flexion and coactivation following eccentric training.

The influence of an eccentric training on torque/angular velocity relationships and coactivation level during maximal voluntary isokinetic elbow flexion was examined. Seventeen subjects divided into two groups (Eccentric Group EG, n = 9 Control Group CG, n = 8) performed on an isokinetic dynamometer, before and after training, maximal isokinetic elbow flexions at eight angular velocities (from - 120 degrees s(-1) under eccentric conditions to 240 degrees s(-1) under concentric conditions), and held maximal and submaximal isometric actions. Under all conditions, the myoelectric activities (EMG) of the biceps and the triceps brachii muscles were recorded and quantified as the RMS value. Eccentric training of the EG consisted of 5x6 eccentric muscle actions at 100 and 120% of one maximal repetition (IRM) for 21 sessions and lasted 7 weeks. In the EG after training, torque was significantly increased at all angular velocities tested (ranging from 11.4% at 30 degrees (s-1) to 45.5% at - 120 degrees s(-1)) (p < 0.05). These changes were accompanied by an increase in the RMS activities of the BB muscle under eccentric conditions (from - 120 to - 30 degrees (s-1)) and at the highest concentric angular velocities (180 and 24 degrees s(-1)) (p < 0.05). The RMS activity of the TB muscle was not affected by the angular velocity in either group for all action modes. The influence of eccentric training on the torque gains under eccentric conditions and for the highest velocities was attributed essentially to neural adaptations.     

Joint-specific power production and fatigue during maximal cycling

Cycling power decreases substantially during a maximal cycling trial of just 30 s. It is not known whether movement patterns and joint powers produced at each joint decrease to a similar extent or if each joint exhibits an individual fatigue profile. Changes in movement patterns and/or joint powers associated with overall task fatigue could arise from several different mechanisms or from a complex interplay of these mechanisms. The purpose of this investigation was to determine the changes in movement and power at each joint during a fatiguing cycling trial. Thirteen trained cyclists performed a 30 s maximal cycling trial on an isokinetic cycle ergometer at 120 rpm. Pedal forces and limb kinematics were recorded. Joint powers were calculated using a sagittal plane inverse dynamics model and averaged for the initial, middle, and final three second intervals of the trial, and normalized to initial values. Relative ankle plantar flexion power was significantly less than all other joint actions at the middle interval (51±5% of initial power; p=0.013). Relative ankle plantar flexion power for the final interval (37±3%) was significantly less than the relative knee flexion and hip extension power (p=0.010). Relative knee extension power (41±5%) was significantly less than relative hip extension power (55±4%) during the final three second interval (p=0.045). Knee flexion power (47±5%) did not differ from relative hip extension power (p=0.06). These changes in power were accompanied by a decrease in time spent extending by each joint with fatigue (i.e., decreased duty cycle, p<0.03). While central mechanisms may have played a role across all joints, because the ankle fatigued more than the hip and knee joints, either peripheral muscle fatigue or changes in motor control strategies were identified as the potential mechanisms for joint-specific fatigue during a maximal 30 s cycling trial.