Blood flow in the triceps brachii muscle in humans during sustained submaximal isometric contractions

The main purpose of this study was to determine the extent to which blood flow through the profunda artery within the triceps brachii muscle may be compromised during maintained low-force isometric fatiguing contractions. Doppler ultrasound techniques were used to record mean blood velocity and arterial diameter of the profunda brachii artery during sustained isometric contractions of 20% maximal voluntary contraction. The arterial diameter did not change throughout the contraction. Thus, blood velocity was considered to be an indicator of blood flow. The mean blood velocity increased initially and then remained constant during the contraction period. When compared to rest [0.06 (SD 0.03) m s^–1] mean blood velocity was significantly larger at the start of the contraction [0.13 (SD 0.07) m s^–1] and larger yet during recovery following the contraction [0.30 (SD 0.14) m s^–1]. Although blood flow through the conduit artery did not drop during the contraction, the post-contraction hyperaemia suggested that circulatory compromise might have occurred at the level of the capillary beds. Electronic Publication     

Human brain activation during sustained and intermittent submaximal fatigue muscle contractions: an FMRI study

During prolonged submaximal muscle contractions, electromyographic (EMG) signals typically increase as a result of increasing motor unit activities to compensate for fatigue-induced force loss in the muscle. It is thought that cortical signals driving the muscle to higher activation levels also increases, but this has never been experimentally demonstrated. The purpose of this study was to quantify brain activation during submaximal fatigue muscle contractions using functional magnetic resonance imaging (fMRI). Twelve volunteers performed a sustained handgrip contraction for 225 s and 320 intermittent handgrip contractions ( approximately 960 s) at 30% maximal level while their brain was imaged. For the sustained contraction, EMG signals of the finger flexor muscles increased linearly while the target force was maintained. The fMRI-measured cortical activities in the contralateral sensorimotor cortex increased sharply during the first 150 s, then plateaued during the last 75 s. For the intermittent contractions, the EMG signals increased during the first 660 s and then began to decline, while the handgrip force also showed a sign of decrease despite maximal effort to maintain the force. The fMRI signal of the contralateral sensorimotor area showed a linear rise for most part of the task and plateaued at the end. For both the tasks, the fMRI signals in the ipsilateral sensorimotor cortex, prefrontal cortex, cingulate gyrus, supplementary motor area, and cerebellum exhibited steady increases. These results showed that the brain increased its output to reinforce the muscle for the continuation of the performance and possibly to process additional sensory information.     

Motor unit activity during long-lasting intermittent muscle contractions in humans

Changes accompanying long-lasting intermittent muscle contractions (30%–50% of the maximal) were investigated by tracing the activity of 38 motor units (MU) of the human biceps brachii muscle recorded from fine-wire branched electrodes. The motor task was a continuous repetition of ramp-and-hold cycles of isometric flexion contractions. During ramp-up phases a significant decline in recruitment thresholds was found with no changes in the discharge pattern. During ramp-down phases the unchanged mean value of derecruitment thresholds during the task was accompanied by increased duration of the last two interspike intervals (ISI). These findings would suggest that during fatigue development the main compensatory mechanism during ramp-up contractions is space coding while for ramp-down contractions it is rate coding. During the steady-state phases the mean value of ISI, as well as the firing variability, had increased by the end of the task in most of the MU investigated . In addition 17 recruited MU were also investigated. These units revealed a lower initial discharge rate and a faster decrease in the mean discharge rate with the development of fatigue. The gradual reduction of the recruitment threshold of already active MU and the recruitment of new units demonstrated an increased excitability of the motorneuron pool during fatigue. A typical recruitment pattern (a first short ISI followed by a long one) was observed during ramp-up contractions in units active from the very beginning of the task, as well as during sustained contractions at the onset of the stable discharge of the additionally recruited MU.     

Influence of electrode position on changes in electromyograph parameters of the upper trapezius muscle during submaximal sustained contractions

The purpose of the present study was to evaluate whether differences could be found in the changes in mean power frequency (MPF) and root-mean-square (rms) due to electrode positions on the upper trapezius muscle during a sustained submaximal task. A group of 25 healthy subjects performed a continuous forward flexion of the right arm at 20% of their maximal voluntary contraction (MVC). Three pairs of bipolar surface electrodes were positioned on a straight line between the spine of the seventh cervical vertebra (C7) and the lateral edge of the acromion. The fourth pair (the caudal position) was placed 2 cm below the midpoint of this line. The analysis of variance revealed significant differences between the electrode positions during the short 20% MVC and during MVC (P?≤?0.01). Furthermore, lower rms values were found when the electrodes were placed symmetrically around the midpoint of the reference line, which confirms previous studies. A statistical model has been developed to discriminate additionally the very small changes in the electromyogram parameters due to electrode position during the sustained 20% MVC. It was found that the slope coefficients of rms and MPF were significantly different from 0 (P?≤?0.01), although to a lesser degree for rms and more clearly for MPF of the caudal position (P?≤?0.05). Furthermore, significant differences were found between several combinations of the uppermost electrodes and the caudal position (P?≤?0.01). From this study, it is concluded that it is important to evaluate several electrode positions on the upper trapezius muscle to be able to represent the behaviour of this muscle accurately during a sustained contraction.     

Handedness but not dominance influences variability in endurance time for sustained, submaximal contractions

The purpose of this study was to compare endurance time and accompanying neuromuscular adjustments when left- and right-handed subjects used the dominant and nondominant arms to sustain submaximal contractions that required either force or position control. Ten left-handed and 10 right-handed healthy adults (21 ± 5 yr) participated in the study. Each subject exerted a similar net torque about the elbow joint during the force and position tasks to achieve a target force of 20% maximal voluntary contraction (MVC) force (56 ± 18 N). MVC force declined to a similar level immediately after task failure for left- and right-handed subjects (27 ± 13 vs. 25 ± 15%, P = 0.9). Endurance time for the position task was similar for the dominant and nondominant arms (task × dominance interaction, P = 0.17). Although the difference in endurance time between the two tasks was similar for left-handed (136 ± 165 s) and right-handed individuals (92 ± 73 s, task × handedness interaction, P = 0.38), there was greater variance in the ratio of the endurance times for the force and position tasks for left-handed (0.77) than right-handed subjects (0.13, P < 0.001; see Fig. 2). Furthermore, endurance time for the force and position tasks was significantly correlated for right-handed subjects (r(2) = 0.62, P < 0.001), but not for left-handed subjects (r(2) = 0.004, P = 0.79). Multiple regression analyses identified sets of predictor variables for each endurance time, and these differed with handedness and task. Hand dominance, however, did not influence endurance time for either group of subjects. These findings indicate that endurance times for the elbow flexors when performing submaximal isometric contractions that required either force or position control were not influenced by hand dominance but did depend on handedness.     

Potassium homeostasis during and following exhaustive submaximal static handgrip contractions

The aim of the present study was to follow local potassium homeostasis during and after exhaustive contractions. Eight subjects performed static handgrip with their right forearm at 10%, 25% and 40% maximal voluntary contraction. Blood flow (venous occlusion plethysmography) and the venous effluent plasma potassium concentration were followed during the contractions and during a 60-min recovery period. Electromyography was registered during exercise (frequency analysis). With all three protocols the blood flow increased significantly during the contractions and the same was true of the effluent plasma potassium concentrations. In the recovery period blood flow and the venous effluent plasma potassium concentration returned to base values within 30 min following 40% maximal voluntary contraction while following 10% and 25% maximal voluntary contraction, venous effluent plasma potassium concentration was still significantly below resting values one hour after the exercise had ceased, indicating a long-lasting uptake of potassium from the blood into the muscles. In line with this a significant potassium deficit was still seen after 1 hour of recovery following 10% and 25% maximal voluntary contraction. It is concluded that the recovery of potassium homeostasis following prolonged low-intensity contractions is a slow process. This may be due to either sequestration of potassium in other tissues with a subsequent slow release and/or insufficient sodium/potassium pump activation. The contraction induced potassium loss may play a major role in muscle performance since it may impair mechanical force production, and it is hypothesized that this may be the origin of low-frequency fatigue.     

Prediction of endurance capacity of quadriceps muscles in humans using surface electromyogram spectrum analysis during submaximal voluntary isometric contractions

The measurement of endurance time (t _lim) is the procedure commonly used to quantify the ability of a muscle to maintain force. The relationship between surface electromyographic (sEMG) manifestations of localised muscle fatigue and t _lim during an effort at 50% of maximal voluntary isometric torque of the knee extensors (vastus lateralis and vastus medialis) until exhaustion was studied in 14 healthy volunteers. It was carried out to test whether changes in sEMG computed over shorter periods than expected t _lim could be used to predict t _lim. Changes in mean muscle fibre conduction velocity, mean power frequency , median frequency , root mean square ), in the relative power in the 6–30 Hz and 30–60 Hz frequency bands were monitored using linear slope and area ratio index as statistical indicators. These indicators were computed over fixed periods shorter than t _lim. The subjects were able to maintain the required force level for [mean (SD)] 78.8 (9.5) s. During the fatigue trial, it was the greatest of the increases in the 6–30 Hz frequency band, recorded for either of the two muscles investigated, that was the only variable which correlated with t _lim. Significant relationships between t _lim and changes in this low frequency band were observed as early as the first 15–30 s of the contraction. These results suggest that sEMG frequency banding may predict mechanical endurance without the need to maintain the contraction until exhaustion. From a clinical perspective, this could be an advantage for patients who might not be able to tolerate contractions to exhaustion. Electronic Publication     

Effects of intermittent hypoxia on the cerebrovascular responses to submaximal exercise in humans

Intermittent hypoxia (IH) has been shown to alter the ventilatory and cardiovascular responses to submaximal exercise; however, the effect of IH on the cerebral blood flow (CBF) response to submaximal exercise has not been determined. This study tested the hypothesis that IH would blunt the CBF response during eucapnic and hypercapnic exercise. Nine healthy males underwent 10 consecutive days of isocapnic IH (oxyhaemoglobin saturation = 80%, 1 h day⁻¹). Ventilatory, cardiovascular, and cerebrovascular responses to cycle exercise (50, 100, and 150 W) were measured before and after IH. Carbon dioxide (5% CO₂), a mediator of CBF during exercise, was administered for 2 min of each exercise stage. Over the 10 days of IH, there was an increase in minute ventilation during the IH exposures (P < 0.05). Although exercise produced increases in middle cerebral artery mean velocity (MCA V _mean), and mean arterial pressure (P < 0.05), there was no effect of IH. Similarly, hypercapnic exercise increased and MCA V _mean (P < 0.05); however, the magnitude of the response was unchanged following IH. Our findings indicate that ten daily hypoxia exposures does not alter the CBF response to submaximal exercise.     

Optimal pedalling velocity characteristics during maximal and submaximal cycling in humans

The aim of this study was to compare optimal pedalling velocities during maximal (OVM) and submaximal (OVSM) cycling in human, subjects with different training backgrounds. A group of 22 subjects [6 explosive (EX), 6 endurance (EN) and 10 non-specialised subjects] sprint cycled on a friction-loaded ergometer four maximal sprints lasting 6?s each followed by five 3-min periods of steady-state cycling at 150?W with pedalling frequencies varying from 40 to 120?rpm. The OVM and OVSM were defined as the velocities corresponding to the maximal power production and the lowest oxygen consumption, respectively. A significant linear relationship (r ² ?=?0.52, P?P?P?P?相似文献   

Altered control of submaximal bite force during bruxism in humans

The control of bite force during varying submaximal loads was examined in patients suffering from bruxism compared to healthy humans not showing these symptoms. The subjects raised a bar (preload) with their incisor teeth and held it between their upper and lower incisors using the minimal bite force required to keep the bar in a horizontal position. Further loading was added during the preload phase. A sham load was also used. Depending on the session, the teeth were loaded by the experimenter or the subject and in one session the subject did not see the load (no visual feedback). The bite force was measured continuously using a calibrated force transducer. In all the subjects, the bite force increased with increasing load. Following the addition of the load, the level of the tonic bite force was reached rapidly with no marked overshoot. The patients with bruxism used significantly higher bite forces to hold the submaximal loads compared to the control subjects. In the control subjects, the holding forces for each submaximal load were identical in the men and the women and were independent of subject maximal bite force. Sham loading evoked no marked responses in biting force. Whether the subject or the experimenter added the load or whether the subject had visual feedback or not were not significant factors in determining the level of bite force. The results indicated that the patients with bruxism used excessively large biting forces for each given submaximal load. This study showed no evidence that the inappropriate control of bite force by patients with bruxism was due to an abnormality in the higher cortical circuits that regulates the function of trigeminal motoneurons in the brainstem. This was shown by a lack of abnormality in coordination of voluntary hand movement with biting force, a lack of abnormal anticipation response to a sham load and a lack of any effect of visual feedback. The results were in line with the hypothesis that afferent input from oral (periodontal or masticatory muscle) tissues does not provide an appropriate control of motor command in bruxism.     

Changes in human alpha-motoneuron excitability during sustained maximum isometric contractions

Experiments were conducted to evaluate the change in alpha-motoneuron excitability during sustained maximum isometric contractions of human triceps surae. A test H-reflex was used to assess motoneuron excitability 10 ms after a conditioning reflex was generated. The test reflex was compared to a reference H-reflex; both test and reference reflexes were of approximately equal amplitudes at the onset of the sustained maximum efforts. Both reflexes were assumed to be influenced by similar descending and peripheral inputs. In addition, the test reflex was influenced by the conditioning reflex. For the 4 subjects tested, the test reflex decreased in amplitude within the first 30-40 s of effort, while the reference reflex remained roughly constant or increased in amplitude. The decline of the test reflex relative to the reference was indicative of an inhibitory effect due to the conditioning reflex. In that the conditioning reflex was always generated 10 ms prior to the test reflex, the two factors most likely responsible for the inhibition would be recurrent inhibition and summation of motoneuron afterhyperpolarization. A combination of these two factors could also account for the associated slowing of motoneuron firing during sustained maximum efforts.     

Plasma potassium concentration and Doppler blood flow during and following submaximal handgrip contractions

The aim of the present study was to investigate the time-course of blood velocity in the forearm during and folllowing isometric handgrip contractions and to reveal a possible temporal relationship between the circulatory response and venous effluent potassium concentration ([K]) not only during contractions but also during the post-exercise recovery period. Contractions of 15% maximal voluntary contraction (MVC) and 30% MVC with and without 3 min of artirial occlusion following the contractions were studied. All contractions induced a significant increase in venous plasma [K] from an average resting level of 4.0 to 5.0 mM during 15% MVC and 5.8 mM during 30% MVC. Blood velocity increased from a resting level of 0.07 to 0.22 m s^-1 diromg 15% and 30% MVC, respectively. MCC of 30% always elicited a larger blood velocity and [K] response than 15% MVC. Following the contractions hyperaemia was elicited. Recovery of the local blood velocity was markedly slower than the K recovery, since [K] remained significantly above resting level for only 25 s following 15% MVC and 45 s following 15 and 30% MVC, respectively. Further, a larger hyperaemia following the occlusion was elicited as compared to the contraction without occlusion, in spite of [K] being lower immediately after the occlusion period than immediately after the contraction. Finally, [K] decreased below resting level in the recovery period while the blood velocity remained elebvated. Therefore, the present study showed that the venous plasma [K] is not causally related to the prolonged post-exercise hyperaemia. The skin temperature remained unchanged during the contractions, while during the recovery period the skin temperature increased for several minutes. The major part of the temperature increase was likely to be due to conductance of heat from muscles to skin surface as a consequence of muscle hyperaemia.     

Rotation of motoneurons during prolonged isometric contractions in humans

Prolonged and weak isometric contractions can result in neuromuscular fatigue. Alternation of discharge of motor units with similar thresholds (termed rotation) could be useful to minimize neuromuscular fatigue by providing periods for metabolic recovery of the contractile elements. In the present study, we investigated the prevalence of motoneuron rotation during prolonged contractions of distal limb muscles. Electromyographic (EMG; needle and surface) was recorded from muscles of the forearm and distal leg. The subject made a slowly increasing isometric contraction to recruit and discharge a motor unit (1) for a prolonged period of time (> 30 min). Sometimes an additional motor unit (2) was recruited in which case the subject relaxed the contraction slightly so that only one motor unit remained tonic. Often it was this newly recruited motor unit (i.e., unit 2) that continued discharging, while motor unit 1 fell silent. Continued contraction would then lead to the resumption of tonic discharge of unit 1 and silence of unit 2. This would complete a rotation between motor units 1 and 2. During prolonged contractions, rotation was observed in approximately 80% of the motor-unit pairs examined. There was no difference in rotation incidence by muscle type. For the unit pairs showing rotation, surface EMG values were significantly higher immediately prior to rotation than after rotation had occurred. Our findings show that rotation of motor units with similar recruitment thresholds during such contractions is common in distal muscles of the arm and leg and may help offset neuromuscular fatigue.     

Distribution of plasma amino acids in humans during submaximal prolonged exercise

Twelve healthy subjects were submitted to an 1 h exercise at two-third of their maximal oxygen consumption. Venous blood samples were drawn before, during and after the exercise. The lactate and amino acid pattern were investigated. The results showed that alanine, methionine and citrulline were significantly changed during the work. The largest difference was that of alanine which accounted for 50% of the total amino acid variation. The data are analyzed in view on the pyruvate metabolism.     

Motor unit recruitment in human biceps brachii during sustained voluntary contractions

The purpose of the study was to examine the influence of the difference between the recruitment threshold of a motor unit and the target force of the sustained contraction on the discharge of the motor unit at recruitment. The discharge characteristics of 53 motor units in biceps brachii were recorded after being recruited during a sustained contraction. Some motor units (n = 22) discharged action potentials tonically after being recruited, whereas others (n = 31) discharged intermittent trains of action potentials. The two groups of motor units were distinguished by the difference between the recruitment threshold of the motor unit and the target force for the sustained contraction: tonic, 5.9 +/- 2.5%; intermittent, 10.7 +/- 2.9%. Discharge rate for the tonic units decreased progressively (13.9 +/- 2.7 to 11.7 +/- 2.6 pulses s(-1); P = 0.04) during the 99 +/- 111 s contraction. Train rate, train duration and average discharge rate for the intermittent motor units did not change across 211 +/- 153 s of intermittent discharge. The initial discharge rate at recruitment during the sustained contraction was lower for the intermittent motor units (11.0 +/- 3.3 pulses s(-1)) than the tonic motor units (13.7 +/- 3.3 pulses s(-1); P = 0.005), and the coefficient of variation for interspike interval was higher for the intermittent motor units (34.6 +/- 12.3%) than the tonic motor units (21.2 +/- 9.4%) at recruitment (P = 0.001) and remained elevated for discharge duration (34.6 +/- 9.2% versus 19.1 +/- 11.7%, P < 0.001). In an additional experiment, 12 motor units were recorded at two different target forces below recruitment threshold (5.7 +/- 1.9% and 10.5 +/- 2.4%). Each motor unit exhibited the two discharge patterns (tonic and intermittent) as observed for the 53 motor units. The results suggest that newly recruited motor units with recruitment thresholds closer to the target force experienced less synaptic noise at the time of recruitment that resulted in them discharging action potentials at more regular and greater rates than motor units with recruitment thresholds further from the target force.