Role of adenosine in regulating the heterogeneity of skeletal muscle blood flow during exercise in humans.

Evidence from both animal and human studies suggests that adenosine plays a role in the regulation of exercise hyperemia in skeletal muscle. We tested whether adenosine also plays a role in the regulation of blood flow (BF) distribution and heterogeneity among and within quadriceps femoris (QF) muscles during exercise, measured using positron emission tomography. In six healthy young women, BF was measured at rest and then during three incremental low and moderate intermittent isometric one-legged knee-extension exercise intensities without and with theophylline-induced nonselective adenosine receptor blockade. BF heterogeneity within muscles was calculated from 16-mm(3) voxels in BF images and heterogeneity among the muscles from the mean values of the four QF compartments. Mean BF in the whole QF and its four parts increased, and heterogeneity decreased with workload both without and with theophylline (P < 0.001). Adenosine receptor blockade did not have any effect on mean bulk BF or BF heterogeneity among the QF muscles, yet blockade increased within-muscle BF heterogeneity in all four QF muscles (P = 0.03). Taken together, these results show that BF becomes less heterogeneous with increasing exercise intensity in the QF muscle group. Adenosine seems to play a role in muscle BF heterogeneity even in the absence of changes in bulk BF at low and moderate one-leg intermittent isometric exercise intensities.     

Muscle use during double poling evaluated by positron emission tomography

Due to the complexity of movement in cross-country skiing (XCS), the muscle activation patterns are not well elucidated. Previous studies have applied surface electromyography (SEMG); however, recent gains in three-dimensional (3D) imaging techniques such as positron emission tomography (PET) have rendered an alternative approach to investigate muscle activation. The purpose of the present study was to examine muscle use during double poling (DP) at two work intensities by use of PET. Eight male subjects performed two 20-min DP bouts on separate days. Work intensity was ～ 53 and 74% of peak oxygen uptake (Vo(2peak)), respectively. During exercise 188 ± 8 MBq of [(18)F]fluorodeoxyglucose ([(18)F]FDG) was injected, and subsequent to exercise a full-body PET scan was conducted. Regions of interest (ROI) were defined within 15 relevant muscles, and a glucose uptake index (GUI) was determined for all ROIs. The muscles that span the shoulder and elbow joints, the abdominal muscles, and hip flexors displayed the greatest GUI during DP. Glucose uptake did not increase significantly from low to high intensity in most upper body muscles; however, an increased GUI (P < 0.05) was seen for the knee flexor (27%) and extensor muscles (16%), and for abdominal muscles (21%). The present data confirm previous findings that muscles of the upper limb are the primary working muscles in DP. The present data further suggest that when exercise intensity increases, the muscles that span the lumbar spine, hip, and knee joints contribute increasingly. Finally, PET provides a promising alternative or supplement to existing methods to assess muscle activation in complex human movements.     

Squeezing the Muscle: Compression Clothing and Muscle Metabolism during Recovery from High Intensity Exercise

The purpose of this experiment was to investigate skeletal muscle blood flow and glucose uptake in m. biceps (BF) and m. quadriceps femoris (QF) 1) during recovery from high intensity cycle exercise, and 2) while wearing a compression short applying ∼37 mmHg to the thigh muscles. Blood flow and glucose uptake were measured in the compressed and non-compressed leg of 6 healthy men by using positron emission tomography. At baseline blood flow in QF (P = 0.79) and BF (P = 0.90) did not differ between the compressed and the non-compressed leg. During recovery muscle blood flow was higher compared to baseline in both compressed (P<0.01) and non-compressed QF (P<0.001) but not in compressed (P = 0.41) and non-compressed BF (P = 0.05; effect size = 2.74). During recovery blood flow was lower in compressed QF (P<0.01) but not in BF (P = 0.26) compared to the non-compressed muscles. During baseline and recovery no differences in blood flow were detected between the superficial and deep parts of QF in both, compressed (baseline P = 0.79; recovery P = 0.68) and non-compressed leg (baseline P = 0.64; recovery P = 0.06). During recovery glucose uptake was higher in QF compared to BF in both conditions (P<0.01) with no difference between the compressed and non-compressed thigh. Glucose uptake was higher in the deep compared to the superficial parts of QF (compression leg P = 0.02). These results demonstrate that wearing compression shorts with ∼37 mmHg of external pressure reduces blood flow both in the deep and superficial regions of muscle tissue during recovery from high intensity exercise but does not affect glucose uptake in BF and QF.     

Skeletal muscle blood flow and flow heterogeneity during dynamic and isometric exercise in humans

The effects of dynamic and intermittent isometric knee extension exercises on skeletal muscle blood flow and flow heterogeneity were studied in seven healthy endurance-trained men. Regional muscle blood flow was measured using positron emission tomography (PET) and an [(15)O]H(2)O tracer, and electromyographic (EMG) activity was recorded in the quadriceps femoris (QF) muscle during submaximal intermittent isometric and dynamic exercises. QF blood flow was 61% (P = 0.002) higher during dynamic exercise. Interestingly, flow heterogeneity was 13% (P = 0.024) lower during dynamic compared with intermittent isometric exercise. EMG activity was significantly higher (P < 0.001) during dynamic exercise, and the change in EMG activity from isometric to dynamic exercise was tightly related to the change in blood flow in the vastus lateralis muscle (r = 0.98, P < 0.001) but not in the rectus femoris muscle (r = -0.09, P = 0.942). In conclusion, dynamic exercise causes higher and less heterogeneous blood flow than intermittent isometric exercise at the same exercise intensity. These responses are, at least partly, related to the increased EMG activity.     

Electromyographic normalization procedures for determining exercise intensity of closed chain exercises for strengthening the quadriceps femoris muscles

The purpose of this study was to compare the electromyographic (EMG) amplitudes of the quadriceps femoris (QF) muscles during a maximum voluntary isometric contraction (MVIC) to submaximal and maximal dynamic concentric contractions during active exercises. A secondary purpose was to provide information about the type of contraction that may be most appropriate for normalization of EMG data if one wants to determine if a lower extremity closed chain exercise is of sufficient intensity to produce a strengthening response for the QF muscles. Sixty-eight young healthy volunteers (39 female, 29 male) with no lower extremity pain or injury participated in the study. Surface electrodes recorded EMG amplitudes from the vastus medialis obliquus (VMO), rectus femoris (RF), and vastus lateralis (VL) muscles during 5 different isometric and dynamic concentric exercises. The last 27 subjects performed an additional 4 exercises from which a second data set could be analyzed. Maximum isokinetic knee extension and moderate to maximum closed chain exercises activated the QF significantly more than a MVIC. A 40-cm. lateral step-up exercise produced EMG amplitudes of the QF muscles of similar magnitude as the maximum isokinetic knee extension exercises and would be an exercise that could be considered for strengthening the QF muscles. Most published EMG studies of exercises for the QF have been performed by comparing EMG amplitudes during dynamic exercises to a MVIC. This procedure can lead one to overestimate the value of a dynamic exercise for strengthening the QF muscles. We suggest that when studying the efficacy of a dynamic closed chain exercise for strengthening the QF muscles, the exercise be normalized to a dynamic maximum muscle contraction such as that obtained with knee extension during isokinetic testing.     

The effect of dynamic knee-extension exercise on patellar tendon and quadriceps femoris muscle glucose uptake in humans studied by positron emission tomography.

Both tendon and peritendinous tissue show evidence of metabolic activity, but the effect of acute exercise on substrate turnover is unknown. We therefore examined the influence of acute exercise on glucose uptake in the patellar and quadriceps tendons during dynamic exercise in humans. Glucose uptake was measured in five healthy men in the patellar and quadriceps tendons and the quadriceps femoris muscle at rest and during dynamic knee-extension exercise (25 W) using positron emission tomography and [18F]-2-fluoro-2-deoxy-D-glucose ([18F]FDG). Glucose uptake index was calculated by dividing the tissue activity with blood activity of [18F]FDG. Exercise increased glucose uptake index by 77% in the patellar tendon (from 0.30 +/- 0.09 to 0.51 +/- 0.16, P = 0.03), by 106% in the quadriceps tendon (from 0.37 +/- 0.15 to 0.75 +/- 0.36, P = 0.02), and by 15-fold in the quadriceps femoris muscle (from 0.31 +/- 0.11 to 4.5 +/- 1.7, P = 0.005). The exercise-induced increase in the glucose uptake in neither tendon correlated with the increase in glucose uptake in the quadriceps muscle (r = -0.10, P = 0.87 for the patellar tendon and r = -0.30, P = 0.62 for the quadriceps tendon). These results show that tendon glucose uptake is increased during exercise. However, the increase in tendon glucose uptake is less pronounced than in muscle and the increases are uncorrelated. Thus tendon glucose uptake is likely to be regulated by mechanisms independently of those regulating skeletal muscle glucose uptake.     

Quadriceps femoris electromyogram during concentric, isometric and eccentric phases of fatiguing dynamic knee extensions

The objective of this study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during fatiguing knee extensions. Thirty young adults were evaluated for their one-repetition maximum (1RM) during a seated, right-leg, inertial knee extension. All subjects then completed a single set of repeated knee extensions at 50% 1RM, to failure. Subjects performed a knee extension (concentric phase), held the weight with the knee extended for 2s (isometric phase), and lowered the weight in a controlled manner (eccentric phase). Raw EMG of the vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscles were full-wave rectified, integrated and normalized to the 1RM EMG, for each respective phase and repetition. The EMG median frequency (f(med)) was computed during the isometric phase. An increase in QF muscle EMG was observed during the concentric phase across the exercise duration. VL EMG was greater than the VM and RF muscles during the isometric phase, in which no significant changes occurred in any of the muscles across the exercise duration. A significant decrease in EMG across the exercise duration was observed during the eccentric phase, with the VL EMG greater than the VM and RF muscles. A greater decrease in VL and RF muscle f(med) during the isometric phase, than the VM muscle, was observed with no gender differences. The findings demonstrated differential recruitment of the superficial QF muscle, depending on the contraction mode during dynamic knee extension exercise, where VL muscle dominance appears to manifest across the concentric-isometric-eccentric transition.     

Electromyographic activity and rate of muscle fatigue of the quadriceps femoris during cycling exercise in the severe domain

This study compared the activation pattern and the fatigue rate among the superficial muscles of the quadriceps femoris (QF) during severe cycling exercise. Peak oxygen consumption (VO(2)peak) and maximal accumulated oxygen Deficit (MAOD) were established by 10 well-trained male cyclists (27.5 ± 4.1 years, 71.0 ± 10.3 kg, 173.4 ± 6.6 cm, mean VO(2)peak 56.7 ± 4.4 ml·kg·min(-1), mean MAOD 5.7 ± 1.1 L). Muscle activity (electromyographic [EMG] signals) was obtained during the supramaximal constant workload test (MAOD) and expressed by root mean square (RMS) and median frequency (MF slope). The RMS of the QF, vastus lateralis (VL) and vastus medialis (VM) muscles were significantly higher than at the beginning after 75% of exercise duration, whereas for the rectus femoris (RF), this was observed after 50% of exercise duration (p ≤ 0.05). The slope of the MF was significantly higher in the RF, followed by the VL and VM (-3.13 ± 0.52 vs. -2.61 ± 0.62 vs. -1.81 ±0.56, respectively; p < 0.05). We conclude that RF may play an important role in limiting performance during severe cycling exercise.     

Dynamic knee extension as model for study of isolated exercising muscle in humans

In an attempt to approach a system of isolated exercising muscle in humans, a model has been developed that enables the study of muscle activity and metabolism over the quadriceps femoris (QF) muscles while the rest of the body remains relaxed. The simplest version includes the subject sitting on a table with a rod connecting the ankle and the pedal arm of a bicycle ergometer placed behind the subject. Exercise is performed by knee extension from a knee angle of 90 to approximately 170 degrees while flywheel momentum repositions the relaxed leg during flexion. Experiments where electromyographic recordings have been taken from biceps femoris, gastrocnemius, tibialis anterior, and other muscles in addition to QF indicate that only the QF is active and that there is an equal activation of the lateral, medial, and rectus femoris heads relative to maximum. Furthermore, virtually identical pulmonary O2 uptake (Vo2) during and without application of a pressure cuff below the knee emphasizes the inactivity of the lower leg muscles. The advantages of the model are that all external work can be localized to a single muscle group suitable for taking biopsies and that the blood flow in and sampling from the femoral vein are representative of the active muscles. Thus all measurements can be closely related to changes in the working muscle. Using this model we find that a linear relationship exists between external work and pulmonary Vo2 over the submaximal range and the maximal Vo2 per kilogram of muscle may be as much as twice as high as previously estimated.     

Relationship between local perfusion and FFA uptake in human skeletal muscle-no effect of increased physical activity and aerobic fitness.

We investigated heredity-independent effects of increased physical activity and aerobic fitness on skeletal muscle free fatty acid (FFA) uptake, perfusion, and their heterogeneity at rest and during exercise. Also, the relationship between local skeletal muscle FFA uptake and perfusion was studied. Nine young adult male monozygotic twin pairs with significant difference in physical activity [229 min (SD 156) average time spent for conditioning exercise per week in more and 98 min (SD 71) in less active twins, P = 0.013] and aerobic fitness [18% (SD 10) difference in maximum O2 uptake] between brothers were studied using positron emission tomography. Submaximal knee-extension exercise increased perfusion, FFA uptake, and oxygen uptake in quadriceps femoris muscles 6-10 times compared with resting values (P < 0.001). More active twins tended to utilize more oxygen, while no differences were found in muscle perfusion or FFA uptake between groups. Mean perfusion and FFA uptake correlated strongly at a whole muscle level, both at rest (r = 0.97, P = 0.03 in more and r = 0.98, P = 0.02 in less active twins) and during exercise (r = 0.99, P = 0.01 and r = 0.94, P = 0.06), but at the voxel level (87 mm3) correlation was only moderate during exercise [r = 0.73 (SD 0.08) vs. r = 0.74 (SD 0.10), P = 0.92] and weak at rest [r = 0.28 (SD 0.13) vs. r = 0.33 (SD 0.21), P = 0.58]. Exercise decreased both perfusion and FFA uptake heterogeneity within the muscles (P < 0.001) similarly in both groups. In conclusion, long-term history of moderately increased physical activity tends to enhance muscle oxidative metabolism, but it does not have any significant influence on the FFA uptake or perfusion rates or their heterogeneity in skeletal muscle. Submaximal knee-extension exercise decreases heterogeneity of muscle FFA uptake and perfusion and improves matching between local muscle perfusion and FFA uptake. Thus it seems that the genetic influence is more important to determine the heterogeneity of perfusion and FFA uptake in skeletal muscle than exercise training.     

Knee joint angle affects EMG–force relationship in the vastus intermedius muscle

It is not understood how the knee joint angle affects the relationship between electromyography (EMG) and force of four individual quadriceps femoris (QF) muscles. The purpose of this study was to examine the effect of the knee joint angle on the EMG–force relationship of the four individual QF muscles, particularly the vastus intermedius (VI), during isometric knee extensions. Eleven healthy men performed 20–100% of maximal voluntary contraction (MVC) at knee joint angles of 90°, 120° and 150°. Surface EMG of the four QF synergists was recorded and normalized by the root mean square during MVC. The normalized EMG of the four QF synergists at a knee joint angle of 150° was significantly lower than that at 90° and 120° (P < 0.05). Comparing the normalized EMG among the four QF synergists, a significantly lower normalized EMG was observed in the VI at 150° as compared with the other three QF muscles (P < 0.05). These results suggest that the EMG–force relationship of the four QF synergists shifted downward at an extended knee joint angle of 150°. Furthermore, the neuromuscular activation of the VI was the most sensitive to change in muscle length among the four QF synergistic muscles.     

Imaging of skeletal muscle function using 18FDG PET: force production, activation, and metabolism

The purpose of thisstudy was to determine whether [¹⁸F]fluorodeoxyglucose(FDG) positron emission tomography (PET) can be used to evaluate muscleforce production, create anatomic images of muscle activity, andresolve the distribution of metabolic activity within exercisingskeletal muscle. Seventeen subjects performed either elbow flexion,elbow extension, or ankle plantar flexion after intravenous injectionof FDG. PET imaging was performed subsequently, and FDG uptake wasmeasured in skeletal muscle for each task. A fivefold increase inresistance during elbow flexion increased FDG uptake in the bicepsbrachii by a factor of 4.9. Differences in relative FDG uptake weredemonstrated as exercise tasks and loads were varied, permittingdifferentiation of active muscles. The intramuscular distribution ofFDG within exercising biceps brachii varied along the transverse andlongitudinal axes of the muscle; coefficients of variation along theseaxes were 0.39 and 0.23, respectively. These findings suggest FDG PETis capable of characterizing task-specific muscle activity andmeasuring intramuscular variations of glucose metabolism withinexercising skeletal muscle.

     

A maximal isokinetic pedalling exercise for EMG normalization in cycling

An isometric maximal voluntary contraction (iMVC) is mostly used for the purpose of EMG normalization, a procedure described in the scientific literature in order to compare muscle activity among different muscles and subjects. However, the use of iMVC has certain limitations. The aims of the present study were therefore to propose a new method for the purpose of EMG amplitude normalization in cycling and assess its reliability. Twenty-three cyclists performed 10 trials of a maximal isokinetic protocol (MIP) on a cycle ergometer, then another four sub-maximal trials, whilst the EMG activity of four lower limbs muscles was registered. During the 10 trials power output (CV = 2.19) and EMG activity (CV between 4.46 and 8.70) were quite steady. Furthermore, their maximal values were reached within the 4th trial. In sub-maximal protocol EMG activity exhibited an increase as a function of exercise intensity.MIP entails a maximal dynamic contraction of the muscles involved in the pedalling action and the normalization session is performed under the same biomechanical conditions as the following test session. Thus, it is highly cycling-specific.MIP has good logical validity and within-subject reproducibility. Three trials are enough for the purpose of EMG normalization in cycling.     

Effects of insulin and exercise on rat hindlimb muscles after simulated microgravity.

This study was designed to examine insulin- and exercise-stimulated glucose uptake and metabolism in the hindlimb muscles of rats after conditions of simulated microgravity. To simulate microgravity, male Sprague-Dawley rats were suspended in a head-down (45 degrees) position with their hindlimbs non-weight bearing (SUS) for 14 days. In addition, rats were assigned to suspension followed by exercise (SUS-E), to cage control (CC), or to exercising control (CC-E) groups. Exercise consisted of five 10-min bouts of treadmill running at the same relative intensity for the CC-E and SUS-E rats (80-90% of maximum O2 consumption). Hindlimb perfusion results indicated that glucose uptake for the entire hindquarter at 24,000 microU/ml insulin (maximum stimulation) was significantly higher in the SUS (8.9 +/- 0.5 mumol.g-1.h-1) than in the CC (7.6 +/- 0.4 mumol.g-1.h-1) rats, signifying an increased insulin responsiveness. Glucose uptake at 90 microU/ml insulin was also significantly higher in the SUS (48 +/- 4; % of maximum stimulation over basal) than in the CC (21 +/- 4%) rats. In addition, exercise-induced increases in glucose uptake for the hindlimbs (133%) and glucose incorporation into glycogen for the plantaris (8.4-fold), extensor digitorum longus (5.4-fold), and white gastrocnemius (4.8-fold) muscles were greater for the SUS-E rats than for the CC-E rats (39% and 1.9-, 1.9-, and 3.0-fold, respectively). Therefore, suspension of the rat with hindlimbs non-weight bearing leads to enhanced muscle responses to insulin and exercise when they were applied separately. However, insulin action appeared to be impaired after exercise for the SUS-E rats, especially for the soleus muscle.     

Glucose uptake is increased in trained vs. untrained muscle during heavy exercise.

Endurance training increases muscle content of glucose transporter proteins (GLUT-4) but decreases glucose utilization during exercise at a given absolute submaximal intensity. We hypothesized that glucose uptake might be higher in trained vs. untrained muscle during heavy exercise in the glycogen-depleted state. Eight untrained subjects endurance trained one thigh for 3 wk using a knee-extensor ergometer. The subjects then performed two-legged glycogen-depleting exercise and consumed a carbohydrate-free meal thereafter to keep muscle glycogen concentration low. The next morning, subjects performed dynamic knee extensions with both thighs simultaneously at 60, 80, and until exhaustion at 100% of each thigh's peak workload. Glucose uptake was similar in both thighs during exercise at 60% of thigh peak workload. At the end of 80 and at 100% of peak workload, glucose uptake was on average 33 and 22% higher, respectively, in trained compared with untrained muscle (P < 0.05). Training increased the muscle content of GLUT-4 by 66% (P < 0. 05). At exhaustion, glucose extraction correlated significantly (r = 0.61) with total muscle GLUT-4 protein. Thus, when working at a high load with low glycogen concentrations, muscle glucose uptake is significantly higher in trained than in untrained muscle. This may be due to the higher GLUT-4 protein concentration in trained muscle.     

Muscle oxygen extraction and perfusion heterogeneity during continuous and intermittent static exercise.

The purpose of this study was to compare the effects of intermittent and continuous static exercise on muscle perfusion, perfusion heterogeneity, and oxygen extraction. Perfusion and oxygen uptake of quadriceps femoris muscle were measured in 10 healthy men by using positron emission tomography and [(15)O]H(2)O and [(15)O]O(2) first during intermittent static exercise [10% of maximal static force (MSF)] and thereafter during continuous static exercise at the same tension-time level (5% static; 5% of MSF). In 4 of these subjects, perfusion was measured during continuous static exercise with 10% of MSF (10% continuous) instead of the second [(15)O]O(2) measurement. Muscle oxygen consumption was similar during intermittent and 5% continuous, but muscle perfusion was significantly higher during 5% continuous. Consequently, muscle oxygen extraction fraction was lower during 5% continuous. Perfusion was also more heterogeneous during 5% continuous. When exercise intensity was doubled during continuous static exercise (from 5% continuous to 10% continuous), muscle perfusion increased markedly. These results suggest that continuous, low-intensity static exercise decreases muscle oxygen extraction and increases muscle perfusion and its heterogeneity compared with intermittent static exercise at the same relative exercise intensity.     

Effects of strength training on submaximal and maximal endurance performance capacity in middle-aged and older men

Effects of a 16-week progressive strength-training program on blood lactate accumulation (LA), maximal workload (W(max)) attained during progressive cycling exercise, maximum half-squat (1RM(HS)), muscle cross-sectional area of the quadriceps femoris muscle group (CSA(QF)), and serum hormone concentrations were examined in 11 middle-aged (46 year old [M46]) and 11 older (64 year old [M64]) men. During the 16 weeks of training, significant increases were observed in 1RM(HS) in M46 and M64 (41-45%; p < 0.001). The muscle CSA(QF) increased (13-11%; p > 0.01) for both groups. The first 8 weeks of training led to significant increases in W(max) (6-11%; p < 0.001) and decreases in submaximal (LA) in both groups, but no further training-induced changes were observed during the subsequent 8 weeks of training. Statistically significant relationships were observed in M64 and in the combined group M46 + M64 between the training-induced changes observed in W(max) and serum testosterone-cortisol and free-testosterone-cortisol ratios, whereas in M46 the respective correlations values did not reach statistically significant levels. These data indicate that strength training results in a significant improvement in maximal and submaximal endurance during the first 8 weeks of strength training in both age groups, related in part to the intensity and the volume of resistance training used and to the training status of the subjects. The relationships found in this study between various indices of cycling testing and serum hormone concentrations after strength training suggest that maximal incremental cycling might be used as an additional test to detect anabolic-catabolic responses to prolonged strength training in middle-aged and older men.     

Disposal of lactate during and after strenuous exercise in humans

Heavy dynamic exercise using both arm and leg muscles was performed to exhaustion by seven well-trained subjects. The aerobic and anaerobic energy utilization was determined and/or calculated. O2 uptake during exercise and during 1 h of recovery was measured as well as splanchnic and muscle metabolite exchange. Glycogen and lactate content in the quadriceps femoris was determined before exercise, immediately after exercise, and after a recovery period. In four male subjects the estimated mean lactate production during exercise was 830 mmol. The splanchnic uptake of lactate during recovery was 80 mmol, and the calculated maximum amount oxidized during the recovery period was 330 mmol. About 60 mmol were accounted for in the body water at the end of the rest period. The remaining 360 mmol of lactate were apparently resynthesized into glycogen in muscle via gluconeogenesis. It is concluded that approximately 50% of the lactate formed during heavy exercise is transformed to glycogen via glyconeogenesis in muscle during recovery and that lactate uptake by the liver is only 10%.     

Reproducibility of eight lower limb muscles activity level in the course of an incremental pedaling exercise.

Despite the wide use of surface electromyography (EMG) recorded during dynamic exercises, the reproducibility of EMG variables has not been fully established in a course of a dynamic leg exercise. The aim of this study was to investigate the reproducibility of eight lower limb muscles activity level during a pedaling exercise performed until exhaustion. Eight male were tested on two days held three days apart. Surface EMG was recorded from vastus lateralis, rectus femoris (RF), vastus medialis, semimembranosus, biceps femoris, gastrocnemius lateral, gastrocnemius medianus and tibialis anterior during incremental exercise test. The root mean square, an index of global EMG activity, was averaged every five crank revolutions (corresponding to about 3 s at 85 rpm) throughout the tests. Despite inter-subjects variations, we showed a high reproducibility of the activity level of lower limb muscles during a progressive pedaling exercise performed until exhaustion. However, RF muscle seemed to be the less reproducible of the eight muscles investigated during incremental pedaling exercise. These results suggest that each subject adopt a personal muscle activation strategy in a course of an incremental cycling exercise but fatigue phenomenon can induce some variations in the most fatigable muscles (RF).     

Inter- and intra-session reliability of muscle activity patterns during cycling

The aim of this study was to determine the inter- and intra-session reliability of the temporal and magnitude components of activity in eight muscles considered important for the leg cycling action. On three separate occasions, 13 male non-cyclists and 11 male cyclists completed 6 min of cycling at 135, 150, and 165 W. Cyclists completed two additional 6-min bouts at 215 and 265 W. Surface electromyography was used to record the electrical activity of tibialis anterior, soleus, gastrocnemius medialis, gastrocnemius lateralis, vastus medialis, vastus lateralis, rectus femoris, and gluteus maximus. There were no differences (P > 0.05) in the muscle activity onset and offset or in the iEMG of any muscles between visits. There were also no differences (P > 0.05) between cyclists and non-cyclists in the variability of these parameters. Overall, standard error of measurement (SEM) and intra-class correlation analyses suggested similar reliability of both inter- and intra-session muscle activity onset and offset. The SEM of activity onset in tibialis anterior and activity offset in soleus, gastrocnemius lateralis and rectus femoris was markedly higher than in the other muscles. Intra-session iEMG was reliable (coefficient of variation (CV) = 5.3–13.5%, across all muscles), though a CV range of 15.8–43.1% identified low inter-session iEMG reliability. During submaximal cycling, the temporal components of muscle activity exhibit similar intra- and inter-session reliability. The magnitude component of muscle activity is reliable on an intra-session basis, but not on an inter-session basis.