Electromyographic activity related to aerobic and anaerobic threshold in ergometer bicycling

Electromyogram power spectrum during dynamic contractions at different intensities of exercise

During dynamic contractions performed on a cycle ergometer, we studied the influence of motor unit (MU) recruitment on the electromyographic (EMG) spectral content by exerting mechanical power of different intensities, which was chosen to remain below the maximal aerobic power (VO2max). The spectral parameters: EMG total power (PEMG), mean (MPF) and median (MED) power frequencies, which are the most representative of the EMG spectral content, were calculated according to the EMG activity of the vastus medialis muscle (VM) and soleus muscle (SOL) of the right leg. For VM and SOL, PEMG increased linearly with exerted power demonstrating an enhancement of MU recruitment. Moreover these relationships were less scattered when exerted power was expressed as a percentage of VO2max. Changes in MPF and MED with varying exercise intensities were different from one subject to another. For a set of subjects, MPF and MED were found to be independent of exerted power. Although VM and SOL muscles are different in fibre type composition, similar results were obtained for both EMG activities. We have concluded that for dynamic contractions performed at different intensities below VO2max, the recruitment of the MU has a poor effect on the EMG spectral content whatever the predominant type of fibre.

Mechanical work and efficiency in treadmill running at aerobic and anaerobic thresholds

Mechanical work, mechanical power, energy consumption and mechanical efficiency were studied in constant-speed treadmill running of 5 min at seven different exercises around aerobic (AerT) and anaerobic (AnT) thresholds. The true efficiency of concentric (positive) mechanical work and gross efficiency of the whole body in seven male subjects were calculated. The total mechanical work was calculated from film through the translational, potential and rotational energy states as the sum of the changes of all the mechanical energy states in all body segments allowing energy transfer between segments and from energy state to state. The total energy consumption was measured by combining aerobic and anaerobic energy production in resting and working conditions. When the speed of the treadmill was increased from the velocity of 10 km h-1 (2.8 m s-1) to 22 km h-1 (6.1 m s-1), the concentric mechanical work per one step increased from 129 +/- 45 J to 228 +/- 82 J (P less than 0.01). Oxygen consumption increased from 2.22 +/- 0.27 1 min-1 to 4.47 +/- 0.24 1 min-1. The amount of blood lactate increased from 0.94 +/- 0.53 mmol l-1 at the lowest speed to 9.90 +/- 2.89 mmol l-1 at the highest speed (P less than 0.001). The true efficiency of concentric work decreased from 74 +/- 14% to 56 +/- 8% (P less than 0.05). At the speed of the AerT, the economy of running, the vertical rise of different body segments and mechanical efficiency of positive work were high. The highest gross efficiency was found at the running speed between the AerT and AnT.

Metabolic effects of glucose, medium chain triglyceride and long chain triglyceride feeding before prolonged exercise in rats

The purpose of this study was to test the hypothesis that oral ingestion of lipids could increase endurance by slowing the rate of glycogen depletion. Trained rats were killed after a 2 h run on a rodent treadmill, following an intragastric infusion of water, glucose, medium chain triglycerides (MCT) or long chain triglycerides (LCT). Glucose and triglycerides were administered in equicaloric concentrations (50 kJ). The results show that oral ingestion of lipids (MCT or LCT) did not reduce glycogen depletion in liver, heart or skeletal muscle after exercise whereas the fat diet increased muscle and heart glycogen stores in resting conditions. In contrast, glucose feeding induced a significant sparing effect on endogenous carbohydrate utilization and reduced physical exercise lipolysis. These data indicated, firstly, that enhanced lipid availability induced by a single lipid meal before exercise was not able to modify the glycogen depletion occurring after exercise and, secondly, that the glucose/fatty acid cycle was not effective in these conditions. The comparison between lipids indicated that the effect on glycogen use of MCT did not differ from that of LCT, and did not seem to be of any particular importance during physical exercise.

Mechanical work and efficiency in ergometer bicycling at aerobic and anaerobic thresholds

Internal and external mechanical work, energy consumption and mechanical efficiency were studied in constant-load ergometer bicycling at five different power outputs below, equal to, and above the aerobic (AerT) and anaerobic (AnT) thresholds. The gross, net and true efficiencies of the whole body in five male subjects were calculated. The work against the external load was defined as the external mechanical work. The internal mechanical work was calculated as the sum of the increments of kinetic and potential energy in all body segments by using methods of film analysis. Total energy consumption was measured by combining aerobic and anaerobic energy production. When the power output of the bicycle ergometer was increased from 146 +/- 15 to 283 +/- 17 W, oxygen consumption increased from 2.20 +/- 0.98 to 4.22 +/- 0.20 l min-1 (P less than 0.001), while the oxygen consumption at rest was 0.30 +/- 0.03 l min-1. The concentration of blood lactate increased from 2.2 +/- 0.4 at the lowest work load to 8.6 +/- 1.2 mmol l-1 at the highest work load (P less than 0.001). The amount of external work done per revolution increased from 139 +/- 20 to 277 +/- 29 J (P less than 0.001), while the amount of internal work per revolution remained almost constant (56 +/- 12 J). The gross efficiency in the present study was 17-20%, net efficiency 18-22% and true efficiency 21-30%, respectively. The highest gross and net efficiencies were reached at the AerT. The lowest efficiencies were obtained at highest work load.(ABSTRACT TRUNCATED AT 250 WORDS)

The aerobic-anaerobic transition: re-examination of the threshold concept including an electromyographic approach

The surface electromyogram (EMG) from the vastus lateralis muscle and the metabolic and respiratory parameters were studied simultaneously during an incremental exercise in order to identify EMG signal modifications during the aerobic-anaerobic transition. Subjects performed an incremental test on the bicycle ergometer from an initial work load of 175 W to exhaustion by steps of 25 W. Ventilatory flow (VE), oxygen uptake (VO2) and carbon dioxide flow (VCO2) were recorded continuously. For lactate concentration determination, venous blood samples were collected during the final 30 s of each step. EMG signals were stored on magnetic tape. They were then converted into successive spectra to allow the study of EMG total power (PEMG) and mean power frequency (MPF) evolutions. A non linear increase in blood lactate reflected by a breaking point at 250 W was observed. A change in VE/VO2 ratio occurred at 275 W. PEMG value showed a non linear increase reflected by a breaking point at 275 W. MPF value increased from the first to the seventh step with a tendency to decrease at the last step. A great interindividual variance of EMG data was observed indicating the difficulty of correlating mean values of EMG parameters with mean values of blood lactate in order to explain sudden lactate increase by fast twitch fibre recruitment. However, comparison of individual EMG data suggests a progressive recruitment of fast twitch fibres as work load increases.

Gas exchange parameters, muscle blood flow and electromechanical properties of the plantar flexors

Sixteen men were tested to determine VO2max (ml X kg-1 X min-1), anaerobic threshold VO2 (ATVO2) and oxygen kinetics (time constant, T.C.) during running on a treadmill. For measuring maximal calf blood flow (maxBF, ml X 100 ml-1 X min-1), venous occlusion plethysmography was employed immediately following a combination of arterial occlusion and toe raising exercise to exhaustion. In addition, supramaximal electrical stimulations were given to determine maximal calf twitch force (Fmax, N), maximal rate of twitch force development (dF/dt) and relaxation (R X dF/dt, N X ms-1) and electro-mechanical delay time (EMD, ms). Results demonstrated that VO2max, ATVO2 and maxBF were all inversely related to T.C. (p less than 0.05). MaxBF and ATVO2 showed the highest correlation (r = 0.89, p less than 0.01). Stepwise multiple linear regression analyses revealed that variance in VO2max (60%) and ATVO2 (84%) could be accounted for by the combined effects of the following peripheral factors: VO2max = 51,25-3.24(dF/dt) + 0.14(maxBF), and ATVO2 = 11.68 + 0.42(maxBF) - 0.2(Fmax). These findings, together with the results of cluster analysis, suggest a tight link between ATVO2 and peripheral blood flow capacity. On the other hand, a moderate correlation (r = 0.64, p less than 0.01) between VO2max and maxBF might be due in part to individual differences in oxygen extraction-utilization capacity during heavy exercise above anaerobic threshold.

Contributions of slow and fast muscles of triceps surae to a cyclic movement

The relative contribution of synergistic muscles has been studied during pedalling on a bicycle. The electromyographic (EMG) activity of the different components of triceps surae (namely soleus or SOL and medial gastrocnemius or MG) has been recorded and analyzed for increasing pedalling speed performed against increasing resistance. The results indicate that SOL IEMG (integrated EMG) increases linearly (y = 2x-12.1; r = 0.98) with increasing load (10-70 N) at constant speed (60 rpm), whereas no change is noted in MG IEMG below 40 N. In contrast, when the pedalling speed is increased (from 30 to 170 rpm) at constant load, MG IEMG shows the largest increase. Furthermore, although in both muscles EMG activity appears earlier in the movement with increases in load and/or speed, the delay between the onset of both EMGs remains unchanged at constant speed and synchronization of MG with SOL is only observed when speed is increased above 140 rpm. These results suggest that the different muscles of the triceps surae make specific contributions to the development of the mechanical tension required to maintain or increase the speed of movement.