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.

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71 +/- 31.3 to 505 +/- 188 microV, p less than 0.01) and mean power frequency (89 +/- 13.3 to 123 +/- 23.5 Hz, p less than 0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike amplitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412 +/- 79 to 972 +/- 117 microV, p less than 0.01) and mean firing frequency (17.8 +/- 5.4 to 24.7 +/- 4.1 Hz, p less than 0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.

Intramuscular and surface electromyogram changes during muscle fatigue

Twelve male subjects were tested to determine the effects of motor unit (MU) recruitment and firing frequency on the surface electromyogram (EMG) frequency power spectra during sustained maximal voluntary contraction (MVC) and 50% MVC of the biceps brachii muscle. Both the intramuscular MU spikes and surface EMG were recorded simultaneously and analyzed by means of a computer-aided intramuscular spike amplitude-frequency histogram and frequency power spectral analysis, respectively. Results indicated that both mean power frequency (MPF) and amplitude (rmsEMG) of the surface EMG fell significantly (P less than 0.001) together with a progressive reduction in MU spike amplitude and firing frequency during sustained MVC. During 50% MVC there was a significant decline in MPF (P less than 0.001), but this decline was accompanied by a significant increase in rmsEMG (P less than 0.001) and a progressive MU recruitment as evidenced by an increased number of MUs with relatively large spike amplitude. Our data suggest that the surface EMG amplitude could better represent the underlying MU activity during muscle fatigue and the frequency powers spectral shift may or may not reflect changes in MU recruitment and rate-coding patterns.

Stride frequency and ventilation at constant carbon dioxide output

To determine the consequences of two different stride frequencies on ventilation (VE) at similar levels of carbon dioxide production (VCO2), eleven male subjects performed two work tests on the treadmill. One test involved walking at a speed of 5 km/hr on a 15% grade while the other consisted of running on the treadmill at 9 km/hr on a 0% grade. Running increased stride frequency by 47%. The running and walking tests resulted in similar VCO2 levels, 1.85 +/- .18 and 1.9 +/- .20 l/min respectively, a non-significant difference. Ventilation during running was 43.73 +/- 6.51 l/min and during walking was 43.26 +/- 6.79 l/min, a non-significant difference. In addition the time constants for oxygen consumption (VCO2), VE and VCO2 were measured. The time constants for VCO2 and VE were not found to differ significantly during either the running or walking test. From our results, it can be seen that VE is more closely aligned to the metabolic state rather than stride frequency. In addition, the coupling of VE and VCO2 during the non-steady state is further indicative that ventilation is linked to the metabolic demands of the body.

Electromechanical changes during electrically induced and maximal voluntary contractions: electrophysiologic responses of different muscle fiber types during stimulated contractions

The electrophysiologic and mechanical responses of a synergistic muscle group composed of different muscle fiber types were studied with respect to excitation frequency and muscle fatigue in five men. The force and evoked action potentials obtained from surface and intramuscular fine wire electrodes were recorded continuously during stimulated contractions of the gastrocnemius and soleus at 20, 50, and 80 Hz. The stimulus voltage was adjusted so that the force generated by high-frequency tetani (50 and 80 Hz) could initially match the force of maximal voluntary contractions (MVCs). The surface and intramuscular EMG signals were digitized at a sampling rate of 10 kHz with 16-bit fast A/D converters and stored on a floppy disk. The digitized data were then processed for every 5 s to calculate evoked potential amplitude and conduction time using an HP 9836 computer. Results indicated that after 30 s of high-frequency stimulation, significantly less force was generated than after a similar period of MVC. During this period of high-frequency force fatigue, considerably greater force was developed at 20-Hz stimulation. The excessive force loss during high-frequency tetanic contractions was accompanied by a marked reduction in the evoked potential amplitude and conduction time (prolongation of the M wave). The recording of intramuscular evoked potentials showed the gastrocnemius muscle to have greater reductions in these parameters. Our results support the hypothesis that force fatigue during high-frequency stimulation results from failure of electrical propagation due to reduced muscle membrane excitability. The observed muscle-fiber-dependent electrophysiologic responses may suggest that the metabolic profile of muscle fibers plays an important role in regulating the muscle membrane excitability during high-frequency stimulation.

Relationship between myoelectric signals and blood lactate during incremental forearm exercise

Five men performed incremental forearm exercise by using a modified hand grip dynamometer while external loads (1 Kg metal plate) were added at each succeeding minute until volitional exhaustion occurred. Myo-electric signals were recorded from the belly of the flexor carpi radialis-palmaris longus. By the use of an LSI-11/23 minicomputer, the digitized data were processed for integrated EMG (IEMG) and mean power frequency (MPF) by means of 512 point fast Fourier transform. Data from individuals revealed that venous lactate correlated highly with IEMG (range: 0.977 to 0.857, P less than 0.001) and MPF (-0.960 to -0.862, P less than 0.001) during the incremental exercise. Group data revealed that none of the parameters tested showed any significant changes between the onset of lactate threshold (LT), i.e., the onset of abrupt increases in lactate, and one minute prior to LT. However, when these parameters were tested between LT and one minute after LT, significant differences were observed for venous lactate (t = 3.16, P less than 0.05), IEMG (t = 3.02, P less than 0.05) and MPF (t = 3.84, P less than 0.05). It was concluded that analysis of myo-electric signals may provide a non-invasive measure of lactate threshold after which dynamic equilibrium of lactate production and utilization becomes unbalanced during the incremental forearm exercise.

[Post-operative suppression of natural killer activity]

The effects of anesthesia and surgical procedures on natural killer (NK) cell activity were studied. The data indicated that splenic lymphocytes taken from mice 1 to 5 days after surgery showed significantly diminished activity of natural killing. When splenic lymphocytes were treated by anti-Thy 1 antibody and complement, NK activity to Lewis lung carcinoma cells was remarkably depressed in normal mice, but was not depressed in laparotomized mice. Peripheral lymphocytes from cancer patients following major surgery also showed depressed NK activity. The suppression was first detected during the operation. In patients undergoing major gastrointestinal surgery, the suppression was at its maximum during first three days and decreased to control levels by the day 7 to 14. Following major thoracic surgery, the suppression continued more longer.

Comparison of oxygen kinetics in young and old subjects

Five older men (aged 60-69 yr) and five young men (aged 21-29 yr) with approximately equal levels of age-corrected VO2 max were compared with respect to oxygen kinetics at equal absolute workloads (100 watts) and at equal relative workloads (45% VO2 max) on a cycle ergometer. At 45% VO2 max, half times for VO2 response to instantaneous transition from unloaded pedalling were 30.0 s and 27.4 s for old and young respectively (t = 0.260, p less than 0.80). No significant differences were found in the VE response and by inference none existed in O2 extraction. Mean half times for heart rate responses at a workload of 100 W were 24.2 s and 20.6 s for old and young groups respectively (t = 0.722, p less than 0.49). Mechanical efficiency estimated from steady state data at 100 W was 19.8% and 20.5% for old and young groups respectively (t = 0.574). The close similarity in responses to submaximal work in old and young subjects of equivalent fitness suggests caution in the interpretation of agewise decrements observed in physiological variables which may be sensitive to physical fitness status.

Anaerobic threshold determination by blood lactate and myoelectric signals

This study examined the potential use of myoelectric signal analysis for determination of anaerobic threshold (AT). Ten college students performed incremental exercise on a bicycle ergometer during which blood samples from either the brachial or radial artery were drawn and corresponding intergrated electromyogram (IEMG) and power spectra also were recorded. Blood AT was determined by observing the changes in arterial blood lactate, Po2 Pco2, HCO3-, and pH. EMG AT was determined by the non-linear increase in IEMG and abrupt increase in frequency band width at 70% of the peak frequency (FRQ70%) which could represent the most active frequency band. The results indicated that a significant increase in FRQ70% occurred immediately after the occurrence of blood AT (p less than 0.05). However, no significant increase could be found in peak frequency. Regression analyses revealed that EMG AT Vo2 correlated significantly with blood AT Vo2 (r = 0.921, p less than 0.001) and with gas exchange AT Vo2 (r = 0.921, p less than 0.001). It was concluded that analysis of myoelectric signals, which reflect motor unit recruitment and discharge frequency, could provide a non-invasive estimate of AT.

Potential for gross muscle hypertrophy in older men

The time course of strength gain with respect to the contributions of neural factors and hypertrophy was studied in five young men and five older men during the course of 8 weeks progressive strength training. Young and old men showed similar and significant percentage increases in strength. However, the neurophysiological adaptations in response to the training were quite different. Increases in maximal muscle activation (neural factors) played a dominant role throughout the training for old subjects, while young subjects showed strength gains due to neural factors only at the initial stage, with hypertrophy becoming the dominant factor after some 4 weeks of training. Our data suggest that the effect of muscle training in the old may entirely rest on the neural factors presumably acting of various levels of the nervous system which could result in increasing the maximal muscle activation level in the absence of significant hypertrophy.