Metabolic correlates of fatigue from different types of exercise in man

Microdialysis and electromyography of experimental muscle fatigue in healthy volunteers and patients with mitochondrial myopathy

Consecutive 60-min microdialysis samples were taken from the tibial anterior muscle in 11 healthy subjects and 4 patients with mitochondrial myopathy before (2-3 samples) and after (3-4 samples, 2 controls and 1 patient excluded) sustained isometric foot dorsiflexions. Before exercise, mean concentrations of lactate, pyruvate, hypoxanthine, urate, aspartate, and glutamate did not significantly differ between controls and patients. After exercise, the controls showed significantly increased concentrations of lactate, pyruvate, and urate, decreased hypoxanthine, and no change in aspartate and glutamate. Similar findings were observed in the patients. Plasma lactate was unchanged. Exercise-induced increase in integrated electromyogram amplitude and rated subjective fatigue were correlated to increased post-exercise lactate concentrations, with no obvious difference between the groups. Microdialysis of skeletal muscle allows the detection and monitoring of biochemical changes in the interstitial space. With the exercise protocol used, however, it was not possible to demonstrate any biochemical difference between healthy controls and patients with mitochondrial myopathy.

Muscle damage and soreness after endurance exercise of the elbow flexors

PURPOSE

This study investigated changes in indirect markers of muscle damage after endurance exercise of the elbow flexors and compared the changes with those after maximal eccentric actions (Max-ECC) of the elbow flexors.

METHODS

Eighteen male students rhythmically lifted (1 s) and lowered (1 s) a light dumbbell (1.1-1.8 kg: 9% of MIF) in 60-180 degrees of elbow joint angle for 2 h (2-h Ex). Maximal isometric force (MIF), relaxed (RANG) and flexed elbow joint angles (FANG), upper-arm circumference (CIR), muscle soreness (SOR), B-mode ultrasound (US), and plasma creatine kinase (CK) activity were assessed before and immediately after, and up to 96 h after exercise.

RESULTS

All measures were altered significantly (P < 0.05) after 2-h Ex in a similar time course to Max-ECC; however, changes in RANG, FANG, CIR, US, and CK (peak: 356 +/- 121 IU.L-1) were significantly (P < 0.05) smaller compared with those after Max-ECC. SOR developed immediately after 2-h Ex and peaked 24-48 h after exercise. MIF dropped to 44.1% of the preexercise level, which was significantly (P < 0.05) lower than that after Max-ECC (58.1%), immediately postexercise. MIF recovered to 79.8% at 24 h, and 97.8% at 96 h postexercise, which was a significantly (P < 0.05) faster recovery compared with that of Max-ECC (73.1% at 96 h).

CONCLUSION

These results showed low-intensity continuous muscle contractions (3600 times) resulted in muscle damage; however, the magnitude of the muscle damage was less severe, and the recovery was faster compared with 12 maximal eccentric muscle actions.

Changes in corticomotor excitability after fatiguing muscle contractions

To investigate whether the type and duration of activity influences corticomotor excitability following fatiguing exercise, we compared motor evoked potential (MEP) responses of the biceps brachii to transcranial magnetic stimulation (TMS) during recovery from two different exercise regimens. Responses were recorded in both the resting state and during a weak contraction. Ten subjects performed a 60-s maximal voluntary contraction (MVC) and, on a subsequent occasion, a sustained 20% MVC to the point of exhaustion. Resting MEP amplitude declined following maximal and submaximal protocols, reaching 34% and 31% of pre-exercise means, respectively (P < 0.001 for both). In contrast, mean facilitated MEP amplitude showed a smaller and more transient decrement following the sustained submaximal effort (64%; P < 0.05), but not the 60-s MVC. Abolition of the postexercise depression in resting MEP amplitude by a weak tonic contraction indicates that decreases in excitability at the spinal level contribute to the reduced corticomotor excitability observed after fatiguing exercise.

Is post-tetanic potentiation, low frequency fatigue (LFF) and pre-contractile depression (PCD) coexistent in intermittent isometric exercises of maximal intensity?

The aim of our experiment was to test the hypothesis that the performance of maximal isometric exercise every 20 s would reduce the intermediate frequency force, i.e. the force that appears while stimulating the muscle at 15 and 20 Hz, and would produce less decrease the force at 10 and 50 Hz, while Pt would increase. Such changes in stimulated force should demonstrate the coexistence of potentiation, low frequency fatigue (LFF) and 'post-contractile depression' (PCD). The quadriceps muscle of 14 healthy men (aged 19-37 years) was studied. The results have shown, that during isometric exercise of maximal intensity there was significant (P < 0.05) decrease in P15 and P20, increase in Pt, however, MVC and P10 and P50 was unchanged (P > 0.05). LFF manifested itself most significantly which is evident from decrease in P20/P50. During recovery after work there was significant increase in LFF and decrease in P50 which is indicative of the manifestation of PCD. Besides, there was significant (P < 0.05) decrease immediately after exercise in RTP20 and RTP50, while no changes in T50 and RT. There were no significant changes (P > 0.05) however, either in RTP20 and RTP50 or in T50 and RT 20 min after exercise if compared to the initial and immediately post-exercise values.

Exercise-induced slow waves in the EEG of cats

The aim of the present study was to investigate the early signs of fatigue. Cats were chronically implanted with electrodes in the frontal and occipital cortical areas, and a thermocouple was inserted into the nasal orifice to record respiratory rate. After a recovery of 10 days, the animals were trained for running on treadmill. On the day before recording, a catheter was tied into one of the common carotid arteries to record arterial blood pressure. The electroencephalogram (EEG), arterial blood pressure, and respiration were recorded continuously. At the time of deceleration of running high amplitude, slow waves appeared both in the sensorimotor and occipital cortical regions. The power spectra showed a significant increase in frequencies of 1-6 Hz in the sensorimotor cortex, and of 1-10 Hz in the occipital cortex, with a great increase in the total power. During rest the pre-running, brain activity reappeared gradually. The arterial blood pressure, the heart rate, and the respiratory rate were elevated during running, but no special changes occurred at the onset of the slow waves in the EEG. The blood glucose level was somewhat higher after the first 2-min running than the pre-running level. It is concluded that the appearance of slow waves in the EEG is an early manifestation of fatigue. The cardiorespiratory changes and the blood sugar concentrations play no role in the slowing of the electrocorticogram. The present results show the involvement of brain mechanisms in the onset of tiredness.