Exercise-Induced Oxidative Stress in Overload Training and Tapering

Tapering can be an effective way of enhancing performance after a period of intensive training, but the mechanisms for this ergogenic effect are unclear. It was hypothesized that overload training will increase oxidative stress through an accumulative effect of repeated high-intensity exercise, whereas tapering will improve the antioxidant defense system and alleviate oxidative stress.

PURPOSE

To study the oxidative stress response to overload training and tapering.

METHODS

A group of eight well-trained male endurance athletes (30+/-6 yr; 73+/-13 kg; 64+/-6 mL.kg.min) performed two 4-wk periods of training in a crossover design. Each period included a 2-wk build-up phase followed either by 2 wk of training at the same load (control) or by a week with a 40% increase in training load (overload) preceding a week with a 60% reduction in training load (taper). Performance was monitored through weekly 15-min cycling time trials preceded by a 45-min preload at 70% Wmax. Blood samples were taken before and after the time trials and analyzed for oxidatively modified heme (OxHm), methemoglobin (metHb), and glutathione redox status.

RESULTS

Cycling time trials induced significant postexercise increases in levels of OxHm (+3.8%; P<0.001) and oxidized glutathione (GSSG: +13.9%; P<0.05) and decreases in metHb (-12.1%; P<0.001), reduced glutathione (GSH: -14.4%; P<0.001), and GSH/GSSG (-29.7%; P<0.001). Tapering was shown to significantly increase performance (+4.9%; P<0.05). Training modifications did not influence resting levels or exercise-induced changes of markers of oxidative stress.

CONCLUSION

A short period of tapered training improves performance but does not seem to be associated with substantial changes in exercise-induced oxidative stress.

Exercise-induced oxidative stress:myths, realities and physiological relevance

Although assays for the most popular markers of exercise-induced oxidative stress may experience methodological flaws, there is sufficient credible evidence to suggest that exercise is accompanied by an increased generation of free radicals, resulting in a measurable degree of oxidative modifications to various molecules. However, the mechanisms responsible are unclear. A common assumption that increased mitochondrial oxygen consumption leads per se to increased reactive oxygen species (ROS) production is not supported by in vitro and in vivo data. The specific contributions of other systems (xanthine oxidase, inflammation, haem protein auto-oxidation) are poorly characterised. It has been demonstrated that ROS have the capacity to contribute to the development of muscle fatigue in situ, but there is still a lack of convincing direct evidence that ROS impair exercise performance in vivo in humans. It remains unclear whether exercise-induced oxidative modifications have little significance, induce harmful oxidative damage, or are an integral part of redox regulation. It is clear that ROS play important roles in numerous physiological processes at rest; however, the detailed physiological functions of ROS in exercise remain to be elucidated.

A new sensitive assay reveals that hemoglobin is oxidatively modified in vivo

Free radical formation in heme proteins is recognised as a factor in mediating the toxicity of peroxides in oxidative stress. As well as initiating free radical damage, heme proteins damage themselves. Under extreme conditions, where oxidative stress and low pH coincide (e.g., myoglobin in the kidney following rhabdomyolysis and hemoglobin in the CSF subsequent to subarachnoid hemorrhage), peroxide can induce covalent heme to protein cross-linking. In this paper we show that, even at neutral pH, the heme in hemoglobin is covalently modified by oxidation. The product, which we term OxHm, is a "green heme" iron chlorin with a distinct optical spectrum. OxHm formation can be quantitatively prevented by reductants of ferryl iron, e.g., ascorbate. We have developed a simple, robust, and reproducible HPLC assay to study the extent of OxHm formation in the red cell in vivo. We show that hemoglobin is oxidatively damaged even in normal blood; approximately 1 in 2,000 heme groups exist as OxHm in the steady state. We used a simple model (physical exercise) to demonstrate that OxHm increases significantly during acute oxidative stress. The exercise-induced increase is short-lived, suggesting the existence of an active mechanism for repairing or removing the damaged heme proteins.

Effect of tapered normal and interval training on performance of Standardbred pacers

Human athletes taper or reduce their training load before a race to enhance performance, apparently because recovery from the effects of fatigue occurs faster than the loss of fitness from the reduced training. However, there appear to be no previous studies of tapering of equine athletes. Our aim in the present study was, therefore, to investigate the efficacy of tapering with Standardbred pacers. We determined the effect of repeated cycles of tapered training on performance of Standardbred pacers. After 8 weeks of jogging and 3 x 2 week cycles of pace work, 19 horses were randomised to a taper and a control group. The taper group completed 5 consecutive 2 week cycles, each incorporating a 7 day taper; some cycles included high-intensity interval training. The control group continued with 5 more cycles of pace work. All horses completed a 2400 m individual time trial after each cycle. Peak and mean speed of the taper group were faster than those of the control group in all cycles; the differences were clear-cut in all cycles for peak speed (overall 4.4%, 95% confidence interval 1.7 to 7.1%), but only in one of the interval-training cycles for mean speed (2.4%, 0.3 to 4.7%). Four horses in the taper group were injured during interval training. Repeated tapering produces a worthwhile enhancement of performance in Standardbreds, but the addition of interval training appears to increase the risk of injury.

Changes in physiological parameters in overtrained Standardbred racehorses

Various changes in physiological parameters are associated with overtraining, which can be a serious problem for human and equine athletes. A 34 week longitudinal study was conducted to investigate the effects of an acute training overload on physiological parameters in 10 Standardbred racehorses. After 24 weeks of training, horses received 8 weeks of increased workload, followed by 2 weeks recovery. Horses performed a 2400 m time trial and a progressive submaximal exercise test on alternate weeks. By the end of the heavy training period, the average time for the final 1200 m of the time trial increased by 4.0% (95% probable range of true value 1.7-5.8) and peak velocity decreased by 6.9% (4.7-8.9), indicating that overtraining had occurred. Acute overtraining coincided with an increase in blood lactate concentration after the time trial and submaximal test. There were also substantial decreases in bodyweight, plasma cortisol concentration and packed cell volume after the time trial, and in the velocity at a heart rate of 200/min (V200). Parameters that showed no clear-cut change with overtraining included maximal and recovery heart rate, basal plasma cortisol, plasma and red cell volume, and markers of skeletal damage (plasma concentrations of creatine kinase and aspartate aminotransferase). Bodyweight, V200, postexercise blood lactate and plasma cortisol concentrations may all be useful for detecting acute overtraining in equine athletes.

Reliability of two 2,400-m time-trial protocols for assessing performance of Standardbred racehorses

OBJECTIVE

To evaluate the reliability of 2 time-trial protocols, 1 that involved use of a pacemaker and 1 that did not, for assessing performance of Standardbred racehorses.

ANIMALS

3- to 7-year-old Standardbred maiden pacers.

PROCEDURE

11 Standardbred pacers were used to determine the reliability of a time-trial protocol that involved use of a galloping pacemaker. All tests were performed by a single driver. Horses were paced for an initial 1,600 m then raced and timed for the final 800 m. They were retested 9 days later. Twenty-two horses were used to determine the reliability of a time-trial protocol that did not involve use of a pacemaker. All tests were performed by a second driver. Horses were paced for the first 1,200 m then raced and timed for the final 1,200 m. They were retested 3 and 8 days later.

RESULTS

Reliability of performance time, expressed as the coefficient of variation (ie, the typical percentage variation in a horse's time between trials), was similar for the 2 protocols (1.0% and 1.3%, respectively).

CONCLUSIONS

The small variations in performance times for the 2 time-trial protocols were similar to those of the best comparable laboratory and field tests of human and equine performance. Both protocols would be suitable for investigating factors affecting performance of Standardbred racehorses.