What the experts say
Nigel Hetherington reviews the latest research material relating to coaching, exercise physiology and athletic development.
Though classically thought of as referring to aerobic fitness, even in sport 'endurance' can be suffixed to many other words, for example: 'speed', 'strength', 'psychological' (mental) all of which may play a role both in a sustained, largely aerobic, activity such as marathon running, cross-country skiing, 90 minutes of football etc. or else provide an important endurance component to some much shorter time-framed activity such as speed endurance for 200m running, the strength endurance for a series of continuous rugby encounters or the mental endurance for 100 shots at a target in archery. Even 'simple' aerobic fitness is affected by a multitude of factors including lung function, heart stroke volume, blood oxygen carrying capacity, muscle function and energy stores, hydration and so on. Furthermore, from a long-term goal-setting perspective, the endurance of both coach and athlete will be severely tested many times as the years go by. It may be important therefore to be aware of 'endurance' in these various forms. This month's review covers an array of reports based on many of these factors.
Recent literature highlights the fact that master athletes (both men and women) are definitely enduring! A study shows that for all 415,000 competitors in the New York City Marathon, over the period 1983-1999, that master athletes endured the best and actually showed improved running times even relative to their younger counterparts. Since the belief is that physical activity may slow the aging process this may mean that masters are now getting younger!!
At the other end of the scale a study based on adolescent Kenyan males found, quite convincingly, that although a scientifically selected group of boys from a 'village' environment were initially 'fitter' (VO2 max) and faster (5k time trial) than an equally carefully selected group of 'town' boys, from near by and living at the same altitude, there was no statistically significant difference in their overall trainability over a 12 week period. The village boys started ahead and finished ahead. The report concludes that the initially higher VO2 max figure was the primary reason for the margin. Village boys improved 56.0 to 59.1 while town boys increased 50.3 to 55.6 (all ml kg-1 min-1). No reason for the initially higher values in the village boys was given though presumably lifestyle may have played a major part rather than a specific genetic factor. It would be very revealing for this study to be continued to establish if the town boys could ultimately match the village boys though continued training.
Another important factor in endurance is the energy cost. A paper identified that in comparing highly, well and non-trained runners that the overall energy cost of running was the same in all three classes (as measured using pulmonary gas exchange). Highly and well-trained athletes are simply better conditioned and will be able to harvest and utilize more oxygen. However, internal mechanical and potential energy measurements did reveal a reduction for highly trained athletes with the suggestion that this related to impact loads i.e. foot-plant biomechanics.
Moving up a gear a study has clearly identified the relevance of aerobic metabolism in maximal speed endurance. Based on cycle ergometer sprints subjects completed either a 10s or 20s maximum sprint followed by 2 minutes of rest than a 30s maximum sprint. Overall the findings indicated that although ATP turnover from anaerobic sources was reduced by around 51% during the second half of a 20s sprint the actual power output only decreased by 28% due to a twofold increase in aerobic energy contribution from 13 to 27% of total ATP turnover. This served to partially compensate for the reduction in the anaerobic pathway. Though we are not sure as to the level of anaerobic conditioning these individuals may have attained beforehand it still appears to be a compelling argument for increasing the aerobic content of training for predominantly anaerobic athletes. Performance in the 30s sprint was least affected by the 10s sprint beforehand over the 20s sprint probably as a consequence of higher muscle acidosis following the 20s sprint.
Predicting endurance performance is always useful for coach and athlete and clearly VO2 max (i.e. the maximum volume of oxygen, measured in ml, that can be used per kg of body weight per minute) is a frequently reported yardstick here. A study from Scandinavia looked at an array of measurements made in a treadmill situation and then compared these to actual competition performances and rankings for 16 cross-country skiers (7 male, 9 female). For the male subjects a high value for the onset of blood lactate accumulation (OBLA, 4mmol l-1) was related to a good performance ranking position. For women the best association was through the respiratory exchange ratio (i.e. CO2 produced to O2 consumed). Oxygen consumption (expressed as l min-1) was most meaningful for both male and female skiers in predicting race results. These findings strongly support the use of appropriate 'lab tests' for performance prediction and, though not stated, are also highly relevant to monitoring improvements brought about through specific training that will result in better race performances.
An interesting study looked at so-called muscle power factors - peak velocity, blood lactate concentration (BLA) and 30m run velocity - and compared them to VO2 max as measures of horizontal and uphill running performance. The report concluded that VO2 max contributed more to uphill running performance than horizontal running.
Since oxygen consumption is inextricably linked to endurance performance it is particularly enlightening to view a recent paper that looked at the decay of VO2 max in older men. The study looked at trained and untrained men of retirement age and followed various regimes ranging from maintenance of activity for a high training group through to an inactive group remaining sedentary. The conclusions from the work were that loss of VO2 max would occur with age at a linear rate with sedentary males but that 'vigorously' trained males could actually retain their values.
Always on the agenda these days is the impact of aerobic exercise on health. A recent study looked at the effect of this for sedentary young men and women. The study found that after just 6 weeks that there were measurable improvements in VO2 max whereas the control group did not change. Similarly the trained subjects retained lower heart rates after an aerobic experiment and subsequently had lower resting pulses. Lower heart rates were also witnessed following a psychological shock. Broader experience also links aerobic training directly to a better ability to protect against age-related coronary heart disease. Though a third group showed measurable aerobic improvement from a weight training only regime (about 50% of that of the aerobically trained group) this was thought to relate to improvement in leg performance and so it could also be postulated that power athletes may benefit health-wise in the long term from increased levels of aerobic exercise.
Many coaches and athletes advocate the taking of pulse rate immediately post-exercise as a means of recording objectively the training effort in terms of % of maximum heart rate (HR). Unfortunately, a recent study demonstrated that due to the rapid recovery of HR following exercise bouts pulse checks made post-exercise might underestimate HR by as much as 20-27 bpm. This sounds like a good argument for the use of a heart-rate monitor during exercise or for a correction factor to be applied afterwards. Pulse rates taken after exercise remain useful for monitoring recovery objectively.
Energy replenishment during endurance activities plays a major part in maintaining output and so it was intriguing to read a report that examined the comparative benefits of consuming a single carbohydrate (CHO) as energy source versus a CHO mixture. The work started with the premise that consuming a mixture of glucose and sucrose or glucose and fructose, at a relatively high rate (1.8g min-1) resulted in a 20-55% higher oxidation rate (1.3g min-1) compared with ingestion of just glucose at the same calorific rate. The latest work by the same group looked at three component mixtures of the sugars mentioned above at even higher rates (2.4g min-1). The results point to an even higher oxidation rate (1.7g min-1) during 150min cycle rides at 62% VO2 max. A complementary study looked at glucose infusion (1g min-1) during a 1-hour time trial working at the same VO2 max as the study above. The results demonstrated that glucose infusion had no effect on the time-trial performance, despite an increased availability of plasma glucose for oxidation and evidence of increased glucose uptake into the tissues. The reason for this is that the endogenous CHO stores were not exhausted during the trial and so the 'top-up' was not necessary.
In Successful Coaching, Issue 12, the potentially dangerous effects of hyponatraemia were discussed. A very recent case report on the South African Ironman Triathlon provides a very poignant reminder of this condition. Based on 371 athletes (62% of all finishers) the athlete who gained the most weight (3.6kg) during the event was the only athlete to develop symptomatic hyponatraemia. During recovery he excreted an excess of 4.6 litres of urine. Clearly athletes need to guard against excessive water intake (dipsomania). On the subject of hydration and endurance exercise a further report from the same group concluded that there was no clinically significant change in serum electrolyte levels and no change in hydration status in runners who experience exercise associated muscle cramping during ultra-distance running. This was based on the study of 72 runners and involved tracking blood levels of a series of markers before, immediately after and 60mins after the race.
Transport of oxygen in the blood is facilitated by iron-containing haemoglobin and many endurance athletes seek to ensure this pathway is maximized by supplementing their diet with iron. However, latest findings suggest that athletes tempted to take iron supplements are well-advised to have their iron levels checked first. A condition known as hereditary haemochromotosis that affects iron metabolism can induce iron overload potentially leading to organ disfunction. In a study of 65 highly trained athletes possible genetic mutations that may cause this condition were found in almost half the cases! Thankfully, none actually appeared to have the disease but the potential seemed to be there. Altitude training is famed for elevating oxygen carrying potential by increasing the number of red blood cells. However, intermittent bouts of exposure to high altitude appear not to be of benefit. A study based on 2-hour exposure to 4100m altitude conditions in a hypobaric chamber each day for 14 days showed absolutely no change in blood indicators or any improvement in performance during a sub-maximal cycle ergometer test.
Finally, back to heart rate monitors and their potential for use in estimating energy cost of exercise. A study has highlighted that use of such devices can provide rough estimates of energy expenditure during running, rowing or cycling when predicted values of VO2 max and HRmax are used. When actual measured values for these parameters were used errors in values were further reduced although energy expenditure for females still showed a 12% over-estimation. Hope reading this did not require too much endurance!!
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About the Author
Nigel Hetherington was the Head Track & Field Coach at the internationally acclaimed Singapore Sports School. He is a former National Performance Development Manager for Scottish Athletics and National Sprints Coach for Wales. Qualified and highly active as a British Athletics level 4 performance coach in all events he has coached athletes to National and International honours in sprints, hurdles as well as a World Record holder in the Paralympic shot. He has 10 years experience as senior coach educator and assessor trainer on behalf of British Athletics. Nigel is also an experienced athlete in sprint (World Masters Championship level) and endurance (3-hour marathon runner plus completed the 24 hour 'Bob Graham Round' ultra-endurance event up and down 42 mountain peaks in the English Lake District). He is a chartered chemist with 26 years' experience in scientific research and publishing.
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