One of the key short-interval puzzlers is finding the combination of intensity, work-interval length and recovery-interval duration which will have the greatest impact on fitness. Fortunately, scientists at the University of Vermont have recently quantified the benefits of different kinds of short-interval work-outs.

In this study, a total of eight athletes tried out the following three work-rest combinations during separate short-interval sessions:
(1) 40 seconds of work followed by 20 seconds of rest (40-20),
(2) 30 seconds of work followed by 30 seconds of rest (30-30), and
(3) 20 seconds of work followed by 40 seconds of rest (20-40).

Each interval workout consisted of 15 total work-rest combinations and therefore lasted for a total of 15 minutes. Two different intensities, 90% V02max and 110% V02max, were used during the work intervals with each of the three work-rest combinations above, so each athlete actually carried out six completely different training sessions. At least three days of rest separated each workout, and all exercise was carried out on a cycle ergometer.

If an athlete's goal is to improve V02max by using one of the six workouts, the best session would clearly be the 40-20 at 110% V02max (for runners, an intensity of 110% V02max is about one-mile race pace, for cyclists it is the best speed that can be maintained for about four minutes). The 40-20 at 110 per cent produced an average oxygen uptake of 47.7 ml/kg/min, while none of the other five workouts could even lift oxygen consumption above 40.

Interestingly enough, in terms of oxygen uptake the 40-20 at 90% V02max was appreciably better than the 30-30 at 110 (for runners, 90% V02max is about 10K race pace, for cyclists it is the best speed which can be maintained for 30-40 minutes). For the purpose of raising heart rate, the superior session would again be 40-20 at 110 per cent, but, surprisingly enough, there was little difference between the 40-20 at 90 per cent and the 30-30 at 110. Each produced an average heart rate of about 155 beats per minute. Maximal heart rate for the eight athletes was around 1 87bpm, so each workout produced average heart rates of about 83 per cent of maximal.



If you're interested in producing a lot of lactic acid during a short-interval session (to boost anaerobic power and increase lactic-acid tolerance) the most outstanding workout would again be the 40-20 at 110 per cent, which raised blood-lactate levels to about 11 mmol/litre at the end of 15 minutes. The 40-20 at 90 per cent and the 30-30 at 110 were about equal in value and were both fairly poor producers of lactate, sending blood concentrations up to only 6 mmol/litre.

Meanwhile, the 20-40 workouts were paltry affairs. For a runner, a 15-minute, 20-40 workout with work intervals at 90% V02max (lOK race speed) is no better than running steadily for 15 minutes at only 62 per cent of maximal heart rate and 40 per cent of V02max. By contrast, the 40-20 at 110 per cent is comparable to running steadily for 15 minutes at 91 per cent of maximal heart rate and 81 per cent of V02max. What a difference small changes in short-interval workouts can make! (European Journal of Applied Physiology, 1992).