Christensen, E. H., Hedman, R., & Saltin, B. (1960). Intermittent and continuous running. Acta Physiologica Scandinavica, 50, 269-286.

The physiological responses of two males to running at 20 km/h on a horizontal treadmill were investigated. Work and rest periods from 5 to 15 seconds in various combinations for a total period of 30 minutes were preformed. A continuous run at the selected pace was also investigated.

Both Ss responded differently to the conditions. One S had less work capacity than the other requiring "lighter" workloads to avoid detrimental fatigue. This observation confirmed the need to individualize training programs if all participants are to receive the best effects from training experiences.

Both Ss reached oxygen uptakes during intermittent running close to or equal to their maximum. For one S the work to rest ratio of 15 seconds to 15 seconds, and for the other 10 seconds to 5 seconds, produced the maximal response.

Several relationships were found among the iterations of duration, distance, and work:rest ratios.

• If total training distance is increased then work periods should be decreased, or the work:rest ratios increased, otherwise lactate will increase.
• For shorter work periods, lactate accumulates for the first five minutes and then levels off, that level increasing as the work period is extended. However, at some point when increasing work periods, lactate accumulation does not level off, indicating excessive work and stress.
• As work:rest ratios are decreased, the period of work should also be decreased otherwise the stress of training (lactate levels) will become excessive.

For both Ss at this running velocity, work periods of 15 seconds were excessive, while 5 and 10 seconds were not.

Under continuous running at 20 km/h, one S could only continue for 3 minutes while the other performed for 4 minutes. The amount of work performed, and therefore the potential to gain benefits from training, became less as the duration of work and accumulation of anaerobic metabolites increased.

With short work and rest intervals, it is possible to perform a substantial volume of high-intensity work supported by primarily aerobic metabolism.

"Two physically trained subjects can run continuously for 3 or 4 minutes respectively on a treadmill at a speed of 20 km/h, reaching maximal values for oxygen uptake and for blood lactic acid. At the end of this time when they have run a total distance 1 and 1.3 km respectively, they will be totally exhausted and will need a comparatively long time for recovery. Running at the same speed, but intermittent with short spells of activity and rest, the character of work will change entirely; despite a marked decrease in oxygen uptake during the actual work periods, the work can be performed without or with only a comparatively slight increase in blood lactic acid concentration, indicating aerobic or practically aerobic work conditions. The trained subjects can stand an effective work time of 15 or 20 minutes respectively, within the experimental time of 30 minutes, and run a total distance of 5 or 6.67 km respectively, without being totally exhausted." (p. 286)

Implications. Several guidelines for planning effective training programs are inherent in this study.

• Training that is exhausting is not necessarily the best or even effective stimulus. Training work volumes decrease with high-intensity work periods of 15 seconds or more at work:rest ratios of 1:1 up to 1:3, and are least when performed as a single continuous work effort.
• It is possible to perform a large volume of high-intensity work by using work and rest periods of 5 or 10 seconds, normally in a 1:1 work:rest ratio.
• As high-intensity work periods extend to 15 seconds and beyond, the requirement for longer rest periods increases disproportionately.
• Intermittent work ("ultra-short" work) of the type explored in this investigation, is the training regime that will allow the volume of high-intensity or competition-specific work to be increased.
• Intermittent work of this type is the only form of training that effectively trains the aerobic component of work at high-intensity.
• The responses to intermittent work are individual. While the work interval (e.g., 10 seconds) might seem to be very short, it could still be too much for some athletes.

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