Four weeks of speed endurance training reduces energy expenditure during exercise and maintains muscle oxidative capacity

Iaia, FM, Hellsten, Y, Nielsen, JJ, et al. Journal of applied physiology, 2009; 106: 73-80.

Background

There are times in every athletes career that necessitate a decrease in training volume, maybe due to unplanned circumstances such as busy periods at work or an injury, but sometimes it will be desirable to reduce volume in order to ‘shed' fatigue ahead of an important race or event. Whatever the reason for the reduced volume, it would be very useful to have a strategy for making the most of your limited training time or at the very least a method for maintaining hard earned fitness!

Methodology

Seventeen moderately endurance-trained male runners with at least four years running experience participated in this study. Subjects were randomly split into two groups; 8 were placed in a control group and 9 were assigned to a speed endurance training (SET) group. Both groups underwent a period of pre-testing followed by a four week training period and finally a post-testing protocol. The control group maintained their existing training which consisted of 4.0 ± 0.4 sessions a week. Each session consisted of 52.3 ± 2.4 minutes of moderate intensity aerobic training. The SET group trained every other day during the four week training block. The training protocol consisted of 8-12 30s intervals with a 3min rest period. Interval intensity was 93 ± 0.1% of average speed from a 30s all out sprint.

The pre and post-training testing protocol consisted of a resting muscle biopsy followed by a treadmill test comprising 4 x 6 minute runs at speeds of 11, 13, 14.5 and 16km.hr^-1. Each run was separated by a 2 minute rest period during which a venous blood sample was taken. VO₂ was measured breath-by-breath throughout the test and heart rate was also recorded. Following the sub-maximal treadmill test an incremental test to exhaustion was undertaken to determine VO_2max. Finally, one week after the lab visit, subjects completed a 10km time trial run on a standard 400m track.

Results

Fig. 1. Oxygen uptake during 4 treadmill stages of 6-min each before (thin line) and after (thick line) 4 wk of speed endurance training (SET group).

The study looked at a large number of physiological and biochemical markers of aerobic metabolism as measured from the muscle biopsy and blood samples. Here is a summary of the findings

• There was no significant difference in mitochondrial oxidative enzyme activity following SET
• Sub-maximal economy was improved at all running speeds in the treadmill test (Fig. 1) and heart rate was lower at all speeds
• VO_2max was unchanged in both groups
• The number of capillaries per muscle fibre was similar pre and post SET, but there was an increase in number of fast-twitch muscle fibres
• 10km performance was not significantly different following training in both groups.

What does this mean?

This study shows that four weeks of SET led to reduced sub-maximal energy expenditure in endurance trained runners. This increase in running economy was not backed up with an improved performance in the 10km run. VO_2max was unchanged suggesting that the % of VO_2max that could be maintained was lower following SET. In other words it looks like a decrease in ‘fitness' was offset by improved economy leading to similar performance.

The authors highlighted this fact and estimated the fraction of VO_2max that was maintained for the 10km time trial. This showed a decrease from 84.1 ± 1.5% to 80.7 ± 1.6% following SET. This is in contradiction to the conclusion that muscle oxidative capacity (the ability of the muscle to utilise oxygen) had been maintained. Although the level of mitochondrial oxidative enzymes measured was unaltered by SET it appears aerobic performance was reduced.

A word of warning though - all subjects had performed "no intense intermittent training" in the 3 months prior to the study. The study offers no insight into the effects if such training sessions were already being incorporated in your program.

In short if a period of reduced volume training is necessary it appears that SET is a suitable training strategy for maintaining performance but it may do so at the cost of a reduction in lactate threshold or the fraction of VO_2max that can be maintained. The most striking finding is the improvement in economy that SET can cause. There is clearly a benefit to including such sessions in a structured training plan.

What this study does offer is an explanation into the concept of an athlete ‘racing' themselves fit. In reality what an athlete following this approach is really doing is ‘racing' themselves economical, while actually losing fitness in the long term. A better strategy for maintaining long term athletic development may be to incorporate specific segments of race pace work within your training program. Using shorter durations at this intensity will give most of the benefits, without accumulating the high levels of fatigue that racing incurs.