Why you should run that last interval

Last Tuesday it was time for another happy interval training with Running Sweden, where I run as one of their coaches. Run happy is their motto and they certainly live up to it - which is why I love being a coach for them. Training consisted of a 5K warmup followed by 6-10x 800m on the track. I like the track because it reminds me of my time as a junior at the athletics club. Needless to say I wasn't particularly good at the high jump, but I have good memories of racing around the track like there was no tomorrow, then lying down on the sun-heated tartan and chat with friends until our trainer forced us to do a proper cooldown...

In 'my' group, we decided to go for 8 intervals. We would go out on a 4min10sec pace (per km) and try to run each next interval faster than that. Said and done. First at 4.03 pace (wow!), then at 4.01. 3.55-3.52. Halfway. Great times thus far, much better than expected, but needless to say it didn't get easier. The last 4 intervals went at 3.51, 3.49, 3.46 and 3.45. Then we faced a tough decision. One more or go home?
 

I have thought about this often. If the training is set at 8 intervals, why would you do nine? Or even, why not just stop after 7?
 

Here is a thought that came up in my mind this Tuesday evening on the track.
 

As a researcher at Karolinska Institutet, my colleagues and I studied the effects of spinal cord injury on skeletal muscle. Due to traffic accidents or falls from great heights, the participants in this study became completely paralysed from the level where the spinal cord got damaged. In the case of our cohort, that often meant that they were paralysed from the neck down.
 

This paralysis obviously has detrimental effects on the muscles that are no longer receiving signals through the nerves. It is not just that the muscle mass diminishes (a process called atrophy, see picture) - the quality of the muscle that remains, also deteriorates. Among other things, the muscle loses its slow muscle fibers and the amount of oxidative enzymes decreases. This causes the muscle to become less resistant to fatigue. Basically it is the exact opposite of what 'we', as athletes, try to accomplish. It doesn't only have consequences on the muscle. These changes in muscle make that the muscle can store less glucose (in the form of glycogen). This glucose instead stays in the blood circulation and goes on to damage other organs. In the end these elevated blood glucose levels can lead to diabetes and cardiovascular disease.
 

MRI image of the cross-section of theupper legs of a healthy subject (upper picture) and a subject with spinal cord injury. The dark matter is muscle, the white matter is fat.

But, most of all, these persons - most of them young and in the prime of their life - cannot walk, let alone run. Have you ever pictured your life in a wheelchair? Would you be able to enter your appartment building without any problem? Your favourite store? Your friend's house? Probably not.
 

So. Why you should run that last interval? Because you can.