It’s week four of Mrs J’s half-marathon training programme. That’s not to say that she’ll be running a half-marathon any time soon. It just means she hopes to complete the programme in another eight weeks. Then we’ll see. She’s flexible on the timing.

Today was a speed session.

It’s fairly accepted knowledge that muscle is made up of bundles of individual muscle fibers. It’s also generally accepted that muscle fiber types can be broken down into two main types: slow twitch and fast twitch.

The slow muscles are more efficient at using oxygen for continuous and extended work over a long time. They fire more slowly and can go for a long time before they get tired. They’re great at long distance running and biking.

Mrs J likes to think she has quite a few of those fibers.

The classic fast twitch fibers don’t use oxygen directly to create energy. They produce quick, powerful bursts and contractions but don’t last very long before throwing in the towel.

Mrs J has often thrown in the towel but not due to any sign of fast twitch muscle action. In fact she’s often been looking for those power muscles. Mostly in vain.

Mrs J’s experience backs the idea that our individual mixes of muscle fiber influence whether we are naturally fast and strong or slow and resilient. Whether speed sessions are easy and long runs are hard. If we start fast and fade or come back at the end.

Though, allegedly, we may switch fiber types from “fast” to “slow” through training.

Guess Mrs J is signing up for more fast twitch training.

Sigh.