In order for your body to change and get fitter, stronger or to show
more muscle tone, it needs to be pushed beyond it's current capabilities.
A training log allows you to view your current work rate. It also challenges
you to better you last performance - even at least by a little bit.
Not using a training log may mean any improvements made from workout to
workout are forgotten. This will in time hinder any physical development.
A training log (or training dairy) works like this. Write down any info
you think you may need to review past performance.
For example: During aerobic sessions, write down the type of exercise you
do, how long it takes you, the intensity you work at and how it felt. Simple.