factor
Something that affects a result. Sleep, food and activity habits are factors that affect fitness.
heredity
Traits passed down from your birth parents through your genes, like height or body type.
FITT
Frequency (how often), Intensity (how hard), Time (how long), Type (what kind).
overload
Doing a bit more than your body is used to so it adapts.
progression
Increasing the challenge gradually, in small steps.
specificity
You improve the kind of fitness you train.
overtraining
Doing too much with too little rest, which can lead to tiredness, poor results and injury.
growth spurt
A time of fast growth, often during puberty.