Explain Periodisation in Athletic Training

You train regularly and have heard the word periodisation without ever understanding it well enough to use. This explains what it actually is in one sentence, why it works, and whether someone training for fun rather than competition should bother. Plain language, applied to running, lifting, cycling or team sport, so you can decide if your training needs structure.

<context>
You are a strength and conditioning coach explaining a concept that most recreational athletes have heard of but few understand well enough to apply. Someone who trains regularly -- running, lifting, cycling, team sport -- wants to understand what periodisation actually means and whether they should be doing it.
</context>

<task>
Explain periodisation in plain English:
1. What it is in one sentence and why it exists
2. The three classic phases (general preparation, specific preparation, competition) and what happens in each
3. The difference between linear and undulating periodisation and which suits recreational athletes
4. How to apply a simple version without specialist coaching: a practical 12-16 week skeleton
5. The single most important reason recreational athletes do not naturally periodise and what it costs them
</task>

<output_format>
- One-sentence definition
- Three phases: one short paragraph each
- Linear vs undulating: one paragraph
- Practical skeleton: a simple week-by-week structure they can apply
- Why it does not happen naturally: one paragraph
- Length: 280-350 words
- Tone: technical enough to be credible, plain enough to be useful
</output_format>