Most injuries give warning signs. Normal pain versus other pain, what a warm-up is for, common injury sites, the first days after an injury and returning.
Training injuries far more often come from pushing through days of discomfort than from a single accident. It hurts only during training at first, then lingers afterwards, then shows up in daily life. What a few days of rest would have settled at the first stage can take months once it reaches the last one. That is why noting any unusual discomfort in a line of your training log helps more than you might expect.
Muscles aching the day after training is normal. Sharp pain felt inside a joint, pain on only one side, or pain landing in exactly the same spot with one specific movement is different.
What you need before training is warmth, not lengthened muscle. Long static stretching from cold can reduce the force you produce immediately after. Raise your temperature with easy movement that takes joints through their range, and leave lengthening stretches for after the session. Five to ten minutes is enough, and doing the activity you are about to do at a light intensity is the best warm-up.
You improve while resting, not while training. Short sleep or insufficient food slows recovery from the same session and raises injury risk. Plans that hit the same area hard every day usually fail, not from lack of discipline but because recovery cannot keep up. A heavy day or two is nothing to worry about, but a run of days with a higher resting heart rate or shallow sleep is a sign that fatigue is building.
Each activity has typical injury sites, and the causes are similar. Most come not from a single accident but from repeating the same movement and suddenly increasing volume. Knowing your activity's weak points helps you catch early signals.
For a sprain or knock with swelling and pain, the usual first aid for the first day or two is to rest the area, apply cold in short repeated sessions, and use compression while keeping it raised above the heart. Kneading it hard or applying heat 'to loosen it' at this stage can increase swelling. Keeping it completely still can also slow recovery, though, so once the pain eases, start moving gently within a pain-free range. If you cannot bear any weight, the area looks deformed or you notice abnormal sensation, give first aid only and get medical care straight away.
The most common mistake after recovering is going straight back to your previous volume because it no longer hurts. Pain disappearing does not mean the tissue is strong enough yet, so start at about half your former intensity and volume and build up over several weeks, roughly as long as you were off. Exercising on painkillers switches off your body's warnings and should be avoided. If you keep injuring the same spot, look for causes such as strength or flexibility imbalances, footwear or your training plan. If you cannot find the cause, a medical professional who treats sports injuries is the quickest route to an answer.
Putting rest days in the plan in advance lets you take them without guilt. Resting only when something hurts means resting after it is already too late. If pain lasts beyond two weeks, or there is swelling or loss of strength, getting it examined is faster in the end than deciding alone.