Before I start I must stress that this is only for injuries you weren't going to see anyone about anyway i.e.
minor pulled muscles and sprains - if you're not sure if your injury is minor or not YOU NEED TO SEE A DOCTOR/PHYSIO
The first 48 hours after an injury is the acute inflammatory phase. The blood supply to the affected area increases and brings substances that promote inflammation to the affected part. Inflammation causes pain, swelling, increased heat and redness.
Treatment during this time aims to reduce inflammation.
Rest - using an injured muscle increases blood flow, so rest is advised during the first 48 hours to minimise this.
Ice - ice reduces blood flow aswell, minimising the presence of inflammatory substances in the injured area.
Compression - reduces swelling
Elevation - reduces blood flow and also minimises swelling
Analgesia - anti-inflammatory analgesics such as advil/ibuprofen obviously reduce inflammation and help the pain.
After 48 hours:
The collagen fibres in your muscles/ligaments under normal circumstances run in straight lines :
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after an injury bonds are formed between the fibres:
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these extra bonds can lead to stiffness and loss of function
Treatment 48 hours after an injury aims to break these bonds.
This is the time to start gently stretching the injured area. It will be a bit uncomfortable but as long as you do it gently it shouldn't cause any problems. Applying heat to the area for 10 minutes prior to stretching causes the fibres to become more pliable and makes stretching easier.
If these simple stretches don't help then ultrasound treatment can also be of use - see a physio
If things aren't getting any better you should be seeing a doctor or physio at this point anyway
And to reiterate - if you are in any doubt about the seriousness of your injury have it properly assessed by a professional BEFORE you try to self treat as gentle stretching does not heal fractures or ruptured ligaments!