When you injure yourself you will quite often have a period of time when you cannot undertake your usual work, home and or fitness activities. What this often results in is a loss of fitness and strength. If your period of pain is only short then you may find you can resume normal activities again with no difficulty.
Unfortunately the longer that your pain lasts, combined with how severe your pain is, the more likely you will find that you cannot immediately resume pre-injury activities, quite simply because of the loss of fitness/strength and the loss of ability of your injured area to “handle the load”.
For example, your lower back may have been handling regular lifting of 30kg but for some reason you have injured it. If you then don’t lift for the next month and then decide to start lifting 30kg immediately you might find that your back, arm and/or leg muscles have gotten weak and you strain your back again (or another area). Your injured area, and/or other areas of your body no longer have the “capacity” to handle lifting the 30kg. You will need to build up the "capacity" of your injured area to handle the load again.
This concept is also appropriate if you are trying to commence a new fitness program and finding that a part of your body wont “co-operate”. For example if you want to start jogging 5 kilometres and haven’t jogged before, or for a while, your knees might keep getting very sore and stopping you from completing the 5 kilometres. Why? Because they haven’t got the "capacity" to handle the 5 kilometres. You need to gradually build up your "capacity" to allow you to complete the 5 kilometres without irritating your knees.
This diagram is a fantastic illustration and is from the following source.
If we use the above 2 examples, then the 30kg lifting or the 5 kilometre jog is the yellow pyramid. The blue pyramid is your current "capacity". In order to build up to the yellow pyramid you need to, in often very small steps, build up your strength and endurance.
How do you do this? Broadly speaking you need to:
Continue doing the painful activity, but at tolerable levels/amounts.
Build up the strength/fitness in the required areas of your body via specific exercises.
See someone who is capable of guiding you through steps 1 and 2.
Bearing in mind that this process can take time to occur. It might even take a couple of months before you start seeing change. The good thing is that our capacity can improve no matter how long we have had the pain or how old we are. You just often have to be very dedicated and patient as trying to force your way up the capacity mountain will usually not work.