Banged Up? Use Load Maximizing Tactics
You need load (weight) to create an adaptation, but there are several ways to get more benefit, with less orthopedic risk, from any given load. These methods allow you to use more joint-friendly loads and still build muscle and maintain strength.
1 – Eccentric Isometrics
This involves lowering a weight slowly (4-5 seconds), holding the stretch in the bottom position for a second or two, then lifting it quickly. Here's an example from Joel Seedman, PhD:
The idea is to accentuate the eccentric (negative) part of the rep, which is known to be largely responsible for the benefits of weight training. However, an additional benefit of slow eccentrics is that for any given weight, the slower you move it, the less total stress on the joint: less weight, more results. Additionally, lowering a weight slowly has some scientific support as being therapeutic for tendinitis.
2 – Occlusion Training
Also known as blood flow restriction training, this involves using a tourniquet placed either high on the hips (for leg training) or high on the arms (for arm training). The idea is to tie the tourniquet, which is usually a knee wrap or elastic tubing, tight enough to restrict venous return, but not so tight as to restrict arterial blood flow to the limb. This means pressure along the lines of a 7 on a 1-10 scale of discomfort.
Here's Mark Dugdale using this technique for calves:
For reasons that are still mostly unknown, occlusion training allows you to reap the benefits you'd normally get with heavy loads by using very light weights (20-30% of 1RM). Occlusion training won't directly improve your strength as much as it'll serve as a stimulus for hypertrophy, but if you've got joint issues that prevent heavy training, occlusion training can be a godsend.
3 – Isometrics
When you perform an isometric contraction, you're creating muscular tension without movement at the joint. While this isn't ideal for increasing muscle growth, it's reasonably effective at helping to maintain current levels of muscle mass. It's also fairly effective at improving strength, particularly at the joint angle(s) being trained, and it's certainly the least irritating type of training for your joints.