I can't do that because...
"I can't do squats because they hurt my knees. What else can I do?"
"I can't do push ups because they hurt my shoulders. What else can I do?"
"I'm not strong enough to do pull ups. What else can I do?"
I get questions like these all the time, and unless the person has a particular injury to be careful with, I typically say, "If you want to get good at something, you have to do that thing." Your body is designed to adapt and improve based on the situations it's exposed to. If you want to become a stronger runner, you need to run. If you want to get better at golf, you need to swing a club. People seem to understand this concept with most things in life, but they assume there is some secret workaround when it comes to resistance training.
There is a trick for getting better at difficult exercises without straining your joints, but it involves working through exercises, not working around them. Any kind of resistance training exercise has two (sometimes three) components: the up part (concentric), the down part (eccentric), and possibly a component where you hold the position before lowering back down (isometric). In the beginning, people are going to struggle with the up part (concentric) of the movement because their muscles aren't quite up to the task yet. Worse yet, your muscles support and protect your joints, so if they can't do their job, then you're going to put an excessive (and possibly dangerous) amount of stress on your joints. This can lead to frustration, joint pain, or even injury because the muscles aren't strong enough to properly support the joint. Most people jump to the conclusion they should then avoid the exercise, but this is a mistake. You're never going to get better at the movement unless you do it, and the way you can do it is by focusing on the eccentric (the down) component of the motion.
Eccentric Training - Not just for elite athletes
Most people don't realize that the eccentric part of the movement is just as important as the concentric. Lowering the weight (or your bodyweight) in a controlled manner is just as important as lifting it for building strength and muscle mass. Studies have shown that high load eccentric training can lead to greater overall strength and muscle mass gains than concentric training alone. This type of advanced training is typically done by athletes because it requires massive weights and a training partner. By lowering a weight with control that you can't normally lift on your own and then having your training partner help you lift it back up, you can improve your concentric strength in the same motion. While this type of high load training is typically reserved for athletes, the same principles can also be applied to beginners.
An example of eccentric training for beginners
Let's look at squats as an example. For years, I've been recommending an exercise called assisted squats for people that tell me that squats hurts their knees. Their knees hurt because their leg muscles are too weak to properly move them through the motion. It causes their form to suffer which focuses excessive strain into their knees. Most people pick the simple solution of skipping squats entirely, but that just means they will never increase their leg strength and they will always have knee pain. To fix the pain and weakness, they need to work through the problem with assisted squats.
Assisted squats is the eccentric version of a standard squat that you can do by yourself. To do them, you hold onto a sturdy object with both hands and then squat down as far as you can. It allows you to lean back deep as you squat down without fear of tipping over. This helps you focus on proper form as you squat down which builds good supporting strength for your knees. Then you can use your arms to help pull yourself back up. Think back to the athlete doing eccentric training with his partner. In this case, the weight the person typically can't handle is his own body weight, and the training partner that helps pull the weight back up is his arms. In time, this eccentric exercise will strengthen his legs enough so that he can finally do normal squats without pain.
Great for any age
I recently did a segment on a local morning show concerning fitness for people over 60. I recommended a few eccentric movements because they are a safe and effective way for severely deconditioned people to basically reclaim their lives. Researchers that have studied aging baby boomers show that they are having increasing difficulty doing simple every day tasks like getting out of bed, going up stairs, getting out of chairs, and picking things up off the floor. As we get older, we can lose as much as 5% of our muscle mass every ten years due to age-related sarcopenia (muscle loss). Even active people can experience this level of atrophy because they get too efficient in their movements.
Instead of squatting down to pick something up off the floor, people typically lock their knees and bend over to pick it up. Instead of squatting down into a chair and lifting back up out of the chair with their legs, they collapse into the chair from standing and rock their way out of their chair using their arms to take the burden off their weakened legs. Over time, this efficient avoidance of effort will cause your unused muscles to slowly melt away. Exercise can not only prevent this atrophy, but for people that have let this muscle loss get too out of control, exercise can also reverse it and help them significantly improve the quality of their lives.
That's why my number one recommended exercise was assisted squats. People don't realize that their legs are their life. It is difficult to think of any activity that can't benefit from proper leg strength. Even simple things like getting in and out of a chair can become difficult if you ignore vital exercises like squats for too long.
Don't ignore any pain
It's important to point out that you still should stop if you feel those old pains in your joints. It is still a sign that something is wrong and your body is warning you to stop. The first thing you should notice when you're doing eccentric exercises like assisted squats, is that you shouldn't feel any of your old joint pain. The people that I've recommended assisted squats to in the past report that even after a few reps, the typical pain in their knees quickly feels better. This is because they're finally doing the movement with proper form and the strain is placed in their muscles rather than in their joints. This will be true of any eccentric exercise, so if you're still feeling stabbing pain in your joints, it either means your form is still incorrect or there is damage in your joint that might require medical attention.
Update 6-5-14: The New York Times just came out with a great article today showing another terrific eccentric exercise that can protect against hamstring injuries. This one is a bit more advanced, so I would mainly recommend it for athletes. Check it out here.