It happens to all of us. Sometimes our bodies physically can't perform an exercise well. Sometimes it legitimately hurts to do something.
These reasons are acceptable grounds for finding another way to get the job done. In lifting, this means finding a new lift that targets the same muscle groups. In cardio, this means finding a new routine that still offers similar heart rate challenges.
Under the resources tab, I have a site listed called Exercise Prescription, or ExRx for short. This has been my lifting bible since I began. It's not loaded with bullshit and keeps explanations succinct.
How to Substitute
Now, if you can't find anything here, Bodybuilding.com has an exercise guide that comes with even more possibilities. It's a cool system that can be sorted by equipment, rating, name and muscle.
When it's time to get the heart rate sky rocketing, virtually anything other than sitting down can do the trick. Unfortunately, not all of us live in beautiful climates that allow for a lot of outdoor activities, and a lot of us live in apartments over others we don't want to piss off with loud banging.
Indoor Options - 1st Floor
Indoor Options - Above Someone
A Note on Alternatives
Those that write greatly respected routines (for example, Mark Ripptoe's Starting Strength) include specific exercises for specific reasons. Because of that, stick with what they have laid out unless you physically cannot do the exercise without harming yourself. Otherwise, there's a good chance you're cheating yourself of potential gains.
Co-host of HeroesForge, founder of Battle to Be Better, gamer, singer and all around happy person.