What is it that drives you to reach a challenging goal, finish a complicated project, or deal with a difficult person? What forces — positive or negative — induce us to go beyond our comfort zones and achieve a little more than we thought we could?

This question has been on my mind ever since I started physical therapy two days after total knee replacement surgery. You, my faithful readers, have followed by progress for the past two-and-a-half months as I, like many people my age, have struggled with a sometimes-painful rehabilitation.

This process has been more difficult than I anticipated, but it’s taught me a lot about goals and what motivates me to strive for them, no matter how uncomfortable. There are many kinds of motivation, and some work better — and promote our mental health — than others.

One of the less healthy motivators is fear. In many traditional religions, theology employed fear of punishment in the afterlife to pressure believers into righteous behavior. If the short-term repercussions of their actions didn’t induce the faithful to do good, long-term — eternal — consequences might do so. Nobody wanted to burn in hell, after all.

Parents sometimes use fear of punishment to make children toe the line. The threat of a timeout or a spanking is sometimes the fastest way to get a result from a youngster. But is it healthy?

Another approach is to create consequences for the child’s behavior; for example withdrawing privileges if he or she doesn’t comply with guidelines. Even young children can respond to this approach — at least sometimes.

Fear — of needing a further, painful procedure to improve my range of motion — wasn’t what helped me make progress in my therapy. The threat of a “manipulation under anesthesia” was in the back of my mind. But it only caused me more anxiety. And far from motivating, anxiety only paralyzes me.

What did work was encouragement (“Come on — you’ve got this!”) and breaking range of motion goals down into small increments. Bending my knee one more degree every session seems a lot more doable than bending it six degrees in two weeks. The results are the same, but progress seems much more achievable. Smaller goals are less daunting.

Another form of motivation is a sense of responsibility. This can also be framed as guilt, but I prefer the former concept.

When my friend’s mother died after a long illness, she was frustrated because her sons hadn’t helped clean the house for visiting family as she’d asked them to. So I stopped by her home and calmly started vacuuming. After a few minutes one of the boys took the vacuum from me and completed the task. “I’m really supposed to be doing this,” he admitted. My goal wasn’t to make him feel guilty. It was simply to remind him he had a job to do. His sense of responsibility motivated him to do it.

Sometimes we say we’ve volunteered for an unpleasant task because of guilt. But I think often our real motivation is a sense of responsibility. Someone needs to do the job, and maybe it’s my turn. (Or course, this sense of duty can go too far; we don’t need to take on every job. Others need to do their part as well.) And when we’ve shouldered the task, we get our reward: Satisfaction of a job well done.

Maybe that’s the greatest motivator of all: Pride. Even if a project is difficult, we take it on because know we’ll feel good about ourselves afterward. I need to remind myself of this when physical therapy feels like a nuisance and I’m beginning to regard my therapists as drill sergeants.

At the end of the process, I’ll have a knee that functions almost like new, and I’ll know I did the best I could. That’s what keeps me going.