When you’re 12 or 13 or 14 and maybe 120 days into it, the school year can seemingly grind to a halt. And though conscientious students would never stop studying for tests or skip doing homework, they will sometimes lose the interest and intensity that previously elevated their work.

They go on what I call “cruise control.” As a result, they get what they call “sucky grades.”

I keep my eyes peeled for the push of that button. When I see it, I explain what they’re feeling, how to fight it, and stress that the opposite - putting the proverbial pedal to the metal - is needed for success in high school and college.

Something similar can happen when you’ve been weightlifting for years. And though conscientious lifters would never stop, they will lift without their previous intensity and interest.

Have you been lifting on cruise control? If so, don’t get too down on yourself.

Death, taxes, and occasionally doing so are all inevitable - but you can do something about the last one.

The something that works for me is altering rep speed, varying the rate in which you move the weight up and down.

My way is less than typical, however.

A personal trainer typically has a lifter alter the standard 2/4 rep speed (moving the weight up in two seconds and down in four) only for the final set of an exercise. So after using 2/4 rep speed for the warmup set and two muscle-building sets of eight-to-12 reps, the lifter reduces the weight and performs 15 to 18 (and sometimes as many as 25) reps at a “fast” pace.

“Fast” means reducing the down time by half, using a 2/2 rep speed.

This recruits slow-twitch muscle fibers to share in the work, giving the fast-twitch muscle fibers a bit of a break without allowing an aerobic one.

While there is nothing wrong with lifting this way, after you’ve been doing so for 40 years as I have, it can become old hat. So I change my mental headgear by changing the rep speed inside individual sets.

It keeps me engaged, intrigued, and intensifies my effort.

Moreover, because of my age and my desire to stay lean for bike riding, my weightlifting doesn’t need to create new muscle. It needs to maintain what I already have.

Since changing rep speed allows me to take both the fast-twitch and slow-twitch muscle fibers to failure while using light weights, this is far more likely to occur.

To explain how to safely and effectively change rep speed, picture yourself doing machine leg presses.

Don’t picture yourself squatting because of the difficulty and danger inherent in that motion. While squatting is the ultimate way to work your legs, your form needs to be impeccable.

If not, the lower back shares the effort, which increases the incidence of injury. Since your form will become loose while altering rep speed inside sets, don’t do so when you squat, deadlift or do any sort of rowing motion.

Pick a weight you could probably do 50 reps if pushed to your absolute limit. For the first few reps, do only the first half the motion and move the weight slowly, maybe 6/8 rep speed.

Gradually increase the range of motion with each subsequent rep while incrementally increasing the pace. Close your eyes and concentrate on pressing your feet forward and keeping your lower back stable.

Eventually do a few reps with a full range of motion at the typical 2/4 rep speed before shortening the motion and quickening the pace.

Notice that a specific number of reps has not yet been suggested. One will never be. That would defeat the purpose.

You need to feel the movement and learn how your body reacts to the changing of rep speed for it to lead to effective weightlifting. So I won’t set a rep target, but I will state the obvious.

A faster pace is far more demanding.

Do a few reps as fast as you can at this point, and you’ll agree. You’ll feel as if you can’t do any more.

But you can.

If you lockout the weight for a few seconds, allow yourself to catch your breath, and go back to performing slower and shorter reps.

By the end of the set like this, your fast-twitch and slow-twitch muscles will be totally spent.

If lifting until you’re “totally spent” is too intense for you, still try changing rep speed, but just notch back the effort. At a reduced intensity, the benefit is now more akin to when a walker who’s used to going three miles at a steady pace does the same distance but frequently changes pace.

More oxygen is processed. More calories are burned. More muscle fibers are engaged.

And isn’t that the desired result for all types of lifting workouts?