Cheating has no clear-cut rules because it varies from one exercise to another and from one individual to another. In general, you want to be able to complete 6- 8 reps on your own with textbook technique before performing cheat reps. Here are some exercises that are either good, decent or bad candidates for cheating. Note: If you can't do even a single rep with good form, any move would be considered dangerous.
5 Best Exercises for Cheating
Experienced bodybuilders can add a few cheat reps to the end of a set as long as the cheating motion is intentional and distributed as evenly as possible over several joints. Remember, heavy weights still increase your risk of injury if your form is poor.
1) Lying Leg Curl
Raising your hips off the bench makes the move easier because it recruits the lower back and glutes, and using a faster rep speed generates momentum.
2) Calf Raise
Use a little bounce at the bottom, a smaller range of motion or speed up the rep.
3) Dumbbell Lateral Raise
Extend your hips and knees slightly to create momentum. (When you do this same move seated, you're essentially unable to generate the same degree of body english, making the seated version far more difficult to cheat on.)
4) Seated Cable Row (or Front Pulldown)
Strict form requires you to keep your torso locked in the upright position, but leaning back slightly as you pull recruits your low-back muscles to help generate momentum.
5) Alternating Dumbbell Curl
When switching arms, whether seated or standing, you can lean into the working-side arm to assist in the lift.