Cheat reps are a common occurrence in gyms across the country. Some people believe that cheat reps have no place in anybody's program, whilst others believe that utilizing cheat reps can actually be beneficial. Truth is that in some ways, both groups are right.
The first thing we need to do is establish what exactly a cheat rep is and what it isn't.
A cheat rep is a repetition of an exercise that is performed without perfect form, so if perfect form equals 100%, and good form is around 85-99%, then a cheat rep would be somewhere between 75-85%
This is the point where that perfect bicep curl has begun to look a little bit like an arm swing, but from a distance it could still be called a bicep curl.
A cheat rep is not an excuse to lift with terrible form, a hunch-backed deadlift is not a cheat rep, it's a liability. Instead a cheat rep for a deadlift might be bouncing the weight slightly between reps to add a little momentum.
The Argument For Cheat Reps
Adding cheat reps in to your workout can work so long as they are added in properly, for example let's say that you are performing three sets of eight reps on the bicep curl. You get to the last set and you can perform all eight reps with great form, even though the last one was a struggle.
You've used a load of muscle fibres to complete the exercise but deep down you know that there's still something left in the tank.
Adding four or five extra reps, even if the form wasn't perfect would undoubtedly require more muscle fibres.
More fibre activation leads to more muscle damage, which leads to greater hypertrophic/strength gains after the workout.
Now obviously you would want to limit the amount of cheat reps per set, and you wouldn't want your form break down too far!
Performing a squat where you hardly bend your knees and calling them cheat reps isn't a good idea. This will actually activate less muscle fibres and the limited ROM could lead to joint issues and injury.
Exercise techniques such as drop sets and negatives (eccentric training) would be a lot less effective if you had to perform perfect reps.
Technically, all eccentric training could be classified as cheat reps. As you are using momentum or a partner to perform the concentric part of the movement whilst only concentrating on the eccentric.
Clearly there are many benefits to eccentric training: It can help produce more force whilst producing less fatigue , leads to greater hypertrophy than regular training , and can actually lower the risk of injury in sports .
If you treat all cheat reps as forms of eccentric training (lowering the quality of the concentric whilst maintaining or even slowing down the eccentric) you can expect some pretty decent results.
The Argument Against Cheat Reps
Humans are notoriously bad at self-reflection, people think that they are more intelligent, more attractive, and more entertaining than they actually are.
People are also very bad at estimating calories burned or calories consumed.
Therefore it is quite possible to believe that a person performing cheat reps would wildly underestimate how bad their reps had gotten. This means that they could potentially be placing themselves at risk of injury, or lowering the effectiveness of their workout.
Another issue is that once someone begins to break form it could negatively affect future performance.
Once you start performing sloppy bicep curls it might be difficult to start over again next session with perfect ones. Obviously the more disciplined lifters won't have this problem, but what about your regular gym goer?
Also while eccentric training and drop sets are very effective, there is also another way to maximise fibre activation that involves perfect form.
Back off sets. This is where you perform your regular allotment of sets and reps (let's say 3 sets of 6 reps at 85% of your 1 rep max), then you have a rest period of around 2-3 minutes and perform a fourth set with only 60-70% of your 1 rep max. But instead of performing 6 reps you attempt to perform 12 or more.
A 2004 study looking into the benefits of back off sets found that they led to increased hypertrophy, increased strength, and a huge increase in muscular endurance (which usually lowers after a strength training program) .
As you've had that rest, you can concentrate on perfect form. But because the weight is so much lighter you can aim for much more reps increasing muscle fibre activation (in a similar way to drop sets).
If used correctly there is absolutely nothing wrong with cheat reps, particularly if you class eccentric training as cheat reps.
However, back off sets seem to be a more elegant solution that provides the same benefits but in a more controlled and safe manner.
Experiment with both and see which one suits you personally. Or possibly find a way to fit both into your program.
 Kelly, S., Brown, L., Hooker, S., Swan, P., Buman, M., Alvar, B., Black, L. 2015. Comparison of concentric and eccentric bench press repetitions to failure. Journal of Strength & Conditioning Research 29(4): 1027-32
 Pope, Z., Willardson, J., Schoenfeld, B., Emmett, J., Owen, J. 2015. Hypertrophic and Strength Response to Eccentric Resistance Training with Blood Flow Restriction: A Pilot Study. International Journal of Sports Science & Coaching 10(5): 919-931
 Petersen, J., Thorborg, K., Nielsen, M., Budtz-Jorgensen, Holmich, P. 2011. Preventative effect of eccentric training on acute hamstring injuries in men's soccer: a cluster-randomized controlled trial. American Journal of Sports Medicine 39(11): 2296-303
 Goto, K., Nagasawa, M., Yanagisawa, O., Kizuka, T., Ishii, N., Takamatsu, K. 2004. Muscular adaptations to combinations of high- and low-intensity resistance exercises. The Journal of Strength & Conditioning Research 18(4): 730-737