An important role as a personal trainer is to not only correct my clients' training mistakes, but to share the information I know about how the body moves and what the body is capable of.
Sometimes, a client keeps asking me to add more, and more, and more weight to an exercise before we have established proper form. Other times, a client is convinced once they feel the weight in their hands that I'm asking them to lift too much. It's my job to help clients identify the difference between what is hard and what is harmful.
The guy who's jerking heavy dumbbells up to his shoulders, arching his back and then swinging the weights back down by his side is not only exhibiting terrible form, but he's not adequately targeting the bicep muscles, and is at serious risk of injuring his back or shoulders. I coach this person to properly isolate the bicep muscle through controlled movement that focuses on the eccentric (unloading) phase of the bicep curl to create optimal muscle recruitment so that when he does his bicep curls, he's not relying on momentum generated elsewhere in his body to complete the exercise.
Conversely, when someone lifts the weight properly but quits after five repetitions because the muscles are starting to ache, they are missing out on challenging the muscles enough to see any notable improvement in strength or fitness. I must encourage this person that when the body works to repair itself after a challenging workout, the muscle fibers thicken and strengthen as the body heals. This is what creates more muscle mass, which will boost metabolism and help change body composition. But only when enough weight is lifted that the client feels challenged between 8-20 reps.
My goal for my clients is to make sure they leave every workout feeling capable and confident. Having a sound understanding of the mechanics of movement and a good eye for proper form are how I make sure my clients both feel and see improvements to their fitness!