A simple statement: You should always train to 'form' and not to 'failure'.
When you train in the gym, how do you know when enough repetitions are enough? I see people really struggle with the last couple of reps all too often and when they struggle they lose form and their body goes into compensation.
The compensation means that the muscles being used for the exercise can no longer survive the load. Your body then recruits other muscles as part of its' survival mechanism, because if it didn't and you were in a survival situation that would be game over for you... now that would be a pretty poor survival mechanism.
It is this compensation when form is lost that causes all the problems when training.
Your body remembers the last 2 repetitions of an exercise (I will go into the science of this another time). This form on the last 2 repetitions is then more likely to be repeated on future attempts.
The real problem is that this reinforces posture: Bringing your head forward when completing the last 2 reps of a bicep curl can encourage a forward head posture.
Another major problem is that it can affect the delivery of sport specific skills: Over recruitment of the pec minor muscle when performing the last 2 reps of shoulder exercises usually leads to poor muscle recruitment of the shoulder complex during skills. That's your pec minor muscle pulling in more than it should on a golf drive, leading to excessive fade or a tennis serve that goes astray. The list is endless.
What's the solution?
It's actually really simple at first glance. Always train to form and never failure.
Of course you need to know what good form is and the use of a spotter to help with this is invaluable.
You might find that your lifting weight is now lower than you thought. Good, you can now stop wearing away your joints, stand taller through good posture and stop developing faulty movement patterns for your sport.
Is the person in this picture demonstrating good form?