Progressive overload relates to an increase in our training volume on an exercise (preferably all exercises!) on a session to session basis. This is required to consistently force the muscle s to adapt and grow. To calculate our training volume for an exercise is simple… if we do 3 sets of 10 reps at 100kg that equates to 3x10x100 which is 3000kg of volume. The next training week we should then aim to increase the volume performed by a small amount for each exercise, so the next week we could increase this to 3x10x102.5kg, which equates to 3075kg of volume.
One of the most common mistakes in training for strength or muscle growth is people increasing volume too quickly by adding too much weight for a given lift. For example, if after our first week of 3x10x100kg we decide to increase to 3x10x110kg that's an increase in volume from 3000kg to 3300kg, a 300kg increase in volume. It is likely two things might happen, firstly you are going to start missing your training targets as the muscles are being asked to adapt at a rate that is unachievable, and secondly if you are looking to increase all lifts in a session that the stress and fatigue caused to the muscles and nervous system will undoubtedly impact on your ability to recover sufficiently and limit progression from session to session.
So outside of simply increasing the weight at a given rep range which is not always possible, how else can we increase volume? We can also increase the number of sets we perform or increasing the number of reps at a given load (or any combination of these things). However, if we are still struggling to increase the number of reps at a given load then we can also include things like drop sets or rest pause sets to increase our training volume.
On an important note, we can also typically perform higher volumes at lighter loads, so if our 5 rep max squat was say at 100kg, this is likely to be ‘harder’ than doing 12 reps at 50kg, yet the latter is more volume per set… this is one reason why to build muscle we need to not just focus on strength, or if we do we typically have to compensate by adding tons more sets to build adequate volume. This can be hard on the body physically, mentally and also big time commitment for most people.
When considering progressing load we also need to also make sure we are consistent with the stimulus we apply to the muscle in order to truly map progression. If you are shortening range of motion, changing tempo, ‘cheating’ reps and losing technique then your measures of progression can become skewed. That's not to say partial reps and cheat reps can't be used to create overload but this should be used sparingly and after the main work is done with consistent tempo and technique.