I've been training relatively heavy (4-7 reps) for the past year or so because of my short attention span and focus, but lately I was considering doing higher rep ranges to give my joints some rest and to go a bit more hypertrophy orientated.
The last time I did relatively heavy dumbbell shoulder press, I used 40kg (88lb) dumbbells for 5 reps, so I estimated my 12RM to be around 36kg. After warming up, I did my first set. To my surprise I could only do 7 reps before reaching failure. So I rested for 3 minutes before trying again with a lower weight. I took the 32kg dumbbells and tried again. I could only do 10 reps. I reduced my poundage by about 35lb (2.2x2x8kg) and only manage to get 10 reps with it?
The same thing happened with my chin ups. Normally I would strap 25-30kg around my waist and do about 4 reps or so, but when I tried using only my bodyweight, I can only crank out about 12 reps (the last couple with flinging legs) before my biceps gave out.
One thing I noted is that the point of failure from when doing heavy weights and lighter weights is different. When I reach failure using lighter weights, there's more burning sensation whereas when I use heavier weights, my muscles just failed me. Does that mean that I don't tolerate lactic acid well? Is it something to do with my conditioning? How do I improve this? Or am I being silly and this is actually normal?