Weight training is one of the most beneficial forms of exercise you can and should do – aged 14 to 99.
It encourages the increase and maintenance of muscle mass which is one of the most underrated factors in health. It maintains insulin sensitivity, prevents heart disease and some cancers, and keeps older people from losing muscle mass and descending into frailty and dependence.
When you perform weight training or resistance exercises to failure your muscle fibres suffer micro tears and then at rest they repair and recruit more more fibres in anticipation of the next bout. This is called hypertrophy. Whilst at rest the body fuses the muscles fibres back together and the size .number of muscle cells increase,The key here is that the muscles must be exercised to failure. The intensity of effort is the single most important aspect. 12 repetitions banged out in 22 seconds no matter what the weight will have little effect. The muscle needs to be under constant pressure for between 45 to 60 seconds and taken to failure to illicit hypertrophy.
So if a bicep curl lifting 2kg (4lb) takes you to failure then you’re achieving hypertrophy. If it takes a 30kg weight then that’s what is necessary.
It’s not rocket science – just science.