When lifting weights try and go slower and more controlled with lighter weights for more muscle mass.
With slower repetition speed you effectively increase intensity of the lifting (concentric) phase while decreasing momentum. While momentum may allow you to lift bigger weights, it basically reduces target muscle stimulation and intensity while also increasing your chances for injury. Researchers in Massachusetts, USA and in collaboration with the YMCA, looked at this phenomenon on middle-aged men and women (mean age 53). They divided these previously untrained people into two groups, all performing 2-3 days per week training in this 8-10 week program.
One group was performing weight training at normal repetition speed (7 seconds) while the other was performing it at slow speed or 14 seconds. To ensure that muscle time under tension was constant, the normal speed group performed 8-10 reps while the slow speed group performed 4-6 repetitions. They conducted two studied and in both, the slow speed group increased strength significantly more than their faster speed counterparts. In study one, the slow speed group showed a mean strength increase of 12 kg while the normal speed group showed an only 8 kg improvement. In the second study, the slow speed showed a 10.9 kg increase while the normal speed showed an increase of 7.1 kg.
This study shows the importance of repetition speed when performing resistance training and further cements the idea that weight must be lifted in a fully controlled manner. This holds especially true when training for strength increases in older adults.