AquaJogger DeltaBells Pro Heavy Resistance Water Weights

I'm a 66 year old woman with knee problems. Ten years ago I used to be fiercely fit but a few knee and ankle injuries and plain old age have slowed me down.For the past 18 months, I've been working out in my condo pool 3-5 times a week, doing a range of aqua fitness moves. (Jumping Jacks, jogging, cross-country ski etc.) At first, I was using webbed gloves. Then I moved to the medium-sized DeltaBells. After that, I jumped to the giant AquaFit Pro Bells (also made by AquaJogger) but they were not rotator cuff friendly for me! So I compromised on these. Maybe it's because I have good muscle memory and some residual strength, or maybe it's because I have worked up to these over 18 months, but I don't find them as challenging as I would have liked. They're great for building endurance if I move quickly enough but strength? Not so much. That said, I recognize that I am not typical in the strength department -- and I must add the quality, like all AquaJogger products, is impeccable.