You don't have to lift heavy weight to get stronger. You can use a lighter weight, but you have to go faster-- like sprinting. The more work that is produced in less time will increase strength. Try this: Use 60% of your maximum and then perform 8 sets of 8 reps, doing the reps as fast as possible. Rest enough time to take in 6 breaths and then fire off another set. It will be a struggle to complete all 8 sets.
Daryl Conant, M.Ed