Although it may seem that if you are working to failure according to the table, you are using the right weight. That may not be the case. In fact, your muscles may have accommodated to a point and may need to be shocked.
To select a weight, increase the weight until you cannot complete the minimum number of repetitions you are looking to do. Then back-off by one increment. Look to increase the weight as the number of To-Failure reps increases.
Never work with weights you can't control. If you need a spotter, get one or select a different exercise.
Regardless of a muscle's ability to complete reps, your supporting structures - bones, joints, ligaments, tendons - may not be up to the task. Listen to your body. Know the difference between simple aches and damage. If you have doubts, see a trainer or physician.
If you are working sub-maximally, you should still be challenging your muscles.
Free Weights, Exercise Machines, Cables
Each method has its advantages and drawbacks, which are summarized below.
Equipment Comparison Chart
Equipment
Pros
Cons
Barbells
Involves dynamic balance; permits unconstrained movement and complex motions involving multiple muscle groups.
Often requires spotter; can limit range of motion; allows one side to carry the load for the other. Can present varying load over arc of exercise.
Dumbbells
All of barbell advantages, plus; allows extended range of motion; isolates each side; typically doesn't require spotter.
Places a lot of demand on the wrist (this may be seen as an advantage); usually decreases total amount lifted for a given exercise; can present a varying load.
Cables
Provides steady load regardless of direction of work; permits working with gravity - high pulleys; good muscle isolation; safe.
Sometimes difficult to maintain; motion is constrained to machine configuration, beneficial complex motions are not available.
Exercise Machines
Safe; easy to adjust load; isolates muscles; no spotter necessary
Motion is highly constrained; unnatural motions can result from improper setup; dynamic balance is not employed or reinforced.
