More on point two: know when stable enough is stable enough. You don’t need to be cargo strapped onto the machine/bench in 3 different places. Just use a setup that doesn’t leave you focusing too much on having to stabilize yourself

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More on point one: important things are resistance profile of the movement and the biomechanics of it. For example, to emphasize the triceps in a press, an ascending resistance profile is optimal, as most of the effort in the top end of a press is in elbow extension.

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