The answer is they are both great, and here's what you should know. When you lift using dumbbells, you have to lift as well as balance which causes your smaller stabilizing muscles to work harder. Machines, on the other hand, keep the weights steady for you, so you don't work your smaller stabilizing muscles as much, but because the machines do the stabilizing for you, you are often able to lift more pounds than with free weights and therefore bulk up faster. Machines make it possible for you to work your larger muscles more. In my opinion it's best to do both. I love the machines for ease of use and stability and form, but I also use dumbbells so that I don't neglect my smaller stabilizers. Keeping your smaller stabilizer muscles fit helps to prevent injury because they are the support network for the larger muscles.
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