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The amount of resistance in dumbbells is easily figured out by using the number printed on them and taking into account your body weight. The weight of resistance machines is a bit more complicated, as it's affected by the length of levers, pulleys and more.
But if you're making the switch from dumbbells to weight machines, here's how to figure out how much weight you should be lifting.
Dumbbells vs. Weight Machines
Dumbbells, or free weights, are a convenient way to begin lifting weights at home. You're able to perform a variety of exercises using dumbbells that more closely mimic everyday movements, allowing you to focus on functional fitness.
You can get started using just one or two sets of dumbbells, available at sporting stores or some department stores. A significant drawback in using dumbbells is that, as your fitness improves, you will need to invest in heavier weights.
Machine weights, on the other hand, have the significant advantage of providing a pattern of movement that helps you maintain better form. Machines also help you to increase your lifting weight by increasing the resistance on the machine, without spending additional money on dumbbells.
In fact, machines may allow you to increase weight significantly in a way you could not without help from a spotter. Machines, however, require monthly gym memberships or a higher upfront cost to purchase a home machine.
Understanding the actual resistance you are training with at a given time is helpful in determining a progressive training plan. Lifting 10 pounds in dumbbells is not the same as lifting 10 pounds on a weight machine. One reason for this is that when performing exercises with dumbbells, your body also provides resistance.
To calculate the actual resistance of a dumbbell exercise, add the weight of the dumbbells to your body's weight. For example, if you are a 150-pound person lifting 20 pounds of dumbbells, the actual resistance is 170 pounds.
To convert the resistance used in dumbbell exercises to those performed with a machine assisting, the principle of adding weight to calculate the resistance is reversed. A machine-assisted exercise requires that you subtract the weight selected on the machine from your body weight.
For example, if you're performing a machine-assisted pull-up, select a weight of 50 pounds, and you weigh 150 pounds, your true resistance is 100 pounds.
Machine Weight Conversion
Typical weight machines, with the exception of the assisted weight machines, calculate resistance based on the resistance provided by the cumulative weight of the weight plates themselves, pulleys, attachments and levers.
The simplest way to convert dumbbell weight to machine weight is to take the weight of the machine at its stated amount. That is, if the plate says 10 pounds, consider it as equivalent to 10 pounds of dumbbell weight.
However, you may also calculate resistance using lifting trials, knowing that your body increases the resistance using dumbbells. For example, if you can lift 200 pounds for 4 reps on the machine, while your 4-rep max with dumbbells is 150 pounds, then 10 pounds on the machine would equal approximately 7.5 pounds of resistance.
Each manufacturer and piece of equipment will have differing conversion formulas, with resistance changing based upon the true weight of each plate, the length of arms, the number of pulleys, and any hooks or straps on the machine. Contacting the manufacturer directly, you can obtain their conversion formula to get the most accurate conversion.