The iso-hold allows you to keep tension on your shoulder muscles longer, which helps spur growth. And since your lower body is moving up and down, your stabilising muscles have to work harder to keep your arms steady. The faster you go, the harder this will be.
There are several ways to use iso-shoulder runners. You can plug it into a total-body circuit. Or you can do for 20 seconds and then rest 10 seconds. Repeat that 5 to 10 times in a row.
Use both weights at once, or go with a single weight in one hand to challenge your opposite obliques.
You can also mix up where you hold the weights – in front of your body, out to your sides, or overhead – or stick to one position the entire time.