Dangling modifiers?

Before you engage in strenuous aerobic exercise, you need to warm up.

Or:

Warm-up activities are necessary for anyone about to engage in strenuous aerobic exercise.

Warm up activities are necessary before engaging in strenuous aerobic exercise. (Place the subject and verb at the beginning of the sentence).

There aren't any dangling modifiers. It is a perfectly coherent sentence, and contains no ambiguities.

It is a general observation, with no overt subject, so there is no confusion over its meaning. "Before doing this, do that". Perfectly alright.