There was a day, not that long ago, when wearing ankle and wrist weights was anything but cool. But now, thanks to stylish designs, celebrity endorsements, and a love for workout efficiency, running with weights is making a comeback. Here's what you need to know about adding weights to your run — the benefits and risks, and how to do it both safely and effectively.

Runners have a bad rap for avoiding strength training. They like to run, so that's what they do. However, resistance work is one of the best things anyone can do to improve their running performance — not to mention reduce their risk of injury. And for people with body composition goals, strength training can support levels of lean muscle mass.

But strap on a pair of ankle or wrist weights, and running automatically becomes a super-efficient cardio and strength workout, right? Yes and no.

The benefits of running with weights

1. It can strengthen your muscles

The main benefit to adding weights is that it makes your muscles work against extra resistance as you run. The result: greater muscle activation and growth. Which muscles the weights target depends on where you wear them.

• Ankle weights will target your glutes, hamstrings, quads, and hip flexors.
• Wrist weights will target your biceps, triceps, shoulders, back, and core.
• Weight vests will target your glutes, hamstrings, quads, hip flexors, back, and core.

2. It can increase your exercise intensity

Wearing weights also places an extra challenge on your cardiovascular system. The greater the demand you place on your muscles during exercise, the more blood and oxygen they need to do their thing — and the faster and harder your heart and lungs have to work to meet your body's nutrient and energy needs.

Wearing a fitness tracker with a built-in heart rate monitor can help you gauge your workout intensity. Remember, the more intense your workouts, the shorter they will need to be.