Kettlebells can help you gain muscle and cardio fitness at the same time. Here's what to know, according to a world-record athlete.

• Kettlebell exercises offer efficient full-body workouts, building strength, endurance, and speed.
• The unique design of kettlebells enhances both cardiovascular fitness and muscle strength.
• Kettlebell training can make workouts enjoyable and offer longevity benefits like better stability. 

If you're not using kettlebells in your workouts, you might miss out on major gains.