Indian Clubs are a well-established training tool that have been rightly referred to as "jump rope for the upper body.”
As anyone who has jumped rope can tell you, technique and rhythm are the most important aspects. All the stamina in the world won't help you if you don't know how to jump efficiently, and Indian Clubs allow you to tap into those benefits for the shoulders and upper body.
They also allow you to build mobility and stability in the shoulder girdle, which helps you keep you shoulders in a strong, safe position. Hours of sitting down during our daily lives and while riding result in poor shoulder posture and Indian Clubs offer a great way to combat that. Better shoulder positions will result in improved posture on your bike—which leads to less shoulder, neck and low back pain and also improves your ability to absorb trail impacts when, not if, you go down.
Last, they are fun to use and require a lot of focus to learn. They can be a real challenge to learn even the basic movements, and odds are you'll find yourself feeling like a fish out of water as you try to get your arms to move in ways they are not used to. They’ll certainly get areas of your brain working that you’re not used to—which is a good thing.
I really like Indian Clubs and have used them to help rehab from a dislocated AC joint on my left shoulder. They have a long history of use and advantages that can't be ignored. In this video I go over some of the basic drills and explain how to get started with Indian Clubs in your routine.