My main sport is windsurfing and mountain biking is my no wind alternative for fitness and fun. High wind windsurfing uses nearly every muscle in your body with a big emphasis on core muscles. I always found MTB to be largely about...or at least limited by lung capacity and quads strength and fitness. My limited experience so far of E MTB is that those limitations no longer apply and a full ride becomes much more of a full body workout with exercise more evenly spread across a wider range of muscle groups. At my age (68) that means a e mtb ride is not only more fun than MTB but more beneficial .