Lifting weights isn't required. Actually bigger muscles can be a disadvantage at times due to the fact that bigger muscles need more oxygen to fuel them. Which means if you don't properly train you can easily gas out. You can see plenty of videos on You Tube of these big bodybuilders gassing out quick, and getting dominated by guys much smaller then them. I do lift weights as part of my training, but nothing super heavy. I use weights more in cross fit type exercises and core building.
It's not "required" but it can certainly help to build strength and stamina, depending on your routine. For example, kettlebells and Olympic-style lifts are common in MMA-oriented conditioning programs.
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Do you have to lift weights for any of these sports?
