Focus your core into working your arms.
The human body is naturally predisposed to assume certain shapes. For example, the infamous T-pose, wherein you stand straight up with your arms shot out to your sides in the shape of a lower-case T. This pose isn’t just for asserting dominance over your friends for TikTok, it’s also a great baseline for exercise. When you do a typical bicep curl with a dumbbell, you’re only using arm, and maybe your shoulder and chest. With the power of the T-pose, you can perform the crucifix curl, putting the power of your entire core into the motion.
This motion can be performed with a free weight, though it arguably works better with a cable machine. As long as you can do curls with it and it matches your weight of choice, though, that’s all that matters. Anyway, the starting position has you on your knees, back and posture straight. Hold one weight straight outwards in your first hand and curl it upwards, while holding your other hand out in the other direction to balance. The trick to this motion is not just to use your hands and arms, but to maintain your balance and base using your core and shoulder muscles.
As you perform curls, make sure to keep the brunt of the weight on your bicep, rather than your wrist. Keep your elbow line with your shoulder as well, because if it drifts inward or outward, it’ll move the focus away from the primary target. Hold each curl for about ten seconds, and keep going until you’ve had enough. Take a break, then switch arms and do it again. You might need to turn around if you’re doing this with a cable machine.