Final answer:
During the upward phase of a bicep curl, the biceps contract to lift the forearm while the triceps relax to allow the motion.
Step-by-step explanation:
During the upward part of a bicep curl, the biceps are contracting and the triceps are relaxing.
The biceps and triceps act as opposing muscles, coordinating movement at the elbow joint. During a bicep curl, when the biceps contract, they make the arm bend at the elbow by shortening their length. This type of contraction is responsible for lifting the forearm and any weights that might be held in the hand. Meanwhile, the triceps must relax to allow this bending motion. Without the triceps relaxing, the elbow could not bend as efficiently because these two muscles would be working against each other.
The dynamic between the biceps and triceps exemplifies how the skeletal muscles function in pairs to facilitate movement, with one muscle contracting while its counterpart relaxes. The balance of forces generated by muscles during exercise, such as the bicep curl, reveals the complexity of biomechanics and muscle physiology inherent in even simple motions.