Final answer:
The concentric phase of the bench press happens as the lifter pushes the bar away from their chest until their arms are fully extended, involving the contraction of muscle fibers in the chest, shoulders, and triceps.
Step-by-step explanation:
The concentric phase of the bench press occurs when the lifter pushes the bar upward from their chest until their arms are fully extended. This phase is characterized by the shortening of the muscle fibers, primarily in the chest, shoulders, and triceps, which are the primary muscles engaged during a bench press. While lifting the weight, the muscles are contracting against the resistance, which can lead to muscle strength gains and hypertrophy over time.