The muscle strain in his calf slowed down his run.

muscle 指的是“肌肉”，是人体或动物体内能收缩产生动力的组织。 strain 是名词，表示“拉伤、扭伤”，指肌肉或韧带因过度用力而受伤的情况。 muscle strain 合起来就是“肌肉拉伤”，指肌肉因过度拉伸或用力而受伤。