Most people believe that team sports help children to become healthier, both mentally and physically.

mentally 是形容词 mental 的副词形式，mental 来自拉丁语词根 ment-（意为"心智、思想"），加上形容词后缀 -al 构成，表示"心理的、精神的"。mentally 作为副词，意思是"在心理上、精神上"。