If possible, I don't want to tire my muscles out needlessly.
I want my muscles to get sufficient rest.
I want to devote my muscles to their training exercises and otherwise use them as little as possible.
ご質問ありがとうございます。
① "If possible, I don't want to tire my muscles out needlessly."＝「可能であれば、筋肉を無用に疲れさせたくありません。」
② "I want my muscles to get sufficient rest."＝「十分な休憩を筋肉に取らせたいです。」
③ "I want to devote my muscles to their training exercises"＝「筋肉をトレーニングに専念させたいです」
"and otherwise use them as little as possible."＝「そしてそれ以外は出来るだけ使わないようにしたいです。」
ご参考に。
アメリカ合衆国 
