I went horseback riding yesterday so my stomach muscles are sore today.
I was riding a horse yesterday and my stomach muscles are killing me today.
この場合、次のような言い方ができますよ。
ーI went horseback riding yesterday so my stomach muscles are sore today.
「昨日乗馬に行ったので、今日はお腹が筋肉痛です」
to go horseback riding「乗馬に行く・乗馬をする」
one's stomach muscles are sore「お腹が筋肉痛だ」
ーI was riding a horse yesterday and my stomach muscles are killing me today.
「昨日乗馬をしていて今日はお腹が筋肉痛だ」
to ride a horse「馬に乗る」
to kill で「人に苦痛を与える」を使っても言えます。
ご参考まで!