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よくある誤解やけど長距離は筋肉落ちるとか心配するの普段ガチで筋トレしてるビルダーくらいやで
当然素人じゃそんなん心配するレベルの筋肉量はない
むしろジョギングは脚の筋肉を落とさないために医師に推奨されている

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4523889/
Historically, it has been assumed that aerobic exercise training has minimal impact on skeletal muscle mass and therefore has received little scientific inquiry compared to resistance exercise. However with the application of high resolution imaging techniques (e.g., computed topography, magnetic resonance imaging) there is a growing body of evidence that aerobic exercise training can induce skeletal muscle hypertrophy in sedentary individuals ages 20 – 80y old