Moreover, irisin increased bone mass in healthy mice [22] and, in hindlimb unloaded mice, a well-known osteo-sarcopenic mouse model of disuse-induced osteoporosis and muscle atrophy, irisin promotes osteogenesis, increases osteocyte viability, and prevents disuse-induced loss of bone and muscle mass [17,23]. Here, FNDC5 is linked to osteoporosis.