Muscle damage is proved to occur after eccentric training (Clarkson, 2002). Moreover, it confers protection to a subsequent session, i.e.: repeated bout effect (Nosaka 2001). A new technology (YoYo technology) has been developed in order to enhance negative phase using inertia (Tesch 1994). Aim of the study is evaluate if lengthening contraction enhanced by inertial flywheel squat will cause symptoms of muscle injury and following protection. Twelve healthy males performed 100 maximal squat repetitions using inertial device. At baseline, after training and up to 4 days after CK blood concentration, Knee extensors strength, muscle soreness and jump performance were measured as markers of muscle damage. Same protocol was repeated after 3 weeks. Statistical analysis was performed using repeated measures ANOVA, using factor Session and Day. No significant Session X Day interaction occurred in all parameters. Single factor Session and Day were significant except in [CK]. Post hoc analysis revealed, compare to baseline and after first trainng, [CK] and soreness increments respectively up to 3 and 4 days, while strength loss and jump height were affected only post training. The second bout revealed significant markers decrements compared to the first one in CK (-50% and -43% after 2 and 3 days), muscle soreness (-2,8; -2,6; -3,4; -2,6 A.U. respectively after 1, 2 ,3 and 4 days) and strength ( 12%; 9%; 15%; 15%; 14% respectively immediately after training, after 1, 2, 3 and 4 days). Inertial flywheel squat induce classical symptoms of muscle injury, even if strength loss and performance could be positively influenced by neural patterns. CK blood activity is relatively lower compared to single joint exercise because of load is divided among more muscles. However, eccentric overload does protect muscle at least up to 3 weeks. Trainers and conditioners should take in account the recovery time course after eccentric inertial squat. Clarkson PM et al., Exercise Induced Muscle Damage in Humans. American journal of physical medicine & rehabilitation, 2002, 81(11)S: S52-69 Nosaka K et al., How long does the protective effect on eccentric exercise induced muscle damage last?. Medicine & Science in Sports & Exercise, 2001, 33(9): 1490-95 Tesch P et al., A gravity-independent ergometer to be used for resistance training in space. Aviation, space, and environmental medicine, 1994, 65(8): 752-6