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Obscurin is a giant sarcomeric protein expressed in striated muscles known to establish several interactions with other proteins of the sarcomere, but also with proteins of the sarcoplasmic reticulum and costameres. Here, we report experiments aiming to better understand the contribution of obscurin to skeletal muscle fibers, starting with a detailed characterization of the diaphragm muscle function, which we previously reported to be the most affected muscle in obscurin (<i>Obscn</i>) KO mice. Twitch and tetanus tension were not significantly different in the diaphragm of WT and <i>Obscn</i> KO mice, while the time to peak (TTP) and half relaxation time (HRT) were prolonged. Differences in force-frequency and force-velocity relationships and an enhanced fatigability are observed in an <i>Obscn</i> KO diaphragm with respect to WT controls. Voltage clamp experiments show that a sarcoplasmic reticulum’s Ca²⁺ release and SERCA reuptake rates were decreased in muscle fibers from <i>Obscn</i> KO mice, suggesting that an impairment in intracellular Ca²⁺ dynamics could explain the observed differences in the TTP and HRT in the diaphragm. In partial contrast with previous observations, <i>Obscn</i> KO mice show a normal exercise tolerance, but fiber damage, the altered sarcomere ultrastructure and M-band disarray are still observed after intense exercise.