Malignant hyperthermia (MH) and central core disease (CCD) are disorders of skeletal muscle Ca²⁺ homeostasis that are linked to mutations in the type 1 ryanodine receptor (RyR1). Certain RyR1 mutations result in an MH-selective phenotype (MH-only), whereas others result in a mixed phenotype (MH+CCD). We characterized effects on Ca²⁺ handling and excitation-contraction (EC) coupling of MH-only and MH+CCD mutations in RyR1 after expression in skeletal myotubes derived from RyR1-null (dyspedic) mice. Compared to wild-type RyR1-expressing myotubes, MH+CCD- and MH-only-expressing myotubes exhibited voltage-gated Ca²⁺ release (VGCR) that activated at more negative potentials and displayed a significantly higher incidence of spontaneous Ca²⁺ oscillations. However, maximal VGCR was reduced only for MH+CCD mutants (Y4795C, R2435L, and R2163H) in which spontaneous Ca²⁺ oscillations occurred with significantly longer duration (Y4795C and R2435L) or higher frequency (R2163H). Notably, myotubes expressing these MH+CCD mutations in RyR1 exhibited both increased [Ca²⁺]_i and reduced sarcoplasmic reticulum (SR) Ca²⁺ content. We conclude that MH-only mutations modestly increase basal release-channel activity in a manner insufficient to alter net SR Ca²⁺ content ("compensated leak"), whereas the mixed MH+CCD phenotype arises from mutations that enhance basal activity to a level sufficient to promote SR Ca²⁺ depletion, elevate [Ca²⁺]_i, and reduce maximal VGCR ("decompensated leak").