Energy analysis investigated the mechanisms of deformation during uniaxial and biaxial extension of plain weft knit fabrics. Redistribution of yarn curvature had the greatest effect on fabric extension and resulted in greater extensibility in the coursewise direction than in the walewise extension. Structural jamming yielded an initial prestress in the direction of extension. Jamming normal to the direction of extension increased fabric modulus at high loads. Jamming had less effect in biaxial extension than in uniaxial extension. Yarn compression increased knit fabric extensibility and reduced the amount of curvature in regions of loop interlocking.