What exercise complements the Jackknife in terms of back extension?

The exercise that complements the Jackknife in terms of back extension is Grasshopper. In the context of Pilates or similar practices, the Jackknife primarily focuses on engaging the core while also emphasizing back extension through the movement of raising the legs and torso from a supine position.

Grasshopper serves as a synergistic exercise that enhances back extension while also incorporating elements of stability and control. It targets the extensors of the spine, similarly to the Jackknife, but it adds different muscle engagement by requiring the practitioner to maintain balance and lift from a prone position. This results in a well-rounded development of the back muscles, reinforcing the principles of strength and flexibility that are essential in Pilates practice.

The other exercises listed have different focal points. Long Box Extension primarily emphasizes spinal extension but does not directly relate to the core engagement aspect found in the Jackknife. Mermaid targets lateral flexion and rotation rather than pure extension. Corkscrew focuses more on the hip and core, with less emphasis on back extension. Therefore, Grasshopper is the most complementary exercise to the Jackknife regarding back extension.