To effectively regress the advanced Mat exercise Boomerang for a client lacking full hamstring flexibility and spinal articulation, what specific modification to the leg position during the rolling phase would best maintain core engagement and safety?

To effectively regress the advanced Mat exercise Boomerang for a client lacking full hamstring flexibility and spinal articulation, the specific modification to the leg position during the rolling phase would be to maintain bent knees, drawing them closer towards the chest, similar to a Rolling Like a Ball position. Boomerang is an advanced Pilates Mat exercise involving rolling the spine back and forth, leg changes, and balance. Hamstring flexibility refers to the capacity of the muscles at the back of the thigh to lengthen, which is essential for maintaining straight legs in this exercise. Spinal articulation is the ability to move each individual vertebra independently and smoothly, vital for controlled rolling. This bent-knee modification directly addresses the lack of hamstring flexibility by eliminating the need for straight legs. Crucially, it best maintains core engagement and safety by shortening the lever arm of the legs. Core engagement is the activation of deep abdominal and trunk muscles to stabilize the body. With bent knees, the mass of the legs is brought closer to the body's center, significantly reducing the muscular effort required from the core to control the momentum of the roll. This allows for greater abdominal activation to achieve and maintain the necessary spinal flexion and C-curve. The shortened lever also promotes safer spinal articulation by reducing the load on the lumbar spine, enabling the client to roll with more control and segmentation, rather than as a rigid block. This prevents uncontrolled movements that could lead to strain or injury to the neck or back.