Short Answer 
The most likely injury accompanying a lumbar spine fracture from a second-floor jump is a lower extremity injury, exacerbated by the impact of landing stiff-legged. Rehabilitation techniques, such as step training, aim to retrain spinal cord circuits to improve mobility following such injuries.
Injury Assessment
Upon landing from a second-floor jump, the most probable concurrent injury to a lumbar spine fracture is a lower extremity injury. This includes injuries such as a calcaneus fracture or ligament injuries like an ACL tear. The force upon landing is magnified due to a stiff-legged position, increasing the risk of these injuries significantly.
Understanding the Forces
When landing stiff-legged from a height, the lower extremities endure immense force, leading to a higher chance of fractures. The bones can only withstand certain force levels, and abrupt impacts can exceed these limits. Techniques such as rolling or parachuting can help dissipate this force over a longer duration, potentially reducing injury severity.
Rehabilitation Techniques
In the context of recovery for spinal injuries, rehabilitation often utilizes methods like step training. This approach involves simulating walking movements to help retrain spinal cord circuits. By sending repetitive sensory inputs, patients may improve their chances of recovering mobility, which is critical following traumatic injuries like fractures.