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For now I would avoid any stretch or end range lumbar flexion if you are experiencing numbness in the calf/shin area when performing those movements. If it feels sketchy its sketchy! The low back and calf numbness sound like symptoms of the sciatic nerve. The sciatic nerve runs off the lumbar spine down the posterior thigh and leg all the way to the foot. Unlike muscles, nerves do not like to be stretched or tensioned. Instead, nerves are supposed to glide or slide within soft tissue nerve beds. These impaired neurodynamics or soft tissue restrictions limit nerve sliding and instead tension the nerve. This can generate symptoms of pain, numbness, and tingling, anywhere along the nerve’s sensory distribution.
Here are some routines to try for the tight TFL and some neurodynamic drills: