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The Iliopsoas (internal hip flexors) have their origin in the lower back. If you have lower back issues, be very careful when doing hip-flexing exercises like sit-ups. Crunches work the abs more in isolation
A normal sit up imposes in excess of 700lb of compression on your lumbar spine or around 3300N. Its interesting that the NIOSH limit for lumbar compression is 3300N! Ideally if you are wanting to train the anterior abs then avoid any lumbar flexion. Performing a curl up will do this. By the way there is no such muscle as a iliopsoas. Psoas and iliacus are two seperate muscles in structure, function and neurally. Because psoas attaches to T12 and every lumbar vertebrae it should not be viewed as a spine stabiliser except perhaps during excessive hip flexor torque. Activation of psoas will place considerable compressive load onto the spine and bending the knees to avoid it, or anchoring the feet doesnt change this.
There aren't any such muscles as the quadriceps or hamstrings, either. The provided link clearly distinguishes between the iliacus and the psoas. But they both function as hip flexors.Otama wrote: By the way there is no such muscle as a iliopsoas. .
I had no idea that sit-ups imposed that much pressure on the spine, though. My back hasn't given me many problems (knock on wood!), but I'll keep that little nugget in mind going forward
My point about the psoas to put it more clearly is that most people view iliopsoas as a muscle, mainly due to texts and other references failing to keep up with research, whereas most would designate hamstrings or quadriceps as a muscle groups which is why they are usually described in the plural. Yes sit ups are nasty and really shouldnt be prescribed to anybody unless specific performance effects are required via such an exercise.