Seems like "between your thumbs" would apply to a "core" muscle group weakness, the erector spinae. I don't know if you've taken anatomy or not but is that what you mean?
Erector Spinae Weakness
Increased risk of lower back injury during lumbar spine extension or stabilization activities. Back extension exercises involving complete lumbar spine range of motion have demonstrated primarily excellent or good results for those with chronic low back pain. Excellent or good results by diagnosis: 76% Mechanical / Strain, 72% Degenerative, 78% Disc Syndrome, 75% Spondylo.
Nelson, B.W. (1993). A rational approach to the treatment of low back pain. J Musculoskel Med, 10(5), 67-82.
Nelson, B.W., O'Reilly, E., Miller, M., Hogan, M. Wegner, J.A., Kelly, C., (1995). The clinical effects of intensive, specific exercise on chronic low back pain: a controlled study of 895 consecutive patients with 1-year follow up. Orthopedics, 18(10), 971-981.
Examples of affected exercises
Straight Leg Deadlift
Squat
Deadlift
Example preventative / corrective exercises:
Back Extension
Cable Row (with spinal articulation)
Stiff Leg Deadlift
I pulled this from a website:
**broken link removed**
I would probably group the reverse crunch in with the group of preventative/corrective exercises for this weakness if the erector spinae is what what you're trying to work. This is a common problem among bodybuilders since alot of them work abs but do not balance it out by working lower back, or at least, I haven't seen people work on them very often. This can cause an instability/imbalance in strength between the two muscle groups that cause frequent lower back problems.
Working your erector spinae would end up making your lower back thicker, not wider or maybe it would. If you're asking what I think you're asking, you may be going back to the discussion about being able to target part of a muscle (lower latissimus dorsi?) vs only being able to work the muscle as a whole. If you are in belief of the former camp, you'd probably end up doing a partial lift of one of your widening exercises or tweaking an arm rotation at the beginning or end of those exercises.
I don't know my kineseology that well but if you find someone that's good with it and you're in the camp that believes in targeting part of a muscle, you'd be asking them about which end of the glenohumeral adduction articulation applies to the portion of the latissimus dorsi that you're thinking of.
I know of a personal trainer to the stars here in LA that swears by one of his tricep training methods, a medial forearm rotation at the end of one-arm kickebacks. The reason that works is because triceps brachii are made up of three heads. Under that theory, that wouldn't work in the case of the latissimus dorsi muscle since there is only the single head. That's where the people in the other camp, (only being able to train a muscle as a whole) would blame genetics. e.g. you can't train your "lower" latissimus dorsi. you'd have to just cross your fingers that training your latissimus dorsi as a whole will give you the shape you want.
I don't think I've seen that debate in this particular forum, as yet (the ability to training part of a muscle vs only being able to train a muscle as a whole). I'm sure a few of you have very set ways on what you think of it.
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I wouldnt gain any width in your lower back. Where your thumbs are at is your waist why gain any width there, it would make your waist look bigger and wider. I think you would wanna keep that tiny little waist it makes everything bigger and wider. Lower back thickness yes, lower back width no. switch to a underand lat pull down to pull those lats down, and do partial deads, to make your lower back thicker, but keep that waistline small you have that going for you, dont ruin that, it would make you look less aesthetic and more blocky, you have that pretty build going for you, keep it.
