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 Abdominal exercises also put some degree of compressive force on the lumbar spine, putting unwanted stress on the lower back. In addition, exaggerated abdominal exercise can cause respiratory problems.[7] A study of twelve exercises concluded that no single exercise covered all abdominal muscles with high intensity and low compression.[8] - High challenge-to-compression ratio
- Crunch with feet anchored
 - Crunch with feet free
 - Bicycle crunch
 - Hanging straight leg raise
 
 
  
 
  | - Low compression, lower challenge
- Crunch with feet anchored
 - Crunch with feet free
 
 
  
 
  |  - High challenge, higher compression
- Straight-leg sit-up
 - Bent-leg sit-up
 
 
  
 
  | - Low challenge-to-compression ratio
 
(not recommended!)- Supine straight-leg raise
 - Supine bent-leg raise
 - Hanging bent-leg raise
 - Static cross-knee crunch
 
 
  
 
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 The benefit of focused training on the "deep core" muscles such as the transversus abdominis has been disputed, with some experts advocating a more comprehensive training regimen.[9] [edit] 
 
 
 
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