I read a Frederic Delavier’s book « Strength Training Anatomy » and I learned good stuff.
For people who don’t have vertebral pathology, arching their back during an exercise isn’t a problem. Arching a little bit the back for exercises such deadlift or squat is necessary because the spine is rounded so arching a little bit the back helps avoid injuries.
But for some people, arching the back during an exercise can be very dangerous :
This concerns people suffering from congenital spondylolysis (absence of welding of the vertebral arch). The lumbar spine’s extension may cause the vertebrae to slide (spondyloystesis), which can seriously compress the nervous elements and cause sciatica.
This concerns people who have demineralization due to age (osteoporosis) or people who haven’t yet completed their growth. The lumbar spine’s extension may cause spondylolysis by fracture of the vertebral arch. This fracture which has broken the posterior attachement system of a vertebrae can slide and seriously compress the nervous elements and cause sciatica.
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