A common cue when squatting and performing other heavy, compound lifts is to "keep your abs tight" or "flex your abs".
Yes, it is true that flexing your abs will increase the stability of your lumbar spine due to cocontractile forces, but that reason alone does not mean it should be done. Part of an evidence-based practice means weighing the risks vs the benefits, and the risks of this commonly advised task are often overlooked.
"However, in light of other research, conscious co-activation of the trunk muscles during the squat exercise may lead to spinal instability and hazardous compression forces in the lumbar spine" (Bressel et al., 2009).
Not only that, but this coactivation will cause the muscles involved to fatigue faster (Gardner-Morse & Stokes, 1998).
Bressel et al. (2009) also supports these findings. Furthermore, Stokes et al. (2011) found that 20% abdominal activation provides no more stability than 10%.
Rather than flexing your abs, it may be more beneficial to concentrate on creating a "big stomach" (pushing your stomach out) through diaphragmatic breathing. This will create more intra-abdominal pressure which will provide up to 31% more support to the lumbar spine compared to resting levels (Hodges et al., 2005). - (cut and paste from
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