to make it tight
To Flex in slang terms mean to "show off" If you flex your muscles, you are posing with your muscles bunched up, looking strong and dangerous - flexing is posing, bragging, showing off.
Your hamstring muscles.
You can flex skeletal muscles voluntary. These are the muscles you can control and move consciously. Smooth muscles and cardiac muscles, which work involuntarily, cannot be consciously flexed.
Hamstring
The sternocleidomastoid muscles are the ones that flex when the intercostals are flexed. This will also cause the scalene muscles to flex.
To flex your chest, you tighten the muscles that are in that area. This might include your arms or back as well, but you tighten the muscles in your chest and abs.
Muscles can contract and shorten
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You flex your muscles to do this and these pull on the tendons which link the muscles to the bones.
brechioradialis
It means he's A manslag
Biceps Brachii flexes the forearm and the Triceps Brachii extends the forearm.