Joel Harper demos the pecs flex.

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Pecs Flex
(Stretches chest and arms)

Sit up straight with your shins and feet tucked under your thighs, and lace your fingers behind your butt, while keeping your arms straight. Lift your hands up, knuckles facing back, while opening your chest wide. Squeeze your shoulder blades together to open up your chest more. You can also interweave your fingers behind your head and pull your hands away from your head. Keep face forward and chin up.