Arms, Chest, Shoulders

Come to a hands and knees position (quadruped) on the mat with your hands directly under your shoulders; fingers facing forward, or slightly inward and knees under your hips. ?
plank position. Keep the abdominals/core engaged to brace the torso. Your head should be aligned with your spine. Your feet.
    Keep the torso rigid and the head aligned with your spine. Do not allow your low back or ribcage to sag or your hips to hike upward. Engage your butt (glutes) and thigh (quadriceps) muscles to help maintain stability and a rigid body.
   Press upward through your arms, straightening the elbows. Keep the torso rigid and head aligned with your spine. Imagine pushing the floor away from you. Do not allow.
position is to keep your fingers facing forward and your elbows close to your sides during the downward phase. This shifts the emphasis from the chest muscles onto the triceps and may reduce stresses.