Plank exercises are static contraction model of exercises that targets various muscle groups. Here we discuss about PLANK ON HANDS (or PUSH UP PLANK). According to me, push-up plank is the resultant of resisted closed-kinematic chain bilateral protraction of shoulders and resisted closed-kinematic chain bilateral hip flexion, using full length of upper and lower extremities and this full length being maintained by extensor muscles (Triceps, Quadriceps) of intermediate joints (elbow and knee joints) of all four extremities.
It is easy to comprehend the role of resisted protraction of shoulders with straightened upper extremities that they would lift the upper trunk (Humeral head and medial end of clavicles lift the upper trunk). But this mechanism alone can only give us a cobra pose (yoga- Bhujangasana) with sagged low back, to stretch the abdominal muscles. In this situation, the distance between the xiphoid process and the pubic symphysis increases. But as the rule of PLANK, the distance between xiphoid process and the pubic symphysis must be kept neutral and that should be endured for some period of time. Ultimately, pelvis has to be lifted up and that is possible with bilateral hip flexion with toes (specifically the metatarsal heads) contacting the ground. To perform this task, Iliacus and Psoas major being the agonists of hip flexion, tend to lift the pelvis using the femoral heads (in this posture, closed kinematic chain hip flexion will result in femoral head going upward instead of femoral condyles moving downward). This hip flexion agonists are reinforced by quadriceps to keep the knee extended, so that the entire length of lower extremity can participate in this pelvis lifting maneuver. During this maneuver, the proximal attachment sites (iliac fossa & lumbar vertebrae) of hip flexion agonists must be stabilized to prevent anterior tilt of pelvis and exaggerated lumbar lordosis. The role of abdominal muscles becomes crucial here to prevent these unwanted effects of Iliacus and psoas major. Recruitment of abdominals:
(1) stabilize the pelvis and lumbar vertebrae
(2) ensures the neutral distance between xiphoid process and pubic symphysis
(3) enables the hip flexors to efficiently push the pelvis upwardly using femoral heads.
Finally, plank posture is maintained by co-activation of Deltoid, Triceps, Pectorals, Serratus anterior, Abdominals, Psoas major, Iliacus, Quadriceps. In fact, the same kinetics and kinematics must occur during dynamic push-ups.