Kinesiology of the Trunk

Kinesiology of the Trunk:

by Brent Brookbush DPT, PT, MS, CPT, HMS, IMT

Prime Mover : The muscle that can produce the most force for a joint action. Generally, this is the muscle that will generate the most force during a joint action when exerting force against an external load.
Agonist : Muscles that perform a joint action, including the prime mover and synergists. That is, unlike the terms prime mover and synergist, agonist refers to all of the muscles that can perform a joint action.
Synergist : Synergists are muscles that assist the prime mover in performing a joint action. By definition, all agonists that are not the prime mover are synergists.
Antagonist : Muscles that oppose the agonists (including the prime mover and synergists) for a given joint action. That is, all of the muscles that can perform the opposing joint action.
Neutralizer : During a joint action, neutralizers are muscles that oppose the unwanted or ancillary joint motions that result from the force generated by the prime mover and/or synergists.
Fixator : Fixators are muscles that act to reduce or prevent movement of joints proximal to the joint in motion.
Stabilizer : The role of a stabilizer is to improve arthrokinematics by maintaining optimal alignment of joint surfaces during joint actions. Most often, these muscles are the most intrinsic muscles of a joint. For example, the muscles of the rotator cuff of the shoulder.

Trunk Flexion

Prime Mover: Rectus Abdominis
Synergists: External Obliques , Internal Obliques , Psoas
Antagonists: Erector Spinae (Iliocostalis, Longissimus, Spinalis) , Latissimus Dorsi
Neutralizers: External obliques and internal obliques when bilaterally contracted neutralize rotation.
Stabilizers: Intrinsic stabilization subsystem (Transverse abdominis , intrinsic muscles of spine, diaphragm, pelvic floor)
Fixators: All musculature of the hip, especially the glute complex.

Trunk Extension

Prime Movers: Erector spinae (Iliocostalis, longissimus, spinalis) , latissimus dorsi , Intrinsic musculature of the spine
Synergists: Intrinsic musculature of the spine
Antagonists: Rectus abdominis , external obliques , internal obliques , psoas
Neutralizers: Bilateral contraction of erector spinae and quadratus lumborum
Stabilizers: Intrinsic stabilization subsystem (Transverse abdominis , intrinsic muscles of spine, diaphragm, pelvic floor)
Fixators: All musculature of the hip, especially the glute complex.

Trunk Lateral Flexion

Agonists: Quadratus Lumborum
Synergists: Ipsilateral external oblique and internal oblique , ipsilateral erector spinae , ipsilateral latissimus dorsi
Antagonists: Contralateral quadratus lumborum , contralateral external and internal obliques , contralateral erector spinae , contralateral latissimus dorsi
Neutralizers: Erector spinae neutralizes flexion force created by internal and external obliques , intrinsic stabilization system creates posterior shear force on lumbar spine to neutralize lumbar extension, rectus abdominis and psoas prevent ancillary extension.
Stabilizers: Rectus abdominis , psoas , intrinsic stabilization subsystem (Transverse abdominis , intrinsic muscles of spine, diaphragm, pelvic floor)
Fixators: All musculature of the hip, especially the glute complex.

Trunk Rotation

Prime Movers: Contralateral external oblique and ipsilateral internal oblique
Synergists: Ipsilateral latissimus dorsi , contralateral psoas
Antagonists: Opposing external and internal oblique
Neutralizers: Erector spinae neutralize the flexion force created by the internal and external obliques , quadratus lumborum neutralizes lateral flexion force created by the internal and external obliques , and rectus abdominis prevent ancillary lumbar extension (anterior pelvic tilt) created by the latissimus dorsi
Stabilizers: Intrinsic stabilization subsystem (Transverse abdominis , intrinsic muscles of spine, diaphragm, pelvic floor)
Fixators: All musculature of the hip, especially the glute complex

Trunk Flexion and Rotation

Prime Movers: Rectus abdominis , contralateral external oblique , ipsilateral internal oblique
Synergists: Ipsilateral latissimus dorsi , contralateral psoas
Antagonists: Opposing external and internal oblique , erector spinae
Neutralizers: Quadratus lumborum neutralizes lateral flexion force created by the internal and external obliques
Stabilizers: Intrinsic stabilization subsystem (Transverse abdominis , intrinsic muscles of spine, diaphragm, pelvic floor)
Fixators: All musculature of the hip, especially the glute complex

Trunk Extension w/ Rotation

Prime Movers: Erector spinae (iliocostalis, longissimus, spinalis) , contralateral external oblique , ipsilateral internal oblique , latissimus dorsi
Synergists – Latissimus dorsi , contralateral psoas
Antagonists – Rectus abdominis , ipsilateral external obliques , contralateral internal obliques , ipsilateral psoas
Neutralizers: Quadratus lumborum neutralizes lateral flexion force created by the internal and external obliques
Stabilizers: Intrinsic stabilization subsystem (Transverse abdominis , intrinsic muscles of spine, diaphragm, pelvic floor)
Fixators: All musculature of the hip, especially the glute complex

Questions, comments, and criticisms are welcomed and encouraged –