The Body Learns to Move in the PATTERNS you teach it- 3rd Posture Principle

The model of a muscle shortening to create a discrete motion in a direction is overly simplistic. As I have previously contended, muscle patterns are unique on a fiber by fiber basis.  The complexity of motion can be described with the three motion subsystems of the StrongPosture^TM model,  Contracting, Connecting and Control (corresponding to Punjabi's Active, Passive and Control subsystems).   Motion patterns result from Connecting subsystem muscle activity constrained by the Connecting subsystem (joint architecture, ligaments, tendon and fascia) and guided by the Controlling subsystem (CNS/PNS).

A new study demonstrates that motion patterns are even more complex. Park, Tsao, Cresswell and Hodges have demonstrated that the deep posture stabilizing muscles do not have discrete functions, but their fibers can have antagonistic function within the same muscle. The researchers looked one of my favorite muscles, the psoas, and showed some fibers were active in flexion and others fibers active in extension.

The Study: Fine-wire EMG electrodes were inserted into the psoas major at the transverse process (PM-t) and vertebral body (PM-v). Activity was then measured in flexion and extension (sagital plane function).

Observation: The two regions of the psoas showed opposite function, with the PM-t engaged and active in extension and PM-v in flexion. In other words, different populations of fibers in the same muscle had antagonistic effects.
A similar effect was noted for fibers inserted into the front and back of the Quadratus Lumborum (QL) where right trunk lateral-flexion was measured

The researchers' conclusion: "These findings provide novel understanding of the unique activation of discrete regions of PM and QL."

SPW Conjecture I: The co-contraction of antagonisic fibers within a muscle plays a significant role in stabilizing and controlling joint motion.
SPW Conjecture II: The conscious control of each degree of motion in StrongPosture^TM exercises trains this stabilization by finding and recruiting unused fibers.

Differential activity of regions of the psoas major and quadratus lumborum during submaximal isometric trunk efforts. Park RJ, Tsao H, Cresswell AG, Hodges PW.J Orthop Res. 2011 Jul 28. doi: 10.1002/jor.21499.