A New Motor Control Theory

There are several major theories on how the brain and nervous system control human movement, they include; inverse dynamics approach, generalized motor program theory, equilibrium point hypothesis and the optimal control approach.  A new alternative approach is called the leading joint hypothesis (LJH) proposed by Professor Natalia Dounskaia and colleagues from Arizona State University, Tempe, Arizona (ASU).  In this article I will explain the LJH and investigate its relationship to the golf swing.  Dr. Dounskaia in her article (Dounskaia, 2010) has articulated these principles very well, so in some cases I will use her descriptions almost exactly, with only the intent of maintaining accuracy.

The LJH is based on the idea that the central nervous system exploits the inertial properties of the limbs for movement organization.  This is specifically relevant to complex motions, especially sports movements, involving movement and control of multiple joints and segments.  In the golf swing for example all parts of the body are involved in moving and controlling the golf club, so that it hits the ball with high speed and excellent precision.

The multi-joint structure of the body and inertial characteristics of the limbs cause motion-dependent mechanical interactions between segments.  In simpler terms, because each body segment has weight and length, the motion of each segment creates forces and torques that must be supported and controlled by the muscles at each joint earlier in the chain.  These torques are called interaction torques.  Not only do muscles have to generate the forces to create the motion in the first place, but they also have to control the forces that are created by the motion itself.  This can be a very complicated process, especially when several joints are involved; it makes motion at all joints interdependent.  These interaction torques escalate very rapidly as movement speed increases and are very influential even at moderate speeds.

The LJH suggests that the nervous system purposefully generates interaction torques to produce the required motion, exploiting the inertial characteristics of each limb to its benefit.  In addition the LJH suggests that joints of a multi-articulate limb play different roles in movement production and control, according to where they are in the joint linkage. 

There is one leading joint that creates the dynamic foundation for the motion of the entire limb.  It produces motion by creating acceleration and deceleration from simple reciprocal muscle activity, similar to single joint motions and largely disregarding other joint motions. 

Dr. Dounskaia states: “The leading role is endowed to a joint that has mechanical advantage in the limb.  Because of relatively high inertia and the increased musculature of the proximal limb (or trunk) segment, the mechanical influence of the proximal joint motion on distal joints is much higher than the influence of the distal motion on proximal joints.  For this reason, the leading join is often the proximal joint that acts similar to a whip handle; a single wave of which can cause complex motion of the cord.”

In contrast to the leading joint, the subordinate joints must control the interactive torques that are created by the motion of the leading joint.  The subordinate joint muscles must monitor the effects of the interactive torques to enhance or impede them and create their own net torques that will produce the required motion.  It is up to the subordinate joints to control movement speed, direction and accuracy.

For example in the golf swing, the wrist is a subordinate joint.  Initially in the downswing the forearm muscles must resist the torque generated by the motion of the club, resisting it from either opening or closing the wrist angle, effectively maintaining a good wrist set angle.  At the appropriate instant in the downswing the wrist muscles turn off releasing the club, allowing the large inertial forces to speed it into impact.  The interaction torque is that caused at the wrist by the inertial properties and motion of the club.  The sensitive forearm and hand muscles don’t become completely passive, they may also quickly act to enhance the club release speed and even guide it more accurately into impact.

A study that validates the LJH and relates directly to the golf swing was done by YK Kim (Kim et. al., 2009) also at ASU.  He studied the horizontal arm swing as initiated by trunk rotation with the goal of creating the fastest hand speed at a target point.  This type of action relates to several sports skills such as the golf swing, baseball and softball batting, tennis backhand and even a Frisbee throw.  Kim compared direct muscle power with interaction power at each joint to determine which of these was dominant.  Note that power is the combination of torque and speed.  He found that muscle power was dominant at the trunk, muscle and interaction power were about equal at the shoulder and interaction power was dominant at the elbow.  It was also found that muscle power and interaction power were opposite in sign.  This means that the muscle power was primarily used to stabilize the joints and control the motion created by the trunk.  This proves that in this particular skill the leading joint was the trunk (core) and the subordinate joints were the shoulder and elbow.  Kim also sectioned the power into 10 time increments and found that there was also a transition in power generation.  Initially the trunk provided the driving power, but in the last 20% of the motion the shoulder took the leading role, adding to increase hand velocity.

This study provides not only credibility to the LJH but also to the kinematic sequence principle as reported by Cheetham et. al., (2008).  It suggests that each joint sequentially takes over during a multi-joint swinging action.  Kim only studied the trunk, shoulder and elbow, so the legs and wrist were not included but nevertheless it is an important study.  These two “joints” should be added in a future study.

The LJH also has significant implications in the training of a skill, implying that the correct action of the leading joint should be learned first and the subordinate joint second.  This will definitely “open a can of worms” in the golf instructional world and is food for a future article.

References

Cheetham PJ, Rose GA, Hinrichs RN, Neal RJ, Mottram RE, Hurrion, PD & Vint PF. Comparison of Kinematic Sequence Parameters between Amateur and Professional Golfers. In D Crews & R Lutz (Eds.), Science and Golf V: Proceedings of the World Scientific Congress of Golf. Phoenix, Arizona.  2008.

Dounskaia N. Control of human limb movements: the leading joint hypothesis and its practical applications.  Exerc. Sport Sci. Rev., Vol 38, No. 4. Pp. 201-208, 2010.

Kim YK, Hinrichs RN, Dounskaia N.  Multicomponent control strategy underlying production of maximal hand velocity during horizontal arm swing.  J. Neurophysiol.  Vol 102, No. 5. Pp. 2889-99, 2009.

 

---