Kinetic Links

I want you to try an experiment at home:  pick up a baseball and throw it (preferably outside, unless you have a girly throw).  On your first throw only use your arms; no hip movement, no drive off the ground, no shoulder whip.  Observe how far it goes.  Now throw the ball again but use your whole body.  Drive the foot off the ground as you release, use a good wind up, activate your double peak (that's a topic for a new day).  Now observe how far it goes.  Unless something went terribly wrong, the second throw probably went a lot further.  Why you may ask; we'll get into that.

Physics 101

Remember back to high school physics class; depending on the quality of the school you attended you may have learned a thing or two about free body diagrams and joint/member analysis.  Imagine the human body composed of rigid bodies/members (bones) all connected via pin joints (joints).  Now this system is free to articulate in all 3 planes of motion, constricted by freedom of articulation of each specific joint.

Taking this idea one step further, these interconnect rigid bodies function as a multibody system - if you disturb one member down the chain it will result in the displacement of a member somewhere further up.  The movement of the multibody system are governed by laws of kinematics and statics; if enough force is applied to a member, that member will move in accordance with the magnitude and direction of external force, as well as its set trajectory based on articulation limitations (ie. a knee joint will have very little medial/lateral movement before something breaks).  If I apply a certain force and velocity Traditionally this kind of modelling is used widely in the Engineering field to predict movements or stress limits of rigid bodies (ie. trusses which support a construction crane), but the human body is nothing but a multibody system so these comparisons hold true (for the most part).

Imagine if the sliding mass were to move in the negative x direction (based on the legend in the centre of the picture).  The prismatic joint limits the sliding mass to only move in the x direction, similar to as how your elbow is only able to bend with 1 degree of freedom.  As the sliding mass moves it pushes the flexible beam.  The movement of the flexible beam causes rotation of the rigid driving body along the revolute joint.  This results in the driving torque shown.  Therefore it can be concluded that the magnitude of the driving torque is dependent on the force applied at the sliding mass.  The greater the force from the sliding mass, the greater the driving torque.  Cases like these crudely model what occurs when a linear force drives a torque; such as in an internal combustion engine when the linear piston motion drives the crankshaft.

Human Applications
"Thanks for the physics lesson, Newton.  How does this help me?"  Like I said earlier the human body can be modelled as a multibody system.  Instead of rigid beams and sliding masses and revolute joints you have bones, skeletal muscles, internal forces developed by skeletal muscles and various types of joints.  When you apply external forces somewhere, a resultant displacement and force will occur elsewhere depending on the magnitude of the applied force.

Back to the baseball example when you push off the ground with your foot you are creating a normal force exerted by the ground onto your foot, with the same magnitude in which you pushed off.  A harder push off results in a greater normal force.  As that force at the foot increases resultant forces are transmitted through the body toward the end output - your arm.  Along with internal reaction forces from muscle activation (ie. hamstrings/quadriceps from the pushoff, deltoid/pec minor during the shoulder activation), the output at the arm will be of a high acceleration/force.  Ignoring throwing mechanics, the harder you push off the further the ball should travel.  The same concept applies to any sport; hockey slapshots, football tackle, throwing a knockout punch in boxing, etc.  Any striking sport coach worth their weight will tell you power in striking comes from the hips and lower body, not just the arms.  For the boxer plant your foot as push off the toes as your punch leaves, rotate the hips and let the arm follow - knockout out ensue.

Pacman knows a little something about knockouts

One thing I haven't mentioned too much are the internal forces of the human body.  What separates us from the machines is that we are able to produce forces internally through the contraction of muscles during activity.  This gives us an assistance in output force based on how efficient we are at contracting muscles.  When you perform a dumbbell curl, for instance, the force generated to overcome the weight of the dumbbell is not magically bestowed upon you; it is generated by the shortening of the biceps brachii and that internal force is what is lifting the weight.  The ability to more efficiently create these forces through muscular contraction is what separates beginner and elite level weightlifters.  Based on this evidence, going back to our throwing example, a person with stronger calves, quadriceps, hamstrings and shoulders will throw the ball much further.  The combination of applying external forces and high internal forces is what will make a person excel in a physical task.  Further explanations of internal forces will be saved for another day.

Going back into the modelling of the human body as a kinetic link, its not just me who's saying this.  Various biomechanical principles are based on this, and it serves as the basis for some chiropractic technique.  In an interview at T Nation many, many years ago chiropractor Dr. Ken Kinakin had this to say about fixing disorders:

I'll keep working on their shoulder, but if the problem keeps coming back I'll drop down to the lower back and work on that. Once that's fixed I'll drop down to the knee and also the ankle. A lot of times a sprained ankle can go all the way up. In that same vein, dentists sometimes use orthotics (insoles for your shoes) to fix the TMJ joint in your jaw. A dropped arch can affect the entire body biomechanically and neurologically.

What Dr. Kinakin is saying that an improperly applied force at the bottom of the kinetic link can lead to disorders way up.  Sprained ankles may result in an altered walking gait, which can apply forces from ground contact at odd angles.  These oddly applied forces can make their way up the link into your shoulder or TMJ.

Researchers, including spine biomechanics professor Dr. Stuart McGill have made the link between kinetic links and resultant forces to the spine.  Altered walking gaits and improper loading patterns to the foot while walking have shown to increase injury causing forces to the spine, which explains why your low back hurts after walking for so long.

Conclusion
So there's a quick introduction into Kinetic Linkages and the human body.  The take home message is that everything in your body is connected.  Our bodies are nothing more than a set of rigid members and joints, controlled via internal forces, or our muscles.  When we apply an external force at one point of this linkage there will be a resultant force elsewhere.  Using this knowledge we can plan on optimizing any athletic movement; boxers will drive off the ground with their feet in order to maximize punching power, football players will big into the field with their spikes and drive forward to achieve that spring to make a tackle; the examples are endless.

In later posts I hope you delve further into this topic and how it pertains to certain sports.  Hockey, Football, Golf, Mixed Martial Arts; anything which requires you to move, can be governed by what has been mentioned today.  Keep this in mind next time you throw a ball, jump onto a box or punch someone in the face.

References:
Kinematic Couple (Kinematic Chain); http://sports.jrank.org/pages/9192/kinematic-couple-(kinematic-chain).html
J. Wittenburg, Dynamics of Systems of Rigid Bodies, Teubner, Stuttgart (1977)
A.A. Shabana, Flexible Multibody Dynamics: Review of Past and Recent Developments, Multibody System Dynamics, Volume 1, Number 2, 189-222; 1997
W. Schiehlen, Multibody System Dynamics: Roots and Perspectives, Multibody System Dynamics, Volume 1, Number 2, 149-188; 1997
C. Shugart, The Symposium of a Lifetime:  An Interview with Dr. Ken Kinakin, Testosterone Nation (2002)
Callaghan*, J.P., Patla, A.E., and McGill, S.M.  (1999) Low back three-dimensional joint forces, kinematics and kinetics during walking.  Clin. Biomech. 14: 203-216.