Preferred Interjoint Coordination – Linear Force Paths

Muscle Contribution to Foot Force Reveals Coordination

To study muscle coordination from measurements of foot force, we must appropriately segment the sources of that foot force. Force produced by the foot on the environment (foot force) is due to the multiple factors of: 1) muscle activity, 2) gravity, and 3) dynamic effects. For example, if one were to measure the force between the foot and a bicycle pedal while the pedal was stationary and the person were as relaxed as possible the force would be due to gravity pulling each leg segment downward (and some small muscle contribution to keep the limb in that posture). This ‘gravity’ component of the foot force is a function of limb posture and the mass of each segment. If the person were to now actively push on the pedal, but the bike doesn’t move because the brakes are on, there would be additional force that would be due to muscle action. Now if the person were to release the brakes and the pedals start to move the acceleration of the limb segments and pedal would yield yet a different force (adding in the inertial component) even for the same muscle and gravity contribution.

We isolate the muscle contribution (Fm in figure) to foot force by controlling the other sources (gravity and inertia). The gravity force (Fp in figure) is constant so we can remove that component by subtracting the initial force measured when the person is (mostly) relaxed. Or more generally we can focus just on the changes in the foot force vector. The inertial contribution (Fi in figure) can be removed by comparing multiple events that have varying muscle contribution but the same inertial contribution. We achieve this by having the person pedal a stationary bicycle that is robotically controlled so that the pedal moves along a cyclical path with constant speed. We then gather a set of measurements made when the pedal passes through a given location in the cycle. The inertial contribution to all members of the set are the same because the limb is accelerating identically regardless of muscle activity level. Subtracting members of that set from each other gives us the effect of changing muscle activation alone. In practice we remove the gravity and inertial effects by considering the changes in foot force vector (path of the vector tip) which we call the ‘force path’ (dashed line in figure).


Kinematically-Constrained Tasks Reveal Preferred Coordination

We conducted several studies revealing the presence of a preferred coordination strategy that later studies have shown is fundamental to human upright posture. In this series of studies, humans were supported by a seat and handles to free the control of foot force from the constraints of whole body posture. The feet were placed in contact with pedals whose motion could be robotically controlled. The foot force vector was measured and the gravitational and inertial contributions removed, revealing only the contribution from changes in muscle activity. The non-disabled adult subjects were asked to push in ‘the most comfortable manner’ to achieve a force magnitude target indicated by graphical force magnitude feedback.

Foot Force Path Linearity

By focusing directly on the contribution of inter-muscle coordination to foot force we discovered an amazingly precise coordination that appeared regardless of whether the pedal was stationary or moving. That coordination caused changes in the foot force vector to have a constant direction across changes in effort level (termed ‘linear force path’). This could be achieved by a constant scaling of relative muscle forces (often called a ‘muscle synergy’) or by some more complex relationship among the muscles. Regardless of the coordinative mechanism, the force output has the attractive feature that force magnitude appears to be controlled independent of force direction. As discussed in other sections, this has important implications for postural control.

Relevant Publications:

Gruben KG, López-Ortiz C: Characteristics of the force applied to a pedal during human pushing efforts: Emergent linearity. J Motor Behavior, 32(2):151–162, 2000.

Gruben KG, López-Ortiz C, Schmidt MW: The control of foot force during pushing efforts against a moving pedal. Experimental Brain Research, 148(1):50–61, 2003.

Gruben KG, Rogers LM, Schmidt MW: Direction of foot force for pushes against a fixed pedal: role of effort level. Motor Control, 7(3):229-41, 2003.

Schmidt MW, López-Ortiz C, Barrett PS, Rogers LM, Gruben KG: Foot force direction in an isometric pushing task: prediction by kinematic and musculoskeletalmodels. Experimental Brain Research, 150(2):245-54, 2003.

Gruben KG, López-Ortiz C, Giachetti RS: Muscular and postural components of foot forces during quasi-static extension efforts. J Applied Biomechanics, 19:239-245, 2003.

Gruben KG, Rogers LM, Schmidt MW, Tan L: Direction of foot force for pushes against a fixed pedal: variation with pedal position. Motor Control, 7(4):366-383, 2003.


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