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**Chris Kirtley** · November 23, 2016, 06:49 AM

Re: Validity of Inverse dynamics for orthosis users

Great question, Ganesh. Most inverse dynamics models assume the joints are frictionless pin-joints (in 2D) or universal joints (in 3D}. Of course an orthosis acts on the limb with 3-point or 4-point control, so strictly speaking the forces at these points would need to be modelled. That's easier said than done, because you probably don't know the magnitude and precise point of application of these forces. Can you forget about that and just model the whole system as a moment? I must admit I don't know!

**Ross Miller** · November 23, 2016, 09:55 AM

Re: Validity of Inverse dynamics for orthosis users

Ganesh,

I'm not very familiar with those devices, but would it be possible to determine a torque-angle relationship(s) for it in isolation? You could then do regular inverse dynamics and calculate/estimate the fraction of the moments that are from the orthosis vs. biological.

Scott Brandon did a similar thing here (chapter 4):

Brace Yourself: Reducing Medial Knee Loading for Treatment of Osteoarthritis

https://qspace.library.queensu.ca/handle/1974/12707

Compared with healthy controls, subjects with medial knee osteoarthritis walk using similar kinematics but greater activation of knee-spanning muscles, especially on the lateral side of the knee (furthest from centerline of the body). Although muscle forces generally compress the knee joint, it had been speculated that this specific activation pattern actually reduces compressive loads across the medial (closest to centerline) side of the knee where osteoarthritis is most common. It is not feasible to measure intra-articular contact forces in biological human knee joints. Therefore, a musculoskeletal simulation was developed to estimate the effect of elevated activation on knee contact forces. A generic musculoskeletal model was adapted to account for subject-specific frontal-plane alignment of the knee joint, and modified to isolate the medial and lateral components of the compressive knee contact load. The model’s predictions were validated using in vivo data obtained from a subject with an instrumented knee prosthesis. Subsequently, the model was used to demonstrate that elevated muscle activation, as found in osteoarthritis subjects, does not protect against harmful medial loads, but actually contribute to an increase in peak load shortly after heel-strike. Since osteoarthritis subjects did not appear to adopt a protective gait pattern, the second half of the dissertation was devoted to the evaluation of a clinical device that is known to reduce medial loads during gait: the knee unloader brace. To accomplish medial unloading, braces apply a frontal plane abduction moment to the knee which, if large enough, would pry open the medial contact surfaces. A novel method was developed to non-invasively compute the abduction moment applied by the brace to each subject’s leg using a mechanical stiffness calibration. The computed brace moment was incorporated into the musculoskeletal model in order to quantify relative contributions of muscle forces, inverse dynamic joint loads, and the applied brace load in reducing medial contact forces for treatment of medial knee osteoarthritis. The model revealed that kinematic, muscular, and inverse dynamic changes were dominated by the large applied brace moment. The medial unloading effect of knee braces could be enhanced by reducing external and muscle forces through gait modification.

Ross

**Ganesh B** · November 25, 2016, 12:26 AM

Re: Validity of Inverse dynamics for orthosis users

Thank you Chris for your insights!
Yes, Orthosis with the limb forms an indeterminate system. So,assuming limb with the orthosis as a single rigid body and no relative motions, is one way to solve the equations as some literature suggests.
I will look into your suggestion. But, I didn't get exactly what do you mean by.."just model the whole system as a moment"

**Ganesh B** · November 25, 2016, 12:27 AM

Re: Validity of Inverse dynamics for orthosis users

Thanks Ross. I will check the literature suggested.

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Validity of Inverse dynamics for orthosis users

Validity of Inverse dynamics for orthosis users

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