Eccentric shoulder external rotation using System 3 Biodex

[Michele LeBlanc]

An undergraduate student of mine is doing her senior Capstone project using our System 3 Biodex to measure maximum torque created in both concentric and eccentric conditions at the shoulder. She is having difficulty with the eccentric external rotation - the corresponding movement is not smooth and is fairly unpredictable.

We know from the literature that others have successfully measured peak torque in this condition and were wondering if anyone who has used the Biodex might help us determine the nature of our problem. We have already spoken with the Biodex folks who suggested that we had the wrong motor type setting or that we alter the torque value. Neither proved to be the solution.

Any suggestions would be greatly appreciated!

Michele

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Michele LeBlanc, Ph.D.
Nena Amundson Professor of Biomechanics
Director, Office for Undergraduate Research
California Lutheran University
(805) 493-3276

[Brian Schulz]

Re: Eccentric shoulder external rotation using System 3 Biodex

I had quite a bit of trouble using a Biodex for eccentric knee extension strength testing. Eccentric mode just didn't seem to work right for recording eccentric peak strength. Biodex suggested that I use the machine in passive, not eccentric mode, and that one change made everything work fine.

-Brian

[Carolynn Patten]

Re: Eccentric shoulder external rotation using System 3 Biodex

Michelle,

Eccentric mode on the Biodex is indeed challenging, even for non-injured, able-bodied individuals. Several years ago, a colleague suggested generating eccentric conditions by operating the Biodex in passive mode and instructing the subject to resist the dynamometer. I have now used this approach for some time across several joint actions and have had great success with it. My group has also published several papers in which we used this approach.

best regards,

Carolynn.

Carolynn Patten, Ph.D., PT

Research Career Scientist
Brain Rehabilitation Research Center
Malcom Randall VA Medical Center

Associate Professor
Departments of Physical Therapy, Neurology, and Applied Physiology & Kinesiology
University of Florida

Gainesville, Florida
e-mail: patten@phhp.ufl.edu

[Glen Lichtwark]

Re: Eccentric shoulder external rotation using System 3 Biodex

We have also used Biodex in passive mode for eccentric contractions, however this is not suitable for contractions that occur at any fast velocities (> 50 degrees/s). This is because the velocity is slowly ramped up and down in passive mode. Therefore if you have a small range of motion (ROM) and/or a faster velocity of contraction (i.e. a faster period of contraction) the actual velocity will vary significantly throughout the ROM and may not even get to the required velocity. The time varying velocity should therefore be closely monitored to ensure that the required velocity is actually met during the contraction.

The eccentric mode will get up to high speeds very rapidly, however it is a bit of a black art to get this to work without being jerky, stopping or even starting in the first place.

Glen Lichtwark, Research Fellow, School of Human Movement Studies, The University of Queensland

[Peter V. Loubert]

Re: Eccentric shoulder external rotation using System 3 Biodex

HI Michele, I suggest you check the thresholds that I believe are build into the system as safety precautions:

There is a minimum threshold for the robotic arm to move in the eccentric direction - set this very low. The minimum threshold protects a patient because if they let up or pull away from the resistance arm it will simply stop. That way if the patient feels pain or has other apprehension they can interrupt the exercise/test simply by letting up or pulling away from the resistance arm.

There should also be a maximum resistance setting that prevents the system from overpowering a patient so much so that it injures them. To get a true maxumum eccentric test you will have to set this very high - higher than their true maximum, or simply turn it off completely (if that is possible). Of course that comes with the risk of injuring your subjects by overloading them, so I suggest you include this in your informed consent and that you only use subjects that are reliable and can personally react in a way that will help them limit that risk.

If either of these thresholds are not adjusted properly the machine can seem to behave erratically ...

I will be curious to hear if that solves the problem for you ...

Pete