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Letters and Responses |
I was pleased to read the article by Patterson et al titled "Material Properties of Thera-Band Tubing" in the August 2001 issue. I feel the article is a valuable addition to our knowledge base for one of the most commonly applied resistive training modalities in physical therapy. Much of the clinical application of elastic resistance is guided only by experience rather than research, due to a lack of basic science literature on elastic resistance. Although the article by Patterson et al significantly adds to our knowledge of elastic resistance, I feel several points in the article should be addressed.
The most common misconception regarding elastic resistance exercise is propagated in the first paragraph of the article when the authors state, "Near the end of the range of the motion, an individual may not be able to complete the desired motion, as muscles may be weaker because they may be in a shortened position at the point at which the resistance is greatest." Although it is true that the elastic resistance increases in force with elongation, the actual torque production of the joint using elastic resistance has been shown to accommodate for the typical "ascending-descending" characteristics of most strength curves of joints in the body.1,2 When applied to the dynamics of a joint, the torque curve of elastic resistance also provides its lowest torque at the beginning and at end range, with peak torque occurring near midrange due to the changing interaction of the lever arm and resistance device.1,2 This fact was clearly demonstrated in the article by Hughes et al,1 but overlooked in the discussion by Patterson et al.
For example, when performing shoulder abduction against elastic resistance, the resistance of the band increases linearly to the full 180 degrees of abduction. However, the shoulder abduction torque peaks at 90 degrees and decreases to near 0 N·m at end range of 180 degrees of abduction, as predicted by Hughes et al1 using the following equation:
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Hughes et al1 calculated shoulder torque using elastic resistance by measuring the force exerted by the band during abduction along with biomechanical analysis to determine the length of the lever arm and the force angle.
As noted by Hughes et al,1 the "band-arm-angle" or "force-angle" is defined as the angle created by the interaction of the elastic device and the lever arm. During shoulder abduction, for example, this force-angle decreases from 180 degrees to 0 degrees at full abduction, giving what I consider to be a smooth ascending-descending torque curve even though the force of the elastic resistance increases.
There are more points that should be corrected in the article by Patterson et al. First, Hintermeister et al3 did not use Thera-Band Tubing for their research on elastic resistance, as was stated in the Patterson et al article. Second, Thera-Band Tubing is not available in 8 colors; only 7 colors, from tan to silver, are available. Detailed material property specifications for both Thera-Band Tubing and Exercise Bands are given in the Thera-Band Clinical Dosing Chart, available at: www.thera-bandacademy.com/IV_Resource_Library/iv.a.2.f.1._clinical_dosing_ch.html.
I want to again commend Dr Patterson and colleagues for their valuable contribution to our body of knowledge and hope to see many more contributions to prove the efficacy of this commonly used modality.
Manager of Clinical Education and Research
Thera-Band® Products
1245 Home Ave
Akron, OH 44310
(ppage{at}thera-band.com)
References
Mr Page does bring up a very interesting comment with regard to issues of joint torque. There are several things that affect the torque on a joint. They include the resistance (either from a constant weight or from elastic tubing), the moment arm of the force (perpendicular distance from the joint center of rotation and the force), and the muscle characteristics (such as the length tension curve and whether the muscle is contracting concentrically or eccentrically). The article by Hughes et al2 shows a torque versus joint angle graph for different colors of Thera-Band Tubing as well as a tension versus % band change and a tension versus joint angle graph. It is not clear, however, what the % strain (% stretch) was for the tubing at each joint angle.
If we consider a hypothetical muscle contracting against Thera-Band Tubing over a range of motion, the 3 above-mentioned factors (externally applied resistance, moment arm, and muscle length) interact simultaneously. There is an increase followed by a decrease in the moment arm as the joint moves from one position to another and a steady increase in tension (force) generated by the stretching elastic tubing.
One further issue to consider is where the muscle is within its length tension curve. It has long been known that a muscle under constant stimulation produces an active force that is maximal at a length close to the resting length of the muscle in the body and decreases at shorter and longer lengths.3–6 In fact, several musculotendon parameters in cadavers and in vivo, including the muscle resting length, have been measured.7,8 All 3 issues must be considered when trying to determine the torque produced at a joint. To illustrate this, consider that during flexion/extension of the elbow, one would expect to see (1) the length of a muscle go from a lengthened to a shortened position with the resting length in the midrange, (2) the moment arm go from zero to maximum and back to zero, and (3) a linear increase in resistance produced by elastic tubing.
The sum of the resistance produced by elastic tubing and the moment arm will be the torque at the joint. In the above example, one would expect the torque to be lower at full elbow extension, peak at 90 degrees of flexion, and decrease at full flexion. However, one must also consider the contractile capabilities of the muscle, which would indicate that it will be more effective at 90 degrees of flexion and less able to contract at full extension and full flexion. Thus, the sum of these 3 parameters will be a net increase from full extension to full flexion and in a general sense would be more indicative of the actual muscle effort at the tissue level.
In addition, one must also consider the anatomy of the muscles that cross a specific joint. The shoulder example is actually a bit more complicated because there are multiple muscles simultaneously active in order to produce shoulder (glenohumeral or total shoulder) abduction. This coupled motion should be considered in the analysis of joint kinematics. In conclusion, our article aimed to quantify the material properties of Thera-Band Tubing. However, we appreciate the opportunity to respond to Mr Page's comments and to discuss the interaction of several issues that contribute to the resulting torque on a joint.
Associate Professor and Biomedical Engineer
Department of Orthopaedic Surgery and Rehabilitation
The University of Texas Medical Branch at Galveston
301 University Blvd, Galveston, TX 77555-0892
(Rita.Patterson{at}utmb.edu)
Assistant Professor
Department of Physical Therapy
The University of Texas Medical Branch at Galveston
Galveston, Tex
Associate Professor and Mechanical Engineer
Department of Mechanical Engineering
Texas A&M University
College Station, Tex
Mechanical Engineering Student
Texas A&M University
Footnotes
* Innovative Sports, Inc, 7 Chrysler, Irvine, CA 92718 
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