For years, we have taught the concept of Thera-Band elastic resistance exercise dosing: proper patient positioning to facilitate the production of a bell-shaped muscle strength curve during isolated single-joint strengthening exercises. The key factor is positioning the origin of the band or tubing in line with the axis of rotation such that the force angle (the angle created by the interaction between the arm and resistance) is as small as possible at the end of the physiological range of motion. Although the resistance of Thera-Band elastic band or tubing increases as its stretched through the range of motion, the resultant torque or strength curve is bell-shaped (Hughes et al. 1999):
In an effort to understand the possible torque curves that can be obtained in single-joint exercises, Italian scientist Andrea Biscarini published an article in the Medical Engineering and Physics journal. Using geometric sketches and equations, Professor Biscarini was able to determine a variety of torque profiles that can be generated with the use of a simple elastic band. Furthermore, these elastic torque profiles reproduce the typical ascending, descending, and ascending-descending human torque curves corresponding to different joint movements.
Dr. Biscarini noted that the versatility of elastic resistance torque curves overcomes a major limitation of traditional weight machines that provide minimal, if any, variations of the resistive torque, noting machine-based cam systems do not replicate true muscle strength curves. In addition, the inertial effects of acceleration of the weight stack may affect the torque curve. This effect was shown in our 2000 study (Page & Labbe 2000, Med Sci Sports Exerc 32(5):S151) below:
Because inertial effects are minimal with elastic resistance, Dr. Biscarini noted that elastic resistance training provides therapists and trainers with more accurate control of the resistance, compared to weight machines.
In conclusion, this article supports the concept of elastic resistance exercise dosing, where properly positioning the elastic band and the patient during exercise will optimize muscle activation throughout the entire range of motion.
REFERENCE: Biscarini A. Determination and optimization of joint torques and joint reaction forces in therapeutic exercises with elastic resistance. Med Eng Phys. 2012 Jan;34(1):9-16. doi: 10.1016/j.medengphy.2011.06.011. Epub 2011 Jul 16.
Up next: PART II: Elastic resistance exercise dosing and knee joint reaction forces
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