Functional Assessment of the Upper Limb: Support for Isotonic Measurement Devices

Abstract

Accurate evaluation of arm strength and function is important to prevent injury, aid rehabilitation, and enhance training. Traditional assessments involve isokinetic devices (e.g., Cybex) to determine post-injury abilities. However, this method is confined to a linear motion and fails to mimic normal isotonic movement patterns. Instruments that measure isotonic motions in three-dimensional space may be more appropriate. PURPOSE: Compare upper limb isokinetic force characteristics to those produced in isotonic actions. METHODS: 35 healthy college students (12 women, 23 men) performed biceps curls and triceps extensions of the dominant arm on one of two machines: Cybex HUMAC NORM isokinetic dynamometer (N=17) or Proteus (N=18), which measures upper limb motion in three-dimensional space using magnetically-mediated resistance. Subjects performed practice trials to minimize learning effects. After completing testing, we used independent and paired-samples t-tests to compare peak force ratios of biceps and triceps generated by the different testing devices. RESULTS: Peak biceps torque on the Cybex was 25.9 ± 8.5 ft-lb; peak triceps torque was 24.3 ± 6.3 ft-lb. On average, it took the biceps approximately 62% longer to reach peak torque than it did the triceps (p<0.001). The isokinetic biceps-triceps strength ratio was 1.07:1 ± 0.22:1. This ratio was different between men and women (p<0.001). Among men, it was 1.28:1 ± 0.16:1. Among women, it was 0.99:1 ± 0.20:1. This ratio was also different when compared to peak power calculated by Proteus (p=0.033). In our sample, isotonic, free-motion testing associated with a higher and more variable biceps-triceps strength ratio: 1.38:1 ± 0.99:1. CONCLUSION: Performance prediction models and return-to-play testing batteries have traditionally captured functional profiles through isokinetic testing. Restricting movement to a limited range of isokinetic motion results in an inaccurate depiction of what a patient or an athlete does outside of the clinic. Isotonic resistance permitting three-dimensional assessment may be able to provide a more optimal analysis of upper limb function, which translates more directly to athletic and therapeutic contexts. More research is needed to understand how these values may help personalize training and rehabilitation programs.