Jeremy M. Huff and Dr. Kenneth L. Knight, Physical Education
Introduction
The use of strength measurements to predict strength gains is commonplace in the rehabilitation community. Athletic trainers, physical therapists and exercise scientists constantly use systems like the Biodex Dynamometer (Biodex Medical Systems, Shirley, N.Y.), to assess the progress of patients undergoing rehabilitation for injuries. Measurements taken on these devices often determine the status of patients permitting or barring them from full activity. Although use of these measurement devices to predict strength gains and functional ability is common, 2,3 some 3,4,5,7question the validity of the measurements. The value of isokinetic training methods has been questioned; 6 however, there has been no argument concerning the reliability of isokinetic testing when testing changes in strength.
Jameson et al. 4 reported low correlations between isokinetic, isotonic, and isometric strength measurements of athletes’ strength at the time of measurement. Stefl et al. 8 reported low correlations (r=.44 and r=.29) between two isotonic measures of strength gains and change in weight lifted during a 10-week weight lifting program. There was no research, however, that addressed the correlation between isokinetic, isotonic and isometric measures of strength change during a generalized training program, with the change in the amount of weight lifted.
As strength is a primary concern in rehabilitation, the ability to accurately predict strength gains is necessary. The concern of this study is to determine whether changes of strength measured isotonicly, isokineticly, and isometricly following a 10-week isotonic training regimen are correlated with each other and with changes in weight lifted. The research will be valuable to the exercise science community to support or refute current use of isometric, isotonic, and isometric strength measurements.
Methods and Procedures
Seventeen subjects (age = 20.46 ± 2.18 yrs, height = 164.63 ± 28.03 cm, weight = 73.41 ± 11.72 kg) were initially tested during this study. The results of 5 of the 17 subjects, however, were disqualified. Of these disqualified subjects, 3 dropped out by not appearing for their final testing and the other 2 were disqualified for not following a weight training routine during the 10-week training period. The 12 remaining subjects included 6 males and 6 females. All were assigned randomly to testing groups that incorporated the isokinetic, isometric, and isotonic testing techniques in different orders. This study was approved by the University IRB, and subjects gave informed consent prior to any testing.
Isokinetic and isometric testing was performed on a Biodex Dynamometer, as single-leg extension, and single-biceps curl exercise. Subjects were tested 3 times per condition, with all results recorded. The highest of the three results was used for statistical evaluation. Isotonic tests were performed on a seated knee extension machine (leg extension), and preachers bench (biceps curl) with measurements recorded as a one-repetition maximum (1 RM) gestimate method. Training weights, initial and final, were taken from the subject’s individual training program. Tests were administered following standard strength testing procedures as presented in Allsen et al. 1 and Dvir. 2 Subjects rested for 2 minutes between tests involving the same limb, and 10 seconds between each of the three-recorded repetitions. Correlations between the changes in the 3 tests and the weight lifted were computed with a Pearson correlation using the NCSS statistical package.
Results
Biceps strength increased about 12% according to the 3 strength tests although the weight they lifted increased 75% (Table 1). Leg strength increases were less but similar; 5%-8% increase in the 3 tests and 46% increase in weight lifted (Table 1).
Pre and post test correlations ranged from low to high. Correlations between the 3 tests were similar for the two muscle groups (Tables 2 & 3), both muscle groups moderate to high during both the pre-test (r=.66 to .94) and post-test (.74 to .89). Correlations between the 3 tests and weight lifted, however, was low to moderate for the biceps during the pre test (r=.13 to .60) and post test (r=.63 to .73) but moderate for the leg (r=.61 to .80 pretest and r=.67 to .85 post-test).
Correlations of change scores, however, were much lower. Correlations among the 3 tests were low to moderate (r=.29 to .50 biceps, -.36 to .48 leg), but correlations between weight lifted and the 3 tests were very low (r= -.04 to .32).
Conclusion
Athletes’ strength as measured on different tests should not be compared, either between athletes or from test to test with the same athlete. Further, the 3 common tests are not good predictors of changes in an athlete’s ability to lift weights. Therefore, a therapist should not make an assumption about what the changes of one strength test would be from the results of another strength test. Judgments about a persons strength changes should be based on changes in the weight they can lift.
References
- Allsen PE, Harrison JM, Vance B. Fitness for Life. 5th ed Madison, WI, Brown & Benchmark, 1993.
- Dvir Z. Isokinetics: Muscle Testing, Interpretation, and Clinical Applications. New York, NY, Churchill Livingstone, 1995.
- Jablonowsky R, Inbar O, Rotstein A, Tenenbaum G. Evaluation of anaerobic performance capacity by the isokinetic ariel computerized exercise system-reliability and validity. J Sports Med Phys Fit 1992, 32:262-270.
- Jameson TD, Knight KL, Ingersoll CD, Edwards JE. Correlation of isokinetic, isometric, isotonic strength measurements with a one-leg vertical jump. Isokinetics and Exercise Science 1997, 6:203-208.
- Knapik JJ, Wright JE, Mawdsley RH, Braun JM. Isokinetic, isometric, and isotonic strength relationships. Arch Phys Med Rehabil 1983; 64:77-80.
- Knight KL, Ingersoll CD, Bartholomew J. Isotonic Contractions May Be More Effective than Isokinetic Contractions in Developing Muscle Strength. J Sport Rehab. In Review.
- Pipes TV. Variable resistance versus constant resistance strength training in adult males. Eur J Appl Phys 1978; 39:27-35.
- Stefl DL, Knight KL, Allsen P, Parcell A, Durrant E, Draper D. Creatine Does Not Enhance Strength Development in Male College Students During a 10-Week Weight Lifting Program. J Athl Train. In review.
I would like to also thank Jody B. Brucker and Mack D. Rubley for their assistance with this project.