Reliability of muscle force measurements with a hand held dynamometer is good to excellent. All ICC values exceeded the criterion of 0.80, indicating good reliability for all ten muscle measurements. These findings corroborate with those of Bohannon . The LOA, however, varies substantially.
Reference values for muscle force for the Dutch working population between 20 and 60 years of age are now made available. Reference values including age gender, weight and height can be calculated with regression analysis as independent predictors for muscle force.
Comparison of the Dutch mean muscle force values to those published by Bohannon and Andrews revealed significant differences between reference values for muscle force values between the assessed populations. Comparison of reference values between populations have not been initiated previously.
Muscle force measurements with a hand held dynamometer exhibit a good reliability as demonstrated by the ICC. The LOA, however, vary substantially. Muscle groups with a relatively low muscle force demonstrate a small range of the LOA while muscle with a greater muscle force exhibit a larger range of the LOA, indicating that measurements of stronger muscles are less precise. Though hand held dynamometers have shown to be a reliable and beneficial instrument for measuring muscle force, a hand held dynamometer may possess some practical limitations. In subjects with high Quadriceps muscle force, it might be impossible to perform a correct measurement. During our study, it was not possible to perform a correct measurement of the Quadriceps muscle in six subjects due to high muscle force as observers were not capable of performing a correct break procedure. As reliability and validity may be affected during these measurements, bias was likely present, which is the reason that these results were omitted from the analysis. The influence of exclusion of these data on reliability, regression formulas, and reference values is very limited due to the considerable sample size. Provided that observers were able to properly perform according to the protocol, the regressions formula for knee extension might be only slightly changed.
In our opinion, a hand-held dynamometer is not suitable for measuring Quadriceps muscle force in stronger subjects.
Reference values for muscle force for the Dutch working population between the ages of 20 and 60 years are now made available. Regression equations illustrate that gender, weight, and height are of major influence on muscle force. The effect of age, however, is limited. In several of the regression analyses, the effect of age was small, though significant, due to the considerable sample size. Regression analysis demonstrated that the effect of aging for subjects aged 20–60 years is larger for lower extremities than for upper extremities. These results are predominantly consistent with previously reported results [3, 9]. Bohannon and Andrews also reported that gender, age, height, and weight are predictors of muscle force and that age correlated significantly, though very limited, with muscle force. Comparison of the outcomes of our study to those earlier exhibited an important difference between reference values. The differences in upper extremity tests, however, were moderate in all cases, whereas most of the lower extremity differences were considerable. For instance, differences in muscle force greater than 100 Newton for knee extension may have clinical consequences as 100 Newton’s may be up to 43 percent of the maximum knee extension force in the Dutch female population. The observed differences, however, exceed 100 Newton. This is all the more remarkable because, in our study, we employed the break method while, in the studies of Bohannon and Andrews, the make method is used. The break method may lead to higher levels in muscle strength measurements . The observed differences in the lower extremity are relevant for clinical practice. It appears to be evident that these differences may probably cause unattainable and/or undesirable training goals to be set and may result in undesired side effects as these external reference values may be too high and, therefore, not suitable for the Dutch population. However, reference values formulated for the United States are, at this moment, utilized in clinical practice and research in the Netherlands.
The results of our study demonstrate that reference values cannot simply be generalized to any country, geographical area, or population. Therefore, it is necessary to generate reference values for different countries or geographical areas. For other physiological tests such as the six minutes walking test reference values for specific geographic reference values are available and indicate considerable differences . Although we did not assess cultural habits or demographic aspects of populations, it is likely that the outcomes of muscle force measurements may be influenced by several such factors. Psychological state or prior experiences related to exertion or physiological responses to exercise, exertion, or pain might have influenced the outcomes . In addition, body composition and weight are related to muscle force as presented in the regression equations. Another potential explanation for the differences between our reference values and those previously reported by Andrews and Bohannon is the difference in time periods. The reference values of Andrews and Bohannon were determined in 1996 and ours in 2010. In approximately 20 years, some characteristics such as BMI may have changed which may affect references values equations.
In our study, we only tested the employed working population between the ages of 20 and 60 years. Our study, therefore, only provides reference values and comparison for this group. Our study does not provide information regarding, for example, unemployed businessmen or housekeepers. Another limitation in our study is that observers were male and female. We did not register whether subjects were tested by male or female observers. The outcomes of measurements may be biased by the gender of the observer.
Reliable muscle force measurements, appropriate and applicable reference values, and accurate knowledge of acquired muscle force in daily living facilitates formulating an effective and accurate rehabilitation process with clear and realistic goals and objective effects.
Although reliable measurements of a person’s muscle force are beneficial, no reliable procedures are currently available for translating isometric contractions or reference values, for that matter, into function. Functional tests probably provide an improved reflection of a subject’s functional muscle force, capacity, or ability for activities of daily living or work. This probably indicates that the role of muscle force should be interpreted with caution and that other variables may also influence activities of daily living. Additional studies are needed to define the specific role and the amount of muscle force required in activities of daily living.