Reference values of maximum isometric muscle force obtained in 270 children aged 4–16 years by hand-held dynamometry

Abstract

Since muscle force and functional ability are not related linearly; maximum force can be reduced while functional ability is still maintained. For diagnostic and therapeutic reasons loss of muscle force should be detected as early and accurately as possible. Because of growth factors, maximum muscle force in children varies with age, which makes detection of force loss difficult. The purpose of this study was to establish reference values for muscle force in children aged 4–16 years, obtained by hand-held dynamometry in 11 muscle groups. In boys muscle force was predicted best by weight whereas in girls weight and age were best predictors. At age 14 boys become significantly stronger for nearly all tested muscle groups. These age-related reference values can be used to quantify muscle weakness in individual muscle groups in children aged 4–16 years and to evaluate the effects of therapy.

Introduction

Muscle function and muscle force are two fundamentally different parameters of the motor system that are closely related and are often tested during neurological examination in children. To determine whether pathological muscle weakness is present or develops during disease and in evaluating the effects of (physio) therapy, reference values for maximum isometric muscle force are needed. Functional grading systems, such as the Hammersmith motor ability score [1], are not suitable to detect force loss at an early stage of disease since there is no direct association between loss of myofibres and functional ability. Another disadvantage of these grading systems is that they are, to some extent, subjective. The Medical Research Council scale (MRC) can determine muscle force. In this scale strength can be classified between 0 (no contraction) and 5 (normal power). Van der Ploeg et al. [2] compared the non-parametric MRC scale with parametric hand-held dynamometry in adult male elbow flexors. They concluded that MRC grade 4 covers a wide range of forces: from 10 to 250 Newton. This inaccuracy makes MRC-grading unsuitable for quantification of normal and slightly decreased muscle strength. To measure muscle force non-hand-held dynamometers like the Cybex (Cybex Division of Lumex, Inc., Ronkonkoma, NY) and Biodex (Biodex, Biodex Corp., Shirley, NY) can be used. These systems consist of a framework that allows a subject to be tested. Because of their size, complicated use and high costs, these dynamometers cannot be used in bedside investigation. Hand-held dynamometry seems to be a good alternative. Most clinically important muscle groups can be measured easily with a dynamometer. Its portable size, rapid application and low price makes it very suitable for use in bedside investigation and in out-patients’ departments. Most studies reporting reference values are performed in adults [5], [7]. Only a few articles published at least 10 years ago report reference values obtained in children [3], [4]. The results show that height and weight are important predictors of muscle force. Since mean weight and height in children have increased in the past decade [8] new reference values are needed.

The purpose of this study is to establish reference values of maximum isometric muscle force in 11 muscle groups obtained with hand-held dynamometry in normal children aged 4–16 years and to provide summed scores for upper and lower extremities as well as for distal and proximal muscle groups. The relationship between anthropometric values and muscle force was investigated also.