The capacity of muscles to generate strength is one of the main features of maturation during child growth. We report paediatric strength values for quantified muscle testing with fixed myometry obtained from 96 children aged between 5 and 17
years. We also described equations for predicting strength of handgrip, elbow flexion and extension, and knee flexion and extension and observed that height is a major explanatory variable for muscle strength.
In previous studies, grip strength has been more extensively studied than other muscle functions. The strength values we report here are consistent with previous work in paediatrics for handgrip [2
] and for other muscle functions tested by handheld dynamometry [4
]. Child muscle strength has rarely been reported as torque, although torque is a more appropriate measure than linear force. Indeed, for the same torque, generated by the muscle during a maximal contraction for instance, the linear force depends on the length of the lever arm, defined as the distance between the application point of force and the rotation axis of the joint. Measuring the torque rather than linear force is therefore particularly important for longitudinal studies involving growing children.
The literature provides evidence of substantial inter-individual variability concerning strength expressed with respect to the age. A significant part of this variability may be due to differences in height among children in a given age class. Indeed, Niempoog et al. [15
] recently indicated that “chronological age alone does not reflect the pubertal stage that leads to different physical performance”. Height was found to be strongly correlated to grip strength in several studies [15
]. Our study suggests that height is a major explanatory variable also for strength of muscle functions other than handgrip and is in line with Parker et al. [18
]. The strong correlations between the different muscle functions are a clue to a global effect of stature on the whole body muscle strength. We found that girls and boys cannot be overall distinguished according to strength when related to height, at least before 17
years of age. A strong relationship between muscle strength and height has previously been reported for pre-pubertal boys and girls [4
] Also, the study of Newman et al. [20
], implies that grip strength is similar in boys and girls under, approximately, 160
cm of height. Thus below 160
cm, norms for girls and boys appear to be the same when using height as the single explanatory variable. Sartorio et al. [21
] observed that “gender differences disappeared when grip strength is normalized for fat free mass in children from 5 to 15”. Since height is closely correlated to lean body mass [22
], it is not surprising that height explains a large part of the variability between individuals. Interestingly, the relationship between strength and height was accurately described by an exponential model, consistent with the observation that "lean body mass is an exponential function of height" [22
The exponential model was also applied to linear force. Compared to torque-height relations, the mean adjusted R2 decreased from 0.825 ±0.035 down to 0.686 ±0.080 due to a larger inter-individual variability that can probably be explained by a lever arm effect.
We found that the dominant hand side was significantly stronger than non-dominant hand side for both sexes for handgrip, elbow flexion (in accordance for both sexes with Bäckman et al. [6
]) and knee extension (only for boys in the same study). Concerning knee flexion, we found that only boys were stronger on the dominant hand side, in contrast with Bäckman et al. [6
] who found this result only for girls. However, the effect of dominance for this muscle function was weak: in our study, the difference between the two sides was only 1
Nm for a mean strength of about 40
Results did not differ between the test session and the retest session. It suggests that the QMT method can be used immediately, without preliminary training, for children in clinical trials. However, a habituation session may be useful to accustom children to the assessment procedures and make them feel confident with the evaluator.
ICCs are relative measures of reliability that have been used in many studies. They are generally good to excellent, as in the present study, particularly because the range of the measures is generally large. However, the assessment of reliability should not be limited to the use of ICC. Standard error of measurements (SEM) is a measure of agreement and serves as an index of absolute reliability; it has been much less widely used for evaluations of the performance of strength assessment techniques. The SEM for handgrip strength was about 20
N in our study, while Moleenar et al. [24
] reported a SEM of about 11
N. The difference may be due to the larger age range, hence the larger range of strengths, of the children in our study. Indeed, when normalized to the mean of the measurements, the relative SEMs for the two studies are similar (10% and 9%, respectively). Meldrum et al. [25
] used the same QMT measurement system with adults, and the standard error of the difference between test and retest reported can be used to compute the relative SEM: the relative SEM was between 3.9 and 12.8% in adults, to be compared to 9.8 to 15.1% for children in our study. This suggests lower reliability of strength measurement in children than adults possibly due to their poorer concentration or motivation through successive visits. However, strength measurement reliability has not yet been formally compared between children and adults.
As underlined by Jaric [26
] in the field of sports medicine, "the primary goal of strength testing has often been to assess the objective value of muscle function independent of possible confounding factors". This is all the more true in the clinical field when the aim is to evaluate neuromuscular involvement in a disease. Moreover, when considering children, early or late maturation needs to be taken into account due to the direct link between body stature and muscle strength. This connection seems also to apply to adults as recently demonstrated by the close relationship between hand circumference, as an indicator of body stature, and grip strength [27
In this study, we used a paediatric population aged from 5 to 17
years to develop models to predict handgrip strength and elbow and knee flexion and extension torque. Several authors have reported strong correlations between strength and height and have proposed models linking the two variables [2
]. Similarly, Van den Beld et al. [3
] observed that "height proved to be a better predictor for handgrip strength than age in children aged 4–11
Our paediatric population counted a rather small number of children and was not necessarily representative of the general population of French children. Reliability was assessed by several indicators. First, there are limitations to using ICC, particularly its interpretation when the data include large inter-individual variability, which is the case for groups of children covering a wide range of growth/maturation stages. In such situations, ICC is only a rough indicator of reproducibility. Second, standard error of measurement (SEM) was used to quantify absolute agreement between test and retest values. Although our results indicate a satisfactory reproducibility between test and re-test results, some children were clearly less motivated during the second evaluation visit than the first. This behaviour could have led to an overestimation of the SEM. This also indicates that evaluation sessions for children should not be complicated, boring or long such that motivation and attention are maintained.
We report here strength values for muscle functions in particular protocol conditions (dynamometer type, body segment positions, number of attempts, maximal value scoring). The predictive equations proposed here are reliable only in the conditions specified. For informative comparisons, test conditions must be the same in patient populations to be assessed and in the normative control population. Note also that the strength values were established for isometric contractions and do not apply to dynamic (concentric or eccentric) contractions.