Participants
Healthy subjects, male and female, aged from 5 to 80 years old were recruited by advertisements in newspapers, websites, and posters. Exclusion criteria were any neurological, neuromuscular or other disorders that could affect muscle strength, any history of injury, disease, pain or discomfort involving the upper limbs in the last two years, and practice of a sport at a national level. Subjects were informed about the terms of the experimental protocol and procedures before giving their written consent. The protocol (namely MyoTools) was approved by the Local Ethics Committee (CPP-Ile de France VI) and aimed to assess muscle strength in several muscle functions (hand grip, wrist extension and flexion, ankle flexion and extension). All subjects gave written informed consent to participate in the measurement sessions.
Anthropometric measurements
The height and weight of the subjects were recorded as well as an estimation of the percentage of body fat mass using an impedance metric scale (Tanita TBF-543). Anthropometric hand data were measured by the experimenter using a standard 1000-mm tape measure. The circumference of the forearm was defined as the perimeter of the largest part of the forearm, located over the bulk of the brachioradialis muscle, at the proximal quarter of the whole forearm length (Fig. 1a). The circumference of the hand (Chand) was measured as the perimeter of the middle part of hand, located at the two major transverse palmar creases ("heart line" and "head line") (Fig. 1b). Hand length was defined as the distance from the tip of the middle finger to the midline of the distal wrist crease (Fig. 1c). All anthropometric data were measured to the nearest millimetre with the forearm and hand in an outstretched and supinated position. Dominant side was defined as the hand with which the subject writes.
Dynamometer description
The Myogrip dynamometer (Ateliers Laumonier, France) is an isometric electronic device specifically developed for measuring grip strength in weak patients (Fig. 2a). It can directly display strength on its screen or be connected to a computer either by wireless, RS232 or BNC connections. Handle size is adjustable in a continuous way. It measures force in kg. It is calibrated on consecutive linear segments to compensate possible non-linear behaviour on the full nominal scale (90 kg). The resulting accuracy reaches 50 g on the whole measurement range with a 10 g resolution. To the best of our knowledge, the metrological performances of this innovative dynamometer are unique.
Calibration quality control
Devices were checked using standardized operating procedures for accuracy, hysteresis and repeatability. The procedure was adapted from the ISO 17025 norm. Twelve weights using M3 class masses were used for checking the calibration (0.2, 0.5, 1, 1.5, 2, 4, 5, 8, 10, 20, 30, 50 kg). Six MyoGrip devices and two Jamar devices were checked for calibration using this procedure. The devices were suspended to a bracket and the masses were directly applied on the handle. The Jamar was unable to accurately detect forces below 5 kg.
Experimental procedure
The subjects were seated on a height-adjustable plinth in order to obtain a right angle at the hip, knee and ankle joints with the legs being vertical and feet flat on the ground (Fig. 2b). The subjects had their shoulders adducted and their testing arm close to their body, with their elbow in full extension.
Subjects were verbally encouraged to produce their maximal grip strength (MGS). Two trials were first recorded, consisting of a 2-4-second maximal contraction, with a 30-second rest period between each trial. If the relative difference between these two MGS was within 10 %, no additional trial was required. If not, additional trials were proposed until two reproducible MGS were obtained. The maximal value of the two reproducible trials was retained for analyses. The contralateral side was then tested according to the same procedure. The first tested side (i.e., right or left) was random.
A subgroup of subjects agreed to return to retest their grip strength. The experimental conditions were the same as in the first session. The evaluator was either the same or another evaluator trained in the experimental procedures. Three evaluators performed the measurements for reliability assessment. The retest session was performed at least one day after the first session or planned within the next 3 months (mean: 31 days).
Jamar and Myogrip devices were checked for calibration before the recording period.
Statistical analyses
Norms were established in kg by age group categories of five years for younger subjects up to 20 years old and then by age groups of 10 years. In order to decide whether norms should be established according to the side tested or to the dominant side, the MGS values between the right and left sides were compared taking into account the dominance effect. The MGS values between the dominant and non-dominant sides in both right-handed and left-handed groups were compared by means of a paired Student t-test.
The difference between test and retest sessions was evaluated by taking into account the rater effect and the side effect for each function using a repeated measurements analysis of variance. The standard error of measurement, the coefficient of variation (CVar) and the limits of agreement according to Bland and Altman [16] were calculated. Correlation between MGS obtained by the MyoGrip and the Jamar dynamometers was tested using a correlation analysis (Pearson). To assess reliability within and between dynamometers, the intra-class correlation coefficient (ICC2,1) was computed as a single measure ICC with a two-way random-effect model (absolute agreement). The ability of the device to discriminate between two measurements was computed as the smallest detectable difference (SDD) according to Beckerman et al. [17]. The agreement between dynamometers was also studied using Bland & Altman plots.
Predictive analyses were performed only on the subjects aged less than 60 to avoid the influence of aging on the model. Indeed, according to norms (e.g., [18]) and functional studies (e.g., [19]), dynapenia becomes significant and accelerates after the age of 60. Stepwise linear regressions were performed to detect the best predicting variables for MGS. Variables tested were height, weight, age, sex, body mass index, percentage of body fat, hand circumference, hand length and forearm circumference. Since hand circumference was found to be the best variable in explaining inter-individual variance, various models using this variable alone were tested to define the best one in terms of explained variance. Predictive equations were applied to the subjects to compute predicted strength values. The statistical analyses were performed using SPSS (v19.0). The limit of significance for all tests was set at p < 0.05.