The measurement of muscle force output is a sine qua non of physical performance evaluation in research and clinical environments. The isokinetic dynamometer is the gold standard1,2,3 as it provides a complete profile of muscle force output, but high cost and lack of portability severely restrict its application. In comparison, the handheld dynamometer (HHD) has been proposed as a low-cost and portable alternative for measuring muscle performance in the clinical setting.3 However, the ease of HHD may lead to misapplication and potentially misinterpretation.
HHDs consist of electronic and/or mechanic force sensors that need to be positioned perpendicular to the tested limb for accurate measurements. The tested individual performs a maximal effort while the tester resists applying an opposing force of equal magnitude to prevent movement. The measured force depends on the distance between the point of application of the dynamometer and the joint’s center of rotation. For a given torque generation, a lower force is measured when the dynamometer is applied further away from the joint. The torque, or moment of force, is recognized by the Greek letter tau (τ) and expressed by the following equation:
where F→ is the applied tester resistance force and d⊥¯ is the perpendicular distance from the location where the force is applied to the evaluated joint. Torque is expressed in units of Newton.meter (N.m), and is the standard measure to assess muscle performance with isokinetic dynamometers.
Previous studies have proposed standard protocols for HHDs,4 in which the device should be placed in a defined distance from a target anatomical reference. However, using an anatomical reference may lead to different readouts depending on the variations in the lever arm of individuals of various sizes. Thus, to consider the raw HHD output as the “muscle force” is misleading, as the measurement depends on the perpendicular distance between the dynamometer and the evaluated joint. Such misapplication can be identified in a recent study from Daloia et al.5 who reported the isometric muscle strength for four joints (shoulder, elbow, knee, and ankle) of boys and girls aged between 5 and 15 years. The authors suggested that their results can be used as reference for muscle strength in the Latin American population. However, without providing the results based on torque values, these measurements are not comparable to any population other than the studies group. Interestingly, Daloia et al.5 cited four studies that reinforce the need to consider torque estimation as a standard of measurements.6,7,8 Eek et al.6 suggested that “to obtain comparable measurements one must either put the device at the same distance from the joint or measure the distance (lever arm) and calculate the torque (force by distance)”. Eek et al.6 also suggest that “In growing children, this [limb length] leads to a change in distance from the joint center, which makes the latter method with calculation of torque more suitable”. Conversely, we also identified several studies that employed HHDs to measure force but without estimation of muscle torque, 8,9,10 indicating a possible misunderstanding of basic biomechanics concepts.
Therefore, this letter highlights the need for considering torque as the absolute, suitable estimate of muscle performance when using HHDs.
The authors thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES): - Finance Code 001.; and V.H.S. is funded by the Jane & Aatos Erkko Foundation.
