Abstract
Dear Sir,
Grip strength measured by dynamometer is an important and easily obtained indicator to evaluate grip function and even overall strength of both healthy subjects and patients. The American Society of Hand Therapists (ASHT) has published recommendations for maximal grip strength (MGS) assessment (Fess, 1992). However, this modus operandi has not always been used, sometimes leading to conflicting results. For instance, concerning elbow position, the ASHT recommend an elbow position of 90°. Several studies have observed higher MGS values for this elbow position compared to full elbow extension (Mathiowetz et al., 1985), while others have found the opposite (Oxford, 2000). Such results are probably influenced by the type of dynamometer used and by the age of the subjects involved. In respect to handle size, although the second handle position of the Jamar was recommended by the ASHT, some researchers have suggested that this setting might not be optimal as the testing standard, depending on gender, hand size or previous experience (Ruiz-Ruiz et al., 2002). Despite the large number of studies of MGS, some methodological issues are thus still not resolved, such as elbow position and handle size. Furthermore, the combination of these two issues has not been studied. We aimed to explore the influence of both elbow position and handle size for three dynamometers: the Jamar (considered by the ASHT as the gold standard), the Martin Vigorimeter and the MyoGrip, a highly precise dynamometer.
We examined a homogeneous sample of 18 young men (age: 23.7 ± 4.3 years; height: 177.2 ± 5.7 cm; weight: 73.3 ± 7.7 kg). All tests using the Jamar were performed at three handle sizes (positions 1, 2 and 3), while all three bulbs of the Vigorimeter were used (small, medium and large). All measurements with the MyoGrip used handle sizes corresponding to the same handle spans as the Jamar. For the evaluation, subjects were seated on a chair facing the evaluator, with their shoulders adducted and their testing arm close to their body. Two elbow positions were tested, either flexed to 90° or extended to 180°. When flexed, the elbow was sustained by an armrest. When extended, the arm was hanging down by the side of their body. The subject’s wrist was in light extension, while their back was in an upright position, with their knees at right angles and their feet flat on the floor. Subjects were given continuous verbal encouragements during all trials in order to motivate them to exert their true MGS. All measurements were performed by the same evaluator. Each subject was tested for both right and left hands, and for all three dynamometers. MGS for each dynamometer was measured twice for each hand, alternating between hands. The MGS was taken as the largest force from the two reproducible trials. Subjects were given rest periods between trials and dynamometers. The order in which the three devices, the two sides, the two elbow positions and the three handle sizes were tested was randomised in order to account for any measurement interaction, learning or fatigue effects. A repeated measurement ANOVA was performed to detect possible effects of device, side, handle size and elbow position and possible interactions between these factors. Unlike previous works, no significant effect of elbow position was found in the present study for three different dynamometers and three handle sizes (Fig 1). In actual tests, physiotherapists have reported that some patients have difficulty holding the elbow flexed at 90°, and gripping without compensations in this elbow position, thus increasing the difficulty in exerting maximal voluntary grip strength. Physiotherapists have also reported that subjects feel less stable when exerting maximal grip force with the elbow flexed.
For all dynamometers, the effect of handle size was significant; MGS generated at the medium handle position was significantly greater than those generated at the first and third positions (Fig 1). There was no significant interaction between different handle sizes and different elbow positions. However, a significant interaction effect between dynamometer and handle size was observed, meaning that the influence of handle size differed according to the dynamometer used. Among other factors, the effect of handle size is primarily dependant on anthropometric data of the hand, which has an effect on the optimal length of the grip muscles as a whole (length–tension relationship).
The Jamar dynamometer is recommended by the ASHT as the gold standard. However, as already underlined by Tyler et al. (2005), ‘this instrument may not be the most appropriate for all patient populations’, particularly because it is neither sensitive to detect reduced strength of weak patients, nor to detect small changes in MGS. For example, in the study of Massy-Westropp et al. (2004), 18 (out of 121 patients) subjects could not be assessed by the Jamar, although their grip strength was detectable by the Grippit, a digital grip handle. This was the main rationale for developing the MyoGrip, which is currently used to assess grip strength of adults and children suffering from various neuromuscular disorders. Since the MyoGrip can communicate with a computer, the strength assessment can be standardised using specific software or even games.
Finally, results of this study suggest that maximal grip strength assessment can be performed with the elbow extended, which seems more convenient in clinical practice. When an experimental study includes subjects with different hand morphology, grip handle size should be adjusted with respect to the hand size of the subjects.