Technical noteEstimation of grip force using the Grip-ball dynamometer
Introduction
Measurement of grip strength is an important component of many different evaluations. For instance, grip strength can be used to assess the effectiveness of a surgical or a therapeutical procedure as well as the effect of a rehabilitation programme [1], [2], [3]. Grip strength has also been used as a measure of general strength in order to determine work capacity [4] and provide insight into patients’ nutritional status [5]. Weak grip strength is also one of the five criteria of frailty identified by Fried et al. [6].
Grip strength is typically measured by means of a dynamometer, such as the Jamar (Sammons & Preston, Bolingbrook, IL, USA), the Lode (Lode dynamometer; Lode BV, Groningen, The Netherlands), and the Martin Vigorimeter (Martin Medizintechnik, Tuttlingen, Germany).The most widely used device is the Jamar, which is recommended by the American Society for Surgery of the Hand (ASSH) and the American Society of Hand Therapists (ASHT) [7]. The Jamar also has a series of published tables of normative data for a range of different populations [8]. Despite the widespread clinical use of the Jamar, the isometric nature of the device may lead to discomfort for some users [9], [10]. For instance, conditions such as rheumatoid arthritis [11], lateral epicondylitis [12], and in particular recovery from carpal tunnel release (CTR) surgery [13], can all lead to lower than expected grip-strength measurements with the Jamar. One of the alternatives to the Jamar is the Martin Vigorimeter, which consists of a bulb attached to a manometer via a tube, with the pressure applied to the bulb displayed on the manometer. The Vigorimeter is highly correlated with the Jamar [14], and is also comfortable to use [9]. Despite these positive points, the Vigorimeter does not actually measure force per se, meaning that it cannot be used to compare results to grip strength norms, for instance an evaluation of frailty using Fried's criteria.
An alternative dynamometer, the “Grip-ball”, was developed in 2008 [15]. This device consists of a supple ball in which pressure and temperature sensors, as well as a data acquisition and communication system have been placed. Communication is performed in real-time via Bluetooth, thus ensuring interoperability with other local devices that could store or transfer the data (computer, tablet, mobile phone, etc.). A comprehensive description of the device can be found in [16], [17]. In addition to the digital acquisition system in the Grip-ball, the other main difference with the Vigorimeter is that the Grip-ball is airtight and is equipped with a valve, meaning that the pressure inside the Grip-ball can be modified. Any changes in the initial pressure inside lead to changes in the stiffness of the ball, thus modifying the corresponding dynamics of grip-strength measurement.
In order for the Grip-ball to be used for grip-strength measurement, a model needs to be developed to predict the grip force applied based only on the pressure measured and the initial pressure inside the Grip-ball. It has already been shown that grip strength measured with the Jamar is highly correlated with the pressure recorded by the Vigorimeter. In the most comprehensive comparison, Desrosiers et al. reported a linear relationship between the measurements obtained from the two devices, with a correlation of r = 0.89 [14]. Given the linear relationship reported by Desrosiers, it was hypothesised in the present study that a similar linear relationship would exist between the force applied to the Grip-ball and the pressure measured by the device. In respect to the effect of the initial pressure in the Grip-ball, a linear relationship would be expected for all pressures, but the slope of this relationship would vary as a function of the initial pressure. The appropriate model would be expected to take the form:where F is the force applied, PA is the pressure applied by the user (the pressure measured by the sensor minus the initial pressure inside the ball), and a is the slope of the relationship that depends on the initial pressure. The aim of the present study, therefore, was to build a unique model to predict the force applied to the Grip-ball based only on the pressure measured by the Grip-ball and its initial pressure.
Section snippets
Experimental protocol
The Grip-ball is currently undergoing a final development to miniaturise the electronic components. During this development stage, the Grip-ball was evaluated using a hybrid device, partway between a Grip-ball and a Vigorimeter, in which the manometer of the Vigorimeter was replaced by the electronics of the Grip-ball. The device was rendered air-tight, but was able to be inflated to different pressures [17]. The hybrid device has previously been tested, with an excellent correlation obtained
Results
A best-fit linear regression equation for PI of 100 kPa (atmospheric pressure) between the force applied and the change in pressure inside the ball was:with PA expressed in kPa and F in kg, and R2 = 0.99.
This equation was then overlaid onto the data obtained from the most highly cited comparison between the Jamar and the Martin Vigorimeter, those of the paper of Desrosiers et al. [14] (Fig. 2). The regression equation obtained for the Desrosiers data was:which corresponds to a
Discussion
The aim of the present study was to build a model for the Grip-ball in order to predict the force applied to the ball using only the pressure recorded. Previous publications using the Grip-ball had provided a technical description [16], a validation compared to the Martin Vigorimeter [17], and a reliability study for different initial pressures [17]. In respect to the relationship between grip force and the pressure inside an object being squeezed, Desrosiers et al. had shown grip force, as
Conclusion
The model developed was able to accurately estimate force based only on the pressure applied, irrespective of the initial pressure inside the Grip-ball. Such a result ensures that the pressure inside the Grip-ball can be regulated according to the stiffness required by the clinician, without adversely affecting the quality of the measurement of grip-force. In respect of the future use of the Grip-ball, it is encouraging that the model works best over the force range that is of the most interest
Conflict of interest
None declared.
Funding
None.
Ethical approval
Not required.
Acknowledgements
This work was supported by the Champagne-Ardenne Regional Council (CRCA) and the European Regional Development Fund under the Collaborative Research Program (Domo-Grip Project, grant reference numbers CRCA E201012437 and FEDER E201013375).
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