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Detecting Submaximal Effort in Power Grip by Observation of the Strength Distribution Pattern

From the Department of Traumatology, Hand, Plastic and Reconstructive Surgery, University of Ulm, Germany and the Department of Hand, Plastic and Microsurgery, Karl-Olga Hospital, Stuttgart, Germany

Correspondence: Dr Joachim Gülke, University of Ulm, Ulm, Germany. E-mail:joachim.guelke{at}medizin.uni-ulm.de

	Abstract

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This study examined patterns of grip strength when maximal and submaximal effort are applied. Using a sensor glove, 50 healthy subjects performed two different power grips. Both maximal and submaximal gripping showed characteristic patterns of strength distribution that were independent of the degree of power applied. Significant differences were also noted in the strength distribution patterns, depending on whether the grip was performed at maximal, or submaximal, strength. The small finger plays a decisive role in this. In maximal strength gripping, the total measured strength is distributed relatively evenly over all four fingers, with each finger contributing between 23% and 27% of the total strength. In submaximal strength gripping, the little finger is involved very little and only contributes between 14% and 15% of the total strength, with the remainder of the gripping distributed relatively evenly between the index, middle and ring fingers, each of which contributes between 26% and 32% of the total.

Key Words: grip strength • hand • sensor glove • sincerity of effort

Strength measurements are a central element of clinical examination of the hand. They play an important role in the clinical appraisal of the effects of injuries and the control of therapy. When using conventional measuring devices, subjectivity is, however, a problem, as the measurements depend on the co-operation and motivation of the patient. Previous studies have tried to distinguish between a fictitious and a genuine loss of strength caused by impairment. Gilbert and Knowlton (1983) examined variations in the gradient of strength curves. They considered not only the increase of strength at the beginning of the grip but also the correlation of average strength to maximum power during the grip. Mentzel et al. (2001) also identified significant differences in the characteristics of the respective strength curves by the evaluation of load–time diagrams during maximal and submaximal gripping.

The TUB-sensor glove, developed at the Technische Universität Berlin (TUB), enables continuous measurement of grip power, which can be depicted in load–time diagrams. The strength distribution pattern of the grip is recorded through ten sensors applied to the palmar surface of the hand (Fig 1) and evaluated on computer. This study was carried out using this glove to determine whether it is possible to assign an individual strength distribution pattern to the different grip sizes of the dynamometer and whether it is possible to recognise submaximal effort, corresponding to a pretended loss of strength, on the basis of characteristics in the strength distribution pattern.

View larger version (74K): [in this window] [in a new window]   Fig 1 Technische Universität Berlin (TUB)-Sensor glove – palmar view.

	MATERIALS AND METHODS

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View larger version (20K): [in this window] [in a new window]   Fig 2 Load–time diagram of a Power Grip II with submaximal effort.

View larger version (18K): [in this window] [in a new window]   Fig 3 Load–time diagram of a Power Grip II with maximal effort.

View larger version (60K): [in this window] [in a new window]   Fig 4a, 4b Power Grip I (broad grip) and sketch of the position of the relevant pressure sensors (D, distal sensors).

View larger version (57K): [in this window] [in a new window]   Fig 5a, 5b Power Grip II (narrow grip) and sketch of the position of the relevant pressure sensors (P, proximal sensors).

For both types of grips, evaluation included data from the sensors on the second to the fifth digit. The strength of each finger was taken as the average value of three grips, held for 20 seconds and detected with the sensor appropriate to that grip. In Power Grip I, the distal sensors were evaluated. In Power Grip II, the proximal sensors were evaluated (Figs 4 and 5).

For both types of grips, the proportion of the total measured strength of all four fingers due to each finger was calculated by taking the average values for the 50 test subjects. These averages were represented on bar graphs, stating the standard deviation. To illustrate the results, the data for submaximal and maximal effort are shown in a diagram (Figs 6 and 7).

View larger version (16K): [in this window] [in a new window]   Fig 6 Strength distribution pattern for the four fingers during Power Grip I (broad grip). For each finger the results of submaximal effort (left) and maximal effort (right) are displayed.

View larger version (16K): [in this window] [in a new window]   Fig 7 Strength distribution pattern for the four fingers during Power Grip II (narrow grip). For each finger the results of submaximal effort (left) and maximal effort (right) are displayed.

	RESULTS

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Figs 6 and 7 show the strengths of the individual fingers as a proportion of the total measured finger strength. The different grips can be differentiated by their individual strength distribution patterns. In Power Grip I, broad grip, the proportion of the strength generated by each finger decreases from radial to ulnar (Fig 6). In Power Grip II, narrow grip, the central rays contribute a higher proportion of the total strength than the peripheral ones (Fig 7).

For power grip performed at maximal effort, the strength is distributed relatively evenly over all four fingers. In Power Grip I, broad grip, the index (26%) and middle finger (26%) contribute a greater proportion of the total strength than the ring (24%) and little (24%) fingers (Table 1 and Fig 6). In Power Grip II, narrow grip, the two central rays, viz the middle (26%) and ring (27%) fingers, contribute a greater proportion of the total strength than the peripheral rays, viz the index (24%) and little (24%) fingers (Table 2 and Fig 7). These different strength distribution patterns are also preserved when the grips are performed with submaximal effort.

	DISCUSSION

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To assess the level of strength applied, Stokes et al. (1995) compared the strength values obtained at the five different settings of a Jamar-Dynamometer. Subjects applying full strength reached maximal strength values in the middle dynamometer grip position, which was illustrated graphically as a bell-shaped curve. In contrast to this, the strength values obtained with a pretended loss of strength appeared on the graph as a horizontal line, because the grip strengths were approximately the same in all grip positions. These findings, however, could not be confirmed by other authors (Hamilton et al., 1994; Hildreth et al., 1989; Janda et al., 1987; Niebuhr et al., 1993; Niebuhr, 1996; Niebuhr and Marion, 1987, 1990; Shechtman et al., 2005).

In another study, dynamometer measurements were combined with EMG measurements for the flexor muscles of the forearm (Niebuhr et al., 1993). While the correlation of a higher EMG frequency with submaximal effort, as described by these authors, could not be confirmed by other authors, it was generally agreed that the amplitude of the EMG signal could be used to assess the level of strength applied (Gilbert and Knowlton, 1983; Hoffmaster et al., 1993; Kroemer and Marras, 1981; Niebuhr et al., 1993; West et al., 1995). Because of the difficulties in interpreting the results and the necessity of repeating the measurements many times, this method was deemed unsuitable for clinical practice.

In the Rapid Exchange Grip Test and the Rapid Simultaneous Grip Test, quick grip repetitions were carried out, either by alternating both hands or by gripping simultaneously (Hildreth et al., 1989; Joughin et al., 1993). This test was intended to make subjects lose control over the level of strength they were applying. Carrying out these tests was, however, a lengthy and complicated process and some subjects were unable to cope. Consequently, the suitability of this method for routine use is questioned (Joughin et al., 1993; Westbrook et al., 2002).

Gilbert and Knowlton (1983) used a measuring apparatus that enabled them to provide a continuous display of strength values in strength–time diagrams. Different criteria were defined to assess the course of the strength curve. They considered not only the increase of power at the beginning of the grip but also the correlation of average power to maximum power during the grip. Other studies showed that the coefficient of variation, i.e. the quotient of the standard deviation of strength values in a single measurement and the average value, is an important evaluation criterion (Chengalur et al., 1990; Dvir, 1999; Fairfax et al., 1995). The fluctuations in the strength curves, both during and between two grips were greater at submaximal than at maximal effort.

With the TUB-sensor glove, the strength of the grip can be measured at up to ten points on the hand. The signals of the sensors are displayed in load–time diagrams. A study on healthy subjects has already shown that the dynamics of the sensor measurements of a grip with maximal effort are significantly different to those of a grip with submaximal effort (Mentzel et al., 2001). The strength curves of gripping with submaximal effort showed significantly greater oscillations than those on gripping with maximal effort. The latter showed a remarkably greater uniformity of strength values during a single grip and, also, during grip repetitions.

In addition to the possibility of examining the dynamics of each sensor, measurement with the TUB-sensor glove also allows for the assessment of the contribution of each finger to the total grip strength of all four fingers. The absolute strength values are of less importance than the relationship of the grip strengths of the fingers to each other and to the total grip strength of all four fingers.

Mitterhauser and colleagues also examined effort-related strength distribution in the hand. They used a two-key dynamometer, the NK DIGIT-Grip, which measured the strength of the index and middle fingers as one value and the strength of the ring and little fingers as another. They found that a greater proportion of the strength being contributed by the ulnar digits, as opposed by the radial digits, signified submaximal effort (Mitterhauser et al., 1997). We have been unable to confirm these results with our data but it has to be borne in mind that completely different grips were being examined in the different test arrangements. A TUB-sensor glove can measure strength via ten sensors with more spatial discrimination than a two-key dynamometer. The subjects can also grip with considerably greater ease as the pressure sensors are attached directly to the fingers and the degree of separation of the individual fingers can be chosen at random.

As we were able to measure the strength of gripping of each finger individually, our study was able to identify the strength distribution pattern between fingers for each of the examined grips. In Power Grip I, with a wide gripping position, the proportions of strength applied by the individual fingers tends to decrease from radial to ulnar. In Power Grip II, with a narrow gripping position, the central rays provide a greater proportion of the total strength than the peripheral rays. These individual strength distribution patterns were stable, irrespective of whether the grip was performed with maximal or submaximal effort. However, differentiation of variations of grip strength is possible for both grips by the variability of the participation of the small finger.

The analysis of grip power by this technique for the purpose defined in this study requires functional integrity of the fingers. This method could also, however, be valuable in providing objective evaluation of grip power in injuries and diseases affecting the proximal parts of the hand, such as the carpus or the wrist, which do not affect the functional integrity of the fingers.

Manuscript received November 19, 2005. Accepted for publication May 23, 2007.

	References

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Chengalur SN, Smith GA, Nelson RC et al (1990). Assessing sincerity of effort in maximal grip strength tests. American Journal of Physical Medicine and Rehabilitation, 69: 148–153.[Web of Science][Medline] [Order article via Infotrieve]

Journal of Hand Surgery (European Volume), Vol. 32, No. 6, 677-683 (2007)
DOI: 10.1016/J.JHSE.2007.05.020


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