Detection of effort maximality in adults performing isokinetic wrist flexion and extension

1. Introduction

Injuries involving the elbow and forearm, especially lateral and medial epicondylitis, are, together with those involving the shoulder, the knees and the back, among the main causes of occupational disease and musculoskeletal disorders [1]. Evaluation of forearm muscle strength should be an essential component to assess the functional status of patient presenting with upper limb disorders. In patients with epicondylitis, forearm muscle strength assessment has been used as an evaluation or research tool [2, 3]. In fact, exercise is considered to be the cornerstone of rehabilitation [4] and althought the dose and type is still not fully clarified, strengthening programs have shown to be effective in epicondylitis treatment [5, 6, 7]. Wrist muscle strength measurement performed using isokinetic dynamometry provides accurate and reproducible findings [8, 9, 10]. Isokinetic test results can serve as guidelines for planning and monitoring strength-training programs, as well as for establishing permanent impairments. However, patient collaboration is crucial in order to generate reliable results. Thus estimation of the level of collaboration, namely maximality of effort, during an isokinetic strength test is of great interest, particularly when medico-legal claims are involved.

A number of parameters such as coefficient of variation (CV), eccentric to concentric ratios (E/CR) and, specially, difference between isokinetic eccentric to concentric strength ratios at high and low velocities (DEC) have been advocated to differentiate between maximal and submaximal efforts using isokinetic testing [11, 12, 21, 22, 13, 14, 15, 16, 17, 18, 19, 20]. Among them, the DEC is the most widely accepted. Specifically, DEC has been proven to be highly effective at identifying feigned maximal muscular effort in different joints and actions in healthy adults [11, 12, 13, 14, 15, 16, 17, 21, 22] and in patients [18, 23, 24].

It has been postulated that the effectiveness of DEC depends on the different central nervous system pathways of activation and control of concentric contractions compared to their eccentric counterparts [25, 26]. It has also been proven that submaximal eccentric strength modulation differs from concentric one [27]. Such fact may explain that in submaximal efforts, i.e. when individuals try to produce less strength than what they are capable of, E/CR increases have been observed, especially at high velocities, which in turn increase the value of the DEC. This increase in DEC in submaximal situations makes it possible to establish, based on studies involving volunteers, cut-off levels to determine whether an isokinetic test hast been submaximal, namely whether the individual has not developed maximal possible strength [11, 12, 13, 14, 15, 16, 17, 21, 22].

To the best of our knowledge, the efficiency of the DEC for detecting submaximal wrist extensors (WE) and wrist flexors (WF) isokinetic strength has not been studied, hence the present study.

As has been mentioned above, DEC is a valid parameter for detecting submaximal effort in different joints. So, the present study hypothesizes that the strength in concentric and eccentric modalities in two velocities could led to a valid DEC results in order to determinate a submaximal effort in wrist muscles.

Therefore, the research question for this experimental criterion standard study was:

Is the DEC a useful tool for identifying submaximal effort during WE and WF isokinetic effort performance?

2. Method

Tables 1 and 2 outline the mean PM values for WE and WF respectively in the two experimental conditions. In both muscle groups, the mean PMs at maximal performance were significantly higher than those registered for the feigned maximal counterpart ( P ⩽ 0.001).

The differences in PM of the WE between maximal and sub-maximal effort (Table 1) were significantly lower in high velocity eccentric measurements than their concentric counterparts ( P = 0.004). Low velocity differences did not yield significant differences. On the other hand, WF analysis did not indicate significant differences regarding maximal and submaximal PM differences at any instance (Table 2).

Table 3 outlines the mean values (SD) of the E/CR for WE and WF in both velocities and conditions (maximal and submaximal). For all modes and velocities, E/CR values at submaximal condition were higher than those found for the maximal condition. Significant differences regarding E/CR values were demonstrated between maximal and feigned maximal contractions in all cases except for WE at low velocity.

Feigned effort DEC values (Table 3) were significantly higher than their maximal effort counterparts, for both WE ( 0.0001) and WF ( P = 0.030). Cut-off values were then calculated using the DEC scores to allow the identification of feigned maximal efforts in case these values were exceeded.

The cut-off score derives from the tolerance intervals for normal distribution of the DEC values. Thus, for a level of confidence (l.o.c.) of 95%, labelling effort as submaximal requires that the DEC exceeds 0.384 and 0.317, for WE and WF, respectively. Obviously, a higher l.o.c. is associated with a higher cut-off score namely, to be confident at 99%, the DEC rises to 0.576 and 0.496, respectively whereas for 90% the cut-off values drop to 0.303 and 0.245, respectively (Table 4).

For each of these cut-off values, Table 5 outlines the corresponding values for sensitivity and specificity. In all maximal effort conditions, the DEC scores were lower than the 99% cut-off value, resulting in a specificity of 100%. Lower cut-off levels (90% and 95%) have not led to a lower DEC specificity in either WF or WE. On the other hand, the sensitivity of the DEC increased as lower confidence levels were selected while being higher submaximal WE effort compared to its WF counterpart.

4. Discussion

This is the first study to assess feigned maximal efforts of WF and WE in healthy adults. The results indicate that DEC can be an efficient tool for detecting submaximal tests.

The main result of this study is that in healthy male volunteers feigned maximal efforts of WF and WE can be efficiently detected using the DEC. Given its associated intricacies this result deserves further elaboration.

This findings become more relevant since injures involving the forearm are very frequent in musculoskeletal rehabilitation and occupational medicine settings. If these injuries involve a muscular component, which eventually leads to weakness, isokinetic dynamometry is the tool of choice for its evaluation. In addition to its high accuracy and reproducibility, the incorporation of different contraction modes and velocities enable a decision regarding patient’s collaboration, a critical condition for the clinical validity of the findings. As shown before, the DEC provides an efficient tool for such objective both in normal subjects [11, 12, 13, 14, 15, 16, 17, 21, 22] and patients [18, 23, 24], but its values are muscle group-specific. Moreover, for vindication of its application in select patient cohorts, its validity in healthy adults regarding the same muscle group is an essential preliminary step, leading to the current study of hitherto unexplored WF and WE muscle groups.

In the present study, the concentric and eccentric maximal effort PM values in WE: 12.96 Nm and 12.56 Nm (10 ∘ /s), 12.84 and 12.58 Nm (40 ∘ /s), respectively, were in the range of the reported strength in healthy men for concentric effort: 11.3 Nm (30 ∘ /s) but lower than the eccentric component: 17.1 Nm at the same velocity [8]. It is important to note at this point that both eccentric PMs (at 10 ∘ /s and 40 ∘ /s) were slightly lower than their concentric counterparts. This finding is in variance with the general observation namely that in normal participants the eccentric strength is higher than its concentric counterpart within the same velocity resulting in an E/CR $>$ 1.0 [28]. However, both E/CRs were very close to unity with 1.09 and 0.98 at 10 ∘ /s and 40 ∘ /s, respectively. Therefore, this slight deviation may be attributed to the relatively low-test velocity [28].

Regarding WF, the maximal effort concentric and eccentric PM mean values (10 ∘ /s: 18.04 Nm; 40 ∘ /s: 18.84 Nm and 10 ∘ /s 20.21 Nm; 40 ∘ /s: 21.11 Nm respectively) were lower than the mean PM values reported in the a previous study (concentric values: 30 ∘ /s: 26.1; 90 ∘ /s: 26.7 Nm; and eccentric mean values 60 ∘ /s: 33.4 Nm) [8]. The gaps noted are due both to the velocity of testing and the use of a different dynamometer. Interestingly, present sample WF tests reveal clearly higher eccentric measurements than concentric ones.

The PM values during the performance of maximal effort were as expected significantly higher than those derived from the sub-maximal condition in all instances and in WF and WE alike. However, this difference was not uniform contraction wise; in WE the mean maximal and sub-maximal PMs at high velocity in eccentric mode were much closer than in concentric mode, an outcome reflecting the more limited central controllability of sub-maximal eccentric contraction control [25, 26, 27]. This finding was not as apparent with respect to WF probably due to a higher voluntary control of these muscles.

Most sub-maximal effort E/CR values in the present study were higher than their maximal effort counterparts. The exception was in low velocity (10 ∘ /s) wrist extensor E/CR. Thus, for WE at 40 ∘ /s the high velocity E/CR was significantly greater in the sub-maximal effort while remaining substantially the same at the 10 ∘ /s test. As highlighted below, these findings provide for a higher power of the DEC in the detection of sub-maximal WE effort, whereas its utility in doing so vis-à-vis WF is slightly lower due to a smaller inter-E/CR difference (see Table 3).

The analysis of the WE and WF DEC (Table 4) showed, as expected, that submaximal effort value were significantly higher than their maximal effort counterparts. The mean DEC value for maximal WE effort (0.01) was well within the DEC value range reported in previous studies performed in other muscle groups ( - 0.007–1.06), whereas WF one ( - 0.013) was slightly lower than the lowest score that was previously registered ( - 0.007 in elbow flexion) [13]. The average DEC values in sub-maximal effort for WE (0.44) was within the range found in previous studies on healthy volunteers, largely between 0.4 and 2.2 [11, 12, 13, 14, 15, 16, 17, 21, 22]. On the other hand, WF DEC sub-maximal effort average value (0.20) was much lower than previously found. One possible explanation is a more refined voluntary control of both concentric and eccentric contractions in forearm wrist flexor muscles.

Based on the maximal DEC values the cut-off scores for admission/rejection zones were calculated (Table 4). These values may be of high significance in relevant medico-legal cases as well as in general clinical practice where isokinetic tests are utilized. However, high sensitivity and specificity are required in order that they are validly applied. At the same time one must bear in mind the interaction between the latter and the pre-set level of confidence (l.o.c.) namely the higher the l.o.c. the higher the specificity while the lower the l.o.c. the higher is the sensitivity, as amply evident from Table 5 which outlines the different sensitivities and specificities obtained using 90%, 95% and 99% l.o.c. Notably the DEC retained its perfect specificity (100%) even at a l.o.c. of 90% for both muscle groups. In addition, given the substantial stability of the specificity of the DEC and given the wide use of 95% as the standard l.o.c. we recommend its application in testing full compliance in testing WE and WF strength.

In the present study the DEC was efficient in testing sincerity of effort using an isokinetic test performed at a very short wrist ROM (namely 20 ∘ ). The use of short ROM tests for this objective has been previously advocated [29]. In particular, they may be highly useful for routine clinical practice when a longer ROM cannot be sustained due to, among other things, pain or joint instability.

However attractive the use of the DEC in the medicolegal arena may be, it is essential to keep in mind that the collaboration of a patient during a diagnostic test can be affected by a wide variety of causes such as pain, fear of suffering pain, fear of being injured, anxiety, depression, misunderstanding of the instructions or of the importance of the test and, obviously, interest in obtaining financial benefit [30]. Therefore, it is very important to have a detailed anamnesis, a complete physical exploration (so as to detect the existence of inconsistencies with the clinic) and other complementary tests that might show the presence of injuries, which could explain any deficiencies, before assessing the patient’s collaboration. It is also of essence to note that the current values provide a baseline only to which future studies of muscular strength in patients with wrist dysfunction should refer. As more recently indicated [23] the DEC values of the uninvolved side of patients impaired with rotatory dysfunction of the shoulder furnished a more valid reference than those derived from normal subjects.

We conclude that the DEC may be a valuable parameter for detecting feigned maximal effort in wrist flexion-extension isokinetic tests in healthy subjects. Further research should focus on patients with injuries involving the wrist, both in medico-legal situations and in routine clinical practice.

Author contributions

CONCEPTION: Joaquim Chaler and Eduard Pujol.

PERFORMANCE OF WORK: Mercè Torra and Eduard Pujol.

INTERPRETATION OR ANALYSIS OF DATA: Joaquim Chaler, Anna Maiques, Salvador Quintana and Eduard Pujol.

PREPARATION OF THE MANUSCRIPT: Mercè Torra and Joaquim Chaler.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Roser Garreta and Joaquim Chaler.

SUPERVISION: Roser Garreta.

Ethical considerations

The Hospital Universitari Mútua de Terrassa Ethics Committee approved this study on 10/01/2010. All participants gave written informed consent before data collection began.

Funding

The authors report no funding.