Isokinetic testing of muscle strength of older individuals with sarcopenia or frailty: A systematic review

1.Introduction

Numerous studies have described muscle strength as decreasing with age. Such decreases are a highlight of geriatric syndromes such as sarcopenia and frailty. As both sarcopenia and frailty typically lead to mobility limitations and a loss of overall self-sufficiency in the later stages; the identification of muscle weakness is important for both mentioned conditions. The identification of sarcopenia and frailty is typically evaluated by handgrip strength in clinical practice [1]. Handgrip strength has practical advantages, as it is easy to measure and interpret relative to available reference values [1, 2]. On the other hand, handgrip strength has disadvantages; it does not focus on muscles underlying mobility limitations [3] or untoward events such as falls [4, 5]. Although research studies on sarcopenia sometimes include isokinetic strength measurements, there is still a lack of consensus as to the what level of isokinetic strength best differentiates individuals who are versus are not sarcopenic. The purposes of this systematic review, therefore were to summarize studies in which isokinetic measures have been used for detecting sarcopenia and to determine an isokinetic strength cut-point for its designation.

2.Methods

The present systematic review is reported in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) [6]. The following electronic bibliographic databases were searched: Web of Science, Scopus, and PubMed. Potentially relevant articles were identified on February 10, 2019. We used the same search string in all the databases (Table 1). After deleting duplicates, article titles and abstracts were examined to determine whether the articles identified by the searches addressed the isokinetic testing of muscle strength and focused on patients/participants aged ⩾ 65. We selected potentially relevant articles and then examined the full texts. Finally, we selected appropriate articles for the purposes of this review. Additionally, we hand-searched the reference lists of eligible articles and of several recently published reviews for additional studies.

We included all the articles from cross-sectional studies, validation studies, and randomized controlled trials in which isokinetic dynamometry was used for diagnosing participants aged ⩾ 65 with sarcopenia, frailty or decreased muscle function. Articles were excluded if they failed to provide procedural specifics. Review articles and articles in languages other than English were excluded. Included articles were examined for study specifics (e.g., design, participants and aims), isokinetic testing specifics, descriptions of how sarcopenia or frailty were described, and key findings. Of particular interest in regard to key findings were the validity, responsiveness, and magnitude of isokinetic measures.

Figure 1.

The PRISMA flowchart of the review selection process.

3.Results

Out of the 1,921 articles identified by the database search, we included a total of 17. An additional two articles were identified by hand searches. Ultimately, 19 articles that met the inclusion criteria were included in the review (Fig. 1).

There were 12 cross-sectional studies [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18], four randomized controlled trials [19, 20, 21, 22], one prospective study [23], one multicenter clinical trial [24], and one validation study [25] that were included. Five studies focused on frail people, 10 focused on people with sarcopenia and the other studies focused on people with muscle functioning problems that were defined differently. The randomized controlled trials examined the influence of several physical activity interventions on different muscle-related measures. One validation study tried to establish the validity of a novel portable trunk-muscle torque measuring instrument [25]. The cross-sectional studies tried to explore different relationships among muscle function or muscle mass and several independent variables, such as plasma levels of interleukin, the extent of apoptosis activation, and bone mass density. The main topic of one study was estimating the reference values of isokinetic measures in females with frailty [18] (Table 2).

The most frequently measured isokinetic strength was that of knee extension, followed by knee flexion. Both were measured with participants in a sitting position. The most commonly reported angular velocity was 60∘/s. Peak torque (Nm) was the most commonly reported measure. Several different diagnostic methods were used to identify muscle problems. The most common method was the sarcopenia algorithm by the European Working Group on Sarcopenia in Older People (EWGSOP) [1]. Validity was demonstrated by a correlation between standardized physical performance tests [7, 11], simple questionnaire to rapidly diagnose sarcopenia (SARC-F) [16], bone mineral density [12], and apoptotic signaling [14], several studies demonstrated that sarcopenic or fragile people were weaker than their healthy peers [9, 8, 13, 15, 17, 18, 23]. One study presented cutoff points for trunk flexion and extension torque [25]. Isokinetic dynamometry was used in 4 intervention studies [19, 20, 21, 22]. Finally, reliability was estimated in 1 study [24]. Summary of the methods in the selected studies is presented in Table 3.

The peak torque (Nm) in knee extension as well as in flexion varied considerably among the studies. In extension, the lowest and highest values in males net moment were 57 ± 24 Nm and 96 ± 25 Nm, respectively; the lowest and highest values in females were 38 ± 14 Nm and 87 ± 18 Nm, respectively. The peak torque in knee flexion also varied; the lowest and highest values in males were 31 Nm (at 95% CI 28–34) and 63 ± 20 Nm, respectively; the lowest and highest values in females were 18 Nm (at 95% CI 16–20) and 50 ± 17 Nm, respectively. The peak torque weighted average in knee extension was 83 ± 23 Nm for males and 60 ± 7 Nm for females. The peak torque weighted average in knee flexion was 47 ± 6 Nm for males and 36 ± 5 Nm for females. All weighted averages were significantly lower than the reference values for the healthy population aged 60–70 (p< 0.001). The results of this analysis are presented in Table 4. There was not enough information to calculate weighted averages in the faster velocity condition (180∘/s). However, the results at 60∘.s-1 were somewhat consistent (Table 5).

4.Discussion

Isokinetic dynamometry is a commonly used procedure for measuring muscle strength in athletes of many sports. Nevertheless, its potential is not fully realized in older people with muscle problems. Especially in people with sarcopenia, isokinetic dynamometry can be informative. This method is useful mainly in measuring lower extremity strength, which is essential for locomotion and is one of the most important factors of self-sufficiency in the elderly population. This study clearly documented that sex-specific isokinetic strength of the knee flexors and extensors at 60∘/s can detect sarcopenia and frailty in individuals; the sarcopenic values (males: knee extension 83 ± 23 Nm, females: knee extension 60 ± 7 Nm, males: knee flexion 47 ± 6 Nm, females: knee flexion 36 ± 5 Nm) are lower than the normative values of a healthy population [26, 27]. On the other hand, we found that isokinetic strength measurements are not consistent across studies; therefore, procedures should be standardized in measuring isokinetic muscle group strength, selecting contraction speeds and reporting values, just as they have been for handgrip strength.

Normative values for the healthy elderly population have been presented for isometric and isokinetic knee flexion and extension and for other muscle groups [26, 27, 28], where the strength of the knee extensors and flexors decreases with age by 2% or more per year [29]. Therefore, it is difficult to determine cutoff points and reference values for multifactorial syndromes such as sarcopenia. However, our reported sarcopenia values (Table 4) are similar to the isokinetic knee extension cutoff points (94.5 Nm in males and 62.3 Nm in females), which have been reported for predicting slow gait speeds [30]. This finding suggests that these cutoff points are useful in forecasting sarcopenia because decreases in muscle strength start to lead to a slower gait speed when there is a clear sign of sarcopenia progression. One of the main limitations of our reported reference values is the absence of isokinetic strength values relative to participants’ body mass; 2 studies [18, 25] successfully found sarcopenia cutoff values (Table 3) for females in knee extension (1.46 Nm/kg-) and for both sexes in trunk flexion and extension. This finding shows that body mass-related strength values might be more informative than the commonly reported raw maximal strength values in representing valid sarcopenia cutoff points.

Selecting muscle groups or muscle chains to test is an important issue in the diagnosis of sarcopenia. We found that most studies focused on the knee extensors and flexors as relevant muscle groups. This selection seems to be justified by knowledge from the rehabilitation field; the knee extensors are the first group of muscles to lose strength with any form of hypokinesis, and knee flexors decrease in strength when knee stability is disrupted. Only one study reported trunk flexion and extension strength [25], two studies reported ankle dorsiflexion and ankle plantarflexion strength [7, 15] and no studies reported hip abduction strength. Although trunk flexion and extension strength have been used to estimate sarcopenia cutoff points [25], we must mention that this test probably has lower validity than the other tests because the complex movements involved in this test are difficult and require more familiarization with the test and because trunk flexors and extensors are often shortened. Regarding ankle muscle groups, we recommend ankle dorsiflexion due to its relationship with physical performance and reported differences between healthy and sarcopenic populations [7, 15]; however, we do not recommend ankle plantarflexion, as its relations and differences have not been reported [7, 15]. The lack of evaluations of the hip abductors is quite surprising because of its relation to falls in the elderly population [31] and the relation between functional muscle activity and the strength ratios of the hip abductor muscles to the knee extensor and flexor muscles [32]. Therefore, we recommend testing of the knee extensors and flexors, which are the most frequently reported muscle groups, and suggest performing a verification study focused on the hip abductors for estimating strength in individuals with sarcopenia.

Seven studies considered multiple speeds of contraction in their approach, where most studies used 60∘/s as the basic speed in reporting strength. Since a higher speed of contraction (180∘/s) showed larger differences between sarcopenic and healthy females [13], the age-related strength decrease is larger in fast contractions than in slow contractions (60∘/s) [29], and the strength decrement in peak torque at 180∘/s may reflect greater losses in type II fiber function [8], we suggest that testing at a higher speed (180∘/s) might provide better reference values that have better relations with functional performance and muscle mass quality. On the other hand, we were not able to find a sufficient number of studies to estimate the cutoff points or reference values for males and females, such as the normative values for isokinetic knee extension and flexion strength at 180∘/s. Therefore, higher contraction speeds during isokinetic testing should be performed in future studies for identifying sarcopenia.

Another finding is that many previous studies reported isokinetic strength in groups of individuals of both sexes, which makes interpretation of the results impossible because the decreases in muscle strength are dramatically larger in males than in females; additionally, females are reported to have less muscle strength than males [27, 33, 34]. Therefore, we strongly recommend reporting isokinetic values by sex.

5.Conclusion

The isokinetic knee extension strength values at 60∘/s that can identify sarcopenia are 83 ± 23 Nm in males and 60 ± 7 Nm in females; in knee flexion, the values are 47 ± 6 Nm and 36 ± 5 Nm, respectively. Therefore, prevention interventions should be applied in potential sarcopenic populations if their strength levels are close to or below these reference values. Future studies should determine the sex-specific sarcopenia cutoff points for hip abductor, knee extensor and knee flexor isokinetic strength in relation to body mass and evaluate whether such values alone can be used as sarcopenia evaluation by itself.