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Original Research

Is it Legitimate to Characterize Muscle Strength Using a Limited Number of Measures?

Bohannon, Richard W

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Journal of Strength and Conditioning Research: January 2008 - Volume 22 - Issue 1 - p 166-173
doi: 10.1519/JSC.0b013e31815f993d
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Abstract

Introduction

The successful completion of everyday activities such a standing from a sitting position requires that an individual possesses sufficient muscle strength (20). It is not surprising then that muscle strength is frequently measured in both clinical and community settings. When information about the strength of a specific muscle action is required, measurement of that strength is certainly warranted. In many cases, however, the intent of measurement is to characterize an individual's overall strength status. The use of a single measure, grip strength, has been suggested for such a purpose (1,4,32). Similarly, the use of a small number of measures has also been advocated for patients with stroke (18) and spinal cord injury (19). The use of a single measure or small set of measures to provide an overall indication of strength is understandable as it greatly reduces test burden. Nevertheless, the legitimacy of the practice is dependent upon the measures' relationship with other measures of strength. Although there is scattered and diverse literature addressing such relatedness, the literature has not been reviewed systematically to establish the legitimacy of characterizing muscle strength with a limited number of measures. The purpose of this systematic review, therefore, was to examine this legitimacy by summarizing the literature describing the relationship between strength measures of different muscle actions.

Methods

Relevant literature was first sought via an electronic search of four electronic databases: Medline/PubMed, Cumulative Index of Nursing and Allied Health, EMBASE, and Science Citation Index. Searches were conducted through 2005 using the terms “muscle” and “strength” in combination with “relationship,” “association,” “correlation,” “index,” “internal consistency,” “factor analysis,” “principal components analysis,” and “cluster analysis.” Identified articles were examined for relevance, as were articles located by examination of their reference lists. Additionally, personal files were searched for pertinent articles. Ultimate inclusion of an article was dependent on its provision of measurements of specific muscle actions (e.g., elbow flexion) from a defined population and its presentation of statistics reflective of a relationship between the actions. Articles were excluded if they only presented data from compound actions (e.g., bench press, leg-press, or squat), provided composite data (e.g., for a limb), used normalized strength (e.g., weak side/ strong side), or described strength relationships for a single joint (e.g., knee).

Relevant articles were abstracted for descriptions of the subjects enrolled, measurements used, muscle actions tested, and statistics reported. Statistical findings were delineated as possible according to limb(s) and side(s) tested.

Results

Twenty-five articles provided information on the relationship between the strengths of different muscle actions. Table 1 summarizes the literature that was identified as relevant and abstracted. The literature addresses the relationship in healthy subjects (7,11,15,21,23,26,27,31) and in patients presenting with an array of diagnoses (3,5,6,8-10,13,14,16,18,21,24,25,28-30), most particularly stroke. Strength data were obtained using muscle testing and various types of dynamometry. Studies included measures obtained from 3-13 muscle actions and from one to all four limbs.

Table 1
Table 1:
Summary of studies addressing the relationship between strength measurements.
Table 1
Table 1:
Continued.
Table 1
Table 1:
Summary of studies addressing the relationship between strength measurements.
Table 1
Table 1:
Continued.
Table 1
Table 1:
Summary of studies addressing the relationship between strength measurements.
Table 1
Table 1:
Continued.

Most authors described relationships simply by documenting correlation coefficients. All reported correlations were positive except for a few presented by Fowler and Gardner (21). The majority of correlations exceeded 0.50; many surpassed 0.90. Relationships were also described according to the internal consistency of strength measures of different muscle actions. Cronbach's α statistics, which were used for this purpose in eight studies, ranged from 0.73 to 0.97. Finally, in four studies, factor analysis was used to examine relationships. The strength of individual actions loaded strongly on a common factor (i.e., limb muscle strength) in each study. The loadings ranged from 0.55 to 0.96.

Discussion

This review addressed whether the strength of a single muscle action or small set of muscle actions should be used to characterize overall muscle strength. The Cronbach's α and factor analysis reported in the literature suggest that strength measures obtained from individual limb muscle actions are reflective of a common underlying construct or factor. It would seem legitimate, therefore, to use the strength of a few muscle actions to characterize the strength of a limb. Demeurisse et al. (18) and many other authors since have done just that with the Motricity Index. Specifically, they have characterized the strength of the upper limb using measures of prehension, elbow flexion, and shoulder elevation and strength of the lower limb using measures of ankle dorsiflexion, knee extension, and hip flexion.

The magnitude and positive direction of most of the correlations also suggest that measurements of strength obtained from some specific muscle actions may be reflective of measurements obtained from some other muscle actions. Several of the results of Fowler and Gardner are exceptions (21). Perhaps the correlations they reported were influenced by the reliability of their measures. Cohen and Cohen have indicated that the maximal achievable validity coefficient is a function of the reliability of the measures used in its calculation (17). The lower the reliability of the measures, the more the correlation between them will be attenuated. Even if certain findings of Fowler and Gardner (21) are considered to be outliers, the correlations between individual actions in other studies are not consistent or high enough to support the carte blanche use of a single action such as grip strength to characterize overall strength. Perhaps single actions can be recommended for more limited generalizations within specific populations; for example, upper limb actions among patients with stroke. In studies included in this systematic review, correlations between strength measures of an upper limb (paretic or nonparetic) ranged from 0.48 to 0.97. A better sense of the adequacy of a specific measure such as grip strength might be gained by a meta-analysis of correlations between grip strength and other specific measures (22). An overall meta-analysis of the data of this systematic review is precluded by the heterogeneity of the data.

The data from individual studies included in this systematic review justify the conclusions presented heretofore. Nevertheless, there are limitations. Despite a thorough and fastidious literature search, relevant articles may have been missed. Of the articles identified, those addressing activities involving multiple muscle actions (e.g., squat) were intentionally excluded as the activities neither identify specific muscle action strengths nor are possible for many individuals (e.g., patients with stroke). Their exclusion does not belie their potential value as indicators of overall strength. Also excluded were articles that focused on relationships between measurements obtained from a single joint (e.g., knee flexion and extension). Although such relationships may be important, they cannot provide insights into the issue of overall strength.

Practical Applications

Practitioners do not need to measure a large number of muscle actions if their intent is to characterize muscle strength. A small number will suffice. Characterization of the strength of a single limb should probably be restricted to a measurement obtained from that limb.

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    Keywords:

    measurement; statistics; dynamometry

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