Open Access Original
Article

DOI: 10.7759/cureus.26106

Verification of Shoulder External Rotators
Strength Measurement Using a Suspension Scale
Atsushi Ueda 1 , Yasuhiro Mitani 2 , Hitoshi Koda 2 , Toshimitsu Omine 2 , Ryuta Inada 3 , Naoyuki Konishi 4 ,Shunsai Mori 5

Review began 05/19/2022
Review ended 06/16/2022
Published 06/20/2022© Copyright 2022
Ueda et al. This is an open access article
distributed under the terms of the Creative
Commons Attribution License CC-BY 4.0.,
which permits unrestricted use, distribution,and reproduction in any medium, provided
the original author and source are credited.

1. Department of Rehabilitation, Hankai Hospital, Sakai, JPN 2. Department of Rehabilitation Sciences, Faculty of Allied
Health Sciences, Kansai University of Welfare Sciences, Kashiwara, JPN 3. Department of Rehabilitation, Shimada
Hospital, Habikino, JPN 4. Department of Physical Therapy, Osaka Isen, Osaka, JPN 5. Department of Rehabilitation,
Miyama Clinic, Amagasaki, JPN
Corresponding author: Atsushi Ueda, a_c.sjnwtpts@icloud.com

Abstract
BackgroundOf the shoulder external rotators, the infraspinatus and teres minor are the key muscles that contribute to
the dynamic stability of the shoulder. It is crucial to properly measure the strength values to evaluate muscle
function and training load for shoulder external rotators. A suspension scale (SPS) can measure the mass ofthe suspended object, and it may be possible to apply it to measure strength. However, the utility of strength
measurements using an SPS has not been clarified in previous studies. In this study, we aimed to investigate
the intra-rater reliability of measuring the strength of shoulder external rotators using an SPS and therelationship between strength measurement using an SPS and a handheld dynamometer (HHD).

Methodology
The participants were 10 healthy males with 20 shoulders (24.5 ± 2.5 years old; height = 172.8 ± 5.4 cm;
weight = 69.6 ± 8.1 kg). Upper extremity strength was measured at 90° shoulder abduction, 90° externalrotation, 0° horizontal adduction/abduction, 90° elbow flexion, and 0° forearm pronation/supination in theprone position. The isometric strength of shoulder external rotation was measured with the SPS and HHD,and one examiner measured the maximum strength value. The intra-rater reliability of the two methods
using SPS and HHD was evaluated using the intraclass correlation coefficient (ICC1,2), standard error ofmeasurement (SEM), minimum detectable change (MDC), and Bland-Altman analysis. The relationshipbetween the SPS and HHD was calculated as the correlation coefficient between the strength values of SPS
and HHD.

Results
The intra-rater reliability of the strength measurement of shoulder external rotators using SPS was ICC 1,2
0.98 (95% confidence interval = 0.95-0.99), and SEM and MDC were 0.3 and 0.9, respectively. The
measurements using SPS had no fixed and proportional biases. A significant positive correlation wasobserved between SPS and HHD (r = 0.94, p < 0.01).

Conclusions
The SPS is an alternative to the HHD for measuring the strength of shoulder external rotators. Thus,
measuring the strength of shoulder external rotators using an SPS may be applied as a cost-effective andportable assessment method for shoulder function.

Categories: Physical Medicine & Rehabilitation
Keywords: portable device, reliability, measurement, strength, shoulder

Introduction
Although the glenohumeral joint (GHJ) has a large range of motion owing to the small size of the glenoid
fossa with the humeral head, the joint structure has minimal bony stability [1,2]. To compensate for GHJ
instability, the rotator cuff, composed of the infraspinatus, teres minor, supraspinatus, and subscapularismuscles, contributes to dynamic stabilization [3]. Particularly, the infraspinatus and teres minor muscles
contribute to suppressing the anteroposterior and superior translation of the humeral head during shoulder
elevation motion [3]. In contrast, weakness of shoulder external rotators such as the infraspinatus and teresminor muscles increases humeral head translations during shoulder motion [4]. Hence, shoulder externalrotator weakness is involved in the onset of shoulder joint disorders, such as rotator cuff injuries [5,6], labral
injuries [5,7], and subacromial impingement [8]. Moreover, Clarsen et al. reported that overhead handballathletes with shoulder external rotator weakness had an increased risk of shoulder disorders [9]. Thus, it is
crucial to properly assess the strength of the shoulder external rotators for physical therapy and injury

How to cite this article
Ueda A, Mitani Y, Koda H, et al. (June 20, 2022) Verification of Shoulder External Rotators Strength Measurement Using a Suspension Scale.
Cureus 14(6): e26106. DOI 10.7759/cureus.26106

prevention.
The isokinetic dynamometer allows us to measure the strength of the shoulder external rotators throughout
the range of motion at various motion speeds and testing modes (i.e., concentric, isometric, and eccentricmodes) [10]. Previous studies have reported that an isokinetic dynamometer has excellent reliability inmeasuring strength [11]. However, they are expensive and not portable [12]. The handheld dynamometer(HHD) is a portable device that is used as a standard method to assess maximum strength without limiting
the measurement surroundings [12]. Hence, it is critical to develop simple methods of strength
measurement that can be used in a variety of fields with various portable devices such as the HHD.
Thus, we developed a simple method to measure the strength of the shoulder external rotators using asuspension scale (SPS). An SPS can weigh the mass of a suspended object. Moreover, they are low-cost andportable. Therefore, measurement of shoulder external rotator strength using an SPS may represent an
inexpensive means to provide accurate measurements of the maximum strength. Hence, an SPS may be used
to measure the strength of the shoulder external rotators. However, in previous studies, the utility ofstrength measurements using an SPS has not been clarified. Thus, we hypothesized that the SPS could be
used as a reliable method for measuring the isometric strength of shoulder external rotators. Moreover,strength measurements of the shoulder external rotators using the SPS correlate with those using the HHD.
The primary aim was to establish the intra-rater reliability of measuring the strength of shoulder externalrotators using the SPS. The secondary aim was to investigate the relationship between the strength values
acquired using the SPS and HHD.

Materials And Methods
Research design
This was a cross-sectional reliability study.

Participants
In total, 10 healthy men with 20 shoulders (age = 24.5 ± 2.5 years; height = 172.8 ± 5.4 cm; weight = 69.6 ± 8.1kg) were included in this study. Patients with shoulder pain and an upper extremity surgical history were
excluded from this study. This study was conducted in accordance with the Declaration of Helsinki and wasapproved by the ethical review board of Hankai Hospital (approval number: 2022-001). Written informed
consent was obtained from all participants.

Methods
To measure the strength of the shoulder external rotators, the SPS (Guangzhou Weiheng Electronics Co.,
Ltd., Guangdong, China) and pull-type HHD (Sakai Medical Co., Ltd., Tokyo, Japan) (Figure 1) were used. The
SPS and pull-type HHD can measure the strength of pulling non-elastic belts attached to both ends of eachdevice. The SPS and HHD can measure from 0.00 kg to 50.00 kg and from 0.0 kg to 150.0 kg, respectively.

FIGURE 1: Suspension scale and handheld dynamometer.

Regarding the experimental setting, to measure the strength of the shoulder external rotators, the2022 Ueda et al. Cureus 14(6): e26106. DOI 10.7759/cureus.26106

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participants were laid in a prone position on the bed in the following positions: 90° shoulder abduction, 90°
external rotation, 0° horizontal adduction/abduction, 90° elbow flexion, and 0° forearm
pronation/supination (Figure 2). Additionally, to maintain the measurement position of 0° shoulder
adduction/abduction, the proximal humerus was placed on a 5 cm platform.

FIGURE 2: Strength measurement of shoulder external rotators using asuspension scale.

The experimental protocol of this study is shown in Figure 3. One non-elastic belt attached to both ends of
the SPS was vertically fixed on the distal forearm, and the other was immobilized on the bar fixed on thefloor (with loading lower extremity of one examiner) (Figure 2). The strength measurement of the HHD was
performed using the same method (experiment setting). Both devices were removed and the experimental
setup was returned to the same condition after each test. Prior to the measurement, the measurement
method instructions (SPS and HHD) were explained to the participants, and each strength measurement waspracticed several times. The isometric strength of the shoulder external rotators was measured for fiveseconds to achieve maximum strength [13]. The measurements were taken twice by a physical therapist withmore than nine years of experience in the rehabilitation of musculoskeletal disorders. Rest time afterstrength measurement was a three-minute break between the strength measurement of each device (SPSand HHD) and trials [11,14]. The order of the tests was determined by randomization to minimize the effectsof fatigue with repeated measurements.

2022 Ueda et al. Cureus 14(6): e26106. DOI 10.7759/cureus.26106

3 of 8FIGURE 3: Experimental protocol of strength measurement.
SPS: suspension scale; HHD: handheld dynamometer

Statistical analysis
To evaluate the intra-rater reliability of the strength measurement for the shoulder external rotators using
the SPS and the relationship between the strength values of the SPS and HHD, a statistical analysis was
conducted using the following procedure. First, to confirm the normality of each measurement data, theShapiro-Wilk test was performed, and normality was defined as a p-value of ≥0.05. Second, the intraclasscorrelation coefficient (ICC1,2), standard error of measurement (SEM), and 95% confidence interval (95% CI)
were calculated to evaluate the intra-rater reliability of the two measurements using SPS and HHD. SEM wascalculated as SD×. The reliability was evaluated as excellent reliability (>0.90), good reliability(0.75-0.90), moderate reliability (0.50-0.75), and poor reliability (<0.50) [15]. The minimum detectablechange (MDC) of the SPS and HHD measurements was calculated as SEM×1.96×[12]. Third, an evaluationof absolute reliability with Bland-Altman analysis was performed to assess the presence of fixed andproportional biases. Fixed bias was calculated as the 95% CI of the mean difference between the two trials foreach measurement. Additionally, a fixed bias was considered positive if 0 was not included within the 95% CIof the SPS and HHD measurements. To determine the proportional bias, we calculated the regression
equation based on the difference in values and mean values of the two methods. If the regression was
significant, the proportional bias was positive. Finally, the correlation coefficient between the strength
values of the SPS and HHD was calculated to investigate the utility of the strength measurement of theshoulder external rotators using SPS. If normality data was confirmed, one correlation coefficient wascalculated using Pearson’s correlation coefficient. Otherwise, it was calculated using Spearman’s rankcorrelation coefficient. Statistical analyses were performed using SPSS Statistics version 27 (IBM Corp.,
Armonk, NY, USA). The significance level was set at 5% (p < 0.05).

Results
The measured values of SPS and HHD and the results of ICC 1,2, SEM, and MDC are shown in Table 1.
Measured values of the SPS and HHD were 5.8 ± 2.3 kg and 6.1 ± 2.4 kg (SPS) and 5.6 ± 2.4 kg and 5.9 ± 2.4 kg(HHD) for the first and second trials, respectively. The Shapiro-Wilk test for all measurement data confirmed
the normality of all data. The ICC1,2 (95% CI) for the SPS and HHD were 0.98 (0.95-0.99) and 0.92 (0.800.97), respectively. The SEM were 0.3 and 0.6, respectively. The MDC were 0.9 and 1.7, respectively.

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Measurement device

Strength valuesa

Intra-rater reliability

Trial 1

ICC [95% CI]

SEM

Minimal detectable change

Trial 2

Suspension scale (kg)

5.8

±

2.3

6.1

±

2.4

0.98 [0.95–0.99]

0.3

0.9

Handheld dynamometer (kg)

5.6

±

2.4

5.9

±

2.4

0.92 [0.80–0.97]

0.6

1.7

TABLE 1: Strength values and intra-rater reliability, standard error of measurement, and minimal
detectable change of suspension scale and handheld dynamometer.
aMean ± standard deviation.

ICC: intraclass correlation coefficient; 95% CI: 95% confidence interval; SEM: standard error of measurement

The results of the Bland-Altman analysis for the SPS and HHD measurements are shown in Table 2. The
Bland-Altman analysis showed that there were no fixed and proportional biases in the measurements of SPS
and HHD.

Fixed bias

Proportional bias

Measurement device
95% CI

Results

Slope of regression line

P-value aResults

Suspension scale-0.52–0.03

No

0.01

0.86

No

Handheld dynamometer-0.90–0.25

No

-0.18

0.17

No

TABLE 2: Bland-Altman analysis of suspension scale and handheld dynamometer.
aValues less than 0.05 were considered statistically significant.

95% CI: 95% confidence interval

A scatter plot of the mean values of the SPS and HHD data is shown in Figure 4. Correlation analysis showeda significant positive correlation between the strength values of SPS and HHD (r = 0.94, p < 0.01).

2022 Ueda et al. Cureus 14(6): e26106. DOI 10.7759/cureus.26106

5 of 8FIGURE 4: Scatter plot of strength values (average of two
measurements) of the suspension scale and handheld dynamometer.

Discussion
This study aimed to clarify the intra-rater reliability of measuring the strength of shoulder external rotators
using an SPS and the relationship between the strength measurement using the SPS and HHD. Our study
found that the intra-rater reliability of measuring the strength using the SPS and HHD were ICC 0.98 and
0.92, respectively. In addition, both devices had excellent reliability. The SEM and MDC of the SPS were
smaller than those of the HHD. The results of the Bland-Altman analysis showed that there were no fixedand proportional biases in measurements using the SPS and HHD. The correlation analysis showed that
there was a strong correlation (positive) between the strength values of the SPS and HHD (r = 0.94, p < 0.01).
Therefore, the study hypothesis was supported by the results.
Regarding the reliability of measuring strength for shoulder external rotators, Papotto et al. reported thatusing an isokinetic dynamometer had an ICC of 0.92 [11]. Additionally, Cools et al. and Furness et al.
demonstrated that the reliability of a push-type HHD was ICC 0.93 and 0.98, respectively [13,16]. A pushtype HHD is a measurement equipment that can assess strength by applying pressure to the sensor. The
reliability of the strength measurement for the shoulder external rotators using the SPS and HHD was ICC
0.98 and 0.92, respectively. Hence, the measurement methods used in our study were similar to those usedin previous studies.
Our study revealed the intra-rater reliability of measuring the strength of shoulder external rotators by using
an inexpensive SPS. Moreover, the results showed that the strength measurements of the shoulder externalrotators using the SPS strongly correlated with those using the HHD. Regarding the clinical aspects ofshoulder external rotators, decreased strength and muscle activity of the infraspinatus and teres minor
muscles in shoulder external rotation were observed in patients with various shoulder disorders such as
subacromial impingement [17], rotator cuff injuries [18], and shoulder instability [19]. Thus, proper
assessment of the strength of shoulder external rotators is fundamental in clinical practice [20]. Our study
indicated that strength measurement of shoulder external rotators using a portable SPS was cost-effective
and an alternative to the HHD. Hence, the measurement method in our study can be used not only in theclinical practice of physical therapy but also in a wide range of fields, such as medical examinations,
exercise, and training instruction for athletes and elderly people in the community.

Limitations of the study
Our study has a few limitations. First, we could not clarify the internal validity of the strength measurementof shoulder external rotators using the SPS, although it strongly correlated with that of the HHD. However,2022 Ueda et al. Cureus 14(6): e26106. DOI 10.7759/cureus.26106

6 of 8to investigate the internal validity of the measuring strength method with a portable device such as the
HHD, previous studies have used a strength value acquired with an isokinetic dynamometer as the gold
standard [21,22]. We could not determine the relationship between strength values using the SPS andisokinetic dynamometer. Second, we could not perform measurements with the SPS and HHD in parallel.
Hence, the concurrent reliability and validity using the SPS were unclear. However, simultaneous
measurement using the SPS and HHD using one belt may be inaccurate due to a bias of the load on a belt.
Therefore, in this study, the measurements using the HHD and SPS were separately performed in accordancewith the previous study [21]. Third, we could not investigate the inter-rater reliability of the strength
measurement using the SPS. Fourth, the study participants were healthy males, and it is unclear whether inthis study the measurements using the SPS could be applied to subjects with other backgrounds, such as
women and elderly people. Therefore, further studies including females and a wider age group are necessary.

Conclusions
This study aimed to investigate the utility of strength measurement of shoulder external rotators using an
inexpensive and portable SPS. Results showed excellent reliability. Additionally, the strength value obtainedusing the SPS showed a strong correlation with that of the HHD. In conclusion, these results indicate that
the SPS can be used as a clinically useful, cost-effective, and portable assessment method for shoulder
function.

Additional Information
Disclosures
Human subjects: Consent was obtained or waived by all participants in this study. The Institutional Ethics
Committee of Hankai Hospital issued approval 2022-001. Human subjects: consent was obtained from all
study participants. The Institutional Ethics Committee of Hankai Hospital issued approval (number 2022001, dated: 15/01/2022). Animal subjects: All authors have confirmed that this study did not involve animal
subjects or tissue. Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors
declare the following: Payment/services info: All authors have declared that no financial support wasreceived from any organization for the submitted work. Financial relationships: All authors have declaredthat they have no financial relationships at present or within the previous three years with anyorganizations that might have an interest in the submitted work. Other relationships: All authors havedeclared that there are no other relationships or activities that could appear to have influenced thesubmitted work.

Acknowledgements
We would like to sincerely thank the participants of our study.

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