The intra‐ and interrater reliability of isometric muscle contractions using a novel fixation system for a hand‐held dynamometer

Abstract

A novel fixation system for a hand‐held dynamometer (HHD) was designed to enable isometric muscle measurements on various muscle groups of strong, healthy individuals in a field setting. The objective of this study was to evaluate the intra‐ and interrater reliability of the system and determine its suitability for use by multiple researchers within large‐scale data collections during field activities. Four researchers tested eight healthy subjects, who each completed eight different maximal isometric muscle strength assessments using the HHD fixation system. Intraclass correlation coefficients (ICC) results were evaluated with a 95% confidence interval. ICC results for interrater reliability demonstrated excellent agreement of all eight measurements tested. ICC results for intrarater reliability demonstrated excellent agreement for six out of eight measurements. This system provides a new opportunity for several different high‐quality maximal muscle strength measurements to be collected by multiple data collectors on large numbers of strong, healthy individuals in a field setting.

Highlights

• This system is a reliable testing procedure that provides a new opportunity for several different high‐quality maximal muscle strength measurements.

• The system allows for laboratory quality data to be collected in the field by multiple data collectors on large numbers of strong, healthy individuals.

1. INTRODUCTION

Hand‐held dynamometers (HHDs) are a low‐cost and portable alternative to traditional in‐lab strength assessment equipment, such as Biodex® or Cybex® systems, for the evaluation of isometric muscle contractions. Intra‐ and interrater reliability assessments have shown HHDs to be reliable when used by experienced clinicians in evaluating patient muscular condition and recovery (Stark et al., 2011). However, the strength of the clinician has been identified as a limiting factor of the HHD when evaluating certain patients and muscle groups (Nadler et al., 2000; Scott et al., 2004; Thorborg et al., 2013; Wikholm et al., 1991). Kelln 2008 reports that while the HHD is a reliable tool for strength measurements in healthy, strong subjects, in some instances, test participants can overpower the tester (Kelln et al., 2008). Techniques including identifying testing postures where researchers can achieve a mechanical advantage over a subject as well as utilizing fixation systems, such as belts, straps, and bars, have been reported on for individual, specific muscle movements, and reliability has been established for these movements (Katoh et al., 2009; Kelln et al., 2008; Scott et al., 2004). In order to successfully collect maximal muscle strength measures outside of a lab environment on healthy, strong participants, we determined a need for a portable, field‐ready HHD fixation solution able to easily and accurately evaluate a large range of lumbar and lower body isometric maximal muscle contractions while removing the physical load burden that is typically placed on the researcher. It is necessary to collect strength measurements in a field context in order to capture the variability introduced by field activities as well as to minimize rest time between challenging field activities and data collection measurements.

The fixation device constructed (Figure 1) called the Strength Evaluation System (SES) (U.S. patent number 10,799,170) provides subject stabilization and enhanced HHD fixation for the following maximal isometric contractions: lumbar extension, lumbar flexion, hip flexion, hip abduction, hip adduction, knee extension, knee flexion, and ankle dorsiflexion. The device is adjustable, enabling researchers to properly position and isolate muscle groups in both male and female adult participants according to the minimum and maximum body segment sizes reported in the 2012 Anthropometric Survey of the U.S. Army Pilot Personnel: Methods and Summary Statistics (Gordon et al., 2016). The system is also designed using straps and rigid beams to support the HHD in appropriate locations during muscle contractions in order to remove much of the physical load and strength requirement of the researcher. The structure of the system also minimizes subject repositioning requirements between muscle group measurements for a faster, easier data collection process. The objective of this study was to evaluate the intra‐ and interrater reliability of the SES and determine its suitability for use by multiple researchers within large‐scale data collections during field activities.

FIGURE 1.

Strength Evaluation System (SES) A. Adjustment along the Y axis B. Adjustment along the Z axis C. HHD attachment for lumbar extension and flexion D. Lap restraint for participant movement reduction E. Thigh restraint for participant movement reduction and hip flexion measurement F. Foam pad for participant comfort G. Shin restraint for participant movement reduction and knee extension measurement H. Foot restraint for ankle dorsiflexion measurement I. Thigh restraint for participant movement reduction and lumbar flexion measurement. HHD, Hand‐held dynamometer; SES, Strength Evaluation System.

2. METHODS

Eight healthy subjects (five women and three men) completed maximal isometric muscle strength assessments using the SES with an HHD (Manual Muscle Tester, Lafayette Instruments). Subjects ranged in age from 25 to 57 years. Four researchers (A–D) administered the full suite of eight strength measurements to each of the subjects twice in 1 day, once in the morning, and once in the afternoon similar to Katoh et al., ‘s 2009 study (Katoh et al., 2009). One subject was excluded from testing with researcher A due to scheduling conflicts. Testing days were scheduled a minimum of 1 day apart and a maximum of 7 days apart to allow for sufficient participant recovery time.

For each testing session, subjects performed a warm‐up of 5 min of treadmill walking at a self‐selected pace followed by eight maximal isometric muscle contractions. Contractions included lumbar extension, lumbar flexion, hip flexion, hip abduction, hip adduction, knee extension, knee flexion, and ankle dorsiflexion. Maximum peak force achieved during each trial was recorded by the HHD. All contractions were performed with the subject in an upright seated position with the hips, knees, and ankles at 90° angles with respect to the sagittal plane. This consistent positioning across muscle contractions minimized the required subject movement, which allowed for improved speed and ease of data collection. Subjects were instructed to cross their arms over their chest for all measures except for the lumbar flexion, hip abduction, and hip adduction measurements, where arms were crossed over the lower back or placed by the their sides. This positioning was employed to remove any participant use of the arms during contractions and improve isolation of the muscle group being tested. Figure 2 shows the position of the subject and the researcher for each contraction type. Each contraction was performed once at 50% effort to practice the motion and warm‐up the muscle group followed by three 3‐s maximal effort contractions with at least 15 s of rest between efforts. These contraction durations and rest periods were deemed appropriate to eliminate the influence of muscular fatigue as literature has shown that an 80% maximal voluntary contraction held for 30 s is necessary to induce muscle fatigue (Stulen et al., 1982), so three‐second contractions are well under this threshold. In addition, assessments of existing isometric muscle testing devices have been conducted previously using three 5‐s trials for each muscle, separated by 5 s of rest (Nadler et al., 2000; Scott et al., 2004). The more conservative strategy used here is again below this threshold.

FIGURE 2.

Researcher, subject, and hand‐held dynamometer (circled) positioning for (A) lumbar extension, (B) lumbar flexion, (C) hip flexion, (D) hip abduction, (E) hip adduction, (F) knee extension, (G) knee flexion, and (H) ankle dorsiflexion.

2.1. Statistical analyses

To assess the interrater reliability of the SES for each of the individual muscle contractions tested, intraclass correlation coefficients (ICCs(2,k)) were calculated using the peak force values achieved during the six 3‐s trials recorded by each researcher (n = 7). Intrarater reliability was evaluated using ICC(2,1) with n = 7 for researcher A and n = 8 for researchers B‐D. All statistics were performed using IBM SPSS Version 24. Resultant ICC values were interpreted according to guidelines identified in Cicchetti (1994) (Cicchetti, 1994) in which values of less than 0.40 are considered poor agreement, values between 0.40 and 0.59 indicate fair agreement, values between 0.60 and 0.74 indicate good agreement, and values between 0.75 and 1.00 indicate excellent agreement.

3. RESULTS

Interrater ICC results (Table 1) demonstrated excellent agreement for all measures with values ranging from 0.83 for the knee extension through 0.95 for the knee flexion and lumbar flexion and extension. Intrarater ICC results (Table 2) demonstrated excellent agreement for all measures with the exception of fair agreement (0.52) for researcher A knee extension and good agreement for researcher C hip adduction (0.61). The average researcher ICCs for all measures were 0.79, 0.89, 0.85, and 0.86 for researchers A‐D, respectively.

TABLE 1.

Interrater ICC(2,K) results with 95% confidence interval (lower bound–upper bound).

Strength measure	ICC
Lumbar extension	0.95 (0.91–0.97)
Hip flexion	0.87 (0.63–0.93)
Knee extension	0.83 (0.69–0.91)
Ankle dorsiflexion	0.94 (0.89–0.97)
Knee flexion	0.95 (0.92–0.97)
Lumbar flexion	0.95 (0.92–0.97)
Hip abduction	0.93 (0.88–0.96)
Hip adduction	0.88 (0.80–0.93)

Abbreviation: ICC, Intraclass correlation coefficients.

TABLE 2.

Interrater ICC(2,1) results with 95% confidence interval (lower bound–upper bound).

Strength measure	Researcher A a	Researcher B	Researcher C	Researcher D
Lumbar extension	0.85 (0.65–0.97)	0.94 (0.85–0.96)	0.92 (0.80–0.98)	0.93 (0.83–0.98)
Hip flexion	0.79 (0.55–0.95)	0.78 (0.52–0.94)	0.90 (0.76–0.97)	0.89 (0.75–0.97)
Knee extension	0.52 (0.22–0.86)	0.87 (0.71–0.97)	0.86 (0.69–0.96)	0.90 (0.77–0.98)
Ankle dorsiflexion	0.81 (0.57–0.96)	0.88 (0.72–0.97)	0.86 (0.70–0.97)	0.81 (0.61–0.95)
Knee flexion	0.86 (0.66–0.97)	0.92 (0.81–0.98)	0.90 (0.76–0.98)	0.91 (0.79–0.98)
Lumbar flexion	0.86 (0.68–0.97)	0.95 (0.86–0.99)	0.96 (0.89–0.99)	0.83 (0.64–0.96)
Hip abduction	0.76 (0.51–0.94)	0.94 (0.86–0.99)	0.84 (0.65–0.96)	0.83 (0.63–0.95)
Hip adduction	0.89 (0.71–0.98)	0.82 (0.61–0.95)	0.61 (0.33–0.89)	0.79 (0.57–0.94)

Abbreviation: ICC, Intraclass correlation coefficients.

^a N = 7.

4. DISCUSSION

The SES was designed and constructed to enable the collection of lumbar and lower body isometric maximal muscle contractions with an HHD during training activities on strong, healthy participants. It was also hypothesized that with the use of the SES, such measurements could be reliably collected by any trained researcher, thereby removing the need for the same researcher to collect all of a subject's data in a repeated measures design. In this study, both inter‐ and intrarater reliability were assessed to determine if the SES was indeed able to meet these requirements. The ICC results for interrater reliability found excellent agreement of all measurements, and all but two ICC results for intrarater reliability found excellent agreement (exceptions are fair agreement in researcher A knee extension, and good agreement in researcher C hip adduction).

Knee extension measures have traditionally been more difficult to assess with the HHD due to high relative strength of the knee extensor muscles (Kelln et al., 2008). In this study, knee extension was one of two measurements that returned an ICC value below excellent. Researcher feedback following this data collection, combined with a visual inspection of the device, suggested that the strap being used to assist in HHD stabilization during this measurement had insufficient strength to withstand repetitive maximal knee extensor contractions. Following this data collection and prior to any further use of the device, the strap material used for HHD support during the knee extension exercise was improved to reduce this movement under pressure. Hip adduction was the other measurement that returned an ICC value below excellent. In this case, an inconsistency was identified with subject muscle isolation during the exercise. Due to the placement of the HHD in between subject's legs, some subjects were able to engage the hip adductor muscles of both legs to “squeeze” the HHD and therefore, increase the force registered for the contraction. Following this data collection and prior to any further use of the device, the instructions for conducting the hip adduction exercise have been updated to disallow the use of the non‐testing leg and eliminate this “squeeze” motion.

In conclusion, this investigation demonstrates the suitability of the SES along with an HHD to be utilized for data collection of lower body and lumbar maximal muscle contractions on strong, healthy soldiers by multiple researchers. ICC results for interrater reliability found excellent agreement of all measurements, and all but two ICC results for intrarater reliability also found excellent agreement with the exception of the knee extension and hip adduction results. Adjustments to the SES device and protocol have been made for future work in order to improve the reliability and repeatability of data collection on these two measurements.

5. PRACTICAL APPLICATIONS

The HHD is a low‐cost, accessible solution for a broad range of coaches, researchers, and practitioners to perform quantitative isometric strength testing in any setting. However, performing these assessments on a strong population, such as athletes or soldiers, can be a challenge and to collect quality measurements data collectors must be well‐trained. Strength differences between the data collector and the individual being tested can be large, and repetitive testing on multiple individuals can make data collection even more difficult. Most groups that would find these measurements useful have limited staff with varying levels of training and personal strength.

The novel fixation device used in this work provides a solution for large‐scale testing on strong populations and allows the staff to reliably spread out the workload of strength measurement data collection. Entire teams or groups of individuals can be tested on several different muscle groups in a matter of hours with limited and even novice staff. Since the development of this fixation system, it has been applied to four large‐scale field studies, testing over 300 military personnel in a wide breadth of measurement conditions, and has provided valuable data on maximal muscle strength that was not accessible prior to its development. The device has also reduced the time required between the completion of a fatiguing event and measurement of strength performance by bringing the test setup directly to the event location and providing a rapid, straightforward collection method.

CONFLICT OF INTEREST STATEMENT

The authors declare that they have no conflicts of interest.