Test-retest reliability of a handheld dynamometer for measurement of isometric cervical muscle strength

Abstract

BACKGROUND:

There is a lack of test-retest reliability studies of measurements of cervical muscle strength, taking into account gender and possible learning effects.

OBJECTIVE:

To investigate test-retest reliability of measurement of maximal isometric cervical muscle strength by handheld dynamometry.

METHODS:

Thirty women (age 20–58 years) and 28 men (age 20–60 years) participated in the study. Maximal isometric strength (neck flexion, neck extension, and right/left lateral flexion) was measured on three separate days at least five days apart by one evaluator.

RESULTS:

Intra-rater consistency tended to improve from day 1–2 measurements to day 2–3 measurements in both women and men. In women, the intra-class correlation coefficients (ICC) for day 2 to day 3 measurements were 0.91 (95% confidence interval [CI], 0.82–0.95) for neck flexion, 0.88 (95% CI, 0.76–0.94) for neck extension, 0.84 (95% CI, 0.68–0.92) for right lateral flexion, and 0.89 (95% CI, 0.78–0.95) for left lateral flexion. The corresponding ICCs among men were 0.86 (95% CI, 0.72–0.93) for neck flexion, 0.93 (95% CI, 0.85–0.97) for neck extension, 0.82 (95% CI, 0.65–0.91) for right lateral flexion and 0.73 (95% CI, 0.50–0.87) for left lateral flexion.

CONCLUSION:

This study describes a reliable and easy-to-administer test for assessing maximal isometric cervical muscle strength.

Keywords

Neck pain reliability muscle strength cervical musculoskeletal pain

1. Introduction

Neck pain is ranked as the fourth most common cause of disability globally [1] and is commonly associated with increased sick leave, health care expenditures [2, 3] and reduced health-related quality of life [4, 5]. At any given time, 12–14% in the general adult population report to have neck pain while the estimated 12-month prevalence ranges between 30% and 50% [6]. A large population-based study in Norway found that the prevalence of chronic neck pain was about 30% in women and 19% in men [7].

There is ample evidence showing that patients with neck pain have reduced cervical muscle strength [8, 9, 10, 11, 12, 13, 14, 15]. Assessment of cervical muscle strength may therefore be a useful indicator of the functional impairment and may also provide a basis for evaluating the effect of exercise interventions as well as other types of treatment in patients with neck pain [16]. Several studies have shown that fixed-frame dynamometers provide a reliable test-retest assessment of cervical muscle strength [13, 14, 17, 18, 19]. However, fixed-frame dynamometers are expensive, space-requiring and impractical for most clinical settings. A viable option is handheld dynamometry, which is easy to use and relatively inexpensive.

Two recent studies indicate that handheld dynamometry can be used to obtain reliable measurements of isometric cervical muscle strength in healthy adults [20, 21]. However, none of the studies assessed reliability within genders and inter-session agreement was only assessed between two measurement sessions. Thus, the extent of any learning effect or other systematic changes in performance or measurement due to repeated testing could not be evaluated. The primary objective of the current study was therefore to further evaluate the test-retest reliability of handheld dynamometry for measurement of maximal isometric cervical muscle strength taking into account gender and the effect of repeated test sessions to assess a possible learning effect.

2. METHODS

2.1 Participants

Thirty women and 28 men participated in the study. Participants were recruited among staff and students at St. Olavs Hospital/Trondheim University Hospital, Norway. Inclusion criterion was age between 20 and 60 years. Eligible participants were excluded if they reported current neck pain, one or more episodes with acute neck pain in the last three months, a history with frequent episodes with migraine or tension headache, or any history with severe neck injury (e.g., whiplash), neck surgery or rheumatic disease. Participant characteristics are presented in Table 1. The study protocol was approved by the Regional Committee for Ethics in Medical Research (project no. 2014/1157) and all participants signed an informed consent before enrolment. The study was carried out according to the latest revision of the Declaration of Helsinki.

Table 1
Participant characteristics

	Women ( $n=$ 30)		Men ( $n=$ 28)
Age (years)	26.4	$\pm$ 7.0 (20–58)	27.4	$\pm$ 7.4 (20–60)
Height (cm)	170	$\pm$ 6 (158–181)	181	$\pm$ 7 (167–194)
Body mass (kg)	66.2	$\pm$ 14.0 (50–120)	79.3	$\pm$ 11.4 (59–110)
Body mass index	22.9	$\pm$ 4.3 (17.5–37.9)	24.2	$\pm$ 2.8 (19.5–29.9)
(kg/m ${}^{2}$ )

Values are mean $\pm$ SD (range).

Table 2

Maximal isometric cervical strength and extension/flexion ratio in women and men measured on three different days

	Day 1		Day 2		Day 3
Women ( $n=$ 30)
Total cervical strength, N	430	$\pm$ 82 (280–616)	449	$\pm$ 80 (327–633)	456	$\pm$ 84 (321–649)
Neck flexion, N	79	$\pm$ 18 (44–118)	87	$\pm$ 22 (48–148)	89	$\pm$ 22 (45–149)
Neck extension, N	131	$\pm$ 27 (94–211)	135	$\pm$ 27 (91–198)	135	$\pm$ 27 (89–214)
Right lateral flexion, N	108	$\pm$ 22 (58–147)	109	$\pm$ 22 (59–153)	112	$\pm$ 23 (72–155)
Left lateral flexion; N	113	$\pm$ 24 (66–154)	119	$\pm$ 24 (79–165)	120	$\pm$ 25 (75–164)
Extension/flexion, ratio	1.69	$\pm$ 0.32 (1.07–2.56)	1.61	$\pm$ 0.38 (1.03–2.79)	1.57	$\pm$ 0.38 (0.94–2.49)
Men ( $n=$ 28)
Total cervical strength, N	774	$\pm$ 132 (477–1003)	810	$\pm$ 132 (583–1017)	806	$\pm$ 148 (560–1076)
Neck flexion, N	160	$\pm$ 34 (99–208)	173	$\pm$ 37 (100–225)	175	$\pm$ 38 (96–227)
Neck extension, N	228	$\pm$ 52 (103–335)	240	$\pm$ 46 (161–341)	239	$\pm$ 48 (151–333)
Right lateral flexion, N	191	$\pm$ 39 (99–266)	195	$\pm$ 41 (123–275)	199	$\pm$ 53 (104–318)
Left lateral flexion, N	195	$\pm$ 37 (108–251)	202	$\pm$ 37 (151–269)	193	$\pm$ 38 (132–271)
Extension/flexion, ratio	1.47	$\pm$ 0.39 (0.83–2.39)	1.45	$\pm$ 0.41 (0.95–2.60)	1.42	$\pm$ 0.39 (0.87–2.38)

Values are mean $\pm$ SD (range)

Table 3

Typical error, coefficient of variation (CV) and change in mean between days 1–2 and days 2–3 for maximal isometric cervical strength and the extension/flexion ratio

	Days 1–2		Days 2–3
Women ( $n=$ 30)
Total cervical strength
Typical error, N	34.9	(27.8–46.9)	21.5	(17.1–28.9)
CV, %	8.9	(6.7–13.3)	4.9	(3.7–7.3)
Change in mean, N	18.7	(0.2–37.1)	6.7	( $-$ 4.7–18.1)
Neck flexion
Typical error, N	7.1	(5.7–9.6)	6.6	(5.2–8.8)
CV, %	7.1	(5.3–3.8)	5.1	(3.8–7.5)
Change in mean, N	7.6	(3.8–11.4)	2.0	( $-$ 1.5–5.5)
Neck extension
Typical error, N	10.3	(8.2–13.8)	9.6	(7.7–12.9)
CV, %	9.6	(7.2–14.4)	6.9	(5.2–10.2)
Change in mean, N	3.7	( $-$ 1.7–9.1)	0.1	( $-$ 5.0–5.1)
Right lateral flexion
Typical error, N	14.2	(11.3–19.1)	9.1	(7.2–12.2)
CV, %	17.3	(12.9–26.2)	12.4	(9.3–18.6)
Change in mean, N	1.3	( $-$ 6.2–8.8)	3.5	( $-$ 1.3–8.3)
Left lateral flexion
Typical error, N	13.4	(10.7–18.0)	8.3	(6.6–11.1)
CV, %	13.7	(10.2–20.6)	8.5	(6.4–12.6)
Change in mean, N	6.1	( $-$ 1.0–13.1)	1.0	( $-$ 3.3–5.4)
Extension/flexion ratio
Typical error, ratio	0.14	(0.11–0.19)	0.16	(0.12–0.21)
CV, %	9.5	(7.4–13.1)	8.9	(7.0–12.3)
Change in mean, ratio	$-$ 0.09	( $-$ 0.16–0.00)	$-$ 0.03	( $-$ 0.12–0.05)
Men ( $n=$ 28)
Total cervical strength
Typical error, N	46.0	(36.4–62.6)	51.3	(40.6–69.8)
CV, %	7.1	(5.5–9.7)	6.6	(5.2–9.1)
Change in mean, N	36.1	(10.8–61.3)	$-$ 4.4	( $-$ 32.5–23.8)
Neck flexion
Typical error, N	13.1	(10.4–17.8)	14.0	(11.1–19.0)
CV, %	8.3	(6.5–11.5)	9.4	(7.4–13.0)
Change in mean, N	12.8	(5.6–20.0)	2.6	( $-$ 5.1–10.3)
Neck extension
Typical error, N	19.1	(15.1–26.0)	12.9	(10.2–17.5)
CV, %	11.7	(9.2–16.3)	5.8	(4.5–8.0)
Change in mean, N	12.3	(1.8–22.7)	$-$ 1.5	( $-$ 8.5–5.6)
Right lateral flexion
Typical error, N	18.8	(14.8–25.5)	20.5	(16.2–27.8)
CV, %	12.2	(9.5–16.9)	12.3	(9.6–17.2)
Change in mean, N	3.8	( $-$ 6.5–14.1)	3.9	( $-$ 7.4–15.1)
Left lateral flexion
Typical error, N	15.4	(12.2–20.9)	18.9	(14.9–25.7)
CV, %	9.7	(7.6–13.4)	10.0	(7.8–13.8)
Change in mean, N	7.1	( $-$ 1.3–15.6)	$-$ 9.4	( $-$ 19.7–1.0)
Extension/flexion ratio
Typical error, ratio	0.15	(0.12–0.20)	0.15	(0.12–0.20)
CV, %	10.6	(8.3–14.8)	9.9	(7.8–13.7)
Change in mean, ratio	$-$ 0.02	( $-$ 0.10–0.06)	$-$ 0.03	( $-$ 0.11–0.05)

Abbreviations: CI, confidence interval; CV, coefficient of variation; N, Newton. Values are mean with 95% CI.

Table 4

Intraclass correlation coefficients (ICCs) between days 1–2 and days 2–3 for maximal isometric cervical strength and the extension/flexion ratio. ICCs was calculated by using mean of all three inter-session measurements and the mean of the 1 ${}^{\text{st}}$ and 2 ${}^{\text{nd}}$ inter-session measurement

	ICCs based on mean of three		ICCs based on mean of 1 ${}^{\text{st}}$ and 2 ${}^{\text{nd}}$
	inter-session measurements		inter-session measurement
	Days 1–2	Days 2–3	Days 1–2	Days 2–3
Women ( $n=$ 30)
Total cervical strength	0.80 (0.61–0.90)	0.93 (0.86–0.97)	0.75 (0.52–0.87)	0.92 (0.84–0.96)
Neck flexion	0.82 (0.46–0.93)	0.91 (0.82–0.95)	0.79 (0.47–0.91)	0.90 (0.80–0.95)
Neck extension	0.85 (0.71–0.93)	0.88 (0.76–0.94)	0.81 (0.65–0.91)	0.87 (0.74–0.94)
Right lateral flexion	0.60 (0.30–0.79)	0.84 (0.68–0.92)	0.46 (0.12–0.70)	0.74 (0.53–0.87)
Left lateral flexion	0.67 (0.41–0.83)	0.89 (0.78–0.95)	0.62 (0.34–0.80)	0.87 (0.75–0.94)
Extension/flexion ratio	0.82 (0.63–0.91)	0.83 (0.67–0.92)	0.76 (0.55–0.88)	0.75 (0.55–0.87)
Men ( $n=$ 28)
Total cervical strength	0.85 (0.64–0.94)	0.87 (0.74–0.94)	0.84 (0.61–0.93)	0.86 (0.72–0.93)
Neck flexion	0.81 (0.49–0.92)	0.86 (0.72–0.93)	0.79 (0.35–0.92)	0.84 (0.69–0.92)
Neck extension	0.83 (0.63–0.92)	0.93 (0.85–0.97)	0.81 (0.61–0.91)	0.93 (0.84–0.97)
Right lateral flexion	0.79 (0.59–0.89)	0.82 (0.65–0.91)	0.73 (0.50–0.87)	0.77 (0.56–0.87)
Left lateral flexion	0.82 (0.65–0.91)	0.73 (0.50–0.87)	0.81 (0.63–0.91)	0.72 (0.48–0.86)
Extension/flexion ratio	0.86 (0.72–0.93)	0.86 (0.73–0.94)	0.84 (0.67–0.92)	0.83 (0.67–0.92)
Total ( $n=$ 58)
Total cervical strength	0.95 (0.91–0.98)	0.97 (0.95–0.98)	0.95 (0.89–0.97)	0.96 (0.94–0.98)
Neck flexion	0.94 (0.81–0.97)	0.96 (0.93–0.98)	0.93 (0.77–0.97)	0.95 (0.92–0.97)
Neck extension	0.94 (0.89–0.96)	0.97 (0.95–0.98)	0.93 (0.87–0.96)	0.97 (0.95–0.98)
Right lateral flexion	0.91 (0.85–0.94)	0.92 (0.88–0.96)	0.87 (0.80–0.92)	0.87 (0.83–0.94)
Left lateral flexion	0.92 (0.86–0.95)	0.91 (0.86–0.95)	0.91 (0.84–0.95)	0.90 (0.84–0.94)
Extension/flexion ratio	0.85 (0.75–0.91)	0.85 (0.76–0.91)	0.81 (0.70–0.88)	0.80 (0.68–0.88)

Abbreviations: CI, confidence interval. ICCs are presented with 95% CI.

2.2 Test protocol

A handheld dynamometer (Lafayette Manual Muscle Testing System, model 01165, Lafayette Instrument Company, US) was used to measure isometric cervical muscle strength with a resolution of $\pm$ 0.1 kg. The accuracy of the instrument is according to the manufacturer $\pm$ 0.1 kg. The same tester (female, height 164 cm, body mass 54 kg) performed all measurements. The test protocol included measurements of maximal isometric strength in neck flexion, neck extension, and right/left lateral flexion. Three trials were performed for each movement direction with a 1 min break between each trial. Each test session lasted 20–25 min and was repeated on three different days with at least 5 days between each session. At the first test session (day 1), the tester demonstrated the exercises and the participants performed 1–2 submaximal warm-up trials to become familiarized with the task and ensure correct execution. During the test trials the participants were instructed to progressively build up maximal force within 3–5 sec while the tester provided a steady isometric resistance. Immediately after each trial the participants ranked their effort on the Borg CR-10 scale with a score of 10 defining maximal effort [22].

For the neck flexion test, the participants sat on a stool with the upper back and head leaning backwards against the wall with the feet placed flat on the floor and the hands resting in the lap (Fig. 1A). The dynamometer was placed just above the eyebrows with the tester positioned in front of the participant, holding the dynamometer with both hands. The participants performed the test by attempting to bend the head forward while imagining that the chin was led against the chest. The back was supported against the wall during the test while the head was allowed to move 1–2 cm from the wall until meeting the steady resistance provided by the tester.

For the neck extension test, the participants were lying on the stomach on an exercise mat with the forehead resting against the mat and the arms along the body (Fig. 1B). The tester was positioned with the feet on each side of the participants’ shoulders, holding the dynamometer with both hands. The center of the dynamometer was placed at the crossing point between an imaginary line from the spine to the top of the head, and from the left to the right of the upper helix of the ear. The participants performed the test by lifting the head 1–2 cm until meeting the steady resistance provided by the tester.

For the test of right/left lateral flexion the participants were seated upright on a stool placed within a doorframe with one shoulder leaning against the doorframe (Fig. 1C). The tester was positioned on the side of the stool with both hands on the dynamometer and the back leaning backward against the doorframe to accomplish firm support. The dynamometer was positioned approximately 1 cm above the upper helix of the ear. The participants performed the test by bending the head to the right/left side, imagining to move the ear towards the shoulder.

2.3 Statistical analyses

Statistical analyses were performed using IBM statistical package for social sciences (version 24.0) and Microsoft Excel. Results are expressed as means with standard deviations (SDs) or 95% confidence intervals (CIs). The mean isometric force (i.e., Newton, N) of the three trials for each neck movement direction within each test session was used in the statistical analyses. Total cervical strength was calculated by adding together the mean maximal isometric force obtained in all movement directions. We also performed additional analyses where we only included the mean isometric force of the 1 ${}^{\text{st}}$ and 2 ${}^{\text{nd}}$ inter-sessions measurement to assess the effect of reducing number of measurements for each movement direction from 3 to 2. Within-subject variation was estimated by the typical error (i.e., standard error of measurement) calculated as the SD of the individual’s differences in strength values obtained on the different days of testing [23]. Coefficient of variation (CV) was calculated by expressing the typical error as a percentage of the mean value [23]. Change in mean was calculated by subtracting the mean values on day 2 from mean values on day 1, and mean values on day 3 from mean values on day 2. Test-retest intra-class correlation coefficients (ICCs [2, 3]) were calculated to describe relative agreement between measurements. Bland-Altman plots were used to illustrate absolute agreement between measurement sessions [24]. The $\pm$ 1.96 standard deviations (SD) indicate limits of agreement between methods and this interval is expected to comprise 95% of the differences between two measurements. Correlation analysis was performed with Pearson correlation while group comparisons were performed with an independent t-test. The flexion/extension ratio was calculated by dividing the neck flexion values by the neck extension values. Statistical significance was accepted at $P<$ 0.05 for all analyses.

3. Results

Table 2 shows total cervical strength and cervical strength in all movement directions in women and men measured on three different days. The extension/ flexion ratio is also presented. As expected, men were considerable stronger than women in all movement directions ( $P<$ 0.001 for all comparisons). Men were twice as strong as women in neck flexion, about 77% stronger in neck extension, and about 78% and 68% stronger in right and left lateral flexion, respectively. The difference in total cervical strength was about 77%. The gender difference in strength remained significant also when normalizing force to body mass ( $P<$ 0.001 for all comparisons; data not shown). Both women and men were strongest in neck extension, i.e., the extension/flexion ratio averaged over the three test session was 1.62 in women and 1.45 in men (Table 2). The extension/flexion ratio tended to differ between women and men ( $P=$ 0.06). Right lateral flexion strength was about 80% and left lateral flexion about 85% of neck extension strength in both women and men.

Table 3 shows within-subject variation in total cervical strength, strength during neck flexion, neck extension, right/left lateral flexion, and the extension/flexion ratio. In women, typical error and CV% were reduced for most measures from day 2 to day 3 compared to day 1 to day 2. This was less consistent among men where the typical error and CV% remained fairly similar between all test days. For both women and men, the change in mean was biased for several of the days 1–2 measurements but none of the days 2–3 measurements (indicated by the 95% CI spanning zero for the days 2–3 measurements). There was no correlation between the change in mean and the change in effort indicated by the scores on the Borg CR-10 scale (Pearsons r $\leqslant$ 0.24 and $P\geqslant$ 0.25 for all correlations).

Table 4 shows ICCs with 95% CI for total cervical strength, strength during neck flexion, neck extension, right/left lateral flexion, and the flexion/extension ratio. In general, ICCs based on days 2–3 measurements were higher than for ICCs based on days 1–2 measurements. Further, ICCs were somewhat higher for neck flexion and neck extension compared to lateral flexion in both women and men. In pooled analyses of data from both women and men, the ICCs were $\geqslant$ 0.91 for all movement directions and marginally higher for days 2–3 measurements than for days 1–2 measurements for most directions as well as for total cervical strength.

We also calculated ICCs based on the average of the 1 ${}^{\text{st}}$ and 2 ${}^{\text{nd}}$ inter-session measurements. Overall, ICCs remained high for the days 2–3 measurements and especially for neck flexion and neck extension.

Figure 2 shows Bland-Altman plots illustrating the agreement between days 1–2 and days 2–3 measurements for neck flexion (A and B), neck extension (C and D), right lateral flexion (E and F), and left lateral flexion (G and H). Overall, the measurement bias (mean difference) was smaller and limits of agreement somewhat narrower for the days 2–3 measurements compared to the days 1–2 measurement.

4. Discussion

The main objective of the current study was to assess the gender specific reliability of measurements of maximal isometric cervical muscle strength using handheld dynamometry. Three measurement sessions were carried out on three different days to enable an evaluation of systematic changes in performance due to repeated testing (e.g., learning effect). In women, there was a systematic reduction in within-subject variation between days 2–3 measurements compared to days 1–2 measurements. This consistency was not observed among men. However, the overall consistency of the repeated measurements was fairly similar between women and men and ICCs were high, ranging from 0.83 to 0.91 in women and from 0.73 to 0.93 in men for the measurements on days 2–3. In both women and men, measurements of cervical strength in neck flexion and extension were more consistent than measurements of lateral flexion.

We are only aware of two previous studies that have investigated the reliability of a handheld dynamometer for measurements of maximal isometric cervical strength [20, 21]. Geary and co-workers [21] recorded maximal isometric cervical strength in a neck flexion, neck extension and lateral flexion in male rugby players by applying a “break” test, i.e., the tester gradually increases the external force and releases the force when the test person no longer can overcome the external force. Testing was performed 2 days apart and inter-session ICCs ranged from 0.80 for left lateral flexion to 0.92 for total cervical strength (i.e., sum of cervical strength in all movement directions), thus the results being comparable to what we observed for men in the currents study. However, compared to the maximal isometric strength values obtained for men in the current study, Geary and co-workers [21] found more than twofold higher values in both neck flexion, neck extension and lateral flexion. This difference can partly be explained by the use of a “break” test instead of a “make” test that was applied in the current study, i.e., in a “make” test the tester gives static resistance resulting in an isometric muscle contraction by the test person. Previous studies have shown that a “break” test result in 10–20% higher force values compared to a “make” test [25, 26]. More importantly, the study by Geary and co-workers [21] only included young well-trained ath- letes (rugby players) with mean age 20 years and body mass 98 kg. In comparison, mean body mass of the male participants was 79 kg in the current study.

Versteegh and co-workers [20] applied a protocol with three repeated measurements of maximal isometric strength in neck flexion, neck extension and lateral flexion in healthy young men ( $n=$ 14) and women ( $n=$ 16). The two first sessions were carried out with a 10 min break in between and used to assess intra-session reliability. The third measurement session was carried out 6–8 days afterwards and used to assess inter-session reliability. They did not report gender-specific reliability but inter-session ICCs was generally high, ranging from 0.92 for neck flexion to 0.95 for neck extension and lateral flexion. Change in mean (i.e., mean difference) ranged from 11.3 N for lateral flexion, 10.6 N for neck extension and 14.0 N for neck flexion, which is comparable to the change in mean observed in the current study. However, compared to Versteegh and co-workers [20] we observed somewhat lower isometric strength in neck flexion and neck extension in both women and men. The difference was less pronounced for lateral flexion. Similar to the current study, Versteegh and co-workers [20] recorded cervical strength in young adults (age 19–37). However, they applied a protocol where the participants themselves provided the necessary resistance to produce the isometric strength values. Since the testers‘ strength may influence the result of a test with handheld dynamometry [27] it may be hypothesized that an approach where the person being tested provide the resistance him or herself will eliminate this shortcoming. The current results may indicate that this approach result in somewhat higher isometric cervical strength values compared to having a tester providing the resistance. However, this question warrants further investigation.

The time required to perform the full protocol per participant per session was about 20–25 min, which may reduce the possibility to implement such measurements in clinical practice. We therefore performed an additional analysis of the ICC where we only included the 1 ${}^{\text{st}}$ and 2 ${}^{\text{nd}}$ inter-session measurement, which reduces the time required to perform the measurements to 10–15 min. The results of this analysis showed that the intra-rater reliability was moderate to excellent according to guidelines by Koo and Li [28], with highest ICCs for neck flexion and extension and somewhat lower values for lateral flexion.

As expected, men were considerable stronger than women in all movement directions. Due to the higher absolute values for the males, the typical error of measurement was larger in men compared to women. However, inter-session ICCs (i.e., relative agreement) were fairly similar between genders and more so for the days 2–3 measurements than the days 1–2 measurements. The gender difference in cervical strength was most pronounced for neck flexion with the males being about twice as strong as the females. In neck extension, the men were about 77% stronger than the women. These gender differences in neck flexion and neck extension strength is similar to those reported by Versteegh and co-workers [20], who found that men were twice as strong as females in neck flexion and 74% stronger in neck extension. Accordingly, the extension/flexion ratio found in the current study was also similar to the ratio reported by Versteegh co-workers [20].

Studies using stationary dynamometers to measure cervical muscle strength have found somewhat higher extension/flexion ratios compared to the current study [9, 13]. However, these differences are likely explained by the participant’s body position during testing. In the study by Cagnie and workers [13], both neck extension and neck flexion was measured in supine/prone positon while Ylinen and co-workers [9] performed all measurements in seated position. In the current study, neck flexion was measured in seated position and neck extension in prone position. Thus, the latter implies that the gravitational force is displaced relative to the cervical neck joints. The mass of the head will therefore add disproportionally to the neck extension load vs neck flexion load compared to the studies mentioned above. It should be noted that measurement in a prone position on the floor may be impracticable for some patients groups (e.g., elderly) and a better alternative may be to perform the measurement with the patient lying prone on a bench.

Ylinen and co-workers [9] investigated neck flexion, extension and rotation strength in women with chronic neck pain on two different test occasions. The results indicated increasing strength from the first to the second test occasion, which is in line with the results in the current study. Another study by Ylinen and co-workers [29] tested neck flexion, extension and rotation strength in young healthy subjects, and found an increase in strength of approximately 2–8% between two different measurements performed on the same day. These results indicate that the increase in neck strength to some extent can be explained by a learning effect (e.g., improved technique) and not an increase in strength capacity per se. However, in the present study the participants were tested on three different occasions, and it is therefore possible that the first test session induced a training effect. It is documented that a small amount of stimuli is sufficient to increase strength in untrained muscles [30]. Thus, findings in previous studies as well as the current findings indicate that intervention studies that target improved cervical muscle strength should include two baseline sessions to be able to record the true improvement in strength.

A strength of the current study is the relatively large sample size with both genders included and three repeated test sessions on three different days, which is recommended in assessment of reliability [23]. The same tester performed all measurements, which reduce the possibility that systematic differences in performing the measurements affected the results. Moreover, the effort by the participants was similar between days and there was no correlation between day-to-day change in force and change in effort. Previous studies have documented that size and strength of the tester may affect the magnitude and reliability of maximal strength measures with a handheld dynamometer [27]. The tester in the current study was a relatively small female (height 164 cm, body mass 54 kg) and we cannot rule out that the reliability will vary with size and strength of the tester. There are several limitations of the current study that should be considered when interpreting the results. First, there was relatively narrow age range in our study sample with most participants being between 20 and 35 years old. Thus, to what extent the current results can be generalized to the general adult population is uncertain. Second, we only assessed intra-rater reliability and further studies should assess the inter-reliability to fully evaluate the reliability of the method. Finally, we did not assess axial rotation strength, which is desirable to fully assess cervical muscle strength.

5. Conclusion

Our results indicate good to excellent reliability for measurement of cervical strength in flexion, extension and lateral flexion in both women and men. Repeated baseline measurements may improve consistency between measurements. This handheld dynamometry protocol represents a viable, easy-to-administer, and reliable option for measurement of maximal isometric cervical strength in research as well as clinical settings.

Footnotes

Conflict of interest

None to report.

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Test-retest reliability of a handheld dynamometer for measurement of isometric cervical muscle strength

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. METHODS

2.1 Participants

Table 1 Participant characteristics

2.3 Statistical analyses

3. Results

5. Conclusion

Footnotes

Conflict of interest

References

Table 1
Participant characteristics