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Abstract

BACKGROUND:

Hand-held dynamometry (HHD) is used to assess muscle strength in various patient populations, but many variations in protocols exist.

OBJECTIVE:

First, to systematically develop a protocol of HHD for all lower limb muscle groups and evaluate intra-rater reliability; second, to validate HHD with fixed dynamometry for the knee flexor and extensor muscles.

METHODS:

Thirty healthy young adults (women: men – 15:15) participated in two testing sessions. HHD of 12 lower limb muscle groups was performed in both sessions, while fixed dynamometry of knee muscle groups was performed only in the second session.

RESULTS:

The intra-rater reliability of HHD was good for five muscle groups and excellent for seven muscle groups (ICC3, $k=$ 0.80–0.96). The criterion validity of HHD ranged from very good to excellent for the knee flexors ( $r=$ 0.77–0.89) and from good to very good for the knee extensors ( $r=$ 0.65–0.78). However, peak moment values for the knee extensor muscles were underestimated by 32% ( $p<$ 0.001).

CONCLUSIONS:

The proposed HHD protocol provides reliable and valid measurements of lower limb muscle isometric strength in healthy adults, which may also be used to test patients with mild muscle strength deficits. However, possible underestimation of absolute strength must be considered when interpreting the results of knee extensors or other large muscles.

Keywords

Muscle strength isometric contraction dynamometer reliability validity

1. Introduction

Skeletal muscle strength assessment is an integral part of clinical examination, goal setting, and treatment planning in patients with musculoskeletal and neurological impairments [1, 2, 3]. Objective quantitative measures of muscle strength, such as fixed and hand-held dynamometry (HHD), are highly sensitive and therefore suitable for evaluation of treatment effectiveness and patient monitoring [4, 5, 6]. HHD proved to be a reliable measure of hamstring strength in athletes with acute hamstring injuries [7]. Its reliability and validity have also been demonstrated for quadriceps muscle in patients after the ACL reconstruction [8]. However, because stronger participants can overpower the tester, HHD may be feasible to measure muscle strength in athletes, mainly in the early stages of recovery from injury or surgical intervention [9].

Since fixed dynamometers are not feasible in certain settings [6], hand-held dynamometers are increasingly used in clinical practice [10]. Compared to fixed dynamometers, they are smaller, cheaper, portable, and easier to use with less time-consuming assessment protocols [11, 12]. However, because fixed dynamometers have been shown to be reliable and valid instruments for assessment of lower limb muscle strength, they are often used as the gold standard reference for other devices [12, 13]. Although HHD is generally reliable and its validity has been previously confirmed by comparisons with isokinetic dynamometry [6, 12, 14], there are many variations in HHD protocols between studies, particularly in relation to patient and tester positions [12]. Other inconsistencies in assessment protocols include, but are not limited to, patient stabilization, lower limb position and muscle activation with respect to gravity, number of measurement trials, rest periods, and use of verbal encouragement [6, 14]. Two testing techniques, i.e. “break” or “make” test, are used, with or without additional stabilization of the dynamometer [14]. Since the majority of previous studies on the reliability of HHD have examined only individual muscle groups [6, 12, 14], there is a lack of systematic and standardized protocols for testing all muscle groups of the lower limbs.

Therefore, the primary aim of our study was to develop a systematic protocol of HHD for all lower limb muscle groups and evaluate its intra-rater reliability in a young, healthy population. The secondary aim was to evaluate the criterion validity of HHD for the knee flexors and extensors by comparing it with fixed dynamometry.

2. Method

2.1 Participants

A convenience sample of 30 healthy young adults was recruited. Inclusion criteria were age between 18 and 25 years and good general health. Participants were excluded if they had a history of injury or current lower limb pain; if they had any existing comorbidities such as neurological, cardiovascular, respiratory, or psychiatric conditions; were pregnant; were professional athletes; or had participated in intensive physical activity in the past 48-h. Participants were asked to avoid any intensive physical activity during the experimental period. The study was approved by the National Medical Ethics Committee of the Republic of Slovenia (0120-48012018/5) before the start of the experiment and was conducted in accordance with the principles outlined in the Helsinki Declaration. All participants gave written informed consent to participate in the study.

2.2 Study protocol

Participants attended two testing sessions separated by 2-d. The HHD was performed during both sessions, while the fixed dynamometry was performed only in the second testing session, following the HHD. In order to reduce fatigue and duration of testing sessions [15], only the dominant lower limb of each participant was assessed. A self-reported lower limb used to kick a ball was considered dominant [16]. All measurements were performed by the same tester (physical therapist, female). Before starting the experiments, the tester was familiarized with the protocol and practiced it on six healthy adults who were not included in the study sample.

2.2.1 Hand-held dynamometry

The hand-held dynamometer (Model-01165, Lafa-yette Instrument Company, USA) was used to assess the strength of 12 lower limb muscle groups. The HHD protocol was compiled and performed from protocols used in previous studies using Lafayette or MicroFET dynamometers [3, 9, 15, 17, 18, 19, 20] and the hand-held pull gage [21]. Participants were tested in positions that eliminate the influence of gravity on muscle groups [5].

The test positions used for HHD are shown in Fig. 1 and described in detail in the Appendix. The order of testing the muscle groups was the same for all participants. HHD was performed in four body positions: 1) supine: hip flexors, hip abductors, hip adductors, ankle dorsiflexors, ankle evertors, ankle invertors; 2) prone: hip external rotators, hip internal rotators, knee flexors, ankle plantar flexors; 3) seated: knee extensors; 4) standing on the opposite lower limb with the upper body supported on a table: hip extensors. When necessary, the participant was asked to stabilize his body position by holding onto the long edge of the table, or stabilization was provided by the tester (see Appendix). A belt was used to stabilize the dynamometer for the assessment of the ankle plantar flexors, knee extensors, and hip extensors, in accordance with previous studies [21, 22, 23].

Figure 1.

Test positions for HHD of the lower limb muscle groups: Hip flexors (A), hip abductors (B), hip adductors (C), ankle dorsiflexors (D), ankle evertors (E), ankle invertors (F), hip external rotators (G), hip internal rotators (H), knee flexors (I), ankle plantar flexors (J), knee extensors (K), hip extensors (L).

Figure 2.

Participant’s position on a fixed dynamometer during measurements of maximal isometric contraction moment of the knee extensors (A) and knee flexors (B) at 90 ${}^{\circ}$ of knee flexion.

All measurements were made using a “make” test. The tester held the dynamometer stationary, perpendicular to the tested limb segment, while a participant applied a maximal force against it [6, 24]. The points at which the dynamometer was placed were marked on the participant’s skin with a ballpoint pen prior to testing to ensure consistent positioning between trials of each session. Participants were familiarized with how to perform maximal voluntary isometric contraction (MVIC) in an exercise trial during the assessment of the first muscle group. They were required to produce a maximal force over a period of 2 s and continue their maximal effort for a further 3 s [5, 25]. To initiate the action, the tester said “Contraction!” and then “Maximum! Push, push, push!” as a standardized verbal prompt during all HHD measurements. One exercise trial was performed for each muscle group, followed by two measurement trials. To minimize fatigue effects, a 30-s rest was given between trials of the same muscle group [26]. The peak forces of the two measurement trials were recorded. If the values differed by more than 15% between the two trials, the third trial was performed [27]. In this case, the measurement that differed the most was eliminated. The mean value from two trials was used for further analysis. The duration of the HHD protocol for all 12 muscle groups was approximately 30 min. The same HHD protocol was repeated after 2 d. Between the first and the second session, the tester and the participants had no insight into the measurement results. For validity assessment, the results of the second session were used.

To calculate the moment (Nm) of the knee extensors and flexors, a distance (m) between the lateral knee joint line and the point of force application (center of the hand-held dynamometer pad) of these muscles was measured for HHD. This value was multiplied by a mean muscle force converted from kilograms to newtons to obtain the moment.

2.2.2 Fixed dynamometry

The knee flexors and extensors were also studied using an isokinetic dynamometer (Humac NORM, CSMI, USA). Measurements of MVIC moment were performed according to the standard positions described in the CSMI manual [28] using gravity correction. Positions were chosen that were as similar as possible to those used in HHD and which have previously shown good to excellent reliability [29, 30, 31]. The knee extensors were tested in a seated position and the knee flexors in a supine position (Fig. 2). Both muscle groups were tested at 60 ${}^{\circ}$ and at 90 ${}^{\circ}$ knee flexion. The order of knee joint positions was randomized by lottery. Stabilization of a participant followed the standard protocol [28], with the exception of the hamstring belt, as it does not provide additional stabilization of the thigh when testing the knee extensors in the seated position (Fig. 2A) or the flexors in the prone position (Fig. 2B). Additionally, participants were asked to stabilize the trunk by holding the grip bars on the side of the base (Fig. 2).

Table 1
Mean peak forces (kg), intra-rater reliability and minimal detectable change for the hand-held dynamometry of all major lower limb muscle groups (calculated for participants of both genders)

Muscle group	Session 1 mean (SD)	Session 2 mean (SD)	ICC (95% CI)	SEM (kg)	MDC ${}_{95}$ (kg)
Hip flexors	20.8 (3.4)	20.3 (3.9)	0.95 (0.89–0.98)	0.8	2.2
Hip extensors	48.2 (14.9)	48.4 (16.5)	0.89 (0.77–0.95)	5.1	14.2
Hip abductors	12.8 (2.6)	13.0 (2.6)	0.92 (0.83–0.96)	0.7	2.1
Hip adductors	15.3 (4.0)	15.3 (3.8)	0.85 (0.68–0.93)	1.5	4.2
Hip external rotators	13.4 (3.5)	13.7 (3.6)	0.96 (0.92–0.98)	0.7	1.9
Hip internal rotators	11.6 (2.5)	12.0 (2.8)	0.92 (0.84–0.96)	0.7	2.1
Knee flexors	15.7 (2.9)	16.1 (3.1)	0.87 (0.72–0.94)	1.1	3.0
Knee extensors	33.6 (9.2)	35.4 (9.7)	0.96 (0.92–0.98)	1.8	5.1
Ankle dorsiflexors	27.2 (5.6)	26.4 (5.0)	0.95 (0.90–0.98)	1.1	3.1
Ankle plantar flexors	50.3 (10.5)	50.5 (11.8)	0.80 (0.57–0.90)	5.0	13.9
Ankle evertors	18.2 (3.2)	19.0 (2.7)	0.86 (0.71–0.94)	1.1	3.0
Ankle invertors	19.7 (3.8)	19.6 (4.2)	0.92 (0.84–0.96)	1.1	3.0

CI $=$ confidence intervals; ICC $=$ intraclass correlation coefficient; MDC ${}_{95}$ $=$ minimal detectable change (95% confidence interval); SD $=$ standard deviation; SEM $=$ standard error of measurement.

The knee extensors were always assessed first, followed by the knee flexors. Participants were given standardized instructions, “On the count of three, push as hard as you can against the lever and maintain that contraction for 4–5 s until I tell you to relax.” Verbal encouragement was given during each trial to ensure maximal effort was achieved. Two exercise trials were performed for each muscle group. The first was performed at the participant’s individual perception of 50% of an MVIC, the second at 75%. Then, the participant performed three maximal trials [29, 30, 31] at a specific knee joint position (60 ${}^{\circ}$ or 90 ${}^{\circ}$ ). The rest between trials of the same muscle group lasted 30 s. After a rest period of 60 s, the same protocol was repeated at the second joint angle. The mean values of the first two and all three trials were used for analysis.

2.3 Sample size estimation

Estimation of minimal sample size at statistical power level of $\geqslant$ 0.90 ( $\beta\leqslant$ 10%) was calculated for repeated measures ANOVA based on standardized effect size for the differences between moments measured with HHD and fixed dynamometry at 60 ${}^{\circ}$ and 90 ${}^{\circ}$ knee flexion (mean of first two trials). By assuming that $\geqslant$ 5% difference in peak torque between measurement methods is of clinical and functional relevance, the estimated minimal number of participants was 28.

2.4 Statistical analysis

IBM SPSS Statistics 25 (IBM Corp., Armonk, New York, USA, 2017) was used to perform statistical tests. Intra-rater reliability was estimated using intraclass correlation coefficient (ICC 3,k) with inter-session consistency agreement. The strength of reliability was rated as follows: ICC $<$ 0.50 poor, 0.50–0.75 moderate, 0.75–0.90 good, and $>$ 0.90 excellent [32, 33]. The associated confidence intervals (CI 95%), standard error of measurement (SEM) and minimal detectable change (MDC ${}_{95}$ ) were calculated [33]. To evaluate criterion validity of HHD for the knee extensors and flexors, Pearson’s correlation coefficient (r) was calculated between HHD (testing session 2) and the fixed dynamometry measurements. Values $<$ 0.25 indicated little or no relationship, 0.25–0.5 fair, 0.50–0.75 moderate to good, and $>$ 0.75 indicated a very good to excellent relationship [33]. Repeated measures ANOVA and post hoc paired $t$ -tests with Bonferroni correction for three comparisons were calculated to test for differences between moments measured with HHD and fixed dynamometry at 60 ${}^{\circ}$ and 90 ${}^{\circ}$ knee flexion (mean of first two trials). Statistical significance was set at $p<$ 0.05.

3. Results

Participants were evenly distributed by gender (15 males, 15 females). Their age ranged from 19 to 25 years (mean 21.7; SD 2.4 years), mean height was 174.5 cm (SD 8.9), and mean body mass was 71.8 kg (SD 9.8). All but one participants were right lower limb dominant. All participants attended both testing sessions.

The intra-rater reliability of the HHD was excellent for seven muscle groups and good for five (Table 1). The ICC value was lowest for the plantar flexors and highest for the knee extensors and hip external rotators. The highest MDC was found for the hip extensors and plantar flexors, and the lowest for the hip external rotators, internal rotators and abductors. The strength of the reliability calculated separately for male and female participants was not substantially different from the reliability for all combined (Table 2). For female participants, reliability was lower for two muscle groups (hip extensors and external rotators), and for male participants, reliability was lower for five muscle groups (hip abductors, adductors and internal rotators, knee flexors, ankle invertors).

Table 2
Comparison of intra-rater reliability of hand-held dynamometry between female and male participants

Muscle group	Females ICC (95% CI)	Males ICC (95% CI)
Hip flexors	0.95 (0.86–0.98)	0.92 (0.76–0.97)
Hip extensors	0.72 (0.17–0.91) ${}^{*}$	0.89 (0.68–0.96)
Hip abductors	0.92 (0.75–0.97)	0.87 (0.62–0.96) ${}^{*}$
Hip adductors	0.87 (0.61–0.96)	0.71 (0.13–0.90) ${}^{*}$
Hip external rotators	0.87 (0.62–0.96) ${}^{*}$	0.97 (0.92–0.99)
Hip internal rotators	0.97 (0.91–0.99)	0.81 (0.43–0.94) ${}^{*}$
Knee flexors	0.79 (0.37–0.93)	0.73 (0.19–0.91) ${}^{*}$
Knee extensors	0.93 (0.79–0.98)	0.97 (0.90–0.99)
Ankle dorsiflexors	0.93 (0.79–0.98)	0.91 (0.72–0.97)
Ankle plantar flexors	0.83 (0.49–0.94)	0.80 (0.41–0.93)
Ankle evertors	0.85 (0.56–0.95)	0.87 (0.62–0.96)
Ankle invertors	0.92 (0.77–0.97)	0.89 (0.67–0.96) ${}^{*}$

CI $=$ confidence intervals; ICC $=$ intraclass correlation coefficient; ${}^{*}$ $=$ lower strength of reliability than for all participants combined.

Table 3

Relationships between peak moments (Nm) of the knee muscle groups obtained with hand-held and fixed dynamometry

Knee muscle group	HHD	Fixed dynamometry
		Knee flexion 60 ${}^{\circ}$				Knee flexion 90 ${}^{\circ}$
	Two trials	First two trials		All three trials		First two trials		All three trials
	Mean (SD)	Mean (SD)	$r$	Mean (SD)	$r$	Mean (SD)	$r$	Mean (SD)	$r$
Flexors	55.2 (12.5)	64.0 (18.1)	0.89	63.8 (17.9)	0.89	43.5 (13.9)	0.78	43.0 (13.8)	0.77
Extensors	121.6 (36.8)	190.3 (42.6)	0.65	189.3 (41.8)	0.65	179.4 (47.1)	0.78	179.2 (46.1)	0.78

HHD $=$ hand-held dynamometry; $r=$ Pearson’s correlation coefficient between HHD and fixed dynamometry ( $p\leqslant$ 0.01 for all correlations); SD $=$ standard deviation.

The criterion validity of HHD for the knee flexors ranged from very good to excellent (Table 3). Correlation with fixed dynamometry was greater when fixed dynamometry was performed at 60 ${}^{\circ}$ of knee flexion. Good to very good correlations between HHD and fixed dynamometry were observed for the knee extensors. Validity was higher when fixed dynamometry was performed at 90 ${}^{\circ}$ knee flexion. Pearson’s correlation coefficients were equal or very similar when a mean peak moment was calculated based on the first two trials or all three trials (Table 3).

The mean values of the peak moments of the knee flexors ( $F=$ 80.08; $p<$ 0.001) and knee extensors ( $F=$ 68.52; $p<$ 0.001) differed significantly between HHD and fixed dynamometry. For knee flexors, post-hoc analyses revealed a significantly higher peak moment with HHD than with fixed dynamometry at 90 ${}^{\circ}$ knee flexion ( $t=$ 7.18; $p<$ 0.001), but significantly lower than with fixed dynamometry at 60 ${}^{\circ}$ knee flexion ( $t=-$ 5.39; $p<$ 0.001). The difference between the mean moments of fixed dynamometry at 60 ${}^{\circ}$ and 90 ${}^{\circ}$ knee flexion was also significant ( $t=$ 12.70; $p<$ 0.001). For the knee extensors, significantly lower peak moment was obtained with HHD than with fixed dynamometry at 90 ${}^{\circ}$ ( $t=-$ 10.81; $p<$ 0.001) and 60 ${}^{\circ}$ knee flexion ( $t=-$ 11.15; $p<$ 0.001). The difference between the mean moments of the knee extensors measured with fixed dynamometry in the two joint positions was not statistically significant.

4. Discussion

A systematic protocol of HHD for 12 lower limb muscle groups was developed and its intra-rater reliability evaluated in young healthy subjects. The mean peak forces obtained for four muscle groups were consistent with other studies with comparable testing protocol (hip abductors: 12.5–13.9 kg [15], ankle dorsiflexors: 17.8–27.5 kg [15], ankle evertors: 17.5–22.4 kg [9, 18], ankle invertors 18.5–22 kg [9, 18]), while they were higher for five muscle groups (hip flexors: 15.3 kg [17], hip external rotators: $\sim$ 42–44 Nm [19], hip internal rotators: 9.1–11.2 kg [9], knee flexors: 6.5-14.4 kg [3, 9], ankle plantar flexors: 36.7–49.4 kg [22]), and lower for three muscle groups (hip extensors: 48.2–79.3 kg [34], hip adductors: 18.3–19.7 kg [15], knee extensors: 208.9 Nm [23]). The differences in measured values may be due to variations in age and gender structure of the samples, use of only the dominant or both lower limbs for the calculation of mean values, number of trials, strength of the testers, dynamometers used and variations in the tester positions and stabilization of the subject.

Intra-rater reliability was good to excellent (ICC $=$ 0.80–0.96). As the strength of reliability did not differ substantially when calculated separately for female (ICC $=$ 0.72–0.97) and male (ICC $=$ 0.71–0.97) participants, we can conclude that HHD is suitable for measuring a wide range of muscle strength in healthy adults of both genders.

Three muscle groups (plantar flexors, knee extensors, and hip extensors) were assessed with a belt-stabilized hand-held dynamometer. When assessing stronger muscle groups in healthy participants or patients with only minor strength deficits, muscle forces can often exceed those that the tester can resist without movement, thus requiring additional stabilization of the dynamometer [14]. In our participants, the use of a belt to stabilize the dynamometer proved effective and reliable for knee extensors assessment, confirming the results of previous studies [23, 25]. In contrast, the use of a belt did not improve dynamometer stabilization during HHD of plantar flexors. Being one of the strongest muscle groups of the lower limb [35], which generates its force through a longer anatomical lever compared to a lever of external resistance [36], the tester is not able to maintain the ankle in the neutral position [37] and provide sufficient resistance [35] during HHD. Although we used a short and wide belt (see Fig. 1J), ankle movement could not be completely prevented. Excessive joint movement is most likely the cause of the lowest reliability and second highest MDC in this muscle group (see Table 1). The measurement may also be affected by the force of upper limbs resisting sliding of the body on a table in the cranial direction [35]. In the studies by Mentiplay et al. [15, 38], the tester held dynamometer in his hand while leaning his upper arm and body against a wall to increase the resistance and stabilization of the dynamometer. The intra-rater reliability of HHD for the plantar flexors in healthy participants (ICC $=$ 0.74–0.87) [15] was similar to ours but excellent (ICC $=$ 0.92–0.97) in post-stroke patients [38]. This suggests that the reliability of HHD for plantar flexors may be higher in participants with weaker muscles. Further research is required to determine the most appropriate position for HHD plantar flexor measurements, particularly in participants with minimal muscle strength deficits. Considering that the force of the tester is applied via a relatively short lever [39], it may be that no HHD protocol feasible in the clinical setting provides the tester with sufficient mechanical advantage to withstand the MVIC of a patient with a small strength deficit in the plantar flexors.

Although the intra-rater reliability of HHD was good for the hip extensors, the MDC of these measurements was the greatest. The main problem observed with HHD for these muscles was the inappropriate shape and size of the dynamometer pad for the posterior thigh region. Most of our participants were uncomfortable at the point of contact between the limb and the dynamometer. We hypothesize that the degree of discomfort negatively affected their ability to exert the MVIC moment [14]. Participant effort has previously been cited as a possible factor in reliability [40] and lack of consistency as a possible source of measurement error [41]. Discomfort could be reduced with a modified, belt-stabilized HHD configuration [23] in which the dynamometer is placed against a rigid surface (e.g., a table leg) so that a padded strap is in contact with the participant’s body segment.

For fixed dynamometry of both knee muscle groups, a knee flexion of 90 ${}^{\circ}$ was used to mimic the position of the knee joint during HHD. However, in the seated position, the peak isometric knee extensors moment is generated at 60 ${}^{\circ}$ of knee flexion [42, 43]. Furthermore, excellent reliability of measuring isometric knee flexors moment with fixed dynamometry in prone position was reported for this angle [29]. Therefore, we performed an additional measurement at this angle. Interestingly, the criterion validity of HHD at 90 ${}^{\circ}$ flexion was highest for the knee flexors when compared to fixed dynamometry at 60 ${}^{\circ}$ rather than 90 ${}^{\circ}$ knee flexion. This finding can be explained by changes in muscle length-tension relationship that likely occurred during HHD due to the lack of pelvic stabilization. During fixed dynamometry, the effective pelvic stabilization provided by the belt prevented participants from increasing force output by performing substitutional hip flexion. In contrast, HHD was performed without pelvic stabilization, which allowed participants to perform substitute actions at the hip and lumbar spine. Hence an elongation of the hamstrings to a more optimal length was achieved [1] which resembled their position during fixed dynamometry at 60 ${}^{\circ}$ of knee flexion. Given that peak moment values obtained with fixed dynamometry at 90 ${}^{\circ}$ knee flexion were 22% lower than those obtained with HHD performed at the same joint angle, further supports this explanation. In addition, muscle spasms in the knee flexors occurred more frequently at 90 ${}^{\circ}$ knee flexion. However, the seated position, which also eliminates the effect of gravity on the measurement results but allows a more optimal muscle length of the knee flexors, was not found to be more valid ( $r=$ 0.66–0.76) [15].

In contrast to the knee flexors, the criterion validity of HHD for the knee extensors was higher when compared to fixed dynamometry at 90 ${}^{\circ}$ knee flexion in our participants, but generally lower than for the knee flexors. We must emphasize that HHD underestimated the peak moment of the knee extensors by up to 32%, confirming the results of previous studies with healthy adults [23, 31]. The underestimation of peak moments and lower criterion validity for knee extensors strength observed in our participants may also be attributed to less efficient stabilization of proximal body segments during HHD (we did not provide specific stabilization of the trunk and pelvis). In the previous studies that reported very good to excellent validity of HHD [15, 23], Hansen et al. [23] used pelvic stabilization belts in both HHD and fixed dynamometry protocols, while Mentiplay et al. [15] instructed participants to stabilize the trunk and pelvis with their hands by holding onto edge of table. Therefore, the absolute peak moment values and the strength of agreement between fixed dynamometry and HHD can most likely be increased by appropriate body stabilization.

Degree of criterion validity of HHD was clearly not affected by calculating the mean peak moment from only the first two rather than all three maximal contractions. Therefore, two repetitions of the maximal contraction on a fixed dynamometer are sufficient for comparison with HHD, even though three repetitions are usually performed for accurate determination of MVIC moment [29, 30, 31]. All in all, the HHD protocol for both knee muscle groups used in our study had good to excellent criterion validity while also being practical and not time-consuming.

Some limitations of our study need to be considered. Inter-rater reliability was not evaluated, and the criterion validity of HHD was evaluated only for the knee muscle groups. Because HHD was performed only on the dominant lower limb, possible differences in measurement properties for the nondominant lower limb cannot be excluded. The results of our study were obtained on healthy young adults; therefore, they cannot be generalized to older adults and patient populations with markedly reduced muscle strength.

5. Conclusions

We developed and evaluated the measurement properties of a new protocol for standardized HHD of 12 lower limb muscle groups. Intra-rater reliability was found to be good for five muscle groups and excellent for seven muscle groups. Criterion validity of HHD was very good to excellent for the knee flexors and good to very good for the knee extensors. This protocol is suitable for testing stronger participants, i.e., healthy adults and may be useful for patients with mild deficits in muscle strength. However, the peak moment values for the knee extensors were significantly underestimated, so absolute muscle strength of knee extensors or other large muscles obtained with HHD should be interpreted with caution. The measurement properties of the proposed HHD protocol need to be determined for different patient populations before its use in clinical practice can be fully endorsed.

Author contributions

CONCEPTION: Tjaša Lipovšek and Urška Puh.

PERFORMANCE OF WORK: Tjaša Lipovšek, Alan Kacin and Urška Puh.

INTERPRETATION OR ANALYSIS OF DATA: Tjaša Lipovšek, Alan Kacin and Urška Puh.

PREPARATION OF THE MANUSCRIPT: Tjaša Lipovšek.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Alan Kacin and Urška Puh.

SUPERVISION: All authors read and approved the final version of the manuscript.

Ethical considerations

The study was approved by the National Medical Ethics Committee of the Republic of Slovenia (0120-48012018/5) before the experiment was started. All participants gave their written informed consent to participate in the study.

Funding

The authors report no funding.

Footnotes

Acknowledgments

The authors have no acknowledgments.

Conflict of interest

The authors have no conflicts of interest to report.

Appendix: Test positioning,stabilization,and dynamometer pad placements for hand-held dynamometry of lower limb

Muscle group	Test positioning			Dynamometer pad placement
	Participant	Tested limb	Tester
Hip flexors	LS, untested lower limb extended; S: holding onto edge of table.	Hip in 90 ${}^{\circ}$ flexion, knee in relaxed flexion.	Stands on the tested side.	Anterior surface of thigh – two finger widths proximal to the base of the patella (measured with the knee extended).
Hip abductors	LS, untested lower limb abducted with leg hanging off table; S: holding onto edge of table.	Hip neutral, knee extended.	Stands on the tested side.	Lateral surface of the leg – three finger widths proximal to the ankle joint line.
Hip adductors	LS, untested lower limb flexed; S: holding onto edge of table.		Stands on the untested side.	Medial surface of the leg – three finger widths proximal to the ankle joint line.
Ankle dorsiflexors	LS, untested lower limb extended, heels on a table; S: holding onto edge of table.	Hip neutral, knee extended, ankle neutral.	Stands at the tested foot.	Dorsal surface of the foot – between the cuneiforms and metatarsophalangeal joints.
Ankle evertors	LS, untested lower limb extended, feet across a table; S: holding onto edge of table.	Hip neutral, knee extended, ankle in relaxed	Lunge at the tested foot; S: manually stabilizing leg medially.	Lateral side of the foot – below the head of the 5th metatarsal.
Ankle invertors		plantar flexion.	Lunge at the tested foot; S: manually stabilizing leg laterally.	Medial side of the foot – below the head of the 1st metatarsal.
Hip external rotators	LP, untested lower limb extended, hands under chin.	Hip neutral, knee in 90 ${}^{\circ}$ flexion.	Stands on the untested side; S: manual stabilization of the pelvis.	Medial surface of the leg – three finger widths proximal to the ankle joint line.
Hip internal rotators			Stands on the tested side; S: manual stabilization of the pelvis.	Lateral surface of the leg – three finger widths proximal to the ankle joint line.
Knee flexors			Stands on the tested side.	Posterior surface of the leg – three finger widths proximal to the ankle joint line.
Ankle plantar flexors	LP, untested lower limb extended, feet across a table; S: holding onto edge of table.	Hip neutral, knee extended, ankle neutral.	Kneeling at the tested foot.	Plantar side of the foot – metatarsal heads. B: around the leg of a table and dynamometer.
Knee extensors	Sitting upright on the edge of the table, feet not touching the floor, arms crossed in front of the chest.	Hip and knee in 90 ${}^{\circ}$ flexion.	Kneeling in front of a participant.	Anterior surface of the leg – three finger widths proximal to the ankle joint line. B: around the leg of a table and dynamometer.
Hip extensors	Prone standing (upper body supported on table), knee of untested lower limb extended with forefoot on floor; S: holding onto edge of table.		Kneeling on the tested side.	Posterior surface of the thigh – as close as possible to the posterior knee joint line. B: around the leg of a table and dynamometer.

S $=$ stabilization; LP $=$ lying prone; LS $=$ lying supine; B $=$ belt to stabilize the dynamometer.

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Reliability and validity of hand-held dynamometry for assessing lower limb muscle strength

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Method

2.1 Participants

2.2 Study protocol

2.2.1 Hand-held dynamometry

Table 1 Mean peak forces (kg), intra-rater reliability and minimal detectable change for the hand-held dynamometry of all major lower limb muscle groups (calculated for participants of both genders)

2.4 Statistical analysis

3. Results

Table 2 Comparison of intra-rater reliability of hand-held dynamometry between female and male participants

5. Conclusions

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

Appendix: Test positioning,stabilization,and dynamometer pad placements for hand-held dynamometry of lower limb

References

Table 1
Mean peak forces (kg), intra-rater reliability and minimal detectable change for the hand-held dynamometry of all major lower limb muscle groups (calculated for participants of both genders)

Table 2
Comparison of intra-rater reliability of hand-held dynamometry between female and male participants