Reliability and validity of the finger flexor dynamometer

Abstract

Introduction

Dynamometric measurement is a low-cost, noninvasive method for diagnosing and evaluating traumatic and degenerative disorders. The purpose of this study was to assess the reliability and validity of a new custom-made finger flexor dynamometer and evaluate the feasibility of the measuring procedure.

Methods

Maximum voluntary isometric contraction force at the distal phalanx of the index finger was measured for 1.5 s in 25 healthy volunteers (18–50 yrs). Test–retest reliability and inter-day reliability were assessed by intraclass correlation coefficient model (2,k) on two and five consecutive days, respectively. Both a single measurement and the mean of three repeated measurements were carried out daily. The standard error of measurement was used to measure the absolute reliability, the smallest detectable change was determined, and the coefficient of variation was calculated for each individual. Construct validity was determined by Pearson coefficient.

Results

Repeated measurement test–retest reliability was excellent according to Munro’s rating scale, with an intraclass correlation coefficient of 0.99 (95%CI: 0.97–0.99), coefficient of variation of 2.6%, and standard error of measurement of 0.4 N. Single measurement test–retest reliability was high to excellent with an intraclass correlation coefficient of 0.94 (95%CI: 0.86–0.97), coefficient of variation of 4.1%, and standard error of measurement of 1.4 N. Smallest detectable change increased from 1.8 N (repeated measurement) to 3.3 N (single measurement). Inter-day reliability intraclass correlation coefficients exceeded 0.93. High construct validity was indicated by a convergent relationship with grip strength (r = 0.85, p < 0.001).

Conclusions

The novel dynamometer provides excellent reliability and construct validity and supports an objective diagnosis of finger limitations by quantifying the flexion force magnitude. Traumatic injuries and follow-up of rehabilitative treatment can be monitored more precisely.

Keywords

Dynamometer fingers physical and rehabilitation medicine reproducibility of results upper extremity

Introduction

The hand is the most advanced musculoskeletal tool of the human being.¹ Full functionality in combination with sufficient strength is essential for conducting various manual activities in everyday life.^2,3 Hand strength is an important and established predictor of health, including loss of physical functionality due to musculoskeletal diseases.^4,5 Furthermore, grip strength has prognostic value regarding all‐cause and cardiovascular mortality as well as cardiovascular disease.⁶

The quantification of pinch and grip strength is a key factor when evaluating a patient’s status, response to treatment, and the functional ability of many patients with upper extremity disorders.⁶ In particular, the outcome of surgical procedures and rehabilitative treatments in patients with traumatic injuries or destructive diseases can be assessed.⁷ Dynamometric evaluation of the hand is considered to be an objective parameter in physical and rehabilitation medicine supporting the diagnosis of diseases, and the noninvasive and inexpensive instruments can be used in a general examination of a patient.^8–10

Nevertheless, subtle impairments of a single finger cannot be adequately identified with these nonspecific hand strength measurements. According to Li et al.,¹¹

a single output of total grip force produced by multiple digits in flexion provides information on the combined function of numerous hand muscles innervated by multiple nerves and therefore does not indicate the force production capacity of individual digits or individual muscles.

Pinch grip and pincer grip likewise include the interaction of more than one finger, with only partial involvement of the distal interphalangeal joint (DIP) and proximal interphalangeal joint (PIP), and do not necessarily assess the isolated motor function of an individual digit.¹¹

Several approaches have been made to evaluate isolated finger strength, multi-finger synergies, and the effect of varying the metacarpophalangeal (MCP) and interphalangeal joint angles on isometric index-fingertip force.^12–14 For instance, Pataky et al.¹⁵ measured the strength of ab-/adduction forces of all fingers simultaneously. Maximal voluntary isometric forces for a variety of finger pulling tasks that varied in the number of fingers and force application locations were determined by Cort and Potvin.¹⁶ Didomenico and Nussbaum¹⁷ compared finger strength in a range of single and multi-digit couplings in an experimental environment simulated seated work.

These experimental studies mainly used complex, nontransportable research apparatus with multiple degrees of freedom or multi-component torque sensors to measure force production of a digit at various points of force.^11,13 However, a comprehensive and time-consuming measuring setup is less suitable for medical diagnostics. Simpler dynamometers have been used clinically: The Rotterdam Intrinsic Hand Myometer (RIHM) was designed to

measure a wide range of muscle groups, such as the abduction and adduction strength of the little finger and index finger, the opposition, palmar abduction (anteposition) and opposition strength of the thumb, and intrinsic muscles of the fingers combined in the intrinsic plus position.¹⁸

It has been successfully used in nerve injury patients to study the functional recovery of the intrinsic muscles of the hand in isolation. Forces are measured by pulling on a broad leather band placed on the digit. The RIHM was not developed to measure forces at the DIP and axial pulling of the entire upper extremity cannot be completely excluded due to a missing handle. The NK Hand Assessment System (NK Biotechnical, Minneapolis, MN, USA) provides measurement of individual digit flexion without being steplessly adjustable in height and distance.¹⁹

No systematic evaluation of motor function of individual digits is currently in clinical use.¹¹

To quantify even subtle impairments and response to treatment by high-precision testing of the isometric flexion force of a single finger, we have developed a portable, noninvasive, easy-to-operate and inexpensive dynamometer that is continuously adjustable and prevents axial pulling of the upper extremity. The dynamometer was designed to measure force at the distal phalanx of a digit in order to record impairments of all joints including MCP, PIP, and DIP, resulting in a reduced maximum voluntary isometric contraction (MVIC).

For the purpose of this study, we have chosen to focus on the index finger.

The main objective of this study was to evaluate the reliability and validity of the new measurement method. In terms of practical implementation, we examined whether a single trial or the average of three trials should be used. The use of reliable methods is indispensable in clinical practice to ensure that changes in the variable of interest are attributable to real differences and are not due to measurement errors.²⁰

Methods

The study was approved by the ethics committee of the German Sport University Cologne. Participation in the examinations was voluntary. The participants received standardized information and gave their written consent. Volunteers were recruited from several scientific institutes in Koblenz, Germany.

Both a single measurement (SM) and a group of three measurements (repeated measurements (RMs)) were carried out for each day. For RM, the mean of three daily trials was recorded at 120 s intervals. Test–retest reliability was assessed on two consecutive days with a total of two and six trials for SM and RM, respectively. Inter-day reliability was performed on five consecutive days with a total of 5 and 15 trials for SM and RM, respectively.

Grip strength (MVIC) was recorded once a day after the assessment of the finger flexion force to avoid measurement-related fatigue of the hand muscles. Throughout the study period, no strenuous sporting or occupational activities involving hand muscles such as climbing or weight lifting were allowed.

Measurement method and dynamometry

The dynamometer was designed to measure the MVIC force at the distal phalanx of a digit. The measuring device consists of a handle and an opposing strain gauge load cell (K25 force sensor, Lorenz Messtechnik GmbH, Alfdorf, Germany) connected to an annular force transducer. These components are adjustable in height and distance (Figure 1). The dimensions of the device were adapted to the 50th percentile of average finger and hand measurements of adults regardless of sex.²¹ MVIC is measured with a resolution of 1 kHz and a sensitivity of 0.1%. Isometric grip strength and elbow flexion strength were assessed by custom-built dynamometers as described previously.²²

Figure 1.

Schematic structure and dimensions of the finger force measuring device. The dimensions of the device used in this study are given in millimeters. The height and distance of the handle and opposing strain gauge load cell are adjustable.

Measuring procedure

The following procedure was used to measure force at the distal phalanx of a digit: In the standing position, the feet were at shoulder width and the hand stretched out in a relaxed posture in front of the body. The shoulder was adducted and neutrally rotated and the elbow slightly flexed in a relaxed posture with the forearm in neutral position.²³ The dominant distal phalanx of the index finger was flexed rapidly against the ring-shaped force transducer for 1.5 s. In order to ensure consistent positioning and identical conditions for RMs, the volar skin fold of the DIP was positioned flush with the edge of this ring (Figure 2). For increased grip, the inner surface of the ring was roughened and slightly fluted radially to avoid position changes during measurement. This configuration allows the assessment of the flexion force, both for the superficial and the deep flexors of the finger, inserting into the medial and distal phalanges, respectively.

Figure 2.

Measuring procedure and finger positioning. The volar skin fold of the DIP is positioned flush with the edge of the ring-shaped force transducer. MVIC force of the index finger is measured for 1.5 s.

The measuring procedure was explained beforehand and, for familiarization purposes, was practiced repeatedly with the nondominant side. All instructions were given in accordance with a standardized protocol to minimize the influence of the examiner and all procedures were conducted by one examiner for all participants.

Anthropometry

Body mass, height, and shoulder width as well as length of finger, hand, forearm, and upper arm were measured using standardized instruments (anthropometer, calibrated scales) in accordance with DIN/EN/ISO 7250-1:2010–06. Body mass index was calculated as the body mass divided by the square of the body height and expressed in units of kg/m².

Statistical analysis

Statistical analyses were performed using SPSS 24 (IBM SPSS, Armonk, NY, USA). Continuous data are given in mean ± standard deviation.

The relative reliability of the measures was assessed using the intraclass correlation coefficient (ICC).²⁴ ICC(2, 1) for SM and ICC(2, 3) for RM with their 95% confidence interval (CI) was calculated on the assumption of an absolute-agreement, two-way random-effects model.²⁵ ICC scores are described with Munro’s classification for interpretation: 0.26–0.49 (low correlation), 0.50–0.69 (moderate correlation), 0.70–0.89 (high correlation), and 0.90–1.00 (excellent correlation).²⁶

The standard error of measurement (SEM) was used to assess the absolute reliability quantifying the precision of individual scores in the dynamometric test and was measured according to the methods proposed by Weir²⁷: $SEM = SDx \sqrt{1 - ICC}$ . The SEM is given in the same unit as the measurement of interest, i.e. Newton. Absolute reliability was additionally assessed using Bland–Altman plots with 95% limits of agreement (LOA = inter-trial mean difference ± 1.96 SD of the between-trial difference).²⁸

The coefficient of variation (CV = (SD of n tests)/(mean of n tests)) was calculated for each individual according to Atkinson and Nevill²⁹ and the mean expressed as a percentage is reported to allow a comparison to be made across different studies and conditions.

The smallest detectable change (SDC) was determined in order to specify the minimal amount of change in the MVIC, which is considered to result from a real difference in the measured force and not to be due to the error in measurement. In accordance with Weir,²⁷ the SDC is expressed as: $SDC = 1.96 x \sqrt{2 xSEM}$

A repeated measures analysis of variance (ANOVA) was performed to determine differences between the follow-up measurements over five days.

Construct validity was primarily determined by Pearson correlation using index finger strength (SM) and grip strength. Additionally, correlation with the length of the forearm, hand, index finger, and elbow flexion strength was determined.

The Shapiro–Wilk test and visualizing the distribution using a histogram were used to evaluate the normality of data obtained in both SM and RM trials. An alpha level of 5% was used to determine the level of significance in differences for all analyses.

Results

Twenty-five healthy volunteers (19 men and 6 women) participated in this study. Characteristics of the participants are listed in Table 1. Adults aged between 18 and 50 were selected to best represent a healthy clinical population. Exclusion criteria were a functional limitation of the hand, musculoskeletal injuries, or previous hand trauma. All the participants reported to be right-hand dominant.

Table 1.

Characteristics of participants.

	Mean	SD
Age (years)	31.4	7.9
Body mass (kg)	88.3	15.6
Body height (cm)	179.4	7.9
BMI (kg/m²)	27.3	3.9
Upper arm length (cm)	38.1	2.5
Forearm length (cm)	28.6	1.8
Hand length (cm)	18.9	1.1
Index finger length (cm)	7.4	0.5

BMI: body mass index; SD: standard deviation.

Mean and standard deviation of participants comprising 19 men and 6 women aged 32.4 ± 8.6 and 28.3 ± 4.1 years, respectively.

Reliability

Measurements for reliability are summarized in Table 2.

Table 2.

Reliability of single and repeated measurements.

	Repeated measurements (RM)				Single measurement (SM)
	ICC	95% CI (ICC)	SEM (N)	CV (%)	ICC	95% CI (ICC)	SEM (N)	CV (%)
Test–retest	0.99	(0.97, 0.99)	0.44	2.59	0.94	(0.86, 0.97)	1.40	4.06
Inter-day (5 days)	0.99	(0.98, 0.99)	0.56	3.97	0.93	(0.88, 0.97)	1.75	4.75

CI: confidence interval; CV: coefficient of variation; ICC: intraclass correlation coefficient; SEM: standard error of measurement.

Test–retest reliability was excellent according to the Munro’s rating scale for RM²⁶ – using the mean of three repetitions – with an ICC of 0.99 (95% CI = 0.97, 0.99) and a CV of 2.59%. Accordingly, absolute reliability was excellent as well, with an SEM of 0.44 N. The SM test–retest reliability was still high to excellent, with an ICC of 0.94 (95% CI = 0.86, 0.97). SM was accompanied by an increased variance, which was reflected in an elevation of the CV by about 1.5 percentage points from 2.59 to 4.06% in comparison to RM. Similarly, the SDC increased from 1.83 N (RM) to 3.29 N (SM).

Inter-day reliability with continuous measurement over five days revealed a comparably high reliability, with an ICC (RM) of 0.99 (95% CI = 0.98, 0.99) and an ICC (SM) of 0.93 (95% CI = 0.88, 0.97), respectively. SDC increased from 2.07 N (RM) to 3.67 N (SM).

A repeated measures ANOVA determined that index finger strength did not differ significantly over the period of five days for either a SM (F(4, 96) = 0.74, p = 0.568) or RMs (F(4, 96) = 0.53, p = 0.718).

Differences between SM and RM are visualized by Bland–Altman plots in Figure 3, showing a considerably decreased variance when RMs are applied. Both SM and RM trials were normally distributed, as assessed by the Shapiro–Wilk test (p < 0.05).

Figure 3.

Test–retest reliability obtained by single and RMs presented as Bland–Altman plots with LOA and scatter plots. (a, c) The differences between two strength measurements conducted two days apart both by SM and a mean of three measurements are plotted against each participants’s mean for these two tests. The 95% LOA are indicated on the plot by the dotted ± 1.96 SD mark and decrease from a (−19.5 N, 19.17 N) to c (−12.48, 12.79 N). The mean ranges from −0.17 N (a) to +0.16 N (c), indicating no systematic bias between the two measurements. The point distribution suggests that variance is considerably decreased when RMs are applied (c). (b, d) Scatter plot of the finger strength achieved on the first trial against the second trial by each person. The mean of RMs shows a narrower point distribution around the regression line (R² = 0.94, y = 0.997x + 0.63) compared to SMs (R² = 0.87, y = 0.893x + 15.2).

Validity

A linear interrelationship between index finger strength (144 ± 28 N) and grip strength (560 ± 120 N) with a correlation coefficient of 0.85 (p < 0.001) indicates high construct validity. Elbow flexion (204 ± 58 N, r = 0.82, p < 0.001) also correlates highly with index finger strength.

Finger strength in addition correlates moderately with anthropometric measurements of the upper extremity, including the length of the index finger (r = 0.65, p < 0.001), hand (r = 0.64, p = 0.001), and forearm (r = 0.59, p < 0.01).

Discussion

Isolated index finger strength is important in functional activities and several occupational settings. For example, industrial hand tools with a pistol grip require repeated or prolonged trigger pulls (e.g. spray painting, rivet guns, sand or water blasting). Another example of a functional activity is the operation of a conventional bicycle brake: flexion force must be applied by the distal pad of the finger via the DIP. Even apparently subtle impairments can therefore lead to relevant occupational disadvantages and disabilities in everyday life. Nevertheless, such impairments of a single finger cannot be sufficiently detected with nonspecific hand strength measurements and can be difficult to diagnose subjectively. The FFD provides an objective parameter of the isometric flexion force with which the functional result can be quantified. Clinically, it allows a precise analysis of therapies and the outcome of surgical procedures in patients with traumatic injuries and follow-up of rehabilitative treatment can be monitored.

Reliability

This study presents a dynamometer that was built to determine the magnitude of the flexion force at the distal phalanx of the finger. The main objective was to evaluate reliability and validity. The dynamometric measurement revealed an ICC of 0.99, suggesting excellent test–retest reliability. Recording of three subsequent trials and calculating the mean MVIC turned out to be the most reliable method and is therefore recommended: This procedure decreases variance of measurement in contrast to SM. The physiological variability of the muscles is hereby arithmetically averaged and adverse variations based on the measuring procedure itself are compensated. Nonetheless, SM test–retest reliability is still high to excellent and is sufficiently suitable for clinical diagnosis. This approach should be applied as well for time-sensitive settings, e.g. for evaluating short-term recovery of MVIC after muscular fatigue in experimental settings.

Despite the various methods described earlier for determining isolated finger strength, none can be compared directly with our dynamometer.^11–15 Measuring finger flexor strength at the distal phalanx according to our approach has not yet been conducted and evaluated in detail. It is important to note that arrangement of components as well as positioning of finger and hand is different compared to similar studies, which is a major and well-known variable for MVIC.¹¹ Cort and Potvin¹⁶ found within-subject CVs between 9.6 and 12.5% for maximum isometric finger pull forces depending on interface characteristics. Didomenico and Nussbaum¹⁷ assessed a CV of 35.9% in females and 38.3% in males for a condition called “Distal Pad Pull”: Persons had to pull with the pad of the finger against a force gauge with their palm facing down. These CVs are considerably higher compared to our dynamometer with a CV of 2.6%.

Reliability of isometric grip strength testing has been sufficiently evaluated and represents a comparable method to some extent.³⁰ Depending on the persons and the particular instrument used, the ICC for grip strength commonly ranges between 0.91 and 0.99.^30–35 Guerra et al.³⁰ reported excellent ICCs between first and second measurements in healthy persons using a Jamar® Plus + Hand dynamometer model (Sammons Preston, Bolingbrook, IL) and the Bodygrip dynamometer (straight handle) with ICCs of 0.96 and 0.98, respectively. Bertrand et al.³² obtained excellent ICCs for maximal grip strength measurements in poststroke patients ranging between 0.97 and 0.99. Our dynamometer provides an ICC in the upper range for healthy patients compared to grip strength measurements accordingly.

Construct validity

Construct validity was determined indirectly based on different hypotheses. First, it was assumed that index finger strength accounts for a fraction of grip strength and should accordingly correlate highly with each other. Second, elbow flexion strength indicates overall strength of the upper extremity and presumably correlates with finger flexion strength to some extent. Finally, extrinsic finger muscles—flexor digitorum profundus and superficialis—are located at the forearm and conduct strength via the metacarpus to the finger. For this reason, correlation with the lengths of the index finger, hand, and forearm was determined. In fact, there was a decreasing correlation in exactly this order, which is anatomically plausible due to the decreasing direct involvement in the flexing process. Validity was altogether assessed by approximation and not by comparison with a “gold standard.” Nonetheless, a correlation coefficient of up to 0.85 (p < 0.001) can be interpreted as excellent, considering the comparison with a measurement method that only partially includes index finger force. Due to a missing gold standard, the recorded measure of construct validity should not be merely interpreted as a fixed value, but rather indicates that there is no systematic error and construction deficit regarding the measuring procedure and the dynamometer itself.

Measuring procedure and positioning of the finger

Predefined hand postures and body positions were suitable for all participants. As a result, all the measurements were qualified for inclusion on the first attempt. Fixed positioning of the finger and unvarying dimensions of the dynamometer are essential for reliability.¹¹ The roughened and radially slightly fluted inner surface of the force transducer ring increased grip and prevented sliding of the finger. Optical monitoring of the volar skin fold of the DIP, which needed to be positioned flush with the edge of the force transducer ring throughout the measuring procedure, proved to be a feasible method for ensuring correct and reproducible positioning. Measurement on the distal phalanx has diagnostic advantages: Distal phalangeal restrictions, in particular injury of flexor digitorum profundus, are entirely recorded and impairments of all joints including MCP, PIP, and DIP result in a reduced MVIC. In combination with a SDC of just 1.21 N, overall diagnostic sensitivity regarding the extrinsic muscles of the finger is increased. In contrast, measurements on the proximal and middle phalanx do not take sufficient account of impairments of the distal phalanx.

Adjustment of the dynamometer

The dynamometer can be adjusted in terms of height and distance to the hand dimensions of each individual. Nevertheless, the constant distance of 6.5 cm between the force transducer ring and the handle, which is essential to prevent axial pulling force of the arm, turned out to be suitable for all participants. Individual adjustment of the handle span affects finger and palmar forces, which can increase up to 30% when the handle span is altered from 4 to 7 cm.^36,37 The comparability of results between persons is increased by using a fixed distance, and this is therefore recommended.³⁸ Comparable results were found for the Jamar Plus+ Dynamometer, which is adjustable to five grip positions from 35 to 87 mm: measurements taken at a single standard handle position are sufficiently accurate to assess grip strengths for all persons.^38,39

Signal sampling

A total measuring time of 1.5 s was sufficient, since MVIC was reached in all trials during this period. Measuring time should be kept as short as possible in order to avoid muscular fatigue due to the measuring procedure itself. Rest intervals of 120 s are recommended to avoid summation of the strain.⁴⁰ As a consequence, RMs are possible without relevant muscular impairments being caused by the dynamometer. Over the period of 1.5 s, the force signal was sampled at a frequency of 1 kHz in order to enable the recording of the rate of force development (RFD). RFD can be used during rapid contractions for characterizing explosive strength in both laboratory and clinical settings. A frequency of 1 kHz is required to accurately identify contraction onset, measure motor response times, and synchronize the force signal with EMG.^41,42 RFD can thus be additionally assessed without adapting the measurement setup.

Limitations

The findings of this study should be interpreted in the light of the following limitations. Only healthy individuals without any functional limitation of the hand, musculoskeletal injuries, or previous hand trauma were included. In order to distinguish between physiological and pathological levels of force, there is a need for age and sex stratified standard values. Reliability was assessed under the condition of a single rater, which is advisable for clinical application. Future studies could evaluate reliability for both right and left hands as well as inter-rater reliability. Lastly, a fixed distance between the force transducer and the handle was used for increased comparability between participants. The effect of varying distances and differently sized handle spans should be analyzed.

Conclusion

The dynamometric finger flexor strength method presented provides excellent absolute and test–retest reliability as well as high construct validity. Recording of three subsequent trials and calculating the mean MVIC was the most reliable method and should be applied. Nonetheless, SM test–retest reliability is still high to excellent and is sufficiently suitable for clinical diagnosis. MVIC measurement on the distal phalanx provides comprehensive assessment of extrinsic muscles, both for the superficial flexors and deep flexors of the finger. The measuring procedure, including posture as well as optical monitoring to ensure correct and reproducible positioning of the finger, turned out to be a feasible method. Furthermore, the dynamometer can be adapted to the fingers DIII-DV as well. Clinically, the dynamometer allows detecting impairments of a single finger, supporting diagnosis of diseases. The precise measuring method allows the quantification of even discrete force changes over time, which allows a better analysis of therapies. In perspective, the outcome of surgical procedures in patients with traumatic injuries and follow-up of rehabilitative treatment can be monitored more precisely.

Footnotes

Acknowledgements

None

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Ethical approval

This study was approved by the ethics committee of the German Sport University Cologne.

Guarantor

KN.

Contributorship

KN designed and developed the FFD, conceived the study, gained IRB approval, and analyzed the data. UR and BB were involved in protocol development and researched literature. DL, DAV, and SW supported the preparation for publication, gaining IRB approval and recruiting participants. All authors reviewed and edited the manuscript and approved the final version of the manuscript.

Informed consent

Written informed consent was obtained from all subjects before the study.

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Welcome

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