Knee Surgery, Sports Traumatology, Arthroscopy
Volume 32, Issue 8 p. 2170-2177
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Open Access

The Calf Raise App shows good concurrent validity compared with a linear encoder in measuring total concentric work

Farshad Ashnai, 

Corresponding Author

Farshad Ashnai

Department of Orthopaedics, The Institute of Clinical Sciences at Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden

Sportrehab Sports Medicine Clinic, Gothenburg, Sweden

Correspondence Farshad Ashnai, Department of Orthopaedics, The Institute of Clinical Sciences at Sahlgrenska Academy, University of Gothenburg, Göteborgsvägen 31, 431 80 Mölndal, Gothenburg, Sweden.

Email: [email protected]

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Jakob Lindskog, 

Jakob Lindskog

Sportrehab Sports Medicine Clinic, Gothenburg, Sweden

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Annelie Brorsson, 

Annelie Brorsson

Department of Orthopaedics, The Institute of Clinical Sciences at Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden

IFK Kliniken Rehab, Gothenburg, Sweden

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Katarina Nilsson-Helander, 

Katarina Nilsson-Helander

Department of Orthopaedics, The Institute of Clinical Sciences at Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden

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Susanne Beischer, 

Susanne Beischer

Sportrehab Sports Medicine Clinic, Gothenburg, Sweden

Unit of Physiotherapy, Department of Health and Rehabilitation, The Institute of Neuroscience and Physiology, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden

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First published: 21 April 2024
https://doi.org/10.1002/ksa.12190

Abstract

Purpose

The linear encoder and the Calf Raise App have been shown to be valid for measurements of plantar flexor muscular endurance in the heel raise test when compared with gold standard equipment. However, the validity of the Calf Raise App has not yet been compared with a linear encoder, an instrument commonly used in clinical and research settings. The purpose of this study was to determine the concurrent validity of the Calf Raise App compared with a linear encoder for the measurement of average heel raise height and total concentric work in the heel raise test.

Methods

Fifty TeamGym athletes (82% females) from an on-going prospective study were included (mean [SD] age: 20 [7] years; body mass index (BMI) = 21.3 [2.5]). Concurrent validity was analysed with single measures intraclass correlation coefficient (ICC) using a two-way mixed effects, consistency model.

Results

Ninety-eight samples were included in the analysis. The mean (SD) average heel raise height and total concentric work measured by the linear encoder and Calf Raise App were 9.9 (1.4) and 7.5 (1.2) cm, and 1728 (584) and 1291 (450) J, respectively. The mean (SD) number of unilateral heel raises was 30 (7.5). The results showed poor to moderate concurrent validity for the measurement of average heel raise height (ICC: 0.62; 95% confidence interval [CI]: 0.48–0.73). Good to excellent concurrent validity was shown for the measurement of total concentric work (ICC: 0.89; 95% CI: 0.84–0.93).

Conclusion

The Calf Raise App shows good concurrent validity in the heel raise test compared with a linear encoder in measuring total concentric work but not average height. While caution is recommended when comparing results from the different instruments, each instrument can be used separately to compare between-limb differences or changes over time in plantar flexor muscular endurance in clinical and research settings.

Level of Evidence

Level III.

Abbreviations

  • CI
  • confidence interval
  • cm
  • centimetres
  • ICC
  • intraclass correlation coefficient
  • SD
  • standard deviation

    • INTRODUCTION

    Plantar flexor muscular endurance is commonly assessed in patients suffering from injuries or problems in the lower leg and ankle. Amongst athletes, for which the prevalence of lateral ankle sprains is 15% [3], strength deficits have been found up to 12 months after injury [5, 12]. The time to full recovery may be influenced by injury severity and preinjury level of activity. Moreover, significant differences in plantar flexor muscular endurance have been reported between individuals suffering from Achilles tendinopathy and healthy controls [11]. It may therefore be important to incorporate objective measurement of plantar flexor muscular endurance in physical assessment, as well as the evaluation of patients presenting with various injuries or problems in the lower leg and ankle.

    To measure plantar flexor muscular endurance in clinical settings, heel raises are commonly used by clinicians. However, using a single parameter, such as a number of heel raises, may not fully elucidate potential deficits in plantar flexor muscular endurance. For example, 63% of patients regained a normal (defined as ≥90% Limb Symmetry Index) number of heel raises 1 year after an Achilles tendon rupture; however, when total concentric work was calculated (where bodyweight and heel raise height are also taken into account), only 23% of the patients showed normal plantar flexor muscular endurance [13].

    The heel raise test, as described by Hébert-Losier et al [8], with the use of a linear encoder, is one of the most commonly used methods to assess plantar flexor muscular endurance in clinical settings and is widely used in research [2, 13, 14]. This protocol has shown excellent test–retest reliability [8]. The use of a linear encoder for the heel raise test has been suggested to be valid in respect of discriminating plantar flexor muscular endurance between injured and uninjured limbs in patients after Achilles tendon rupture [13]. When compared with gold standard three-dimensional (3D) motion capture data with the heel as a reference marker, Andreasen et al. [1] reported a measurement error of 2.9% for the linear encoder. However, when the pelvis was used as a reference marker, the measurement error was 24.7%, implying that the linear encoder overestimates the vertical displacement of the centre of gravity from its true value. In addition, while the use of a linear encoder is common in clinical settings due to the fact that it is more affordable than gold standard equipment, it may still be too costly for some rehabilitation clinics. Recently, a free-to-use smartphone/tablet application, the Calf Raise App, was developed to measure various aspects of the heel raise test, including heel raise height and total concentric work. This may facilitate objective measurement of plantar flexor muscular endurance in clinical and field settings. The Calf Raise App uses video recording to track the displacement of a marker placed on the subject's foot and has been shown to have good to excellent validity (intraclass correlation coefficient [ICC]: 0.84–1.00) when compared with force plates and 3D motion capture data [4, 7]. In all, both the linear encoder and Calf Raise App have been reported to be valid for the measurement of heel raise height, but no comparison of the two instruments has yet been conducted. Comparisons of various aspects of the heel raise test recorded via a linear encoder and Calf Raise App are needed to determine how accurately the results from the two instruments can be compared in future research, as well as in clinical and field settings.

    The purpose of this study was to determine the concurrent validity of the Calf Raise App compared with a linear encoder for the measurement of average heel raise height and total concentric work in the heel raise test. Based on the measurement errors between pelvis markers and the heel linear encoder [1], it was hypothesised that the Calf Raise App would underestimate heel raise height and total concentric work compared with the linear encoder. Findings from this study could provide clinicians and researchers with useful information when deciding which instrument to use for measurements in the heel raise test and when comparing their results with reference values in the scientific literature.

    MATERIALS AND METHODS

    This study was designed as a validity study, including participants from an on-going prospective study that includes TeamGym athletes (ClinicalTrials.gov: NCT05323773). Data analysis was performed using results from baseline testing. All participants submitted a signed consent (by both legal guardians for participants aged <15 years) after receiving written and oral information about the study and possible risks (delayed onset muscle soreness) associated with the testing protocol. The current research adheres to the Declaration of Helsinki and was approved by the Swedish Ethical Review Authority (registration number: 2022-00045-01).

    This study was reported using the Guidelines for Reporting Reliability and Agreement Studies (GRRAS) [10].

    Participants

    The first 50 TeamGym athletes who performed the heel raise test were included in this study. Inclusion criteria were age ≥13 years, regular participation ( ≥ 1 session/week) in TeamGym training and being free of injury in the lower extremity that may be aggravated by performing the heel raise test. Participant characteristics are presented in Table 1.

    Table 1. Demographic data of 50 included study participants.
    Age (years) 20 (7) 13–43
    Height (cm) 166 (10) 149−195
    Weight (kg) 59 (11) 38−92
    Females/Males 82%/18%
    • Note: Values are presented as “mean (SD) with minimum—maximum” for age, height and weight.
    • Abbreviations: cm, centimetres; kg, kilogram.

    Procedure

    Before performing the heel raise test, participants underwent a brief interview addressing recent injuries to ensure that testing would not exacerbate any existing injuries. Thereafter, participants were queried about their age, height, weight and biological sex. Participants were barefoot for all tests, and all tests were conducted by the same examiner (F. A.), who is an experienced physiotherapist. The order of foot testing (left first or right first) was determined individually, as each participant was asked to guess which hand contained an object while the examiner held a small object behind his back.

    Heel raise test: The heel raise test is a measure of the muscular endurance of the plantar flexors [8] and has shown excellent test–retest reliability, with an ICC of 0.96 and a standard error of measurement of 2.2 heel raises [8]. In the present study, the test was performed according to the protocol used by Hébert–Losier et al. [8] on a board with an inclination of 10° (i.e., starting from a slightly dorsiflexed position). A metronome was used to maintain a pace of 30 repetitions/minute (1 second up, 1 second down). A linear encoder (MuscleLab™, Ergotest Innovation AS) was used to measure the height of each heel raise (cm). For data obtained from the linear encoder, one heel raise was added to the number of heel raises of each test due to the fact that linear encoder software systematically excludes the first heel raise of each test. Thus, the following formula was used to calculate total concentric work, where the last factor represents the standard gravity of Earth:

    The encoder was attached with white athletic tape on the heel and around the ankle of the participant. The participants were instructed to perform as many heel raises as they could, as high as possible, to the set pace, with the knee and trunk kept straight and the support of one finger on each hand on a wall in front of the board. Heel raises were performed unilaterally with the nonweight-bearing limb kept slightly to the side of the board to avoid disturbing the working limb. Participants were also asked to keep the pressure on the medial side of the foot in order to minimise excessive inversion at the top of the heel raises. The test was terminated if, after a warning, the participant was unable to perform heel raises according to the instructions or chose to stop due to muscular fatigue. Participants were allowed five repetitions on each side to practise the pace and instructions before performing the actual test. Two minutes of rest was given between the completion of testing on the first side and familiarisation on the other side. Average heel raise height and total concentric work for each leg were used for further analysis.

    Heel raise height was measured with the Calf Raise App (version 1.5) and the linear encoder simultaneously. A small piece of black tape was placed on the distal part of the lateral malleolus (Figure 1), to be recorded with an iPad Pro (3rd generation, Apple Inc. iPadOS 15) placed on the side of the participant, 1 m away, with the camera lens at about the same height as the lateral malleolus during heel raises. The total concentric displacement was divided by the number of heel raises to calculate the average heel raise height and total concentric work for further analysis. The number of heel raises was controlled by visual inspection of the recording of each test.

    Details are in the caption following the image
    Figure 1
    Open in figure viewerPowerPoint
    Attachment of reference markers for the linear encoder (white) and Calf Raise App (black). The picture is of the first author and was taken during drafting of the manuscript.

    Statistical analysis

    All statistical analyses were performed using IBM SPSS Statistics (28th version). The level of significance was set at 5%.

    The concurrent validity of the Calf Raise App was analysed using single measures ICC (95% confidence intervals [CI]) using a two-way mixed effects, consistency model (model = 3, form = 1) and was interpreted as poor (<0.5), moderate (0.5–0.75), good (>0.75–0.9) and excellent (>0.9) [9]. Bland–Altman plots were used to examine limits of agreement between the two instruments, presented with a mean difference and 1.96 × SD, with the mean difference ±1.96 × SD representing the upper and lower limit of agreement, respectively [6]. Measurement error (expressed in percent) was calculated for the average heel raise height and total concentric work independently by dividing the mean difference between the means from the linear encoder and the Calf Raise App by the mean from the linear encoder, that is, for total concentric work:
    where LE = linear encoder and CR = Calf Raise App.

    In order to be able to detect an ICC of 0.75 (reflecting good agreement [9]) between the Calf Raise App and the linear encoder (80% power, 5% significance level), a sample size of 10 participants was needed. However, the heel raise tests of 50 participants were measured to increase the chance of detecting a larger variability of measurements across samples and to enable sensitivity analyses.

    RESULTS

    The results for the right leg of two participants were deemed incorrectly registered due to higher average heel raise height than peak heel raise height in the Calf Raise App and were therefore excluded from the analysis. Thus, the analysis included 98 samples (left leg = 50 tests; right leg = 48 tests). Results from the heel raise test are presented in Table 2.

    Table 2. Results from 98 heel raise tests from 50 participants.
    Linear encoder Calf Raise App
    Average heel raise height (cm) 9.9 (1.4) 7.1–14.0 7.5 (1.2) 5.1–12.7
    Total concentric work (J) 1728 (584) 785–3052 1291 (450) 502–2258
    Number of heel raises 30 (7.6) 11–52
    • Note: Values are presented as "mean (SD) with minimum–maximum.”
    • Abbreviations: cm, centimetres; J, joule.

    Average heel raise height

    The validity of heel raise height measured with the Calf Raise App was poor to moderate (ICC: 0.62 [0.48–0.73]) when compared with heel raise height measured with the linear encoder. In 95 of 98 tests, the Calf Raise App recorded a lower heel raise height than the linear encoder (mean difference: 2.5 cm ± 2.2) (Figure 2). The measurement error for heel raise height was 24.2%.

    Details are in the caption following the image
    Figure 2
    Open in figure viewerPowerPoint
    Bland–Altman plot showing the variability of measurements for average heel raise height. cm, centimetres; CR, Calf Raise App; LE, linear encoder.

    Total concentric work

    The validity of total concentric work measured with the Calf Raise App was good to excellent (ICC: 0.89 [0.84–0.93]) when compared with total concentric work measured with the linear encoder. In 95 of 98 tests, the Calf Raise App recorded lower total concentric work than the linear encoder (mean difference: 437.0 ± 477.4 joules) (Figure 3). The measurement error for total concentric work was 25.3%.

    Details are in the caption following the image
    Figure 3
    Open in figure viewerPowerPoint
    Bland–Altman plot showing the variability of measurements for total concentric work. CR, Calf Raise App; J, joule; LE, linear encoder.

    DISCUSSION

    The main finding of the current study was that the Calf Raise App showed good to excellent concurrent validity for the measurement of total concentric work. Furthermore, the hypothesis that the Calf Raise App would underestimate average heel raise height and total concentric work compared with the linear encoder was confirmed. The need for a direct comparison with respect to the validation of the Calf Raise App as an alternative to the linear encoder has previously been indicated in the scientific literature [4]. The current study is the first, to the authors’ knowledge, to directly compare results between the Calf Raise App and the linear encoder.

    Average heel raise height

    There are various ways to measure different dimensions of the heel raise test in clinical and research settings. The analysis showed poor to moderate concurrent validity (ICC: 0.62) for the Calf Raise App in measuring average heel raise height, where average heel raise height was underestimated in most cases compared with the linear encoder. There are several reasons that can explain these findings. Mainly, the two instruments record heel raise height using different reference markers (linear encoder = the heel; Calf Raise App = the lateral malleolus). Moreover, the linear encoder measures displacement in any direction from its attachment, whereas the Calf Raise App only measures vertical displacement. This means that a forward movement and/or inversion of the heel at the top of a heel raise will be registered as vertical displacement by the linear encoder, potentially overestimating the true vertical displacement. Conversely, the use of the lateral malleolus as a reference (albeit, using the Calf Raise App) has been found to entail good to excellent concurrent validity (ICC: 0.84–1.00) in previous studies [4, 7].

    Total concentric work

    The analysis showed good to excellent concurrent validity (ICC: 0.89) in measuring total concentric work. The measurement error of the Calf Raise App was 25.3%, which equals a mean difference of 437 joules compared with the mean total concentric work recorded with the linear encoder and is similar to the measurement error of 24.7% between the linear encoder and 3D motion capture data reported by Andreasen et al. [1]. The reasons for differences in heel raise height measurements between the linear encoder and Calf Raise App also apply to measurements of total concentric work. However, differences in heel raise height do not lead to the same magnitude of difference in total concentric work, as heel raise height only constitutes one of three variables in the calculation (heel raise height × number of heel raises × bodyweight).

    Clinical implications

    In order to compare measurements from two different instruments on different occasions, potential measurement differences need to be within an acceptable range with respect to differences that are clinically relevant. Based on the results, comparison between the Calf Raise App and linear encoder should only be done using total concentric work. To decrease the risk of over- or underestimation of the plantar flexor muscular endurance of an individual to a degree that may be clinically relevant (approximately 400 joules) [13], one may expect the results from the Calf Raise App to be about 450 joules lower than results obtained from a linear encoder. However, this difference may be influenced by the weight of the patient and number of heel raises performed, and further research is warranted to gain insight into the extent to which these factors may affect differences in total concentric work. Moreover, examiners should pay attention to excessive inversion and/or forward movement of the heel during the heel raise test, as this may increase the difference in results between the two instruments. In the Bland–Altman limits of agreements (Figures 2 and 3), all but a few cases fall within the 95% limits, indicating a high level of statistical agreement. However, the authors do not recommend comparison between the Calf Raise App and linear encoder using heel raise height due to the, at best, moderate concurrent validity found in the current study. Each instrument can, however, be used separately to compare between-limb differences or changes over time in plantar flexor muscular endurance expressed in average heel raise height, as well as total concentric work. In addition, it is fair to recommend that the lateral malleolus be chosen as the measurement point instead of the heel when measuring average heel raise height and total concentric work in the heel raise test.

    A number of methodological considerations were made before drawing any conclusions from the results of the current study. One limitation was the proportion of sexes in the study population. Only nine (18%) males were included, and potential effects from sex-related differences in the degree of inversion at the top of a heel raise cannot be ruled out. However, the proportion of male TeamGym athletes registered for competition in Sweden between the fall of 2021 and spring of 2022 was 12% (Swedish Gymnastics Federation, personal communications), indicating fair representation of sexes in the current study. Due to the insufficient number of male participants, a sensibility analysis based on participants’ sex could not be performed. Future research that includes a higher proportion of male participants is needed to appropriately assess the potential effects of sex on the validity of the Calf Raise App compared with the linear encoder.

    Another limitation may be that the heel raise test was performed barefoot, which may entail differences in the range of plantar flexion and inversion motion compared with performing the heel raise test in shoes that may limit the range of motion. However, shoes were not allowed in the current study as TeamGym is mainly practiced barefoot. Therefore, the results of this study may not be generalised to athletes who wear shoes during the heel raise test. Furthermore, the inclusion of athletes from a single sport may affect the generalisability of the results of this study to other sports. Conversely, a homogenous population, in terms of sports participation, may increase the likelihood of consistency in the way heel raises are performed, as heel raises are a common exercise amongst TeamGym athletes.

    The current study is the first to compare the Calf Raise App and the linear encoder in measuring heel raise height and total concentric work, and further research is thus warranted to confirm the conclusions drawn in the current study. In addition, the two instruments can be used to measure other aspects of plantar flexor muscular performance with different test protocols (e.g., peak concentric power), for which future research may have different implications.

    The major strength of the current study is the use of a common test with excellent reliability [8]. Another methodological strength is the use of the same examiner in all trials. Lastly, using a larger than needed sample size to examine the limits of agreement between results obtained from the two instruments provided additional information about the variability of measurements, which proved to be helpful for considerations regarding the clinical implications of the findings.

    To summarise, the linear encoder has been used in previous research, and results obtained in clinical settings using this instrument can easily be compared with reference values in the scientific literature. In contrast, comparisons from results obtained with the Calf Raise App are not as straightforward. On the other hand, the Calf Raise App is free to use and may allow a larger number of clinicians to systematically assess and evaluate the plantar flexor muscular endurance of their patients. Clinicians may therefore decide which instrument best fits with their goals and resources.

    CONCLUSION

    The Calf Raise App shows good concurrent validity in the heel raise test compared with a linear encoder in measuring total concentric work but not average height. While caution is recommended when comparing results from the different instruments, each instrument can be used separately to compare between-limb differences or changes over time in plantar flexor muscular endurance in clinical and research settings.

    AUTHOR CONTRIBUTIONS

    Farshad Ashnai, Jakob Lindskog, and Susanne Beischer contributed to planning the project. Farshad Ashnai was responsible for the data acquisition and statistical analysis. Farshad Ashnai, Jakob Lindskog, and Susanne Beischer drafted the manuscript. All the authors contributed to the interpretation of data, critically revised the manuscript and approved the final version of the manuscript.

    ACKNOWLEDGEMENTS

    The authors would like to thank the Swedish Gymnastics Federation and the coaches of the involved TeamGym clubs for their help in recruitment of participants. Ultimately, we would like to thank the athletes for their voluntary participation. This work was supported by the Research and Development primary healthcare Region Gothenburg and Södra Bohuslän, under Grant RHS-2022-00802.

      CONFLICT OF INTEREST STATEMENT

      The authors declare no conflict of interest.

      ETHICS STATEMENT

      The study was approved by the Swedish Ethical Review Authority (registration number: 2022-00045-01). All the included participants provided written informed consent. The person in Figure 1 (first author) has given consent for publication.

      DATA AVAILABILITY STATEMENT

      The dataset used and/or analysed is available from the corresponding author on request.

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