Test–Retest Reliability and Concurrent Validity of FysioMeter C-Station Assessing Lower-Limb Muscle Strength via Isometric Mid-Thigh Pulls

Abstract

The isometric mid-thigh pull (IMTP) is a reliable method to assess lower limb isometric muscle strength. A portable device (FysioMeter C-station) could serve as a suitable alternative. The reliability and concurrent validity of the FysioMeter C-station have not been evaluated for the IMTP unilateral test. The aims of this study were to (1) examine the between-session reliability of the unilateral IMTP test for the left and the right legs using the C-station, and (2) explore the concurrent validity of these measures when compared to gold standard force plates (AMTI). Twenty healthy, recreationally active males (age: 23 ± 4 years, height: 1.81 ± 0.60 m, weight: 79.6 ± 10.8 kg) participated and completed test sessions one week apart. The participants performed the following: (i) three bilateral IMTPs on dual force plates (AMTI_BI); (ii) three unilateral IMTPs on each leg on a single force plate (AMTI_RIGHT, AMTI_LEFT); and (iii) three unilateral IMTPs on each leg on the C-station (CS_RIGHT, CS_LEFT). The peak force was measured in all the testing modalities and an Intraclass Correlation Coefficient (ICC) and Pearson Correlation Coefficient (PCC) were used to evaluate the reliability and validity. The C-station showed good between-session reliability for CS_LEFT (ICC = 0.84) and CS_RIGHT (ICC = 0.85). A strong concurrent validity (PCC ≥ 0.82) was found for the C-station compared to the gold standard (AMTI). The C-station appears to be reliable for measuring unilateral IMTP in recreationally active males. Furthermore, strong concurrent validity of the C-station compared to the gold standard was demonstrated.

1. Introduction

Assessing lower limb isometric muscle strength in athletes may help guide practitioners to develop tailored training programs, which can lead to enhanced performance and reduced injury risk [1,2] To best conduct these assessments, it is imperative to use equipment and methods that are valid and reliable [3].

Multi-joint tests are commonly used to quantify lower limb muscle strength in athletes. The isometric mid-thigh pull (IMTP) is one key outcome measure, and studies have shown a moderate to high correlation between the performance in IMTP tests and dynamic movements such as vertical jumping, short sprints, and change of direction [4]. IMTP tests have also been used in athletes in both unilateral and bilateral conditions, with unilateral IMTPs often used to evaluate strength imbalances between the lower limbs [4,5]. Additionally, isometric testing offers a quicker and potentially less fatiguing alternative to traditional one-repetition maximum (1RM) testing [6].

Most previous studies have used force plates to measure the force production during an IMTP, which is considered the gold standard. The relative reliability in these studies for within-session intraclass correlation coefficient (ICC) ranged from 0.97 to 0.99, and for between-session ICC, it ranged from 0.95 to 0.97 [4,7,8]. In addition, some studies used weightlifting straps to reduce the impact of handgrip strength as a limiting factor in the IMTP [6,9,10]. However, the traditional force plates can be expensive, hard to transport, and difficult to operate [11].

Cheaper alternatives to the force plates, such as strain gauges, exist, but a novel device called the C-station has recently been developed to evaluate lower limb isometric strength [12], reaction time [13] and balance [14]. The C-station (FysioMeter, Aalborg, Denmark) is cheaper and easy to operate compared to traditional force plates. Pilot observations suggest the C-station can measure unilateral IMTP, and the device is highly portable, as it fits into a standard trolley (55 cm × 40 cm × 25 cm). The C-station uses a Nintendo Wii balance board (WBB) (Nintendo, Kyoto, Japan) and simple intuitive software to measure force. These advantages may allow sports teams to conduct more in-season monitoring of athletes. However, the reliability and validity of a unilateral IMTP performed on the C-station is unknown.

The purpose of this study was to (1) examine the between-session reliability of unilateral IMTP in both the right and left legs using the FysioMeter C-station in healthy adults between 18 and 35 years of age who partake in recreational sports, and (2) explore the concurrent validity of this test modality compared to traditional force plates (AMTIs). The addition of a bilateral IMTP was used as a control test for the between-session reliability, as it is a well-established testing procedure in the isometric testing literature. We hypothesized that the FysioMeter C-station would show a good to excellent test–retest relative reliability (ICC > 0.75) and that the concurrent validity would be strong compared to the AMTI force plates.

2. Methods

2.1. Participants

To estimate the needed sample size, a statistical power analysis was conducted in G*Power (Version 3.1.9.7, Heinrich Heine Universität Düsseldorf). “ANOVA: repeated measures, within factors” was chosen as the test design in the program. In this study, the effect size was set to 0.35, as previous studies had calculated Cohen’s d to 0.27–0.46 for peak force in IMTP_UNI tests [8,15]. The significance level was set at 0.05 and the power to 0.8. The analysis indicated that a minimum of 15 participants had to be recruited to show a valid significant result. To account for potential dropouts, 20% was added to the calculated number of participants, and as a result, the aim was to recruit 18 participants.

Twenty participants (age: 23 ± 4 years; height: 1.81 ± 0.06 m; weight: 79.6 ± 10.8 kg) from different sporting backgrounds (football n = 14; tennis n = 3; golf n = 1; running n = 1; CrossFit n = 1) were recruited to this study. Most participants had prior experience with resistance training (n = 17). All participants participated in recreational sports and were over 18 years old. Participants were excluded if they had any injuries influencing their ability to perform the test protocol. All participants provided written informed consent, and the study procedure was aligned with the Declaration of Helsinki and the North Denmark Regional Committee for Health Research Ethics (journal number: 2023-000206). Furthermore, this study followed the guidelines for reporting reliability and agreement studies (GRRAS) [16].

2.2. Design

A repeated measures test design was used, consisting of two test days separated by one week. Participants were randomized and counterbalanced for the completion of three different exercises, and for the right and left leg in the unilateral exercises to decrease a potential order effect because of fatigue [17]. The same order of exercises for each participant was repeated on day 2. The exercises consisted of a unilateral IMTP for both the right and left leg on FysioMeters C-station, a force plate (AMTI model OR6-5 biomechanics platform), and a bilateral IMTP on dual force plates, where ground reaction force (GRF) was measured (Figure 1).

Each participant was tested at approximately the same time of day to mitigate the potential impact of circadian rhythm variations [18]. Participants were asked to match their food and fluid consumption before each test and to avoid strenuous exercise 48 h before each test [7]. Height and body weight were measured on test day 1. Participants completed a baseline questionnaire regarding their primary sport, preferred kicking leg, and resistance training experience.

2.3. Procedure

The IMTP test procedure replicated a standardized protocol previously published (Comfort et al., 2019). Before the test procedure began, the barbell and squat rack (11-001, Er Equipment, Denmark) were adjusted to the participants’ body measurements (Figure 2B,C). In contrast, the C-station had a corresponding harness where the barbell was fastened, which was also adjusted to the participants’ body measurements (Figure 2A). For both procedures, this was an iterative process. Participants would take a stance that looked like the start of the second pull phase in the clean lift, with the bar at mid-to-upper-thigh level. The bar would then be adjusted to a level that allowed the participant to obtain the optimal knee (125–145° extension) and hip (140–150° extension) angles for maximal force production [4]. As a result, the upper body position was an upright torso with the shoulder girdle retracted and depressed and shoulders slightly behind or above the vertical plane of the bar. A reference point was established on the wall for the participants to focus on, ensuring that they maintained an upright position and to eliminate possible vision confounders [19]. The participants were instructed to use a pronated grip on the barbell at shoulder width and were equipped with lifting straps, to ensure that grip strength was not the limiting factor while performing the IMTP [10]. The lower body position for the bilateral IMTP was with the feet centered under the bar at approximately hip width, with knees under and in front of the bar with the bar in contact with the upper thighs. For the unilateral IMTP, the foot was placed in a position where the participant could maintain their balance [4]. When the participant obtained the correct position, the exact measurements of foot placement, bar height, grip width, and joint angles of knee and hip were recorded to ensure the same position was reproduced on test day 2 [4].

2.4. IMTP Protocol

Prior to the test, participants performed a standardized warm-up protocol. The general part of the warm-up included 10 body-weight lunges with each leg and 10 body-weight squats. After the general warm-up, a specific IMTP warm-up was used, which consisted of three progressively graded practice/familiarization IMTPs at 50%, 70%, and 90% of perceived maximum effort [4,5,20].

After the warm-up, participants performed a standardized IMTP protocol, replicating previous research [4]. Participants performed three maximal effort (i.e., 100%) trials. Each trial lasted 5 s, and trials were separated by two minutes. If participants were unable to maintain their balance, or if a trial did not meet the required effort, defined as being within 250 N of the highest recorded trial, an additional trial was conducted. This process continued until three valid trials were completed. Before the trial, participants were cued to push their feet into the ground as fast and hard as possible, which was initiated by the countdown “3, 2, 1, pull” [15]. Furthermore, participants were instructed to stand still without pulling the bar for at least 1.5 s before initiating the maximal pull to allow the calculation of body weight based on the associated force-time data. Trials were only accepted if they had a stable baseline force trace, as well as no visible countermovement [4]. The unilateral IMTP followed the same procedure as the bilateral IMTP. However, only one leg was in contact with the force plate, while the unsupported leg was flexed 90° at the knee joint. Each participant performed three unilateral IMTPs per leg on both the C-station (CS_RIGHT, CS_LEFT) and the AMTI force plates (AMTI_RIGHT, AMTI_LEFT), and three bilateral IMTPs on the AMTI force plates (AMTI_BI). GRF data were sampled at 100 Hz with the C-station (FysioMeter, Aalborg, Danmark) and recorded with a Surface Pro 7 laptop using FysioMeter software version 5.0.1. The GRF data measured with AMTI force plates (AMTI model OR6-5 biomechanics platform) were sampled at 1000 Hz for 10 s, interfaced with a desktop computer (Dell, Texas), and recorded using Mr. Kick II (version 2, Knud Larsen, Department of Health Science and Technology, Aalborg University).

2.5. Data Analysis

To calculate the participants’ absolute strength, their bodyweights were subtracted from the GRF exerted on the force plates [21]. When participants exerted force on the force plates, their peak force was defined as the moment when the produced force reached its highest point. From the three maximum effort trials, the trial with the highest peak force was selected for further analysis. The data were extracted from the program Mr. Kick II.

2.6. Statistical Analysis

Statistical analysis was performed using SPSS software version 27 (IBM Corp. Released 2020. IBM SPSS Statistics for IOS, Version 27.0. Armonk, NY, USA: IBM Corp). The dependent variable measured was peak force. The Shapiro–Wilk test was used to test for normality in all conditions. A two-way repeated measures ANOVA was used to determine whether there was an interaction between the independent variables and a dependent variable. The independent variables were the tests (AMTI_RIGHT, AMTI_LEFT, CS_RIGHT, CS_LEFT and AMTI_BI) and days (day 1 and 2). A paired samples t-test was used to compare (i) peak force on day 1 and day 2 for between sessions, and (ii) peak force for AMTI_RIGHT and CS_RIGHT and peak force for AMTI_LEFT and CS_LEFT, on both day 1 and day 2. The p-value threshold for statistical significance was defined as p = 0.05.

To examine the reliability of the C-station and the AMTI force plates, a test–retest analysis was made for between sessions. This was performed by an intraclass correlation coefficient analysis (ICC_3,1). A two-way mixed effects model with absolute agreement was used, and the values of the ICC_3,1 (single measure) were assessed by an identification scale, where 0–0.49 is poor reliability, 0.50–0.74 is moderate reliability, 0.75–0.89 is good reliability, and 0.9–1 is considered excellent reliability [22]. While the ICC is a good measure of reliability, it is sensitive to sample heterogeneity. To counter this problem, coefficient of variation (CV) and standard error of measurement (SEM) were calculated to evaluate the absolute and relative reliability [23,24]. It has been argued that a CV < 10% reflects an acceptable reliability for dynamic and isometric strength tests [25]. The concurrent validity between the C-station and the AMTI force plates was examined with Bland–Altman plots with 95% limits of agreement (LOA), as well as Pearson correlation coefficient analysis (PCC). The values of the PPC were assessed by an identification scale where 0.00–0.1 is negligible, 0.1–0.39 is weak, 0.40–69 is moderate, 0.70–0.89 is strong, and 0.9–1.0 is a very strong correlation [26]. Furthermore, the minimal detectable change (MDC) was calculated in the following way: $MDC=1.96\times SEM\times \surd 2$. The MDC% was calculated as follows: MDC% = MDC/average of the means obtained from the 2 tests $\times$ 100.

For all tests, mean and standard deviation were calculated. In addition to the paired t-tests, Cohen’s d effect sizes were used to determine the degree of significant differences between sessions, if any were observed. Effect sizes were interpreted as follows: trivial < 0.19, small 0.20–0.59, moderate 0.60–1.19, large 1.20–1.99, and very large 2.0–4.0 [27].

3. Results

All twenty recruited participants were included in the final analysis. The Shapiro–Wilk tests showed that all the data were normally distributed (all p’s > 0.05).

3.1. Between-Session Reliability

Descriptive between-session statistics for the AMTI_RIGHT, AMTI_LEFT, CS_RIGHT, CS_LEFT, and AMTI_BI performed on day 1 and 2 are reported in

Table 1. Between-session reliability of IMTP for AMTI_RIGHT, AMTI_LEFT, CS_RIGHT, CS_LEFT, and AMTI_BI (n = 20).

	Day 1	Day 2	Diff	p	d	ICC [95% CI]	CV (%) [95% CI]	SEM (%)	MDC (%)
Tests	Mean ± SD (N)	Mean ± SD (N)	Mean ± SD (N)
AMTIRIGHT	1687.5 ± 369.6	1805.4 ± 460.3	117.9 ± 226.5	0.031	−0.28	0.82 [0.57–0.93]	7.7 [4.9–10.5]	2.9	8.0
AMTILEFT	1744.1 ± 391.6	1802.3 ± 454.9	58.2 ± 173.8	0.150	−0.14	0.91 [0.79–0.96]	6.7 [4.7–8.8]	2.2	6.1
CSRIGHT	1581.9 ± 327.1	1658.4 ± 493.0	76.5 ± 222.1	0.140	−0.18	0.85 [0.66–0.93]	6.8 [4.3–9.3]	3.1	8.5
CSLEFT	1595.8 ± 353.4	1643.1 ± 477.9	47.3 ± 237.0	0.383	−0.11	0.84 [0.64–0.93]	7.6 [4.6–10.5]	3.3	9.1
AMTIBI	2176.2 ± 463.6	2160.4 ± 478.6	15.8 ± 216.2	0.747	0.03	0.89 [0.76–0.95]	6.2 [4.1–8.4]	2.2	6.2

Mean = Mean peak force; SD = Standard deviation of the mean; N = Newton; p = Significance level; d = Cohen’s d; ICC = Intraclass correlation coefficient; CV = Coefficient of variation; CI = Confidence interval; SEM = Standard error of measurement; MDC = Minimal detectable change; BI = Bilateral.

. The ICC for AMTI_RIGHT, CS_RIGHT, CS_LEFT, and AMTI_BI showed good between-session reliability (ICC = 0.82–0.89), while AMTI_LEFT showed excellent reliability (ICC = 0.91). The paired samples t-test showed that AMTI_RIGHT was significantly higher on day 2 compared to day 1 (p = 0.031). The effect sizes ranged between trivial and small for all the IMTP tests (d = 0.03–0.28).

3.2. Concurrent Validity

Descriptive statistics for concurrent validity for the AMTI_RIGHT, AMTI_LEFT, CS_RIGHT, and CS_LEFT performed on day 1 and 2 are displayed in

Table 2. Concurrent validity of unilateral IMTP for AMTI and C-Station on day 1 and 2 (n = 20).

	AMTIUNI	C-Station	Diff	p	d	PCC
Test	Mean ± SD (N)	Mean ± SD (N)	Mean ± SD (N)
Right leg day 1	1687.5 ± 369.6	1581.9 ± 327.1	105.6 ± 210.24	0.037	0.30	0.82
Left leg day 1	1744.1 ± 391.7	1595.8 ± 353.4	148.3 ± 229.10	0.009	0.40	0.82
Right leg day 2	1805.4 ± 460.3	1658.4 ± 493.0	147.0 ± 247.5	0.016	0.31	0.87
Left leg day 2	1802.3 ± 454.9	1643.1 ± 478.0	159.2 ± 280.7	0.020	0.34	0.82

SD = Standard deviation of the mean; N = Newton; p = Significance level; d = Cohen’s d; ICC = Intraclass correlation coefficient; CV = Coefficient of variation; CI = Confidence interval; SEM = Standard error of measurement; MDC = Minimal detectable change.

. Strong concurrent validity was observed for all the tests (PCC = 0.82–0.87, CV = 8.62–11.1%). The paired samples t-test showed that the right leg and left leg were significantly higher when measured on the AMTI force plates compared to the C-station on day 1 and day 2 (p < 0.05 for all), and the effect sizes were small (d = 0.3–0.4 for all).

Bland–Altman plots with LOA investigating concurrent validity are displayed in Figure 3. A systematic bias was observed for all the conditions, ranging from 106 N to 159 N.

4. Discussion

The current study is the first to examine the between-session reliability of the FysioMeter C-station when evaluating the IMTP. It is also the first study to explore the concurrent validity of the FysioMeter C-station compared to AMTI force plates for IMTP assessment. A number of important findings were revealed. First, good relative reliability was demonstrated for CS_RIGHT (ICC = 0.85), CS_LEFT (ICC = 0.84) and AMTI_RIGHT (ICC = 0.82), while AMTI_LEFT showed an excellent relative reliability (ICC = 0.91). Second, absolute reliability was acceptable for all the measurements (CV = 6.2–7.7%). Third, when comparing the validity of the C-station to the AMTI force plates, the C-station showed a strong correlation for both right leg and left leg on day 1 and day 2 (PCC = 0.82–0.87). Fourth, the peak force was significantly higher when tested on the AMTI force plates compared to the C-station for all the conditions.

The excellent reliability observed for both the AMTI platform and the C-station is consistent with the existing literature featuring the performance of unilateral (ICC = 0.95, CV = 4.91) and bilateral IMTPs (ICC = 0.96, CV = 4.61) [7,8]. In addition, a previous study investigated the between-session reliability of the WBB when testing the isometric maximal force of the plantar flexors in a similar population, which reported excellent reliability (ICC = 0.91, SEM = 52.8 N) [28] Another study investigated the between-session reliability of the WBB when performing an isometric hamstring strength test, as well as an isometric quadriceps strength test, in a similar population, and found an excellent reliability (ICC = 0.91, CV = 7.2–8.8%) [29]. Interpreting these data together, the ICC, CV, SEM, and MDC values of the IMTP performed on the C-station in the current study appear to be in accordance with several previous studies [7,8,28,29], which indicates that the use of the IMTP and C-station is a reliable tool to measure lower limb isometric peak force.

A significantly higher value for AMTI_RIGHT on day 2 compared to day 1 was observed in the current study, which is contrary to previous research [7]. This could be attributed to a potential learning effect, given that none of the participants had any prior experience performing the IMTP [23]. However, no significant differences were observed for the other tests, which could indicate that a learning effect was not the cause of a significant increase in peak force. A way to minimize the potential learning effect that could occur is to include familiarization sessions. A study investigated the effect of familiarization sessions a minimum of 48 h before an isometric squat test. They found a significant increase in peak force production from familiarization sessions 1 and 2 to session 3, before stabilizing [30]. Based on these findings and data from the current study, we recommend that future studies should implement a familiarization session.

Strong concurrent validity was observed when comparing the C-station to the AMTI force plates with a PCC analysis. Furthermore, when comparing the C-station values for CV, SEM, and MDC with the AMTI force plate values, these findings support the strong concurrent validity. This is consistent with a previous study, in which the concurrent validity of the WBB compared to an isokinetic dynamometer during the execution of isometric plantar flexion was examined in healthy university students. The PCC analysis showed strong concurrent validity for peak force (PCC = 0.72), which is similar to the findings of the current study [28]. Furthermore, a study compared the WBB to ID during an isometric quadriceps and hamstring strength test and found a strong correlation between the devices (PCC ≥ 0.69) [29].

Significant differences in peak force performed on the AMTI force plates compared to the C-station could be attributed to the variations in the equipment. Different bars were used for the IMTPs performed on the C-station and on the AMTI force plates. The bar used for the IMTP performed on the AMTI force plates was a standard Olympic barbell with a rough surface for enhanced grip, while the barbell used for the IMTP on the C-station had a smooth surface, supplying no extra grip. Even though lifting straps were used, some participants reported that they felt a degree of grip loss during the IMTP, which could explain the higher force produced when performing the unilateral IMTP on AMTI force plates. Furthermore, some participants expressed that they felt more stable in the unilateral IMTP performed on the AMTI force plates compared to the unilateral IMTP performed on the C-station. This could have been caused by the fixation of the barbell in the squat rack compared to the harness used for the C-station, where more movement of the barbell could occur. As a result, a bigger demand for balance and stability appears to be needed when performing the unilateral IMTP on the C-station.

4.1. Limitations

There are some limitations in the current study. One of them is the limited population sample, which limits the generalizability of the findings. For future studies, a more diverse sample could enhance the applicability of the findings.

Furthermore, the lack of bilateral testing on the C-station could be a limitation. The addition of a comparison between bilateral and unilateral tests across different devices could provide additional insights into the consistency of the C-station for a wider range of strength assessments.

Another limitation of the current study is the lack of familiarization sessions. By implementing one familiarization session or more, it would be possible to eliminate a potential learning effect and obtain more representative data.

The difference in equipment used for the C-station and the AMTI force plates could be seen as a limitation, as it could influence the participants’ ability to make the same movement on both systems. To account for this, future studies should make use of the same setup on both measuring systems.

The lack of longitudinal data could be seen as a limitation. Future studies should investigate long-term reliability to ensure the device’s consistency over extended periods, particularly in a clinical or rehabilitative setting.

4.2. Practical Applications

Our findings suggest that the C-station is a reliable tool to measure isometric force production and evaluate the muscle strength of the lower limbs using the IMTP. The C-station is potentially an accessible alternative for clinicians due to being portable, user-friendly, and less expensive compared to stationary equipment. This could be helpful for practitioners, coaches, and clinicians aiming to serially monitor athletes throughout the season, in multiple locations. The calculations of CV, SEM, and MDC in the current study are also useful to ensure that the increase or decrease in lower limb strength when performing the IMTP during a season is beyond measurement error and clinically meaningful. Our data will help with the practical interpretation of IMTP results.

5. Conclusions

The FysioMeter C-station showed excellent within-session reliability on both day 1 and day 2, and good between-session reliability. Furthermore, the C-station showed good concurrent validity compared to the AMTI force plates. These findings may help to guide practitioners in the field when screening IMTP in patients and athletes.