Introduction

Resistance training (RT) has been considered an effective strategy for improving muscle strength and overall wellness in older adults1,2. In a geriatric setting, prescribing RT variables, namely, volume (sets x repetitions) and intensity (load), is crucial to maximize strength and power and prevent the incidence of falls2,3,4. Traditionally, the number of repetitions per set is predetermined, while the load is prescribed based on the direct or indirect measurement of the one-repetition maximum (1RM)5,6. However, more recently, a novel RT approach implemented in a geriatric context showed the clinical utility of monitoring movement velocity to determine and individualize the relative loads and number of repetitions per set in older adults2,6,7. The authors showed that velocity-monitored RT provides accurate information in prescribing training volume and intensity, particularly in managing the degree of fatigue2,6,7.

Moreover, a velocity-monitored RT approach allows coaches and researchers to accurately determine the effect of RT and assess the actual changes after an intervention by measuring the velocity reached against the same absolute load (kg) at two different moments (e.g., pre and post-training)8. However, a high precision and reliable assessment of movement velocity is crucial for all these procedures to be efficient and accurately inform coaches and researchers regarding the actual changes. Therefore, considering this critical premise, various velocity measurement devices have emerged over the past few years, including, but not limited to, linear velocity/position transducers, optoelectronic systems, and accelerometers4,9,10,11,12,13.

Recent studies with older adults have measured movement velocity during resistance exercises, such as the leg and chest press, to determine the training load through the load-velocity relationship analysis14,15,16. In the studies by Marques et al.15,16, the authors used the T-Force System (Ergotech Consulting, Spain), which consists of a linear velocity transducer (LVT) connected to a computer and interface and measures the velocity through the recording of electrical signals proportional to the cable displacement17,18,19. Alternatively, Marcos-Pardo et al.14 used the Chronojump System (Boscosystem, Spain), which is a linear position transducer (LPT) that transfers the data through an interface to a computer and calculates the velocity through the numerical differentiation of time-displacement data. Although the T-Force and Chronojump systems seem valid and reliable for measuring lifting velocity during resistance exercises in strength-trained young adults5,11,20,21,22,23, it remains unknown whether these devices present the same validity, agreement, and reliability levels when used by the older population in resistance machines, like the leg press and chest press.

In this regard, the different characteristics of the two velocity measurement devices can lead to different velocity outputs and random errors since LVT enables a direct velocity measurement23,24, and LPT indirectly estimates velocity through the differentiation of the displacement data concerning time data25,26. Therefore, given the differences between devices in the methods used for calculating movement velocity, more research is needed to elucidate researchers and coaches regarding the accuracy of both devices in measuring velocity when coupled to resistance machines and when testing older adults. Given that the movement of the cable in both devices is completely linear when performing the repetitions due to the linear vertical displacement of the weight stack plates of the machines, it is not expected that there will be substantial velocity measurement errors due to potential oscillations that might occur when, for example, the repetitions are performed with free weights. On the other hand, it is crucial to analyze each device’s sensitivity in measuring the movement velocity along different relative loads, given that one device might be more accurate than other in measuring velocity against high relative loads (low velocities) or low relative loads (high velocities). Importantly, as older adults reach lower concentric velocities and have a range of velocities narrower than young adults in load increments of 5% of 1RM in the leg and chest press (~ 0.03 m⋅s− 1)14,15,16, this analysis would allow an understanding of whether these devices can accurately assist in testing, prescribing, and monitoring velocity-monitored RT in geriatric contexts, detecting possible differences between LPT and LVT when used by older people. Furthermore, as Chronojump LPT costs ~ 600€/663$ and T-Force LVT ~ 2500€/2765$20,23, it is crucial to understand whether using a less expensive device ensures valid and reliable velocity measurements so those who have budget limitations can find an affordable solution to implement velocity-monitored RT in geriatric settings.

Based on the previous considerations, the present study aimed to analyze the validity and reliability of the T-Force and Chronojump systems in measuring the movement velocity in the leg and chest press machines when performed by older adults. We hypothesized that both devices would present high validity and reliability levels in measuring movement velocity in both exercises when performed by this population.

Results

Inter-device reliability, validity, and agreement

The ICC values suggest excellent inter-device relative reliability (ICC average of 0.96 in the chest press and 0.98 in the leg press), and the CV value’s acceptable inter-device absolute reliability (CV average of 2.6% in the chest press and 2.2% in the leg press) (Table 1).

Table 1 Inter-device reliability in measuring mean velocity.
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Figure 1 shows very high r² values (0.97–0.98) and low SEE values (0.01–0.02 m·s− 1) suggesting a robust association between devices; furthermore, a low systematic bias (average of 0.03 m·s− 1 in the chest press and 0.02 m·s− 1 in the leg press) and LOA (range between 0.00–0.06 m·s− 1 in the chest press and 0.00–0.05 m·s− 1 in the leg press) were observed.

Fig. 1
figure 1

(A) Upper panels show the average mean velocity in the chest and leg press obtained by T-Force and Chronojump systems (each device is represented with a different color). (B) Middle panels represent the association between devices through the coefficient of determination (r2) and standard error of the estimate (SEE). (C) Lower panels show the Bland Altman plots depicting the systematic bias and random errors (each plot depicts the average difference and 95% limits of agreement [dashed lines]).

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Table 2 shows significant mean differences between devices only for velocities at 40% 1RM (p < 0.05), with the T-Force LVT presenting higher velocities than the Chronojump LPT.

Table 2 Mean velocity differences between devices (mixed-effects model).
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As shown in Fig. 2, participants tended to reach higher mean velocities in the T-Force than the Chronojump for all relative loads and exercises.

Fig. 2
figure 2

Individual mean velocity data measured by the T-Force and Chronojump devices at 40% 1RM (A), 50% 1RM (B), 60% 1RM (C), 70% 1RM (D), and 80% 1RM (E) in the leg and chest press.

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Intra-device reliability

The ICC values suggest excellent intra-device relative reliability (average of 0.95 in the T-Force and Chronojump) and the CV value’s acceptable absolute reliability (average of 4.7% in the T-Force and 5.1% in the Chronojump) (Table 3). The range of MDC values for the spectrum of relative loads analyzed (40-80% 1RM) was between 0.03 and 0.07 m·s− 1, demonstrating similar sensitivity in both devices.

Table 3 Intra-device reliability in measuring mean velocity.
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Discussion

This study aimed to assess the validity and reliability of the T-Force and Chronojump systems in measuring movement velocity in the leg and chest press machines when performed by older adults. To the best of our knowledge, this study is the first to measure the validity, reliability, and agreement levels of movement velocity on strength machines when performed by older people. The main results indicated that both devices presented high validity, reliability, and agreement in measuring the movement velocity in both resistance exercises in older adults, thus confirming our study hypothesis.

Evaluating the reliability of a device or test allows researchers to understand the variations in the results27. The intra-device analysis enables the comparison of velocity outcome measures from the same device under identical intervention conditions made by the same participant11,20. Conversely, the inter-device analysis enables the comparison of velocity outcome measures from a given participant when simultaneously using two or more devices26.

Previous studies with strength-trained young adults have already shown the absolute reliability of mean velocity in both systems in the bench press (CV values of 4.3% in the T-Force and 4.5% in the Chronojump)23, as well as in the bench press and squat (CV values of 1.9% in the bench press and 2.5% in the squat when using the T-Force and 4.3% in the bench press and 3.9% in the squat when using the Chronojump)20. Although the CV values of our study were considered with acceptable absolute reliability (< 10%), they were slightly higher than the results of the previous studies. The minor differences may be due to participants’ characteristics (older adults present lower concentric velocities than young adults) and the type of exercises performed. Regarding relative reliability, both devices showed excellent ICC values, corroborating previous findings (ICC values of 0.99 for the T-Force and Chronojump)20.

In addition to the reliability analysis, we also conducted the validity and level of agreement analysis to understand the accuracy of the measurements achieved by the devices20,28. Here, the results showed high validity, which is in line with the data reported by Pérez-Castilla et al.23, showing a Pearson correlation coefficient (r) of 0.99 between the Trio-OptiTrack system and the T-Force and Chronojump systems in the bench press with physically active men. Likewise, Courel-Ibáñez et al.20 revealed high correlation values (around 0.99) between the T-Force and Chronojump systems.

Furthermore, the Bland-Altman plots revealed low systematic bias and random errors (~ 0.03 m·s− 1) from Chronojump compared to T-Force. Previous findings with strength-trained young adults also demonstrated a low systematic bias between devices (~ 0.05 m·s− 1) in the bench press20,23 and full squat20. In our study, the mixed-effects model showed a significant difference between devices only for velocities at 40% 1RM, with the T-Force exhibiting higher velocity outputs than the Chronojump. These results indicate that the T-Force measures the repetitions with higher mean velocity outputs, which is a finding corroborated in a previous study24 and suggests that as the relative loads increase, the differences between devices in measuring mean velocity tend to decrease. Therefore, these findings suggest that one might be more confident obtaining similar mean velocity values when training with relative loads between 50 and 80% 1RM in the leg and chest press with older adults.

Possible reasons for the lower measurement errors in T-Force when compared to Chronojump might be related to less raw data manipulation (i.e., direct velocity measurement) and the use of an analog-to-digital data acquisition board with 14-bit resolution, which allows the elimination of noise when executing the movement12,29. Moreno-Villanueva et al.26 also reported significant differences (p < 0.001) between LPT and LVT. Although that study used the T-Force, the authors used a different LPT (ADR encoder), variable (mean propulsive velocity), strength exercise, and statistical approach, which limits the comparison with our results. Therefore, as different velocity measurement devices may give different mean velocity outputs, each device must be independently analyzed for its validity and reliability, especially in geriatric settings where research is scarce.

Regarding the practical implications, the current study allows coaches, researchers, and practitioners to understand that the T-Force and Chronojump devices can be valuable and accurate during velocity-monitored RT in geriatric settings. Previous studies with older adults have shown that measuring movement velocity in real-time during RT sessions enables an effective individualization of the training volume (through the analysis of velocity loss) and intensity (through the velocity reached in the first repetitions)2,6. Thus, by monitoring movement velocity during each RT session in real-time, one can obtain accurate knowledge of the relative loads being used, the level of effort, and the effects of the training21. Notably, the current study indicates that for a change to be considered clinically meaningful after RT and not a fluke due to measurement error, the mean velocity (measured on the T-Force and Chronojump devices) must increase more than 0.05–0.07 m·s− 1 for loads between 40 and 60% 1RM and 0.03–0.04 m·s− 1 for loads between 70 and 80% 1RM in both exercises. Such information might be considered by coaches and researchers when interpreting physical performance changes following velocity-monitored RT in older adults; in this way, it is possible to evaluate velocities reached against the same absolute loads and accurately report possible differences found before and after the training program.

This study has some limitations that we must address. First, although we estimated the sample size to obtain a statistical power of at least 80%, it would be essential to include a larger sample size and more age ranges, from middle-aged adults to the oldest old, to generalize the utility of both devices to measure movement velocity when working with these populations. Furthermore, the generalization of results is limited by using only two exercises. Second, as only two velocity measurement devices were used, it would be essential to consider analyzing the validity and reliability of more devices to measure movement velocity in resistance machines, such as accelerometers, apps, and optoelectronic systems. Third, as the current study only has focused on the mean velocity due to being the variable most used in practical contexts with the older population, the reliability and validity analysis for other velocity variables (e.g., peak velocity) could be essential to understand their practical utility in the older population. Fourth, a between-day reliability analysis would allow an understanding of whether these results do not substantially vary between days in this type of population. Fifth, a test-rest analysis with repeated procedures (e.g., one week apart) could be important in future research to detect mean velocity variations within participants over time. Finally, understanding both devices long-term validity and reliability and exploring the impact of different testing protocols (e.g., different ranges of relative loads, number of repetitions performed per testing set, and execution techniques), as well as other exercises (e.g., with varying groups of muscle recruited) and resistance machines (e.g., with a different number and distribution of levers), would be important to expand the current findings.

In summary, this study shows that the T-Force and Chronojump devices are highly valid and reliable for measuring movement velocity in the leg and chest press exercises in older adults. Considering this evidence, coaches, researchers, sports-related professionals, and practitioners can use both devices to accurately assess, prescribe, and monitor the training load during velocity-monitored RT in older adults. When coaches and researchers do not have budget constraints, they might consider using the T-Force system to measure mean velocity in geriatric settings, as it proved to be an extremely reliable device. On the other hand, when budget constraints are present, the Chronojump LPT can be a valid and reliable alternative to the T-Force LVT, considering the low measurement errors and similar mean velocity values observed during the execution of the leg and chest press exercises in older adults, especially for relative loads between 50 and 80% 1RM.

Methods

Study design

In a cross-sectional study design, the participants performed six sessions for three weeks (two per week, interspersed by 48 h of rest). In the first week, we familiarized the participants with each exercise and procedure to enable an adequate adaptation and improve the exercise technique. Specifically, we instructed the participants to perform the concentric repetitions with the maximal intended velocity, followed by a controlled eccentric action, with a coach constantly supervising the movement by placing the hands on the machine’s arms (chest press) or the platform handles (leg press). On verbal command, the participants extended their elbows or knees as fast as possible and slowly returned to the initial position. In the second week, they performed a progressive loading test until reaching the 1RM in the leg and chest press. Last week was dedicated to performing three repetitions against five loads (40, 50, 60, 70, and 80% of 1RM) in the leg and chest press. Different exercises and load intensities were selected to test the validity and reliability of the T-Force and Chronojump systems at different velocities. An experienced researcher and one strength and conditioning coach supervised all sessions and testing procedures. The Ethical Committee of the University of Beira Interior approved the study (approval number: CE-UBI-Pj-2021-078), which follows the ethical norms of the Declaration of Helsinki. This study was prepared following the “Strengthening the Reporting of Observational Studies in Epidemiology” (STROBE) guidelines30.

Participants

The sample size was estimated based on an expected reliability level of 0.95 and a minimum acceptable reliability level of 0.80. With an alpha value of 0.05, a minimum sample size of 16 participants was required to obtain a power of 80%31. Therefore, eighteen older adults (6 men and 12 women, 79.9 ± 8.5 years; 1.53 ± 0.10 m; 66,1 ± 13.7 kg, 1RM chest press: 29.4 ± 12.6 kg; 1RM leg press: 71.9 ± 14.2 kg) were recruited. Figure 3 shows the flowchart of the recruitment process. Inclusion criteria were men or women aged ≥ 60 from residential care facilities or day centers who could collaborate with the research team. Exclusion criteria were severe cognitive decline, musculoskeletal injuries in the previous six months, and terminal illness. Eighteen participants completed the chest press tests, while seventeen completed the leg press tests. One participant failed the leg press tests because of an illness unrelated to study procedures. All participants signed written informed consent and received detailed information regarding the study procedures.

Fig. 3
figure 3

Study flowchart.

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Experimental procedures

The first testing week was used for anthropometric measures and to determine the 1RM during a progressive loading test in the chest and leg press exercises following the procedures described by Marques et al.15,16. The general warm-up consisted of 10 min walking on a treadmill (2–4 km/h). In the seated leg press, participants placed their feet on the platform shoulder width apart, hands on the side handles, and knees flexed at 90°. The specific warm-up consisted of two sets of seven and five repetitions with 20 and 30 kg, respectively. The test started with 20 kg and progressively increased by 10 kg. In the seated chest press, participants placed the handles in the middle of the chest, with their shoulders abducted and with a neutral grip. The specific warm-up consisted of seven and five repetitions with 5 and 7.5 kg, respectively. We set the initial load at 5 kg and progressively increased it by 2.5 kg. Participants performed three repetitions, whenever possible, with the concentric phase executed with the maximal intended velocity and the eccentric phase at a controlled velocity. If participants could not perform a correct repetition, the weight was decreased by 1–5 kg and 1–2.5 kg in the leg and chest press, respectively. This procedure was conducted until the 1RM load was reached. The inter-set rest was 3–5 min7,32.

In the second testing week, participants performed three repetitions with the maximal intended velocity against five loads (40, 50, 60, 70, and 80% of 1RM) in an incremental order in both exercises (inter-set rest of 3 min). Participants were encouraged to perform all concentric repetitions with the maximal intended velocity against all weights, followed by an eccentric phase at a controlled velocity.

Data acquisition

A seated leg press machine (L050, BH Fitness, Portugal) and a seated chest press machine (L070, BH Fitness, Portugal) were used to perform all tests. The mean velocity (i.e., the average velocity across the entire concentric phase) was measured simultaneously through the T-Force and Chronojump systems33,34. We excluded the repetition that deviated ± 0.03 m·s− 1 from the target mean velocity associated with the relative load used during the test (40, 50, 60, 70, and 80% of 1RM). In older adults, a deviation of 0.03 m·s− 1 represents, on average, a difference of 5% of 1RM in the leg and chest press15,16. In general, the excluded repetition was the first in both exercises, presenting, on average, the slowest mean velocity. The tendency for lower mean velocities in the first repetitions might be related to the greater inertia to overcome at the beginning of the movement in both machines when the joints (knees or elbows) are flexed (“sticking point”)35. In cases where the three repetitions did not deviate ± 0.03 m·s− 1 from the target mean velocity, we removed the slowest repetition. Using the abovementioned criteria, we ensured that the mean velocities reached by every participant accurately matched the programmed relative load. For analysis, we averaged the mean velocity of the remaining two repetitions. Figure 4 illustrates the connection between both devices and machines.

Fig. 4
figure 4

The upper panels show both resistance machines ((A) Chest Press, (B) Leg Press), and the lower panels illustrate the connection between T-Force and Chronojump devices and the resistance machine ((C) left view – T-Force device; (D) central view – both devices; (E) right view – Chronojump device).

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The specific characteristics of devices are described below:

  • The T-Force dynamic measurement system (Ergotech Consulting, Murcia, Spain) is an isoinertial dynamometer that consists of a cable-extension LVT interfaced with a personal computer through a 14-bit resolution analog-to-digital data acquisition board and custom software (version 3.70). Instantaneous bar velocity was sampled at a frequency of 1000 Hz and subsequently smoothed with a fourth-order low-pass Butterworth no phase shift and 10 Hz cut-off frequency.

  • The Chronojump system (Chronojump Boscosystem, Barcelona, Spain) is an isoinertial dynamometer that consists of a cable extension LPT interfaced with a personal computer and custom free software (version 2.3.0). Time and displacement data were sampled at a frequency of 1000 Hz and subsequently smoothed using a Butterworth filter with a 10 Hz cut-off frequency.

Statistical analyses

The Shapiro-Wilk test and Q-Q graphs were performed to check the normality of the data. The validity was analyzed through linear regressions (coefficient of determination [r²] and the standard error of the estimate [SEE]). The mean difference between devices (bias) and 95% limits of agreement (LOA) was calculated through the Bland-Altman method36. Furthermore, a linear mixed-effects model with devices and exercises as fixed effects and participants as the random effect estimated the differences between devices in measuring the mean velocity from 40 to 80% 1RM. This statistical approach allows for modeling the influence of individual characteristics (within-subject variability) when analyzing the differences in mean velocity between devices37. The relative reliability was assessed using the intra-class correlation coefficient (ICC) model 1,k (one-way random effects, absolute agreement, multiple raters/measurements) with a 95% confidence interval. The ICC values were interpreted as < 0.50, poor; 0.50–0.75, moderate; 0.75–0.90, good; >0.90, excellent38. The standard error of measurement (SEM) and the coefficient of variation (CV) analyzed the absolute reliability. The inter-device SEM was calculated as the standard deviation of the difference between devices divided by √239. The intra-device SEM was calculated as the standard deviation of the mean of the two repetitions x √1-ICC36. The CV (%) was calculated as the SEM divided by the mean, and values < 10% were considered acceptable40. The minimal detectable change (MDC) was derived from the SEM (√2 x SEM x 1.96)36. Statistical analyses were performed in SPSS v28 (SPSS Inc., Chicago, IL, USA) and Microsoft Excel v2407 (Microsoft Corporation, Redmond, WA, USA). Figures were generated using GraphPad Prism v9 (GraphPad Inc., San Diego, CA, USA) and SPSS.