The application of range of motion (ROM) tests lacks standardization regarding the number of repetitions and warm-up protocols. Stretch training studies show a plateau in ROM gains after four to five repetitions, with diminishing returns thereafter. This study aimed to determine if this acute effect also occurs in common ROM tests. Inconsistency in measurement protocols across interventions, normative data surveys, and reliability studies necessitates a standardized approach. The acute effect of stretching, particularly prominent in the first five repetitions, significantly impacts the comparability of ROM test results. Therefore, investigating the repetition-dependent acute effect of stretching on ROM during ROM tests is crucial for establishing reliable and comparable measurements. The study focuses on multi-joint movements, as they are primarily limited by muscle-tendon units, unlike single-joint movements restricted by bones, mass, or ligaments. Five frequently used multi-joint ROM tests were selected, primarily evaluating trunk mobility: Fingertip-to-Floor test, Lateral Inclination test (left and right), Retroflexion of the trunk (modified after Janda), Shoulder test (modified after Janda, left and right), and the modified Thomas test (left and right). This study aimed to investigate the repetition-dependent acute effect of stretching on ROM during these tests and identify potential plateau formation.

Existing literature lacks consensus on optimal repetition numbers and warm-up procedures for ROM assessments. Studies on stretch training consistently demonstrate an acute effect, showing increased mobility, stretch tolerance, and reduced passive torque after a few repetitions, with gains diminishing with increasing repetitions. Four to five repetitions are often recommended for stretch training due to minimal gains beyond this point. However, the application of these findings to ROM assessment remains unclear. Current practice varies widely, with studies employing one to three repetitions, sometimes with warm-up, and others without specifying repetition numbers or warm-up protocols. This lack of standardization hinders the comparability of results. The inconsistent application of warm-up, repetitions, and averaging in interventions, normative data, and reliability studies necessitates a standardized procedure. The significant acute effect of stretching, particularly within the first five repetitions, underscores the need for a uniform protocol to improve comparability and control for factors such as stiffness from prior sports activity.

This prospective study included 22 healthy sports students (10 males, 12 females; mean age 25.3 ± 1.94 years). Exclusion criteria included relevant musculoskeletal surgeries or diseases affecting joint mobility. Two raters, trained and experienced in exercise physiology, conducted the measurements. Five ROM tests were performed in a randomized order: Fingertip-to-Floor (FtF) and Lateral Inclination (LI) tests using a tape measure, and Retroflexion (RF), Shoulder (ST), and Modified Thomas (TT) tests using a digital inclinometer. Each test involved 20 repetitions, each held for three seconds at the maximum ROM. A three-second break separated each repetition. Statistical analysis involved ANOVA/Friedman tests with multiple comparisons to detect acute effects and non-linear regression to identify plateau formation. The significance level was set at 5%. BiAS version 11.08 and R were used for data analysis. A pilot study nature meant no formal power analysis was performed. Individual regression analysis assessed the ROM gain in each subject. To analyze changes within 20 repetitions, either repeated-measures ANOVA or the Friedman test (depending on data normality) with multiple comparisons (Bonferroni correction) was used. Mean standard error of measurement was calculated for each repetition to evaluate measurement accuracy. Non-linear regression with random effects (a*exp(-b*x) + c) was applied when a positive sign test for ROM gain was found. Linear regression was performed if the non-linear model was inadequate. The ethical committee of the Goethe-University approved the study (2018-46).

Except for the right-side Modified Thomas test (TT-right, p=0.93), all ROM tests showed significant flexibility gains within 20 repetitions (p<0.001 for most, p=0.009 for RF). Multiple comparisons revealed significant ROM gains after 5-8 repetitions for most tests (19 repetitions for ST-right). Individual analysis showed consistent ROM gains for FtF in all volunteers and >80% in LI-left/right and ST-left/right. Non-linear regression successfully modeled FtF, RF, LI-left/right, ST-left, and TT-left, indicating a gradual decline in ROM gain (plateau formation). A linear regression was performed for ST-right and TT-right due to inadequate non-linear model fits. The mean standard error of measurement varied across tests and sides, with larger errors observed in ST-right and TT-right. Predictions were made for the number of repetitions needed to reach certain percentages of maximal ROM gain (Table 2).

The findings confirm the hypothesis that ROM tests, similar to stretching exercises, elicit a repetition-dependent acute effect. The observed gradual decline in ROM gain and plateau formation are consistent with established findings in stretch training literature. The number of repetitions required to reach a plateau varies among tests, with some reaching a plateau earlier (RF) possibly due to fatigue or differences in movement limitations (e.g., ligamentous limitations in RF). The shorter stimulus exposure in the ROM tests (seconds) compared to static stretching (seconds to minutes) might explain the differences in plateau formation timing compared to previous studies. Although the tests use static stretching, the duration is much shorter than in static stretching protocols. The discrepancies between active and passive ROM tests (maximal vs sub-maximal torque) and the variation in the duration of stimulus exposure might account for the different results observed. The lack of a uniform individual effect might be due to the small magnitude of the effects in relation to measurement accuracy, particularly noticeable with the digital inclinometer's integer measurements. The variability in individual results is larger for the digital inclinometer compared to the tape measure. The significant differences in the contralateral tests (ST and TT) might be explained by differences in hand/footedness or variations in test protocols. The study's results primarily apply to young, healthy adults, and future studies should examine other populations (elderly, impaired individuals).

This study demonstrated an acute effect in most ROM tests, characterized by a gradual decline in ROM gain and plateau formation. This acute effect, similar to that observed in stretch training, should be considered when performing and interpreting ROM assessments in young, healthy adults. Future research should explore this effect in diverse populations and investigate the optimal number of repetitions for maximizing accuracy while minimizing the effort involved in conducting ROM tests.

The study's focus on healthy young adults limits the generalizability of the findings to other populations (e.g., elderly, individuals with musculoskeletal impairments). The relatively small sample size may affect the statistical power, and the use of different measurement tools (tape measure vs. inclinometer) introduced potential variability. The digital inclinometer's integer-based measurements might have reduced sensitivity in detecting small changes in ROM.