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. 2025 Nov 17;25(12):e70087. doi: 10.1002/ejsc.70087

Partial Range, Full Gains? The Effect of 8 Weeks of Partial Range of Motion Training at Long Muscle Lengths on Elbow Flexor Hypertrophy and Strength in Trained Individuals

Tim Havers 1,2, Niklas Wagner 1, Steffen Held 3, Stephan Geisler 1, Thimo Wiewelhove 1,✉
PMCID: PMC12621570  PMID: 41247250

ABSTRACT

We investigated the effects of initial partial range of motion (pROMinitial; 0°–70°) and full range of motion (fROM; 0°–140°) training on elbow flexor hypertrophy and strength in resistance‐trained individuals. Thirteen individuals (males: n = 11, 26.6 ± 4.0 years, 89.2 ± 16.7 kg, and 183.3 ± 10.0 cm; females: n = 2, 24.0 ± 1.4 years, 75.5 ± 12.3 kg, and 168.0 ± 4.2 cm) completed a randomized within‐subject study, performing unilateral preacher curls with each arm assigned to one condition over 8 weeks. Muscle thickness at 50% and 70% of the distance between the acromion and cubital fossa, maximal voluntary contraction (MVC) at elbow angles of 40° and 100°, and one‐repetition maximum (1RM) were measured pre‐ and postintervention. Bayesian analyses were employed to infer effects from posterior distributions. Results showed similar improvements in muscle thickness at 50% humeral length between conditions, whereas pROMinitial exhibited trivial to small superiority at 70% elbow flexor length (standardized mean difference [SMD] = 0.10 and Bayes factor = 4.87). Additionally, MVC at 100° (SMD = 0.24 and Bayes factor = 3.02) and 1RM (SMD = 0.17 and Bayes factor = 1.95) demonstrated greater but negligible improvements with fROM, with weak to moderate evidence supporting the hypothesis of differential effectiveness across interventions. These findings suggest that pROMinitial may offer modest benefits for regional hypertrophy, particularly at longer muscle lengths. The results indicate that both training modalities can induce beneficial adaptations, with pROMinitial offering slight advantages in specific contexts.

Clinical Trial Registration

This study was registered at German Clinical Trials Register with the registration number DRKS00035811

Keywords: cross‐sectional area, muscle strength, partials, resistance exercise, training intervention

Key Points

  • Partial range of motion training in the initial phase (pROMinitial) induced slightly greater hypertrophy in the distal elbow flexors compared to full range of motion (fROM), although the effect sizes were trivial to small.

  • Strength gains varied by testing angle: fROM training improved one‐repetition maximum (1RM) more effectively, whereas pROMinitial showed a trend toward greater strength gains at shorter muscle lengths (MVC100°). However, the absolute estimated group differences in strength outcomes were minimal.

  • In trained individuals, both ROM configurations were effective in inducing hypertrophy, but pROMinitial may offer an advantage in emphasising muscle stretch and tension in the lengthened position.

1. Introduction

In health, rehabilitation, recreational, and competitive sports, resistance training is fundamental for improving strength and muscle hypertrophy, which in turn enhances athletic performance and overall well‐being (Roberts et al. 2023; Schoenfeld et al. 2021a; World Health Organization 2020). To optimize these outcomes, extensive research has focused on key training variables—intensity, volume, frequency (i.e., amount of training sessions per week), and rest periods—leading to the development of specialized protocols designed to promote muscle hypertrophy (Grgic et al. 2022; Iversen et al. 2021; Schoenfeld et al. 2021b; Schoenfeld et al. 2016).

One approach that has gained increasing attention is the manipulation of range of motion (ROM) during exercises. ROM can be classified into full range of motion (fROM) and partial range of motion (pROM) (Kassiano et al. 2023b; Wolf et al. 2023). fROM involves executing an exercise through the complete range available at each joint (McBride 2016), whereas pROM restricts the movement to a specific segment of this full range. Recent research has focused on training protocols that apply partial range of motion (pROM) during either the initial phase of concentric movement at long muscle lengths (pROMinitial, also called ‘lengthened partials') or the final phase at short muscle lengths (pROMfinal) (Goto et al. 2019; Kassiano et al. 2023a; McMahon et al. 2014; Pedrosa et al. 2022, 2023; Sato et al. 2021; Werkhausen et al. 2021; Wolf et al. 2025a). Several investigations comparing pROMinitial and pROMfinal generally report greater muscle hypertrophy at longer muscle lengths (Kassiano et al. 2023a; McMahon et al. 2014; Moreno et al. 2024; Pedrosa et al. 2022, 2023; Sato et al. 2021).

Recently, five studies have compared pROMinitial to fROM resistance training (Goto et al. 2019; Kassiano et al. 2023a; Pedrosa et al. 2022; Werkhausen et al. 2021; Wolf et al. 2025a). Although strength gains generally tend to be specific to the ROM used (Kassiano et al. 2023a; Pedrosa et al. 2022), three of these five studies reported greater muscle hypertrophy with pROMinitial (Goto et al. 2019; Kassiano et al. 2023a; Pedrosa et al. 2022), whereas Werkhausen et al. (2021) and Wolf et al. (2025a) found similar hypertrophic responses between conditions. Notably, Werkhausen et al. (2021) employed a concentric‐only leg press protocol with a markedly limited ROM (a 9‐degree change in knee angle), which may have contributed to its divergent results, given that the lengthening phase of an exercise is considered highly stimulatory for muscle hypertrophy (Schoenfeld et al. 2017; Schoenfeld et al. 2022; Warneke, Lohmann et al. 2023).

Importantly, most studies investigating ROM manipulation have predominantly been conducted in untrained populations (Kassiano et al. 2023a; Pedrosa et al. 2022; Werkhausen et al. 2021), with few studies focusing on resistance‐trained individuals (Goto et al. 2019; Wolf et al. 2025a). For example, Goto et al. (2019) observed significant differences in elbow extensor hypertrophy between fROM and pROMinitial, whereas Wolf et al. (2025a) reported no significant differences in either elbow flexor or extensor thickness. Wolf et al. (2025a) employed an ecologically valid design in which subjects performed a variety of upper‐body exercises (e.g., flat bench press, dumbbell row, overhead triceps extension, supinating curl, incline machine chest press, single‐arm pulldown, and cable pushdown) under both pROMinitial and fROM conditions, with the assignment of conditions to each arm randomized. This multifaceted exercise prescription makes it difficult to ascertain whether the lack of significant hypertrophic differences is attributable to the advanced training status of the subjects or to the specific exercise protocols that may differentially engage the elbow flexors and extensors.

To resolve these discrepancies and contribute to a more comprehensive understanding of ROM manipulation in resistance training, we aimed to investigate the effects of pROMinitial versus fROM on elbow flexor hypertrophy in resistance‐trained individuals. Based on the limited yet suggestive evidence in the literature, we hypothesized that pROMinitial would induce greater hypertrophy of the elbow flexors compared to fROM in trained subjects.

2. Materials and Methods

2.1. Study Participants

The study sample consisted of healthy men and women aged 18–35 years recruited from the area of Bamberg and Erlangen (Germany). All subjects were required to complete a data protection declaration, a consent form, and a Physical Activity Readiness Questionnaire (PAR‐Q). The sample size was determined in advance by conducting a power analysis using G*Power software and following the recommendations of Beck (Beck 2013). An alpha level of 0.05, a power of 0.8, and an effect size f of ∼0.3 were used based on previous work (Kassiano et al. 2023a; Kassiano et al. 2023b). The calculation determined the required sample size of 16 subjects. This is equivalent to a minimum of eight subjects in a within‐subject design to achieve adequate statistical power. The inclusion criteria for subjects were as follows: age between 18 and 35 years, with at least 2 years' experience of resistance training, with an average of two training sessions per week, and no use of anabolic steroids or other muscle‐building substances (self‐reported). Individuals who did not attend at least 85% of the training sessions were excluded from the final analysis. Subjects were not allowed to perform any additional isolated biceps training during the intervention period. However, they were permitted to continue their regular resistance training routines for other muscle groups. Due to the unilateral within‐subject design, any indirect elbow flexor activation was equally distributed between both arms, ensuring comparable training exposure outside the study intervention. All subjects were instructed to maintain a high protein intake (approximately 1.6 g/kg body weight) throughout the intervention. In addition, all subjects were informed of and consented to the intervention procedures. This study was conducted in accordance with the Declaration of Helsinki.

Figure 1 illustrates the recruitment process leading up to the analysis, including all drop‐outs. A total of 11 male subjects (26.6 ± 4.0 years; 89.2 ± 16.7 kg; and 183.3 ± 10.0 cm) with 6.0 ± 4.0 years of strength training experience and 4.0 ± 1.3 days of training per week and two female subjects (24.0 ± 1.4 years; 75.5 ± 12.3 kg; and 168.0 ± 4.2 cm) with 6.5 ± 4.9 years of strength training experience and 5.5 ± 0.7 days of training per week completed the entire procedure and were included in the final analysis.

FIGURE 1.

FIGURE 1

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Flowchart showing the recruitment process leading to the analysis, including all dropouts.

2.2. Study Design

This study employed a randomized within‐subject design to examine the impact of range of motion (ROM) on elbow flexor muscle growth and strength. Subjects were randomly allocated—via an external process using randomizer.org—to perform biceps curls with one arm using partial ROM at long muscle lengths (pROMinitial, 0°–70°) and with the other arm using full ROM (fROM, 0°–140°). This approach ensured that the investigators remained blinded to each arm's assigned ROM until after postintervention assessments.

The study spanned 10 weeks (see Figure 2). During the first week, two pretests were conducted to assess the reliability of the measurement protocols; only the data from the second pretest were used in subsequent statistical analyses. The assessment battery included measurements of upper arm circumference using a tape measure, ultrasound‐based evaluations of elbow flexor muscle thickness as well as isometric (MVC) and dynamic (1RM) strength tests.

FIGURE 2.

FIGURE 2

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Schematic illustration of the study design. The study lasted 10 weeks. The pretest was conducted 1 week before the start of the resistance training intervention and the posttest was conducted 1 week after the end of the intervention. The testing protocol remained consistent throughout and included ultrasound assessments of elbow flexor muscle thickness at 50% and 70% of muscle length, and muscle strength assessments including maximal voluntary contractions (MVC) at 40° and 100° of elbow flexion and 1RM testing.

The training program lasted 8 weeks (weeks 2–9), with postintervention data collected in week 10. Subjects trained twice a week for a total of 16 sessions. In each session, each arm was trained according to its assigned ROM: the pROMinitial arm performed biceps curls within a range of 0°–70°, whereas the fROM arm operated within a range of 0°–140°. Subjects performed five sets per arm to muscular failure, using a load determined during the pretest phase. All sessions were meticulously documented, recording both the weight used and the number of repetitions performed in each set.

2.3. Resistance Training

In the week following the initial testing phase, subjects began their training. Each subject trained unilaterally, with one arm assigned to partial range of motion at long muscle lengths (pROMinitial, 0°–70°) and the other to full range of motion (fROM, 0°–140°). The training program consisted of two sessions per week, with a minimum of 48 h of rest between sessions, and was performed on a GYM80 preacher curl machine equipped with a pin‐load system.

At the start of each session, subjects completed a standardized warm‐up consisting of two sets of bilateral preacher curls at 100% of their unilateral 1RM, with a one‐minute rest interval between sets. After the warm‐up, they performed five sets of biceps curls to momentary concentric failure for each arm. In the first set, subjects were required to perform between 10 and 15 repetitions. If fewer than 10 repetitions were completed, the load was reduced by 0.5 kg per missing repetition in the subsequent set. Conversely, if more than 15 repetitions were performed, the load was increased by 1 kg in the following session. This autoregulation system ensured that subjects consistently trained to failure within the target repetition range.

The starting load was set at 65% of the unilateral 1RM for each arm. Given the different ROM conditions, subjects frequently completed a different number of repetitions in the pROMinitial condition compared to the fROM condition. To account for this variation over time, the aforementioned autoregulation system adjusted the load based on the number of repetitions performed.

Subjects maintained a cadence of approximately 2 seconds during the eccentric phase and 1 second during the concentric phase for both ROM conditions. Training sessions alternated between arms: one arm was trained to failure, followed by a 30‐s rest before the other arm was trained to failure. There was a two‐minute rest period between sets, and the arm initiating the first set alternated between sessions. All weights and repetitions were carefully recorded. The training volume (sets x repetitions x load) of each session was calculated for each condition.

For the fROM condition, the range of motion was defined as the subject's maximal possible elbow flexion (approximately 0°–140°). In the pROMinitial condition, a visual and tactile marker was applied by placing an unmovable resistance on the weight stack of the preacher curl machine, thereby restricting elbow flexion to 0°–70° (see Figure 3).

FIGURE 3.

FIGURE 3

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Standardization of ROM for pROMinitial (0°–70°). (a) 70° elbow flexion on the preacher curl machine and (b) visual and tactile marker used to restrict the range of motion to 70° elbow flexion. The immovable marker was securely attached to the weight stack of the preacher curl machine in a standardized position, ensuring that 70° of elbow flexion was the maximum allowable range.

2.4. Elbow Flexor Muscle Thickness Measurement

A B‐mode ultrasound system (Mindray DP‐50, Mindray Medical International Ltd, Shenzhen, China) equipped with an 8.5 MHz linear probe (Mindray 75L53EA, Mindray Medical International Ltd, Shenzhen, China) was used to assess muscle thickness at depths ranging from 4.6 to 6.5 cm. Elbow flexor muscle thickness was measured at 50% and 70% of the distance from the acromion to the cubital fossa of each humerus. The acromion was selected as the proximal reference point, in line with previous studies (Pedrosa et al. 2023; Sato et al. 2021), whereas the cubital fossa was chosen as the distal reference due to its higher reproducibility of measurements.

Muscle thickness measurements were taken with subjects in the supine position, arms at their sides, and forearms in a neutral relaxed position. Ultrasound transmission gel was applied to the probe head, which was positioned horizontally along the long axis of the elbow flexor muscle, ensuring no compression of the underlying tissues. The muscle thickness recorded in the ultrasound images was analyzed using the device's measurement function. Three images were captured at each measurement site, and data were stored on a USB drive. The ultrasound settings were individually adjusted to ensure clear, reproducible images of the entire muscle. All ultrasound scans were performed by the same technician, who was blinded to the arm allocation, to ensure consistency.

2.5. Measurements of Muscle Strength

Before performing the maximal strength tests for elbow flexors, all subjects were required to complete an identical warm‐up protocol. The warm‐up followed the NSCA guidelines for pretest exercises (Haff and Triplett 2016). Initially, subjects performed two sets of biceps curls with light to moderate weights for 10 repetitions. This was followed by two heavier sets of five repetitions before attempting the first 1RM. Subsequently, isometric maximal voluntary contractions (MVCs) were performed on both arms for the elbow flexor at elbow joint angles of 40° and 100° using a custom‐designed apparatus. The apparatus consisted of an incline bench set at a 65° angle to replicate a preacher curl machine, two inelastic ropes, a goniometer, and a dynamometer (Force Gauge FG‐5100). One rope was attached at one end to an immovable resistance positioned 70 cm high, with the other end connected to the dynamometer. A second strap connected the dynamometer to the subject's hand. Subjects sat on an elevated platform, positioning themselves as close to the bench as possible to ensure the armpit was firmly against the edge of the bench. The upper arm of the tested side maintained full contact with the bench during the procedure while gripping the strap. The distance between the bench and the subject was adjusted to achieve elbow angles of 40° or 100° with the rope under tension. Subjects then performed an MVC at the specified joint angle, with only one attempt allowed per arm. The nontested arm was secured behind the back to prevent interference with the measurement.

After completing the MVC tests for both arms, the 1RM test was conducted. Subjects were seated on the preacher curl machine (gym80 3010 biceps machine), with a maximum of three attempts allowed to determine the one‐arm 1RM. Since the machine only allowed adjustments in 5.0 kg increments, 0.5 kg plates were used for finer adjustments. A minimum rest period of 3.5 min was given between attempts to ensure adequate recovery.

2.6. Statistical Analysis

All statistical analyses were performed using R (version 4.4.2). Prior to the main analyses, the reliability of all outcome measures was evaluated using two baseline assessments. Test–retest reliability was assessed with two‐way mixed‐effects intraclass correlation coefficients (ICC [3,1]), coefficients of variation (CV), and standard errors of measurement (SEM). Reliability was classified as excellent (ICC > 0.90), good (0.75 ≤ ICC ≤ 0.90), or acceptable (ICC < 0.75) according to Koo and Li (2016).

To compare differences in volume load over time between conditions, we fitted a linear mixed effects model using restricted maximum likelihood with volume load as the dependent variable. Fixed effects included training session and condition (fROM vs. pROMinitial), whereas random intercepts and slopes were included for each subject to account for individual differences. Model estimation was performed using the lmer function from the lme4 package in R. Statistical significance for this analysis was defined as p < 0.05.

Further analyses were performed within a Bayesian framework (van de Schoot et al. 2021). Bayesian inference was chosen for its ability to quantify uncertainty, incorporate prior knowledge, and provide direct probability estimates for model parameters (Swinton and Murphy 2022). The analyses were implemented using the brms package, which interfaces with Stan for full Bayesian sampling (Bürkner 2018). A Bayesian hierarchical linear mixed model was used to assess the effect of training condition (fROM vs. pROMinitial) on the dependent variables. The model included a random intercept for each subject to account for repeated measures (Bürkner 2018). The primary fixed effects of interest were condition (fROM vs. pROMinitial), timepoint (Pre vs. Post), and their interaction (condition × timepoint), which allowed us to examine whether the effect of training differed between groups over time. Inferences were drawn from the posterior distributions of the model parameters. The strength of evidence for each effect was assessed using Bayes factors (BFs), which quantify how much more likely the observed data are under the alternative hypothesis compared to the null hypothesis (Lee and Wagenmakers 2013). BFs were interpreted using standard classifications: BF < 3 = weak evidence, 3 ≤ BF < 10 = moderate evidence, 10 ≤ BF < 30 = strong evidence, and BF ≥ 30 = very strong to decisive evidence (Schad et al. 2023). Informative prior distributions were derived from meta‐analyses on related research in strength and conditioning (Swinton et al. 2022). The modeling process included prior predictive checks to verify reasonable prior assumptions, posterior predictive checks to assess model fit, and simulation‐based calibration to ensure robust Bayes factor computation (Schad et al. 2023). Although the primary focus of this study was to assess differences between training conditions (fROM vs. pROMinitial), within‐condition changes (pre vs. post) were also analyzed to contextualize the findings. These within‐condition effects were reported descriptively and assessed using Bayes factors, but no formal hypothesis testing was performed on these changes alone. By adopting this Bayesian hierarchical approach, the analysis provides a probabilistic interpretation of training effects rather than relying on dichotomous significance testing (van de Schoot et al. 2021).

3. Results

Volume load significantly increased over time (β = 24.98 [regression coefficient, indicating the rate of change], SE = 6.76 [standard error, measuring the precision of β], t = 3.69 [t‐value, indicating the strength of the effect], and p < 0.05), whereas pROMinitial exhibited a significantly higher baseline training load (β = 95.93, SE = 37.43, t = 2.56, and p < 0.05), but no significant interaction effect was found (p = 0.40) (Figure 4).

FIGURE 4.

FIGURE 4

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Overview of training load over time.

All outcome measures demonstrated acceptable to excellent reliability. For elbow flexor muscle thickness at both 50% and 70% of the upper arm length, intraclass correlation coefficients (ICC [3,1]) exceeded 0.99, coefficients of variation (CV) were below 2%, and standard errors of measurement (SEM) were under 0.01 cm, indicating excellent reliability. MVC at 40° showed good reliability for both arms (ICC (3,1) > 0.75, CV < 13%, and SEM < 1.0 kg). At 100°, MVC values demonstrated good reliability for the left arm and excellent reliability for the right arm (ICC (3,1) > 0.89, CV < 8%, and SEM < 0.4 kg). 1RM test also showed excellent reliability across both arms (ICC (3,1) > 0.95, CV < 11%, and SEM < 0.6 kg).

The descriptive pre–post hypertrophy and strength outcomes are presented in Table 1.

TABLE 1.

Descriptive neuromuscular outcomes pre and post intervention.

Parameter Timepoint fROM pROMinitial
Muscle thickness 50% humeral length (cm) Pre 2.90 ± 0.92 2.94 ± 0.97
Post 3.01 ± 1.01 3.08 ± 1.00
Δ (%) 2.87 ± 4.87 5.06 ± 4.77
Muscle thickness 70% humeral length (cm) Pre 2.94 ± 0.53 3.01 ± 0.61
Post 3.06 ± 0.55 3.24 ± 0.67
Δ (%) 4.38 ± 4.25 7.60 ± 4.67
MVC 40° (kg) Pre 42.45 ± 12.05 41.09 ± 11.11
Post 40.84 ± 13.54 39.15 ± 10.81
Δ (%) −3.84 ± 16.25 −4.23 ± 14.71
MVC 100° (kg) Pre 28.35 ± 6.16 28.26 ± 6.81
Post 29.52 ± 7.14 29.91 ± 6.73
Δ (%) 3.81 ± 6.45 6.46 ± 8.05
1RM (kg) Pre 35.23 ± 13.08 35.04 ± 13.32
Post 43.88 ± 16.22 41.77 ± 15.42
Δ (%) 28.01 ± 17.78 21.59 ± 13.58
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Initial analyses of within intervention changes across our outcomes of interest are illustrated in Figure 5.

FIGURE 5.

FIGURE 5

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Comparative distribution plot of the estimated standardized mean difference of interventions across outcomes. Density plots illustrate estimates and uncertainty of standardized mean difference changes across the two conditions. Threshold describing the magnitude of improvements are obtained from strength and conditioning‐specific data (see statistical analyses).

Overall, there was no substantial evidence for a time effect in MVC at 40° (estimated difference: −0.15 [–0.90 to 0.61], posterior probability: 35.6%, and BF = 1.81). In contrast, MVC at 100° (0.41 [–0.27 to 1.06], posterior probability: 88.9%, and BF = 8.03) and 1RM (0.49 [–0.28 to 1.27], posterior probability: 89.3%, and BF = 8.38) provided moderate evidence for performance improvements over time. Regarding hypertrophy, both elbow flexor muscle thickness at 50% (0.10 [0.01 to 0.19] and posterior probability: 98.4%) and at 70% humeral length (0.13 [–0.02 to 0.27] and posterior probability: 95.3%) showed strong to very strong evidence for positive time effects (BF > 30).

Estimated mean group differences are presented in Table 2. Results were inconsistent across outcomes with point estimates close to zero and Bayes factors (1.17–4.87) in general providing weak to moderate support for the H1 hypothesis of different improvements across interventions.

TABLE 2.

Estimated group differences from Bayesian linear mixed models with informative neutral priors.

Outcome Estimated group difference Posterior probability Bayes factor
Elbow flexor muscle thickness 50% humeral length (cm) 0.04 [−0.09 to 0.17] 72.7% 2.66: Weak evidence for H1
Elbow flexor muscle thickness 70% humeral length (cm) 0.10 [−0.10 to 0.31] 83.0% 4.87: Moderate evidence for H1
MVC 40° (kg) −0.14 [−0.90 to 0.61] 36.3% 1.75: Weak evidence for H1
MVC 100° (kg) 0.24 [−0.47 to 0.93] 75.1% 3.02: Moderate evidence for H1
1RM (kg) 0.17 [−0.63 to 0.94] 66.1% 1.95: Weak evidence for H1
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4. Discussion

The primary aim of this study was to investigate the effects of two distinct range of motion (ROM) configurations—full range of motion (fROM) and initial‐phase partial range of motion (pROMinitial)—on elbow flexor hypertrophy in trained individuals. It was hypothesized that training with pROMinitial would result in greater muscle growth compared to fROM.

Bayesian analyses of hypertrophic outcomes revealed weak evidence supporting the alternative hypothesis (H1) for elbow flexor muscle thickness at 50% humeral length, whereas moderate evidence was found for muscle thickness at 70% humeral length, suggesting a potential advantage of pROMinitial over fROM. Similarly, both MVC40° and 1RM exhibited weak evidence for H1, whereas MVC100° provided moderate evidence for the superiority of pROMinitial.

To date, six studies have directly compared fROM and pROMinitial (Goto et al. 2019; Kassiano et al. 2023a; Pedrosa et al. 2022; Werkhausen et al. 2021; Wolf et al. 2025a). Three of these studies—examining the gastrocnemius (Kassiano et al. 2023a), knee extensors (Pedrosa et al. 2022), and elbow extensors (Goto et al. 2019)—reported significantly greater hypertrophy with pROMinitial resistance training compared to fROM resistance training. In contrast, the remaining studies found no significant differences in knee extensor thickness (Werkhausen et al. 2021) or in elbow flexor and extensor hypertrophy (Wolf et al. 2025a). Our findings align partially with this body of evidence. Specifically, we observed moderate certainty that elbow flexor thickness at 70% humeral length increased to a greater extent in pROMinitial compared to fROM after 8 weeks (4.38% vs. 7.60%). However, the standardized mean difference (SMD = 0.10) suggests only a trivial‐to‐small effect. Additionally, no meaningful differences were found in elbow flexor thickness at 50% humeral length (SMD = 0.04), implying that although the distal region may have benefitted from pROMinitial, the proximal region exhibited similar hypertrophic outcomes across conditions.

Supporting these findings, Kassiano et al. (2023a) and Pedrosa et al. (2022) observed greater hypertrophy in pROMinitial compared to fROM in untrained women following 12 weeks of lower‐body RT. Similarly, Goto et al. (2019) reported greater elbow extensor hypertrophy with pROMinitial (45°–90°) than fROM (0°–120°) in resistance‐trained men. However, other studies found no significant differences (Werkhausen et al. 2021; Wolf et al. 2025a). Notably, Werkhausen et al. (2021) employed a concentric‐only leg press protocol that failed to induce significant hypertrophy, raising questions about its suitability for comparing pROM and fROM. Similarly, Wolf et al. (2025a) implemented a multiexercise RT program (e.g., bench press, dumbbell row, overhead triceps extension, and supinating curl), which complicates the isolation of hypertrophic responses in the elbow flexors and extensors. In contrast, our findings provide more targeted evidence in partial support of pROMinitial.

Overall, existing evidence indicates that both untrained and trained individuals can benefit from pROMinitial, but the magnitude and mechanisms of these adaptations are not fully understood. Studies in animal models have shown that prolonged stretching can promote longitudinal adaptations, such as the addition of sarcomeres in series or increases in fascicle length as well as radial hypertrophy (Warneke et al. 2023). However, direct evidence for sarcomerogenesis in humans following resistance training remains limited and has so far been confined to eccentric training protocols and specific muscles, such as the biceps femoris (Andrews et al. 2024). Notably, Andrews et al. (2024) reported sarcomerogenesis in the human biceps femoris following a 9‐week Nordic hamstring exercise program. Interestingly, structural changes resulting from longitudinal hypertrophy can remodel muscle architecture and may ultimately also contribute to increases in overall muscle cross‐sectional area (Jorgenson et al. 2020; Warneke et al. 2024; Warneke et al. 2023).

Whether resistance training emphasizing the lengthening portion of the range of motion is sufficient to induce longitudinal hypertrophy, in addition to radial hypertrophy, remains equivocal (Wolf et al. 2025b). Training status may also influence the magnitude of these adaptations. Resistance‐trained individuals, such as bodybuilders, typically possess high baseline muscle mass and may exhibit blunted anabolic signaling responses to resistance training (e.g., reduced AMPK and Akt phosphorylation (Coffey et al. 2006; Lopez et al. 2021)), thereby limiting their hypertrophic potential. Although a relative advantage of pROM initial may still apply to well‐trained individuals (i.e., individuals who have progressed through years of structured resistance training), its effects could be attenuated due to a physiological ceiling. Thus, it remains uncertain not only whether longitudinal adaptations occur in response to pROMinitial training but also whether such adaptations contribute meaningfully to overall muscle growth or plateau earlier in trained individuals. Therefore, we recommend that future studies employ longer intervention periods with multiple intermediate assessment points and, where feasible, incorporate direct microstructural measurements (e.g., muscle biopsy, microendoscopic techniques, or validated imaging protocols) to more conclusively determine whether and how longitudinal adaptations, including sarcomere addition, contribute to muscle growth in humans.

In our study, subjects in both conditions trained within the same repetition range, adjusting the load to maintain muscle failure within the range of interest. Interestingly, although the evidence was not strong enough to support a clear difference, pROMinitial consistently achieved higher training volume loads than fROM throughout the intervention period (see Figure 3). Thus, the observed difference in hypertrophy may partly reflect the higher training volume achieved in the pROMinitial condition. This is likely to be due to the reduced ROM which allowed higher loads per repetition. Given that pROMinitial was restricted to the ROM that maintained maximal muscle stretch in this exercise, time spent in the stretched position differed between conditions. In fROM, the elbow flexor spent only half the duration of each repetition in its fully stretched position, whereas pROMinitial maintained the stretch throughout the entire movement. Despite the difference in time under tension, hypertrophic outcomes were comparable to fROM, with a modest advantage observed at 70% humeral length. A potential explanation is that prolonged time under tension in the stretched position enhances muscle growth. Increased passive tension at greater muscle lengths contributes to overall mechanical tension—a primary driver of hypertrophy (Innocenti and Galbusera 2022). Moreover, the sustained mechanical tension is known to activate the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, a key regulator of muscle protein synthesis (Rindom et al. 2019). Thus, the longer accumulated duration in the stretched position and the higher total tension in pROMinitial may explain the observed hypertrophic trends, despite differences in total work performed. This suggests that hypertrophy may not be solely dictated by ROM but rather by the cumulative time spent in a stretched position under moderate resistance. These findings align with studies comparing pROMinitial to pROMfinal (Kassiano et al. 2023a; McMahon et al. 2014; Pedrosa et al. 2022, 2023; Sato et al. 2021), where pROMinitial consistently demonstrated superior hypertrophy, particularly in the distal region of the elbow flexors (Pedrosa et al. 2023; Sato et al. 2021).

For strength outcomes, we observed weak evidence for differences in MVC40° (SMD = −0.14) and 1RM (SMD = 0.17) between pROMinitial and fROM, whereas MVC100° (SMD = 0.24) provided moderate evidence for H1. Notably, both groups experienced reductions in MVC40° strength, which may seem counterintuitive given the principle of specificity (Kassiano et al. 2023a; Pedrosa et al. 2022). However, lower reliability (ICC > 0.75, CV < 13%, and SEM < 1.0 kg) in MVC40° compared to MVC100° (ICC > 0.89, CV < 8%, and SEM < 0.4 kg) and 1RM (ICC > 0.95, CV < 11%, and SEM < 0.6 kg) may contribute to these findings. Nonetheless, fROM demonstrated greater increases in 1RM than pROMinitial, which aligns with the principle of specificity, as fROM involved frequent training across the entire ROM (Kassiano et al. 2023a; Pedrosa et al. 2022). Conversely, MVC100° showed a trend favoring pROMinitial, suggesting that training in a more restricted ROM may still enhance strength at shorter muscle lengths. However, whether pROMinitial (0°–70°) truly provided greater specificity for MVC100° remains debatable as the trained range of motion did not fully encompass the 100° position. Instead, this advantage may be explained by the prolonged time under tension in the lengthened position, which could have promoted greater hypertrophy, particularly in the distal elbow flexors. Indeed, percent changes in MVC100° were higher in pROM initial (6.46%) than in fROM (3.81%), which corresponds with slightly greater regional muscle growth. These results suggest that increased muscle mass, likely reflecting a greater accumulation of contractile tissue, may partly account for improved force production at MVC100° (Folland and Balshaw 2021). However, it is important to note that these interpretations are based on percentage changes, posterior probabilities, and Bayes factors. Given that the absolute estimated group differences in strength outcomes were small (ranging from −0.14 to 0.24 kg), claims of superiority for one condition over the other in terms of strength outcomes should be made with caution.

This study has several limitations that should be acknowledged. First, the relatively small sample size may have limited the statistical power to detect potential differences between pROMinitial and fROM. Although the sample size met the requirements of the power analysis, a larger cohort would provide more robust findings. Second, the study duration of 8 weeks may have been insufficient to capture the full extent of muscle hypertrophy, particularly in well‐trained individuals. Adaptations to novel training stimuli—such as training with a restricted range of motion and performing all sets to failure—may initially be predominantly neural, potentially limiting the magnitude of hypertrophy observed within this timeframe (Gabriel et al. 2006). Future research should consider longer intervention periods to better assess the long‐term effects of pROMinitial training. Third, the findings of this study are specific to the elbow flexor muscles. As the elbow flexor muscles include the biceps brachii, brachialis, and brachioradialis, which could not be distinguished within the ultrasound measurement, it is unclear whether different elbow flexor muscles respond differently to pROMinitial. Lastly, the results are only applicable to resistance‐trained individuals. Previous research suggests that training status influences hypertrophic adaptations, with untrained individuals generally exhibiting greater muscle growth in response to novel training stimuli. This may explain the discrepancies between the present diminished findings and prior studies conducted on untrained populations. Future research should explore whether the efficacy of pROMinitial varies across different training levels.

5. Conclusion

Both fROM and pROMinitial effectively induced elbow flexor hypertrophy. However, pROMinitial resulted in slightly greater hypertrophy at 70% humeral length, suggesting a potential advantage for distal muscle growth. Although fROM produced greater gains in 1RM, pROMinitial showed a trend favoring MVC100°, although the estimated absolute group differences in both outcomes were small. Overall, both training approaches appear effective for promoting muscle hypertrophy and strength adaptations.

Funding

The authors received no specific funding for this work.

Ethics Statement

This study was approved by the Ethics Committee of the IST University of Applied Sciences (Düsseldorf, Germany) under approval number 032024IST233. All procedures were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Consent

Informed consent was obtained from all individual participants included in the study.

Conflicts of Interest

The authors declare no conflicts of interest.

Permission to Reproduce Material From Other Sources

All figures and tables are original and have not been reproduced from other sources.

Acknowledgments

We extend our gratitude to all subjects for their contributions to this study. The authors declare no conflicts of interest related to this study.

Havers, Tim , Wagner Niklas, Held Steffen, Geisler Stephan, and Wiewelhove Thimo. 2025. “Partial Range, Full Gains? The Effect of 8 Weeks of Partial Range of Motion Training at Long Muscle Lengths on Elbow Flexor Hypertrophy and Strength in Trained Individuals,” European Journal of Sport Science: e70087. 10.1002/ejsc.70087.

The first two authors contributed equally to this article.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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