Ⅰ. Introduction
Flexibility is a critical component of muscle function. Reduced flexibility is especially problematic in fast twitch muscles like the hamstring, which crosses multiple joints (Meden et al., 2015). Because of these characteristics, the hamstring is especially prone to tightness, which can lead to an increased risk of strains and overuse injuries (Lempke et al., 2018). The hamstring's structural vulnerability is a significant concern for athletes, as tightness can reduce range of motion, impair performance, and increase the risk of injury (Kaneda et al., 2020). Addressing hamstring tightness through appropriate evaluation and treatment is therefore essential for both injury prevention and optimal physical performance (Medeiros et al., 2016).
Many physical therapists have explored various methods and conducted studies on hamstring relaxation. These efforts include techniques such as muscle energy techniques (MET), active release technique (ART), and static stretching. MET increases muscle length through gradual changes in connective tissue, facilitated by biomechanical and neurophysiological mechanisms. ART improves flexibility, athletic performance, and recovery by precisely addressing adhesions and tension within soft tissues. Static stretching enhances muscle length by utilizing autogenic inhibition to activate the golgi tendon organ, allowing for sustained elongation (Ahmed et al., 2023; Albertin et al., 2020; Gaur et al., 2021). In addition to these techniques, foam rollers have been widely adopted as a simple yet effective tool for myofascial stretching. They are often used either as an adjunct to or a replacement for regular static stretching. Foam rollers contribute to muscle and fascia relaxation, and they have been shown to improve flexibility (Mohr et al., 2014).
Additionally, eccentric contraction based relaxation methods have been explored. Previous studies have shown that a 7 week program of eccentric stretching significantly improved tendon flexibility. Moreover, even a single session of eccentric stretching was found to be effective in increasing the length and reducing the tension of the hamstring muscles (Brockett et al., 2001; Duclay et al., 2009). Although research on muscle relaxation techniques that involve eccentric contractions with equipment such as the reformer or sling remains limited, the reformer is recognized for its adaptability. Equipped with a carriage and springs, it supports a range of exercises intended to improve flexibility, address body imbalances, and strengthen muscle function. Furthermore, it accommodates both open and closed kinetic chain (OKC and CKC) movements, which contribute to spinal and pelvic stabilization, muscle strengthening, and posture enhancement. The reformer is also well suited for rehabilitation exercises, as it increases joint range of motion and is effective in preventing injuries and alleviating pain (Vaquero et al., 2016). The sling, by creating an unstable surface with ropes suspended from the ceiling, has been demonstrated to be effective in exercises targeting stabilization, neuromuscular control, and muscle strengthening. Previous studies using slings have shown their effectiveness in pain management and increasing flexibility (Li et al., 2020; Li et al., 2022).
To date, numerous studies have been conducted on hamstring rehabilitation using traditional physical therapy methods. However, there is a lack of research examining the effects of modern trends, such as reformer and sling exercises, on changes in knee angles and hamstring muscle tension following muscle contraction. Therefore, this study aims to compare the effects of relaxation techniques according to the type of exercise chain by using the reformer, which induces eccentric contractions in an OKC format, and the sling, which does so in a CKC format. Additionally, the study will examine the effects of direct muscle compression stretching through the use of a foam roller, evaluating changes in muscle length and tension.
Ⅱ. Methods
1. Participants
This study was conducted with 30 healthy adults in their 20s (15 males and 15 females) enrolled at B university in Busan, South Korea. The participants were selected after receiving sufficient explanation about the study’s purpose and methods, and they voluntarily provided their informed consent. The inclusion criteria were as follows: individuals without musculoskeletal disorders, including leg related issues, within the last six months, those who were not engaged in regular exercise but were able to participate in the intervention, and individuals without any cardiovascular conditions.
The 30 participants were randomly assigned to one of three groups: 10 in the foam roller group, 10 in the sling group, and 10 in the reformer group. This study adhered to the ethical guidelines outlined in the Declaration of Helsinki and was approved by the Institutional Review Board (IRB) of Daegu University (Approval No. 1040621-202408-HR-061). The general characteristics of the participants are presented in Table 1.
Table 1. General characteristics of the participants
2. Experimental procedure
Prior to the intervention, knee angles for each group’s participants were measured using the passive knee extension test (PKET), and the biomechanical properties of the hamstring were assessed with the MyotonPRO. The final result for each measurement was determined by averaging three trials. All interventions lasted for two minutes each, with the reformer and sling exercises consisting of four sets of 10-second contractions, repeated three times for a total of 120 seconds, with a one-minute rest between sets. After the exercise, PKET and MyotonPRO measurements were repeated using the same method.
3. Measurement
1) Passive knee extension test (PKET)
In this study, the PKET was performed to evaluate the range of motion of both knees and changes in hamstring tension before and after exercises in the three groups. First, participants were instructed to lie comfortably on a bed, with their hip and knee joints of the measured leg flexed at 90 degrees, and their hands supporting the hamstring. If the participant's arms were too short to provide adequate support, a wedge was placed under the torso to adjust for arm length. A bar was positioned in front of the participant's thigh to maintain the 90 degree hip flexion. The ankle was kept in a neutral position, and the examiner passively extended the knee by holding the heel, measuring the angle at which resistance, or "end feel," was observed in the hamstrings (Reurink et al., 2016). The knee angle was measured using a goniometer, with the fulcrum placed on the lateral epicondyle of the femur, and the moving arm aligned with the lateral malleolus, to measure the degree of knee extension (Reese & Bandy, 2016; Nelson & Bandy, 2004) (Fig 1).
Fig 1. Passive knee extension test
The PKET is a reliable test with high intra rater reliability (ICC= 0.97~0.98) (Youdas et al., 2005). All measurements were first conducted on the dominant leg, followed by measurements on the opposite leg, and the results were obtained by averaging three measurements performed by a single examiner.
2) MyotonPRO (Myoton AS, Tallinn, Estonia)
In this study, MyotonPRO was used to assess changes in hamstring tension in both legs before and after exercises across the three groups. Participants lay comfortably in a prone position on the bed, and a marker was placed at the midpoint of the biceps femoris muscle, between the ischial tuberosity and the popliteal fossa (Gönen et al., 2024) (Fig 2). The measurements were performed by a single examiner who positioned the MyotonPRO perpendicularly to the muscle, applied a pre force of 0.18 N, followed by a mechanical impulse of 0.4 N with a duration of 15 ㎳. The probe was set to vibrate three times (with a delay of 0.7 s, an interval time of 0.8 s and a tap time of 15 ㎳) and the final result was obtained by averaging three measurements. MyotonPRO is a noninvasive device that measures the biomechanical properties of muscles-namely, frequency (muscle tone), stiffness, decrement, relaxation time, and creep by applying an external mechanical stimulus that induces vibrations (Nguyen et al., 2022).
Fig 2. MyotonPRO
Frequency represents muscle tone and reflects the muscle’s resistance to passive stretching or relaxation, measured in Hertz (㎐), where a higher frequency value indicates higher muscle tone. Stiffness reflects the resistance to deformation, with a higher stiffness value indicating greater muscle stiffness. Decrement represents the ability of the muscle to return to its original state after deformation, where a lower decrement value indicates higher elasticity (Gönen et al., 2024). Relaxation time refers to the time required for the muscle to recover to its original form after voluntary contraction or the removal of external force. Creep refers to the gradual lengthening of the muscle when subjected to sustained force (Núñez et al., 2023). Additionally, MyotonPRO demonstrates high inter and intra rater reliability in measuring muscle frequency and tension variables (Lettner et al., 2024).
4. Intervention
1) Reformer
All participants received a thorough explanation of the reformer intervention before proceeding. A long box was placed on the reformer, and participants were instructed to lie prone on the box, with their sternum positioned at the edge of the box. Both arms were crossed over the chest, and the medial condyles of the elbows were pressed against the box to maintain a slight thoracic extension of approximately 30 degrees. Participants were instructed to maintain a neutral pelvic position by engaging their abdominal muscles, drawing the umbilicus toward the spine, while keeping their knees flexed at 90 degrees and positioning the reformer straps at the center of their feet. Additionally, a small gap, roughly the width of one finger, was maintained between the participant’s knees and the long box (Fig 3).
Fig 3. Reformer exercise (knee flexion)
The reformer resistance was adjusted using red springs for male participants and yellow springs for female participants. During exhalation, participants were instructed to slowly extend their knees over a duration of 5 seconds, and during inhalation, to slowly bend their knees back over another 5 second period. One repetition consisted of these movements, and the exercise was performed for 4 repetitions per set, with a total of 3 sets. Throughout the exercise, participants were guided to maintain a neutral alignment of the spine and pelvis, as well as the correct positioning of the knees. A 1 minute rest was provided between each set (Park et al., 2020).
2) Sling
After thoroughly explaining the sling intervention method to all participants, the exercise was conducted. Participants lay supine on a bed positioned centrally within the sling, with their hands placed beside their hips and palms facing upward. The suspension point of the sling was positioned near the ankles, and straps were securely fastened around both heels. The height of the sling straps was adjusted to match the level of each participant's anterior superior iliac spine (Fig 4).
Fig 4. Sling exercise (knee flexion)
While ensuring that the pelvis remained lifted by pressing firmly into the straps, participants were instructed to slowly bend their knees to 90 degrees during exhalation over a 5 second period, and to slowly extend their knees during inhalation over another 5 seconds. One repetition consisted of these movements, and the exercise was performed for 4 repetitions per set, with a total of 3 sets. Throughout the exercise, care was taken to ensure that the knees and feet remained aligned, and pelvic movement was controlled. A 1 minute rest was provided between each set.
3) Foam roller
After thoroughly explaining the foam roller intervention method to all participants, the exercise was conducted. Participants were seated with their legs extended, placing their arms behind their torso for support. After shifting their weight backward, a foam roller was positioned under both hamstrings. Participants slowly rolled the foam roller from the ischial tuberosity to the popliteal fossa, maintaining the stretch for 2 minutes (Fig 5) (Pérez-Bellmunt et al., 2023). In this study, the intervention time for all exercises was uniformly set at 2 minutes. After completing the intervention, a 5 minute rest period was provided, followed by reassessment using the same method as the pre-intervention measurements.
Fig 5. Form roller exercise (stretch)
5. Data analysis
The statistical analysis of the data in this study was performed using the SPSS ver. 22.0 (IBM Co., Armonk, NY, USA) software. The homogeneity test of this study was conducted using the Shapiro-Wilk test. A paired t-test was conducted to compare pre and post intervention measurements, thereby identifying the statistical significance of changes before and after the intervention. To compare differences between groups, a one-way analysis of variance (ANOVA) was used to analyze intergroup differences for each measurement site. Post-hoc analysis was conducted using the Bonferroni correction to determine significant differences between groups. The significance level (α) was set at .05.
Ⅲ. Results
1. PKET before and after intervention
Table 2 shows significant within group differences in both knees of the reformer and sling groups, and the right knee of the foam roller group (p.05). In Table 3, the between group comparison revealed significant differences in both the left and right knees of the reformer group (p<.05).
Table 2. Comparison between pre and post for passive knee extension test (unit: ˚)
L; left, R; right
Table 3. Comparison between groups for passive knee extension test (unit: ˚)
2. MyotonPRO before and after intervention
In Table 4, the within group comparison showed significant differences in relaxation time and creep in the left hamstring of the reformer group, as well as in creep in the right hamstring (p<.05). In the sling group, significant differences were found in stiffness, relaxation time, and creep in the left hamstring (p<.05), while in the right hamstring, significant differences were found in stiffness and relaxation time (p<.05). For the foam roller group, significant differences were found in creep in the left hamstring and in relaxation time in the right hamstring (p<.05). In Table 5, a significant difference was observed in the comparison between groups for the left hamstring's relaxation time, with the reformer and sling showing a greater difference compared to the foam roller (p<.05).
Table 4. Comparison between pre and post for MyotonPRO
F; frequency, S; stiffness, D; decerment, R; relaxation time, C; creep
Table 5. Comparison between groups for MyotonPRO
F; frequency, S; stiffness, D; decerment, R; relaxation time, C; creep
Ⅳ. Discussion
This study was conducted to compare the effects of reformer, sling, and foam roller exercises on knee angles and hamstring tension in 30 healthy adults in their 20s.
Static stretching with a foam roller increases muscle length by activating autogenic inhibition in the golgi tendon organs (Lim et al., 2014). Proprioceptive neuromuscular facilitation (PNF) stretching promotes muscle relaxation by reducing reflex activity while simultaneously inducing voluntary muscle contraction, thereby increasing the range of motion (Farquharson, 2010). This has been reported as a result of stretching the musculotendinous unit, which involves both viscoelastic elements of muscles and connective tissues, as well as neurological reflexes and voluntary muscle contraction (Lim et al., 2014). These neurological reflexes occur when stretching forces are applied to the golgi tendon organs, increasing tension in the musculotendinous unit and triggering muscle relaxation (Sharman et al., 2006). Additionally, this effect is thought to be due to the activation of not only the muscle fibers of the agonist and antagonist but also the sensory receptors (Lempke et al., 2018).
In this study, the PKET results showed that the reformer group exhibited the greatest changes, with an average difference of more than 7 degree in both knees, suggesting the largest improvement (p<.05). A significant difference was also observed in the reformer group when comparing between groups (p<.05). This is likely because the reformer exercises promoted significant voluntary muscle contractions and reduced neurological reflex elements, thus enhancing muscle viscoelasticity and activating sensory receptors. On the other hand, the foam roller group showed the smallest change (p<.05), with an average of 2 degree, which is likely due to the focus on separating adhesions in the fascia between muscle layers, a mechanism associated with autogenic inhibition of the golgi tendon organs and self myofascial release (Cheatham et al., 2015; Schleip, 2003). The sling group, with an average change of 4 degree (p<.05), likely benefited from voluntary contractions in closed chain exercises and the muscle energy technique (Liebenson, 2007).
In terms of MyotonPRO measurements, stiffness is a biomechanical property that reflects resistance to external forces altering muscle shape, and an appropriate level of stiffness is essential for maintaining dynamic stability (Villanueva et al., 2020). If stiffness is too high, it indicates a lack of muscle flexibility and increases the risk of injury, whereas if stiffness is too low, it suggests muscle weakness (Jo et al., 2022). Increased muscle tension can elevate intramuscular pressure, impair cardiovascular function, and restrict blood circulation to the muscles (Wu et al., 2021). In a study on soccer players, it was found that the hamstrings of the injured leg showed higher frequency and stiffness compared to the uninjured leg. This was attributed to the need for greater eccentric knee flexor torque before the foot contacts the ground, to counter horizontal ground reaction forces (Núñez et al., 2023).
In this study, the sling group showed a significant decrease in stiffness based on MyotonPRO measurements (p<.05). The reduction in stiffness in the sling group can be explained by the improved joint stability through coordinated contraction of the agonist and antagonist muscles, a secondary effect of the enhanced eccentric hamstring torque, and better blood flow and reduced intramuscular pressure compared to the reformer and foam roller groups. The decrease in stiffness after the sling exercises aligns with previous studies that reported reduced stiffness values after exercise, likely due to decreased strength and efficiency of fully contracted muscles (Lall et al., 2022). Previous studies comparing exercise methods based on hamstring contraction also reported a decrease in the stiffness-value in the eccentric contraction group. This was attributed to the development of a more complete musculotendinous structure that enhances the muscle’s adaptability to various forces, thereby increasing the cushioning effect (Uysal et al,. 2021). However, this study did not assess joint stability or blood circulation, and therefore lacks sufficient evidence to support these findings. Further research is needed to explore these aspects based on the results of this study.
In MyotonPRO, relaxation time refers to the time taken for muscle recovery after mechanical stress, and creep represents the gradual elongation of muscle under applied stress. In elite professional soccer players, lower relaxation time and creep values were observed in the distal part of the injured hamstring, indicating reduced eccentric force production (Núñez et al., 2023). A more flexible musculotendinous system can absorb external forces over a greater distance and time due to the cushioning effect, allowing for better shock absorption (Roberts, 2016). Lower relaxation time and creep values in the injured leg's hamstrings suggest a less flexible musculotendinous system that absorbs external forces over a shorter distance and time. Additionally, a previous study on massage guns found that increased muscle relaxation after use led to a small increase in intramuscular volume, resulting in higher relaxation time values for the hamstrings (Skinner et al., 2023).
Throughout this study, both relaxation time and creep values were reduced in all three groups-reformer, sling, and foam roller (p<.05). These findings imply that voluntary muscle contractions may have temporarily impaired flexibility and decreased the muscles' ability to absorb impact. Notably, significant differences in left leg relaxation time values were found between the reformer and sling groups when compared to the foam roller group (p<.05). This variation could potentially be explained by the voluntary contractions involved in reformer and sling exercises, which might reduce muscle viscoelasticity, alter neurological reflexes, and temporarily diminish intramuscular volume. As a result, collagen deposition in concentrically trained muscles may be denser and more compact, whereas eccentrically trained muscles tend to be more linear and less dense. These differences can be attributed to variations in density and collagen types (type Ⅱ versus types Ⅰ and Ⅲ) (Uysal et al,. 2021).
However, this study has several limitations. The results reflect only immediate effects, limiting their generalizability. Additionally, gender differences were not considered, and the intensity of the exercises was not quantified. To gain a more comprehensive understanding of the effects of different exercise methods on muscle tension, future research should address these limitations by considering long term effects, including gender differences, and quantifying exercise intensity.
Ⅴ. Conclusion
This study was conducted to determine the effects of hamstring exercises using a reformer, sling, and foam roller on knee angles and MyotonPRO measurements in 30 healthy adults in their 20s.
Based on the findings of this study, if the primary objective is to enhance changes in knee angles, open-chain exercises using a reformer are recommended. However, if the consideration extends to the biomechanical aspects of the hamstring, closed-chain exercises using a sling are advisable. Therefore, based on the results of this study, it is believed that the applications of the reformer and sling can be appropriately considered and utilized in the rehabilitation of the hamstring in clinical practice.
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