Effects of keiser-assisted Nordic hamstring curl training on hamstring flexibility, mechanical properties, and pitching performance in high school baseball pitchers

Abstract

[Purpose]

This study aimed to investigate the effects of 10-week Keiser-assisted Nordic hamstring curl (NHC) training on the mechanical properties, muscle strength, and flexibility of the hamstrings, as well as shoulder range of motion (ROM) and pitching performance in high school baseball pitchers.

[Methods]

Sixteen high school baseball pitchers performed Keiser assisted NHC training for 10 weeks. The pre- and post-training measurements included the mechanical properties of the hamstrings (muscle tone, stiffness, elasticity, creep) measured using MyotonPRO, isometric muscle strength, trunk and shoulder ROM, and pitching performance (velocity and spin rate).

[Results]

After training, hamstring muscle tone, stiffness, and elasticity significantly decreased (p < 0.01), whereas creep and isometric strength significantly improved (p < 0 0.001) (effect size dz = 0.84-2.90). Additionally, the trunk flexion range significantly increased (p < 0.001, dz = 2.12), and the glenohumeral internal rotation deficit in the throwing shoulder significantly improved (p < 0.05). The pitching velocity (p < 0.001, dz = 2.61) and spin rate (p < 0.01, dz = 0.89) significantly increased.

[Conclusion]

Keiser-assisted NHC training improved hamstring mechanical efficiency, isometric strength, and trunk flexibility, with concurrent gains in pitching velocity and spin rate. As this was a single-group pre-post study without a control group, these findings require confirmation through controlled trials.

INTRODUCTION

Baseball is a sport that requires sophisticated movement and technical proficiency; in particular, the pitching motion is a key element in determining the outcome of a game [1]. Pitching is not limited to upper limb movements and involves a harmonious kinetic chain motion requiring the coordination of the lower limbs, trunk, and upper limbs [2].

The mechanism of overhead pitching comprises six phases: wind-up, stride, arm cocking, acceleration, deceleration, and follow-through [2,3]. The importance of the lower limbs in pitching performance is prominent during the first four phases [4]. Specifically, the kinetic energy generated in the hip joint directly influences ball velocity [5]. If the hip range of motion (ROM) is inadequate, the kinetic energy transferred from the lower limbs to the upper limbs decreases, negatively affecting pitching mechanics [6].

The hamstrings, which are involved in the functional movement of the hip and knee joints, play a decisive role in adjusting the stride length of the stride leg during pitching, particularly during the stride and deceleration phases3. Tight hamstrings shorten stride length and restrict the trunk flexion required for knee extension and weight transfer, thereby reducing the force and deceleration capacity of the stride leg [7,8]. The ability of trunk flexion to accelerate rapid pitching is closely related to hamstring flexibility9, and pitchers with faster velocities exhibit higher angular velocities of the pelvis and trunk flexion as the range of trunk flexion increases [10].

Conversely, a lack of hamstring flexibility forces compensation through increased total shoulder ROM and excessive rotation to achieve the ROM required for throwing, which places additional stress on the shoulder and increases the risk of injury [4,7,11]. The flexibility and strength of the hamstrings are crucial in physical activity as they are crucial in regulating the pelvic and trunk positions relative to the femur [12]. While flexibility and strength are the foundations for improving all physical abilities [13], balancing them is difficult because of their contradictory natures; excessive flexibility can even lead to decreased strength [14].

Nordic hamstring curl (NHC) training is widely used to prevent and rehabilitate of strains by strengthening the eccentric contractions of the hamstrings [15]. NHC training involves lowering the upper body toward the ground with knees flexed at 90°, resisting hamstring lengthening and inducing knee extension [16]. This triggers neuromuscular and morphological adaptations such as increased muscle fiber length and improved length tension relationships [17].

However, conventional NHC training has a limitation in that the “break-point” phenomenon occurs, which is a failure to maintain muscle contraction until the end phase owing to a rapid increase in torque as the moment arm increases during execution [18]. This restriction prevents the provision of a sufficient eccentric load in the maximal stretch state, which is essential for improving flexibility [19]. To address this limitation, the Keiser-assisted machine is used, which enables consistent and controlled eccentric contractions across the entire ROM through pneumatic resistance [15]. This provides biomechanical advantages that maximize structural changes and functional flexibility by maintaining muscle activity up to the maximal stretch interval of the hamstrings [19].

Although rapid pitching velocity is a critical factor in performance, research on pitching mechanisms has primarily focused on the upper limbs. As more than 50% of the total force during pitching is generated by the lower limbs and trunk, kinematic research is essential [20] however, objective data remain insufficient. In particular, research identifying the relationship between hamstring function and pitching performance and analyzing the effects of NHC training in adolescent baseball players is limited. Therefore, this study aimed to analyze how improvements in hamstring flexibility and strength through NHC training using a Keiser-assisted machine affects pitching performance, providing objective information to develop effective rehabilitation programs for sports professionals.

METHODS

Participants

This study was conducted on 16 elite high school baseball pitchers from a metropolitan city in South Korea. The participants were skilled athletes with > 8 years of playing experience. The selection criteria were as follows: (a) overhead pitchers; (b) individuals with excessive shoulder external rotation ROM; (c) those with no history of lower limb or shoulder injuries within the preceding 6 months; (d) those with no experience in eccentric hamstring loading training within the preceding 1 month; (e) those with no history of musculoskeletal surgery involving the lower limbs or shoulders; and (f) those with no pain or stiffness in the lower limbs or shoulders after pitching. As the participants were minors, a full explanation of the study’s purpose and procedures was provided to their guardians and coaches, and the study proceeded with voluntary consent (Table 1). This study was approved by the Institutional Review Board (IRB) of Dongshin University (1040708-202505-BM-023).

Table 1.

General characteristics of the study participants

Characteristic	Values (n = 16)
Age (yr)	16.8 ± 0.8
Height (cm)	174.1 ± 5.8
Weight (kg)	76.3 ± 8.3
BMI (kg/m2)	25.1 ± 1.39
Experience (yr)	8.1 ± 1.8
Throwing Shoulder (Right / Left)	13 (81.2%) / 3 (18.8%)

Values are means and SD. or number (%), BMI: body mass index

Study Procedures

This study was designed to investigate the effects of a 10-week Keiser-assisted NHC training program on pitching performance at the JW Baseball Center in a metropolitan city in South Korea. All training procedures were supervised by experts who strictly adhered to the Youth Resistance Training Safety Guidelines established by Faigenbaum et al.21 The training frequency was progressively increased: once a week in the 1st week, twice a week in the 2nd week, and three times a week from the 3rd to the 10th week. The variables measured before and after the 10-week training program included the muscle tone (F), stiffness (S), elasticity (D), creep (C) of the hamstrings; isometric muscle strength; trunk flexion flexibility; shoulder internal and external ROM; and pitching performance (ball velocity and spin rate).

Sample Size Calculation

The sample size was calculated using G*Power v3.1.9.7. Based on an effect size of 0.8, a significance level (α) of 0.05, and a power of 0.8, the minimum required number of participants was determined to be 15. Accordingly, 16 elite high school pitchers who met the selection criteria were included.

Keiser-Assisted NHC Training

NHC training in this study was conducted using a Keiser Functional Trainer (Model 3020; Keiser Co., USA), which provides smooth and consistent pneumatic resistance throughout the ROM. The exercise protocol was designed based on the studies by Mjølsnes et al. [22] and Burrows et al. [15]. The participants started in a kneeling position with the trunk and hips fully extended and the knees flexed at 90°. The Keiser machine was set to 20% of the participant’s body weight as the assisted weight. While holding the bar connected to the device, participants performed eccentric contractions by extending their knees to the maximum range at a constant angular velocity for 5 s. A 10-s rest interval was provided between each repetition. To exclude the influence of concentric contractions, the participants were instructed to return to the starting position by pushing off the ground with their hands after each repetition. The training intensity was progressively increased by adjusting the number of sessions, sets, and repetitions (Table 2).

Table 2.

Keiser-assisted Nordic hamstring curl training program

Adaptation stage (1-2 weeks)	Improvement stage (3-4 weeks)	Maintenance stage (5-10 weeks)
Keiser-assisted NHC performed for 10 sec; 5-6 reps × 2 sets; 1-2 sessions/week	Keiser-assisted NHC performed for 10 sec; 6-10 reps × 3 sets; 3 sessions/week	Keiser-assisted NHC performed for 10 sec; 8-12 reps × 3 sets; 3 sessions/week
Load: Assisted bodyweight (approx. 20%)

reps: repetitions

Hamstring Muscle Mechanical Properties test

The biomechanical and viscoelastic properties of the hamstrings were measured on both sides (stride and pivot legs) before and after the 10-week training program using MyotonPRO (Myoton AS, Tallinn, Estonia). MyotonPRO quantifies the state of biological soft tissues by applying mechanical stimuli using a probe and analyzing the induced oscillations [23]. The measured biomechanical properties included tone (F), stiffness (S), and elasticity (D), and measured the viscoelastic property was creep (C). Measurements were performed with the participants in the prone position, marking the midpoint of the muscle belly of the biceps femoris. The average values of the three measurements were used for the final data analysis.

Hamstring Muscle Strength Test

The isometric strength of the hamstrings was measured using a digital hand-held dynamometer (Commander Echo, JTECH Medical, USA). Participants were positioned prone, with the knee flexed at 160°. Following the signal “Flex your knee,” the participants performed and maintained a maximal isometric contraction for 3 s. The measurement was repeated three times with 10-s rest intervals, and the average value in pounds (lb) was used [24].

Trunk Flexibility Test

The sit-and-reach test (SRT) was performed using a sitand-reach box. The participants sat with their knees fully extended and soles against the box. They were instructed to slowly reach forward as far as possible and hold their position for 2 s. The furthest point reached by the fingertips was recorded, and the highest score from two trials was used for analysis [25].

Shoulder Rotation ROM Test

The internal and external rotational ROM of the shoulder were measured using a K-Move (Kinvent Move, Kinvent Physio, France) equipped with a gyroscopic inertial sensor. The device was fixed 5 cm proximal to each participant’s wrist. The zero point was set with the participant’s shoulder at 90° flexion and the wrist in a neutral position. After a 5 s preparation period following the signal, the angle maintained for 3 s at the end of ROM was measured. The measured data were analyzed using the K-Move V3 system [26,27].

Pitching Performance Test

Pitching velocity and spin rate were measured using Rapsodo PITCHING 2.0 (Rapsodo Pte Ltd., USA). This system utilizes a 240 fps camera and a 50 kHz radar to capture the state of the ball in real time. Measurements were taken at a regulated distance of 18.4 m (60 ft 6 in). The participants performed 10 full-effort pitches, and the highest velocity and maximum spin rate among the recorded pitches were used for the analysis [28].

Data Analysis

Statistical analyses were performed using SPSS version 22.0. Data normality was confirmed using the Shapiro-Wilk test. The participants` general characteristics were analyzed using descriptive statistics. A paired t-test was conducted to determine significant changes in the variables before and after the 10-week Keiser-assisted NHC training program. The significance level (α) for all statistical tests was set at 0.05. In addition, effect sizes (Cohen’s dz) and 95% confidence intervals (CIs) for the mean pre-post difference were calculated for each outcome to express the magnitude of the change.

RESULTS

Changes in Hamstring Muscle Mechanical Properties

Analysis of the hamstring mechanical properties after 10 weeks of training confirmed significant improvements in all indicators. Muscle tone (F) in the stride leg significantly decreased from 17.19 ± 1.91 to 15.28 ± 1.24 Hz (p < 0.001), and in the pivot leg from 17.75 ± 2.69 to 15.58 ± 1.37 Hz (p < 0.01). Stiffness (S) significantly decreased in the stride leg from 357.19 ± 34.56 to 307.69 ± 46.21 N/m (p < 0.01) and in the pivot leg from 368.63 ± 43.94 to 303.06 ± 45.78 N/m (p < 0.001). Elasticity (D) significantly decreased in the stride leg from 1.26 ± 0.13 to 1.08 ± 0.13 arb and in the pivot leg from 1.26 ± 0.10 to 1.06 ± 0.17 arb (p < 0.001). Conversely, creep (C) significantly increased in the stride leg from 0.98 ± 0.05 to 1.05 ± 0.08 arb (p < 0.001) and in the pivot leg from 0.96 ± 0.07 to 1.03 ± 0.08 arb (p < 0.01) (Table 3). Effect sizes were large to very large for all measures: tone (stride dz = 1.06, 95% CI [−2.87, −0.95]; pivot dz = 0.84, 95% CI [−3.54, −0.80]), stiffness (stride dz = 0.88, 95% CI [−79.39, −19.61]; pivot dz = 1.15, 95% CI [−95.89, −35.25]), elasticity (stride dz = 1.53, 95% CI [−0.24, −0.12]; pivot dz = 1.52, 95% CI [−0.27, −0.13]), and creep (stride dz = 1.15, 95% CI [0.04, 0.10]; pivot dz = 0.85, 95% CI [0.03, 0.11]).

Table 3.

Pre- and post-training hamstring mechanical properties

Variable	Side	Pre-test	Post-test	t	P-value
Muscle Tone (F) (Hz)	Stride leg	17.19 ± 1.91	15.28 ± 1.24	4.23	0.001***
	Pivot leg	17.75 ± 2.69	15.58 ± 1.37	3.38	0.01**
Stiffness (S) (N/m)	Stride leg	357.19 ± 34.56	307.69 ± 46.21	3.53	0.01**
	Pivot leg	368.63 ± 43.94	303.06 ± 45.78	4.61	0.001***
Elasticity (D) (arb)	Stride leg	1.26 ± 0.13	1.08 ± 0.13	6.13	0.001***
	Pivot leg	1.26 ± 0.10	1.06 ± 0.17	6.09	0.001***
Creep (C) (arb)	Stride leg	0.98 ± 0.05	1.05 ± 0.08	-4.58	0.001***
	Pivot leg	0.96 ± 0.07	1.03 ± 0.08	-3.40	0.01**

Values are means and SD. F: frequency, S: stiffness, D: decrement (logarithmic), C: creep (Deborah number).

^**P < 0.01,

^***P < 0.001.

Changes in Hamstring Muscle Strength

Analysis of hamstring strength showed a significant increase from 62.63 ± 7.41 lb pre training to 71.56 ± 7.54 lb post-training (p < 0.001) (Table 4). The effect size was very large (dz = 2.90, 95% CI [7.29, 10.57] lb).

Table 4.

Changes in hamstring muscle strength

Variable	Pre-test	Post-test	t	P-value
Hamstring Strength (lb)	62.63 ± 7.41	71.56 ± 7.54	-11.58	0.001***

Values are means and SD.

^***P < 0.001.

Changes in Trunk Flexibility

Trunk flexibility significantly increased from 8.54 ± 4.12 cm pre-training to 12.69 ± 3.39 cm post-training (p < 0.001) (Table 5). The effect size was very large (dz = 2.12, 95% CI [3.11, 5.19] cm).

Table 5.

Changes in trunk flexibility

Variable	Pre-test	Post-test	t	P-value
Trunk Flexibility (cm)	8.54 ± 4.12	12.69 ± 3.39	-8.50	0.001***

Values are means and SD.

^***P < 0.001.

Changes in Shoulder Rotation ROM

In the throwing shoulder, external rotation significantly decreased from 106.51 ± 4.95° to 105.5 ± 4.03° (p < 0.05), while internal rotation significantly increased from 75.04 ± 5.36° to 76.02 ± 4.79° (p < 0.05). In contrast, no significant changes were observed in non-throwing shoulderS (Table 6). The effect sizes were medium (external rotation dz = 0.54, 95% CI [−2.01, −0.01]°; internal rotation dz = 0.62, 95% CI [0.14, 1.82]°).

Table 6.

Changes in shoulder rotation range of motion

Shoulder	Motion	Pre-test (°)	Post-test (°)	t	P-value
Throwing	External Rotation	106.51 ± 4.95	105.50 ± 4.03	2.155	0.048*
	Internal Rotation	75.04 ± 5.36	76.02 ± 4.79	-2.493	0.025*
Non-throwing	External Rotation	95.23 ± 4.17	95.29 ± 3.9	-0.149	0.884
	Internal Rotation	85.51 ± 4.80	85.87 ± 4.58	-0.204	0.841

Values are means and SD.

^*P < 0.05.

Changes in Pitching Performance

Pitching velocity and spin rate significantly increased from 127.44 ± 9.27 to 133.06 ± 8.79 km/h (p < 0.001) and 2089.62 ± 289.39 to 2170.75 ± 263.66 rpm (p < 0.01) (Table 7), respectively. The effect size was very large for velocity (dz = 2.61, 95% CI [4.47, 6.77] km/h) and large for spin rate (dz = 0.89, 95% CI [32.32, 129.94] rpm).

Table 7.

Changes in pitching performance

Variable	Pre-test	Post-test	t	P-value
Ball Velocity (km/h)	127.44 ± 9.27	133.06 ± 8.79	-10.43	0.001***
Spin Rate (rpm)	2089.62 ± 289.39	2170.75 ± 263.66	-3.543	0.003**

Values are means and SD.

^**P < 0.01,

^***P < 0.001.

DISCUSSION

This study aimed to identify the effects of a 10-week NHC training program using a Keiser-assisted machine on the mechanical properties and strength of the hamstrings in elite high school pitchers and to analyze the biomechanical mechanisms through which changes in trunk flexion flexibility and shoulder ROM contribute to ball velocity and spin rate.

Appropriate flexibility serves as a biomechanical foundation for generating powerful contractions by maximizing muscle length within the joint’s ROM [29]. General stretching increases the length of the musculotendinous unit (MTU) [30], but such increases in compliance have limitations that can alter the muscle length-tension relationship and decrease force production ability [31].

To overcome this, NHC training significantly increases muscle volume and fascicle length through eccentric contractions [32]. In particular, NHC training strengthens maximal force by increasing the number of parallel sarcomeres [33] and fundamentally improves force-velocity and force-length relationships [19,34] by stimulating the formation of sarcomeres-in-series to extend muscle fascicle length [19,32,34].

Quantitative evaluation of the hamstring mechanical properties using MyotonPRO showed that muscle tone (F), which refers to mechanical tension was significantly decreased in both the stride (p < 0.001) and pivot (p < 0.01) legs. Stiffness (S), which represents the degree of resistance to deformation also significantly decreased in the stride (p < 0.01) and pivot (p < 0.001) legs. Elasticity (D), which represents the ability to return to the original state, also significantly decreased on both sides, confirming a higher elastic efficiency (p < 0.001). In contrast, creep (C), - a viscoelastic property representing the degree of progressive elongation under constant stress, was significantly increased in the stride (p < 0.001) and pivot (p < 0.01) legs. The consistent resistance provided by Keiser-assisted NHC training effectively stretched the muscles and tendons, serving as a powerful stimulus for improving ROM [14]. This training promotes muscle mobility by enhancing the elasticity and plasticity of connective tissue, facilitating the activation of agonist muscles, and reducing co-activation of antagonist muscles [35]. Consequently, NHC training in this study appeared to have lowered the resting tone and stiffness of the hamstrings while increasing their capacity for progressive elongation.

These reductions should not be considered unequivocally beneficial. Because the degree of hamstring stiffness aids in elastic energy storage and force transmission during the stretch-shortening cycle of the pitching motion, the observed decreases are best interpreted as a normalization of the excessive passive stiffness commonly reported in pitchers with limited hamstring extensibility, rather than as a uniformly favorable adaptation. As the stiffness and active force output were not measured during dynamic pitching, the optimal stiffness-compliance balance for this population remains to be established.

Analysis using a digital dynamometer revealed a significant increase in isometric hamstring strength (p < 0.001). As a closed kinetic chain exercise, NHCs increase joint accessibility through axial loading and stimulates mechanoreceptors to improve proprioception and kinesthesia [36]. The resistance stress mechanism that maintains high contractile tension is a major factor in strength improvement [14]. This eccentric training is thought to induce neuromuscular adaptation that expand the fascicle length and increases the muscle volume of the biceps femoris long head, enabling the generation of higher torque at longer muscle lengths [34,37,38]. This optimized muscle fiber architecture positively influences the force-velocity and force-length relationships [34,38].

The results of the SRT showed significant improvements in trunk and hamstring flexibility (p < 0.001). During trunk flexion, the hamstrings serve as the primary muscles that are elongated while resisting hip flexion [39], and NHC eccentric training is considered to increase the ROM by reducing muscular resistance during stretching [40,41]. This is interpreted as NHC eccentric training promoting “sarcomerogenesis”, in which new sarcomeres are added in series to the distal ends of hamstring fibers, thereby expanding the total fascicle length and raising the physical limits of the muscle [11,42].

Consequently, sufficient trunk flexibility enables efficient energy transfer during pitching, thereby enhancing ball velocity. However, when flexibility is inadequate, energy is lost in the process of transferring rotational energy from the lower extremities to the upper limbs. This energy loss concentrates excessive stress on the arm muscles and medial ulnar collateral ligament of the elbow, potentially increasing the risk of shoulder and elbow injuries [43,44]. This repetitive stress increases the stiffness of the anterior soft tissue, leading to a glenohumeral internal rotation deficit (GIRD) [45,46], which manifests as a characteristic pattern of increased external rotation and decreased internal rotation compared with those on the non-throwing side [44].

The results of this study demonstrated that the external rotation of the throwing shoulder decreased significantly (p < 0.05), whereas the internal rotation increased significantly (p < 0.05), indicating an improvement in asymmetrical ROM. This suggests that the enhanced hamstring and trunk flexibility achieved through 10 weeks of training may have alleviated the compensatory burden concentrated on the shoulder, potentially modulating abnormal adaptive changes. Consequently, improvements in flexibility may help to reduce the torque and compressive forces applied to the shoulder, which could have implications for injury prevention [2].

In parallel with these changes, the pitching velocity (p < 0.001) and spin rate (p < 0.01) increased significantly. A plausible interpretation is that gains in hamstring flexibility and trunk flexion capacity improved kinetic-chain efficiency [42], allowing energy to be transferred more completely to the upper limbs during pelvic-trunk separation [47,48], thereby raising ball velocity [49]. Simultaneously, as the compensatory action of the shoulder was suppressed by securing trunk flexibility, the asymmetry in the shoulder rotation range (GIRD) was alleviated, which might have contributed to the improvement in the spin rate by allowing more stable and efficient movement of the arm and fingers at release. Because stride length, pelvic and trunk rotation, pelvic-trunk separation, muscle architecture, and electromyographic activity were not measured, these kinetic-chain explanations remain as hypotheses and require direct biomechanical confirmation.

Beyond baseball, the findings are relevant to exercise science; by maintaining tension across the full ROM, machine-assisted eccentric loading improves hamstring extensibility, isometric strength, and trunk mobility together, rather than trading one for another. As the hamstrings are among the most commonly injured muscles in sprinting, jumping, and change-of-direction tasks, this approach may also aid in injury prevention in physically active populations beyond overhead athletes. Physiologically, the concurrent decrease in passive stiffness and increase in isometric strength reflect favorable remodeling of the muscle-tendon unit, an adaptation broadly relevant to conditioning, rehabilitation, and performance in the physical-activity and exercise-science fields.

This study had several limitations. A single-group prepost design was used, without a control group; therefore, the changes cannot be attributed to NHC training alone, as concurrent baseball training, seasonal adaptation, or normal growth and maturation may also have contributed. In addition, the small, single-region sample (n = 16) limits generalizability. Biomechanical and electromyographic measures were not used in the study, preventing verification of the proposed mechanisms or separation of the effects of flexibility and strength [50]. Randomized controlled trials including larger sample sizes and biomechanical analyses are warranted. In conclusion, considering that the independent contributions of flexibility and strength could not be determined, Keiser-assisted NHC training is best applied as a combined conditioning component rather than a means of targeting either quality alone; future studies should isolate these factors and incorporate biomechanical analyses to refine its practical application in adolescent pitchers.