Extended Abstract
1.    Introduction
Low back pain (LBP) is one of the most prevalent musculoskeletal disorders worldwide and is associated with disability, psychological consequences, and substantial socioeconomic costs (1–4). Individuals with LBP often demonstrate altered movement strategies, including impaired trunk motor control, changes in lower-limb kinematics, increased postural sway, altered center of pressure displacement, and reduced trunk muscle performance (5). These biomechanical adaptations may influence the distribution and regulation of external forces during functional activities such as walking. Walking requires coordinated interaction between the lower limbs and the trunk to effectively control external loads, including tensile, shear, and rotational forces (6, 7). Alterations in lower-limb mechanics and hip muscle function have been associated with low back disorders and may contribute to abnormal gait patterns in individuals with LBP (8–10). Previous studies have reported differences in ground reaction force characteristics, including force peaks, impulse, loading rate, and time to peak force, among individuals with LBP compared with healthy populations (11). Various exercise-based interventions, including general exercise programs, the McKenzie method, Pilates, core stabilization, and motor control exercises, have been investigated for improving movement impairments associated with LBP (12–15). Recently, corrective exercise approaches based on the National Academy of Sports Medicine (NASM) model have gained attention due to their emphasis on flexibility, muscle activation, and neuromuscular control. However, the effects of NASM-based corrective exercises on gait-related biomechanical variables, particularly ground reaction forces, remain unclear. Therefore, the present study aimed to investigate the effects of a NASM-based corrective exercise program on ground reaction forces during walking in individuals with hamstring tightness and low back pain.
2.    Methods
This quasi-experimental laboratory study employed a pretest–posttest design with a control group. Based on previous studies and an a priori sample size estimation using G*Power software, 15 participants per group were considered sufficient to achieve a statistical power of 0.80 at a significance level of 0.05. Accordingly, 30 men aged 20–30 years were recruited through purposive sampling from patients with hamstring tightness and low back pain who were referred to public clinics and private centers in Ardabil, Iran. Participants were evaluated and diagnosed by an orthopedic specialist according to the inclusion and exclusion criteria. The participants were assigned to two groups: an experimental group consisting of individuals with hamstring tightness and low back pain, and a control group consisting of healthy individuals without low back pain or hamstring tightness. Inclusion criteria for both groups included male sex, age between 20 and 30 years, no participation in regular exercise programs during the previous six months, no involvement in competitive or professional sports, the ability to sit, stand, and walk independently without assistive devices, no history of musculoskeletal surgery in the trunk or lower extremities, and absence of neurological, musculoskeletal, or orthopedic disorders that could limit physical activity. Specific inclusion criteria for the experimental group included hamstring tightness confirmed by the Active Knee Extension (AKE) test and a history of low back pain lasting at least three months. The control group was required to have no history of low back pain and no hamstring tightness based on the AKE test. Participants were excluded if they had previously participated in structured rehabilitation programs for low back pain, used analgesic or anti-inflammatory medications during the intervention period, experienced injury, withdrew consent, or missed more than two consecutive training sessions. The study protocol was approved by the Ethics Committee of Mohaghegh Ardabili University (IR.UMA.REC.1403.090), and all participants provided written informed consent after receiving a complete explanation of the study procedures.
Hamstring tightness was assessed using the AKE test. Participants were positioned supine, and the examiner placed the hip in 90° flexion. Participants were instructed to actively extend the knee, and the knee extension angle was measured using a goniometer. The axis of the goniometer was aligned with the lateral femoral condyle, with the stationary arm aligned with the femur and the movable arm aligned with the lateral malleolus. The test was considered positive when participants reported a strong stretching sensation in the posterior thigh or knee before reaching the final 25° of knee extension.
The experimental group participated in an eight-week NASM corrective exercise program consisting of 24 training sessions (three sessions per week, 60 minutes per session), whereas the control group did not participate in any structured exercise program and maintained their usual daily activities. Participants in both groups were instructed to avoid additional exercise programs during the study period. The NASM program included diaphragmatic breathing, baby rock, prone on elbows, rolling, side lying and oblique sit exercises, tripod position, kneeling to squat, and Czech get-up exercises, which were progressively introduced according to the stages of the program.
Ground reaction force data were collected in the biomechanics laboratory of Ardabil using a Bertec force plate (40 × 60 cm). Participants walked along a 20-m walkway at a controlled speed of 2.3 m/s, with the force plate positioned in the middle of the walkway. Walking speed was monitored using a stopwatch to ensure consistency among trials. Ground reaction force signals were filtered using a fourth-order Butterworth low-pass filter with a cutoff frequency of 20 Hz. The analyzed variables included peak three-dimensional ground reaction forces, peak positive free moment, and time to peak values for the mediolateral (Fx), anteroposterior (Fy), and vertical (Fz) components. All force variables were normalized to body mass and expressed as a percentage of body weight. Data normality was assessed using the Shapiro–Wilk test, and homogeneity of variances was examined using Levene’s test. A two-way repeated-measures analysis of variance was used to evaluate the effects of time (pretest and posttest), group (experimental and control), and the time × group interaction. Bonferroni post hoc tests were performed when significant interactions were observed. Statistical significance was set at α < 0.05, and all analyses were conducted using SPSS version 26.
3.    Results
The results indicated a significant main effect of time on peak mediolateral ground-reaction forces at heel contact (p=0.024, d=0.201) and during the push-off phase (p=0.001, d=0.633). A significant main effect of time was also observed for peak anteroposterior ground-reaction forces at heel contact (p=0.011, d=0.249) and during push-off (p=0.001, d=0.533). Likewise, peak vertical ground-reaction forces showed a significant main effect of time at heel contact (p=0.001, d=0.459) and during push-off (p=0.002, d=0.496) (Fig. 1). A significant time × group interaction was found for peak anteroposterior ground-reaction force at heel contact (p=0.001, d=0.399) and for peak vertical ground-reaction force during push-off (p=0.010, d=0.254). Significant time × group interactions were also detected for time-to-peak of the anteroposterior ground-reaction force at heel contact (p=0.006, d=0.276) and for time-to-peak of the vertical ground-reaction force during push-off (p=0.016, d=0.224).

4.    Discussion
This study investigated the effects of NASM-based corrective exercise training on ground reaction forces during walking in individuals with hamstring tightness and low back pain. The findings demonstrated significant changes over time in peak ground reaction forces across the mediolateral, anteroposterior, and vertical directions at both heel contact and push-off phases. These findings suggest that lower-limb loading patterns changed during the intervention period; however, the specific contribution of NASM training should be interpreted based on the observed time × group interactions. Alterations in the anteroposterior and vertical components may reflect modifications in force absorption and generation strategies throughout the gait cycle, as these components play important roles in controlling forward progression and managing external mechanical loads.
The mediolateral component of ground reaction force represents an important aspect of frontal-plane loading during locomotion and may influence lower-limb joint loading patterns (26). In the present study, significant changes in peak mediolateral ground reaction forces were observed at heel contact and push-off, indicating alterations in mediolateral loading strategies following the intervention period. These findings are partially consistent with previous research by Jafarnezhadgero et al., who reported reductions in peak mediolateral forces following sand-surface training in individuals with foot pronation (31). Together, these findings suggest that corrective exercise interventions may contribute to modifications in lower-limb force distribution and gait-related loading patterns. Vertical ground reaction force is one of the primary external forces acting on the body during locomotion and contributes to mechanical loading of the ankle, knee, hip, and spine (27, 28). In this study, peak vertical ground reaction forces showed significant reductions at both heel contact and push-off phases. These changes may indicate alterations in impact absorption and force-generation strategies during walking. However, reductions in vertical ground reaction force alone cannot directly confirm improvements in tissue tolerance or reductions in injury risk.
The anteroposterior component of ground reaction force, particularly its braking phase during early stance, reflects the interaction between the foot and ground and contributes to deceleration control following foot contact (32). The present findings demonstrated significant changes in peak anteroposterior force and time-to-peak values. Previous studies have suggested that alterations in the timing of force development may affect lower-limb loading characteristics (33–35). Nevertheless, anteroposterior force characteristics are influenced by several factors, including walking speed, step length, and confidence during walking (36). Therefore, these factors should be considered when interpreting changes in this variable. The propulsive phase during push-off is primarily influenced by plantarflexor muscle function and contributes to forward progression (6, 32). Accordingly, the observed changes may reflect adaptations in gait mechanics and force-generation strategies following the training program. The current study also demonstrated a significant effect of time on vertical loading rate. Loading rate represents the speed at which external forces are transmitted to the musculoskeletal system and has been considered an important biomechanical parameter related to repetitive loading conditions (35). The observed reduction in loading rate may indicate changes in impact absorption strategies during walking. This finding is consistent with previous studies reporting that gait characteristics, including stance duration and walking speed, can influence loading rate values (38, 39). However, further investigations using additional biomechanical variables are required to clarify the mechanisms responsible for these changes.
Regarding the free moment, a significant time × group interaction was observed for peak positive values. The free moment provides information about rotational control during locomotion and has been associated with transverse-plane loading of the lower limb and joints (41–44). The observed changes may indicate modifications in rotational control strategies following NASM training. Previous studies have demonstrated that corrective exercise programs can improve balance, lower-limb strength, and movement patterns (46, 47). Additionally, comprehensive corrective training has been shown to improve balance and plantar pressure distribution in individuals with lower-limb abnormalities (48). Therefore, improvements in neuromuscular coordination, muscle function, and movement strategies may represent possible explanations for the observed changes in free moment and loading characteristics. Nevertheless, these mechanisms require confirmation through direct assessments such as kinematic or electromyographic analyses. Overall, the findings suggest that NASM-based corrective exercise training may influence specific ground reaction force characteristics during walking in individuals with hamstring tightness and low back pain. These changes may reflect adaptations in lower-limb loading strategies and movement control. Future studies with larger sample sizes and comprehensive biomechanical assessments are recommended to further investigate the mechanisms underlying these adaptations. 
5.    Conclusion
The findings of this study suggest that NASM-based corrective exercises may influence specific biomechanical characteristics of walking in individuals with hamstring tightness and low back pain. The intervention was associated with changes in ground reaction force components in the mediolateral, anteroposterior, and vertical directions, as well as alterations in vertical loading rate and peak positive free moment. These changes may reflect adaptations in lower-limb loading strategies and movement control during walking. Although the findings indicate potential benefits of NASM exercises as a non-invasive approach for improving gait-related biomechanical characteristics, further studies with larger sample sizes, longer follow-up periods, and additional biomechanical assessments are required to confirm the underlying mechanisms and clinical effectiveness.

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Ethics Committee of Mohaghegh Ardabili University (IR.UMA.REC.1403.090). Written informed consent was obtained from all participants prior to participation. 
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. 
Authors' contributions
Makwan Jabbar Ali and Mohsen Barghamdi contributed to the conception and design of the study, methodology development, and interpretation of findings. Makwan Jabbar Ali was responsible for data collection and project management. Mohsen Barghamdi performed the data analysis and supervised the research process. Both authors contributed to manuscript preparation, critical revision, and editing of the manuscript. All authors reviewed and approved the final version of the manuscript.
Conflicts of interest
The authors declare that there is no conflict of interest regarding the publication of this article.