Background and Purpose
Over the past decades, various types of eccentric exercises have been developed and used for facilitating improvements in musculoskeletal function and prevention or rehabilitation of muscle and tendon injuries. One of these exercises is the forward lunge, which is broadly used within the sports community amid training and recovery as it is better suited to the movement patterns of everyday tasks and different sports, activates multiple muscle groups simultaneously, and requires less specialized equipment. Various types of forward lunges have long been used when prescribing exercises for the lower extremities depending on a person's specific needs.
In recent years, instability resistance training has gradually gained importance over traditional type in the field of sports training and fitness activities because of its simplicity (since it requires only body weight as a load), its specificity, and its high transferability to actions in athletic competition. Performing closed-kinetic-chain exercises like forward lunge with an instability device such as TRX suspension trainer system can alter movement patterns and affect motor control requirements differently A review of the research background shows that despite the increasing tendency of people to use instability devices, not many studies have been conducted in this area and most research has been done in the area of muscle activity, while there is no study on movement patterns or joint coordination, and due to the lack of information or mixed results [2,4], the different aspects of this way of exercise are still in an aura of uncertainty, leading to confusion for instructors or clinicians when designing exercise programs. Therefore, the aim of this study was to analyze coordination of the lower limb joints in performing forward lunge with and without using TRX.
 
Methods
A sample of 22 recreationally active female adults (mean height 164.9 ± 4.2 cm; mean body weight 60.6 ± 8.4 kg; mean age 25.45 ± 5.2 years) was recruited through a convenience sampling approach. Upon arrival at the laboratory, a 15-minute session of warm-up exercises was conducted and reflective markers (14-mm spheres) were placed (with double-sided tape) bilaterally over the landmarks [3]. Then, the random allocation of performance order was applied to each participant, who completed three repetitions of each exercise including conventional forward lunge, forward lunge by placing hands or foot inside the TRX rope. While performing exercises, kinematics of movements were collected using an 8-camera motion analysis system at 250 Hz (Oqus 5+, Qualisis). Analyzing motion data was conducted using The MATLAB software (The Mathworks Inc., Natick, MA). The lower limb was modeled as a composition of four fixed rigid segments, namely the pelvis, thigh, leg, and feet. Middle repetition of each exercise was interpolated and normalized to 100% of the movement cycle using spline. Then, angular displacement and velocity, range of motion, continuous relative-phase (CRP) angles and coordination variability of the joints of right leg of the subject, which correlates with the leading leg in all variations of forward lunge were calculated based on previous researches. In order to conduct a comparison of obtained values across different joints/exercises/phases repeated measures analysis of variance was employed. A post-hoc analysis was conducted utilizing Tukey's multiple comparison tests in order to identify notable variations. All statistical procedures were conducted using the SPSS platform for Windows (version 24. 0, SPSS Inc., located in Chicago, IL) and the predetermined level of significance was set at P < 0.05 for all statistical tests.
 
Results
The obtained results show that knee range of motion is greater than hip and ankle range of motion in all exercises, but FLL shows decreased knee and ankle range of motion and increased hip ROM. However, no significant interaction between exercises was found. RMS values of angular velocity for lower extremity joints across demonstrates higher angular RMS values for hip and lower values for knee and ankle in comparison of FLL with other variations. That means the hip moves faster than the knee and ankle when doing the FLL exercise. The difference in angular velocity of joints between FL and FLH was significant. RMS and MARP values of CRP couplings show an in-phase mode during 100% of movement cycle. Analysis of coordination variability (CV) in sagittal plane for the joints shows a significant difference between exercises and phases.
 
Conclusion
The study analyzed the displacement curves and found similar patterns in all three types of lunges, with FLL targeting the hip and FLH targeting the foot flexors. While all three types of lunges are recommended for improving knee joint range of motion and muscle function, FLL is recommended for improving hip extensor function and FLH for improving foot flexor function and ankle joint range of motion. Also an in-phase movement is found for all three types of lunges for the hip/knee, hip/ankle, and knee/ankle joint pairs during the entire cycle of motion in the sagittal plane. The study also found that closed-kinetic chain exercises require more synchronized joint movement, while open-kinetic chain exercises allow for more independent movement. The study suggests that closed-kinetic chain exercises are more effective for improving lower limb muscle function.
 
Article Message              
Lunge is a closed kinetic chain exercise and is an essential part of most exercise programs. It is known as a knee-dominant movement. However, when performing the lunge exercise with the TRX strap, the range of motion in the hip joint increases, and the range of motion in the ankle joint decreases. Therefore, it can be said that the TRX lunge exercise can be more effective than other types of lunges in restoring the performance of the hip flexors in individuals diagnosed with weakness in the hip flexor muscles. Similarly, for individuals diagnosed with weakness in the plantar flexor muscles, the FLH exercise will be more effective. In addition, due to the increased variability in the ascending phase of various types of lunges, it is better for individuals with balance problems or muscle weakness to stand next to a support and ask for help if they slip or fall.
Ethical Considerations
This study was granted approval by the institutional ethics commission (IR.KHU.REC.1400.026).
Authors’ Contributions
Conceptualization: Fahimeh Khoshmaram, Heydar Sadeghi, Ronald Snarr
Data Collection: Fahimeh Khoshmaram,
Data Analysis: Fahimeh Khoshmaram, Heydar Sadeghi
Manuscript Writing: Fahimeh Khoshmaram,
Review and Editing: Fahimeh Khoshmaram, Heydar Sadeghi, Ronald Snarr
Responsible for funding: Fahimeh Khoshmaram, Heydar Sadeghi
Literature Review: Fahimeh Khoshmaram
Project Manager: Heydar Sadeghi
Any other Contributions: Fahimeh Khoshmaram, Heydar Sadeghi, Ronald Snarr
Conflict of Interest
We, the authers, declare that no conflicts of interest related to this research paper.
 
Acknowledgments
The author(s) would like to express their sincere gratitude to all the participants for their valuable time and cooperation in this study.