An underlying reason for the greater hip abductor strength gains in the prior studies (7,9) may stem from the type of the participants. The study performed by Ferber et al. (9) had patients with PFPS as a subject population. Another study, that also used patients with PFPS, showed a 21.0% hip abductor strength increase as a result of implementing 8 weeks of rehabilitative exercises (a combination of 4 weeks of hip-focused exercise and 4 weeks of functional weight-bearing exercises) (7). However, the study performed by Baldon et al. (5) that examined women individuals did not have a PFPS condition. Recent clinical studies reported lower hip abductor strength in women with PFPS compared with women without PFPS (1,4,16). Therefore, the greater hip abductor strength increases observed in the 2 studies (7,9) may be primarily because of lower hip abductor strength baseline associated with the PFPS condition. In another words, women who have a lower hip abductor strength baseline are more responsive to hip-focused neuromuscular exercises than women who already have an average or within normal level of hip abductor strength.
Because this study demonstrated a strong association between trunk and hip–focused INT compliance and isokinetic hip abductor strength changes, the compliance is imperative for those who have lower hip abductor strength such as women with PFPS. Yet, several previous studies struggled in achieving high neuromuscular compliance and noted it as a limitation (23,28). To successfully incorporate neuromuscular training, the achievement of understanding and acceptance for the training from health care providers, coaching staff, and each athlete was advocated (11). Additionally, it was stated that providing variety and progression to improve neuromuscular training may be a key to enhance neuromuscular training compliance. In this study, a qualified instructor (G.D.M., who holds a certified strength and conditioning specialist certification) supervised every neuromuscular training session. The coaching staff members and the athletes were receptive of his instruction. Additionally, the INT program was designed to offer upto 13 diverse exercises per session and was divided into 5 phases. Each of the INT exercises was individually progressed in phases by increasing the degree of difficulty. Each phase of the progressive INT was designed to bring more challenges, which was intended to enhance the intensity of each exercise throughout the duration of the study. The wide variety of exercise selections and periodized progression may have assisted in maintaining 78.9% of compliance rates of the intervention group in this study.
Another interesting discussion point is how long the neuromuscular training effects can be retained. A study was conducted to test the retention of motor movement, which was approximately 15 minutes of balance training to women handball players (15). The balance training was given a minimum of 3 times per week in preseason (5–7 weeks) and once a week during in-season (for ∼5 months). This study found that the participants improved balance ability after 8 weeks, and the gained balance ability was maintained approximately 6 months after the completion of the program. Furthermore, a recent study evaluated motor learning retention ability of the lower extremity (25) by implementing a 10–15-minutes ACL prevention program for 3–4 times per week. This study assessed jump landing movement patterns in 14-year-old girls and boys in 2 groups as follows: 3-month and 9-month durations. This study reported that the 9-month group retained the learned landing movement patterns for 3 months after post-ACL prevention training program, whereas the landing movement patterns of the participants in the 3-month group returned to their original movements after 3 months. According to this result, it seems that a motor skill is better retained with higher dosage and longer duration training (25). Cumulatively, the current results add to the literature that there seems to be a certain threshold of exercise to influence the desired adaptations for injury risk reduction or reductions in symptoms. Future work focused on the dosage thresholds is warranted in helping guide future intervention design.
The limitations to this study should be stated. The participants in the noncompliance group had higher hip abductor peak torque at pretesting compared with the participants in the moderate- and high-compliance groups, which can be perceived as a confounding variable. However, to minimize the pretesting value differences among the groups, the change values, posttesting values minus pretesting values, were used for the analysis. Additionally, sample size of each compliance group was small; therefore, analysis to detect a difference among each group was not performed in the current analysis. This needs to be addressed in future research. One aspect of neuromuscular training that would influence the magnitude of muscular strength development is intensity. Compliance and dosage (number of exercises, frequency and length of session, and duration of intervention) can be measured objectively. However, it was challenging to measure a level of intensity from the participants although the qualified instructor (G.D.M., who holds a certified strength and conditioning specialist certification) was present for every training session. Furthermore, concentric hip abductor strength measurements were taken instead of eccentric strength. Because ACL injury and PFPS pathologies occurred in closed kinetic chain motions, it was theorized that lower extremity muscles eccentrically contracts (3,24). Thus, an eccentric hip abductor muscle strength measurement was desirable instead of concentric hip abductor peak torque measures. Last, the participants in the intervention group performed routine strength training once a week in addition to the trunk and hip–focused INT. Therefore, the routine strength training might have possibly contributed to the hip abductor peak torque changes.
A significant positive correlation was observed between the hip abductor strength improvement and the trunk and hip–focused INT exercise compliance. The hip abductor strength improvement responses seemed to be directly related to the dosage of the specific trunk and hip–focused INT. In comparison with other studies, it seems that hip abductor strength responses may be sensitive to a subject population. More specifically, the greater hip abductor strength gain may be obtained if the initial hip abductor strength has a deficit. Because those who have hip abductor strength deficit seemr more responsive to neuromuscular training, participation and practice of the trunk and hip–focused INT exercises are believed to be highly beneficial. Adaptations from trunk and hip–focused training that improve hip abductor strength and recruitment may be protective against high knee abduction or valgus loading during dynamic movements and potentially reduce ACL and PFPS risk in young women athletes. Future studies are warranted to include biomechanical measures and injury outcome data along with hip abductor strength changes.
Ten weeks of trunk and hip–focused INT, a series of exercise programs aiming to improve trunk and hip strength and power and dynamic stabilization, demonstrated the increases in isokinetic hip abductor strength among young women athletes. The participants who complied with the trunk and hip–focused INT with high level demonstrated significant hip abductor peak torque compared with control group, but not between moderate and noncompliance groups. The observed isokinetic hip abductor strength increases were directly associated with compliance of the trunk and hip–focused INT. Therefore, participating in the trunk and hip–focused INT seemed to be an effective neuromuscular training intervention to increase hip abductor strength in young women athletes when compliance with the training program was high. Because the significant difference and strong association between the trunk and hip–focused INT compliance and hip abductor strength changes has been established, health care and strength and conditioning professionals should pay attention to compliance when incorporating a neuromuscular training into their current training program and should consider tracking participation, performance of and completion of given exercises. Because hip abductor weakness is linked to PFPS, hip abductor strength enhancement may potentially help to improve the existing PFPS condition or provide prophylactic effects for future PFPS development.
The authors would like to thank Ms. Catherine P. Starnes for her statistical expertise and guidance for this project. The authors would like to acknowledge funding support from National Institutes of Health/NIAMS grants R01-AR049735, R01-AR055563, and R01-AR056259.
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