Our world is becoming increasingly aged, approximately 1.4 billion people aged 60 and above in 2022, and it is expected that this number will double globally by 2050 (WHO, 2022). The growing aging population has exacerbated health challenges related to aging, the overall muscle capacity decreases in elderly individuals, including the pelvic floor, partially leads to the occurrence of urinary incontinence, fecal incontinence, and constipation (Talasz et al., 2018; Ma and Chen, 2020; Levaillant et al., 2023; Salari et al., 2023; Zhang et al., 2023). There are various reasons for elderly people to develop urinary and defecation dysfunction, one of which is that the excretion of feces and urine is controlled and regulated by the neuromuscular system. For elderly individuals who have not completely lost urinary and fecal control, five physical rehabilitation methods, including exercise training, magnetic stimulation, electrical stimulation, vibration stimulation, and biofeedback, have been shown to enhance pelvic floor muscle strength and improve urinary and defecation dysfunction (Elfatah et al., 2023; Alouini et al., 2022; Mazur-Bialy et al., 2020; Brusciano et al., 2020; Höder et al., 2023; Jin et al., 2023; Yang J. et al., 2024).

For muscle rehabilitation training for elderly people with different types of urinary and defecation dysfunction, a “one size fits all” strategy should not be used. Personalization is a extremely important issue (Pinto et al., 2020; Davies et al., 2020; Sarrió et al., 2021; Suhonen et al., 2022). Taking constipation as an example, strategies based on individual risk factors of elderly constipation patients have been proven to reduce the occurrence of constipation (Huang et al., 2015). However, the muscle prioritization for the rehabilitation of elderly individuals with urinary and defecation dysfunction is still not fully understood. It is necessary to explore the quantitative relationship between different rehabilitation training methods and urinary and defecation control ability, in order to provide guidance for personalized rehabilitation training practices.

Finite element analysis (FEA) is an appropriate research methodology for investigating complex relationships between rehabilitation training and pelvic floor function, as it is capable of precisely simulating pelvic floor complex structures and intricate biomechanical behaviors. One of the key advantages of the FEA is that it eliminates the need for human experimentation, thereby ensuring a high level of safety, while also facilitating the execution of numerous experiments. For instance, muscle parameters were varied to observe the corresponding changes in outcome indicators. This approach is not only cost-effective but also highly efficient (Wang et al., 2024a; Chen et al., 2010). As an advanced tool for biomechanical research, FEA has been widely applied in obstetrics and gynecology, highlighting its contributions in simulating pelvic floor dysfunctions (Moseley et al., 2025). Notable studies have simulated female pelvic floor analysis (Xu et al., 2023; Soliman et al., 2023), pelvic organ prolapses in women (Silva et al., 2024; Yang M. et al., 2024; Xue et al., 2023), urethral support functions (Peng et al., 2016), levator ani muscle injuries during vaginal delivery and pelvic floor disorders (Zhou LX. et al., 2020), and the mechanical mechanisms of posterior vaginal prolapse (Qiu, 2017). These previous applications have contributed to the development of pelvic floor simulation models for elderly individuals.

Other studies (Ren et al., 2015; Bhattarai and Staat, 2018; Liu et al., 2022; Gordon et al., 2019) have developed pelvic floor models for female individuals aged 55, 63, and 70 years. However, these existing finite element models are limited to simulating the pelvic floor muscles and tissues neglecting the abdominal, hip, and back muscles, which are critical for rehabilitation training beyond the pelvic floor and play significant roles in urinary and fecal control. Consequently, the current female pelvic floor finite element models offer limited representation of the full spectrum of muscles involved in urination and defecation. Furthermore, these models typically do not address the unique anatomical and physiological characteristics of elderly populations. Few studies have focused on rehabilitation outcomes and the role of muscle prioritization, especially in elderly women with pelvic floor dysfunctions. Therefore, a more comprehensive composite model is needed, one that integrates not only pelvic floor muscles but also abdominal, hip, and back muscles, to provide more accurate rehabilitation guidance tailored for elderly women.

We previously established to establish a finite element model in elderly males, using FEA to explore the quantitative relationship between different rehabilitation training methods and urinary and defecatory control ability (Wang et al., 2024b). The research findings can elucidate the muscle prioritization for rehabilitation training in elderly men suffering from various types of urinary and defecation dysfunction. However, marked physiological and anatomical differences exist between male and female pelvic floors. For example, women have a greater colonic length, a larger rectal cross-sectional area, and lack a prostate but possess female-specific structures such as the uterus, vagina, urethral detrusor muscle and urethrovaginal sphincter. Functionally, females are more prone to deficiencies in the maximum voluntary contraction (MVC) strength and endurance of the external anal sphincter and puborectal muscle, whereas males more commonly exhibit elevated muscle tone and incomplete relaxation (Notenboom-Nas et al., 2023). These differences alter muscle support mechanisms and mechanical load distribution, making it unfeasible to accurately assess female-specific conditions using male model data alone. Therefore, it is necessary to construct a female-specific finite element model to enhance the biological and clinical relevance of pelvic floor simulations. Such a model would allow a direct comparison with male models under the same analytical framework, thereby clarifying sex-based differences in rehabilitation, guiding sex-specific optimization of clinical treatment strategies, and providing more effective personalized rehabilitation plans for elderly patients with urinary and defecatory dysfunction.

This study aims to clarify the relationship between muscle function in elderly women and rehabilitation training, prioritizing the identification of key muscles essential for urinary and defecatory rehabilitation. As early as 2011, Chen et al. (2011) constructed a FEA of the pelvis and levator ani muscles by performing pelvic CT and MRI scans on healthy elderly nulliparous female volunteers. This approach avoided the disadvantage of relying on cadaveric specimen dissection previously used to obtain models. This study aims to collect CT and MRI data from a normal elderly female volunteer and establish a composite finite element model of the pelvic floor. Following the model construction, the study will conduct quantitative validation of the finite element model, to ensure its validity and guarantee the accuracy of the finite element analysis. Similar to the validation method used in the Peng Y’S study (Peng et al., 2016), we compared whether the Valsalva movement process simulated by the patient’s finite element model and the Valsalva movement in the patient’s actual dynamic MRI produced consistent changes (including ARA, RVA, and waist circumference) as a validation method. Muscle material properties, such as the material modulus, are typically related to their physical characteristics. The material modulus describes the ability of muscles to deform under force. Research indicates that the material properties of muscles, including the elastic modulus, may change following rehabilitation training (de Sousa et al., 2023; Peng et al., 2016). Muscle contraction can cause changes in the internal structure of muscles, increasing their resistance to deformation, reflected in an increase in elastic modulus (Morin et al., 2022; Stafford et al., 2017; Aljuraifani et al., 2018; Creze et al., 2018). Under various simulation plans of FEA, the study will proportionally alter muscle material properties to simulate the rehabilitation effects of five physical therapy methods (Peng et al., 2016; Dias et al., 2017; Brandão et al., 2015). For the observed outcome measures, this study selected retrovesical angle (RVA) and anorectal angulation (ARA), which are used to assess the ability of elderly individuals to voluntarily control urination and defecation (Zhou and Lai, 2021; Guo et al., 2015). Among them, RVA is the angle formed between the base of the bladder and the long axis of the urethra in the mid-sagittal plane, and ARA is the angle between the longitudinal axis of the anal canal and the posterior wall of the rectum above the levator ani muscle. Ultimately, the study aims to quantify the relationship between rehabilitation training methods and urinary and fecal control by comparing the changes in outcome indicators under different muscle material properties parameter settings. The results of this study will provide answers to our research question regarding how to implement personalized and targeted rehabilitation training for elderly individuals with, taking into account different genders and types of conditions urinary and defecation dysfunction.

The study targets a highly relevant issue in elderly women—urinary and fecal dysfunction—and proposes personalized rehabilitation guidance using biomechanical modeling. Compared with previous studies that focused solely on pelvic floor muscles, this research firstly developed a comprehensive 3D pelvic-thigh model in elderly women. The findings highlight the critical role of integrating muscle groups beyond the pelvic floor, including the abdominal, hip, and back muscles, in maintaining control of urination and defecation and achieving functional recovery. This comprehensive approach enhances the practical relevance of the study.

In constructing the present model, a healthy elderly female dataset was deliberately selected to provide a geometrically intact baseline for biomechanical simulation. In this study, the elastic modulus of muscles was proportionally adjusted to approximate rehabilitation-induced adaptations within a finite element framework. Studies have shown that the elastic modulus is associated with neuromuscular control (Dickson et al., 2024; Wolff et al., 2020), and rehabilitation training not only alters the active contractile capacity of muscles but also induces passive mechanical changes, which can be observed at both the single-fiber and fiber-bundle levels (Noonan et al., 2020; Kawama et al., 2024). In addition, oportional modulation of material parameters has also been adopted in previous FEA studies (Dias et al., 2017), supporting its feasibility as an exploratory strategy.

The model extends beyond traditional pelvic floor simulations because it was constructed based on data from dynamic MRI, CT, and static MRI data, and its accuracy was validated through geometric comparison with key anatomical landmarks, including the waistline, RVA, and ARA. While the model showed good geometric agreement with key pelvic floor anatomical structures, the observed waistline variation of 8.28% was slightly higher than anticipated. This discrepancy can be attributed to the complex nature of soft tissue deformation, which may explain the larger variation observed in waist circumference compared to ARA and RVA. The resulting model showed good geometric agreement with key pelvic floor anatomical structures in a representative elderly female, supporting its preliminary applicability in simulating pelvic biomechanics. These findings suggest potential utility for understanding mechanisms underlying urinary and fecal control, although broader validation is needed.

The study employed RVA and ARA as the sole indicators for assessing urinary and fecal control. At rest, the puborectalis muscle continuously pulls the rectum forward to maintain an appropriate ARA (Zhou and Lai, 2021), which generally ranges from 90° to 127°, and normally increases to 120°∼152.4°during defecation (Zhou and Lai, 2021; Kobi et al., 2018; MR Group, 2022). Failure of the ARA to increase, or paradoxical narrowing during defecation, indicates pelvic floor muscle relaxation, loss of tone, or impaired coordination. Conversely, when weakening of the puborectalis muscle leads to an ARA exceeding the normal range, fecal incontinence may occur, often associated with reduced pelvic floor activity or pudendal nerve injury (Elmalı, et al., 2024). RVA reflects the integrity of bladder neck closure, with a normal resting range of 90°∼120° (Liu et al., 2013). In pelvic floor dysfunction, the RVA may abnormally widen to 160°∼180°during straining (Fielding et al., 2000), and in patients with stress urinary incontinence, the RVA often exceeds 140° (Li and Yan, 2015), suggesting decreased urethral closure pressure and impaired urinary continence. The results showed that as the muscle material coefficients gradually increased from 0.05 times to 1 times or decreased from 2times to 1 times, both RVA and ARA values progressively approached the normal range. These biomechanical changes suggest a potential mechanism whereby targeted reinforcement of the pelvic floor, abdominal, hip, and back muscles through rehabilitation interventions may enhance pelvic floor functional capacity and thereby improve urinary and fecal dysfunction.

While RVA and ARA are significant imaging biomarkers, urinary and defecation dysfunction are complex clinical symptoms whose severity is closely linked to patient subjective experience, quality of life, and neural control. Alterations in these two parameters alone do not fully capture the comprehensive clinical functional enhancement. In this study, RVA and ARA served as the primary outcome measures because they are the most readily quantifiable biomechanical indicators within the finite element modeling framework. Thus RVA and ARA should be interpreted as surrogate biomechanical markers rather than comprehensive clinical endpoints.

For the first time, this study provides preliminary evidence on the potential muscle prioritization of pelvic floor rehabilitation benefits for elderly women. For constipation management, computational findings suggest that rehabilitation targeting the levator ani muscle might be prioritized when muscle properties fall below baseline levels (1 time), while cases with above-baseline muscle properties may benefit more from comprehensive pelvic floor muscle engagement. Regarding fecal incontinence, sub-baseline muscle conditions suggest the need for integrated training targeting pelvic floor muscles along with rectus abdominis, and erector spinae muscles, whereas supra-baseline conditions may warrant concentrated pelvic floor muscle rehabilitation. For urinary incontinence cases, combined pelvic-hip muscles rehabilitation appears advantageous regardless of baseline muscle status. These observed associations between muscle material coefficients and rehabilitation outcomes highlight the importance of condition-specific muscle prioritization in elderly women’s pelvic rehabilitation, these findings may contribute to the development of more tailored rehabilitation nursing recommendations. It should be emphasized that our interpretation focused on the general trends observed when material properties were below or above baseline, rather than the extreme values of parameter variation. The broader range (0.05–2.0) was applied primarily to test model robustness, while clinical rehabilitation is unlikely to reach such extremes.

Recent clinical research by Ojukwu et al. (2022) demonstrated that hip exercises alone, as well as combined abdominal, hip, and pelvic floor muscles training, significantly enhance pelvic floor muscle activity. This may be attributed to the synergistic contractions induced by adductor and gluteal muscle engagement, which facilitate the co-contraction of pelvic floor muscles and the striated urethral sphincter, thereby strengthening pelvic floor muscles contraction and improving neuromuscular control (Marques et al., 2020). These findings are consistent with our simulation-based results, which suggest that pelvic floor-hip combined training may offer more comprehensive benefits for elderly women with urinary incontinence. In contrast, clinical and imaging studies directly addressing the prioritization of specific muscle groups in constipation and fecal incontinence rehabilitation are limited. Currently, research on pelvic floor muscle training (PFMT) for elderly women primarily focuses on the overall efficacy of PFMT, rather than the sequence or prioritization of individual muscle group interventions. As such, direct clinical comparisons with the prioritization patterns observed in our simulations remain exploratory.

Compared to our previous findings (Wang et al., 2024b), there were notable differences in the key muscles for rehabilitation of urinary and defecation dysfunction between elderly men and women. In elderly men, the levator ani muscle and external anal sphincter are the critical muscles for improving defecation dysfunction. In contrast, for elderly women, the levator ani, pelvic floor, rectus abdominis, and erector spinae muscles play more significant roles in improving defecation dysfunction. In terms of urinary incontinence, for elderly men, training should primarily focus on the levator ani and pelvic floor muscles, while for women, combined training of the pelvic floor muscles and hip muscles, or a comprehensive approach targeting the pelvic floor muscles, rectus abdominis muscle, hip muscles, and erector spinae muscle, is more effective in restoring urinary control. Specifically, rehabilitation training for urinary and defecation dysfunction in elderly men focuses on strengthening localized muscles, such as the levator ani muscle and external anal sphincter, and the benefits of such interventions are concentrated on localized improvements in the core muscle groups. In contrast, rehabilitation training for elderly women emphasizes the synergistic action of multiple muscle groups, focusing on overall functional recovery. Compared to their male counterparts, the neuromuscular system in elderly women undergoes more severe degeneration (Stalling et al., 2024). The decline in estrogen levels leads to a reduction in neuromuscular function in postmenopausal women, negatively affecting muscle coordination (Willoughby et al., 2024; O'Bryan et al., 2025). Studies have shown that while elderly men tend to exhibit greater muscle size, strength, and functional performance, elderly women demonstrate a smaller cross-sectional area of the lateral thigh muscle, lower knee extensor torque, and poorer force steadiness in muscles around the hip joint (Guo et al., 2023). Anatomically, the pelvic floor muscles in women are more vulnerable, as they are heavily influenced by pregnancy and childbirth. Therefore, rehabilitation training for elderly women requires special attention to synergistic training of the muscles of the pelvic floor, abdomen, hip, and back, ensuring a comprehensive approach to improve their functionality.

These findings suggest that sex-based differences may influence muscle prioritization in pelvic floor rehabilitation, underscoring the value of further investigation into gender-specific rehabilitation strategies. Clinical phenomena also confirm that gender differences lead to different types of diseases in patients. In clinical practice, elderly men are more prone to defecatory dyssynergia, whereas women are more likely to experience insufficient defecatory propulsion (Abe et al., 2023). Women are also more susceptible to stress urinary incontinence, while men more commonly present with urge urinary incontinence (Hu et al., 2024). Bjarnason-Wehrens et al. (2007) proposed in 2007 that sex-based issues should be considered in cardiac rehabilitation, recommending customizing programs specifically for women with ischemic heart disease. Similarly, the anatomical and functional differences between men and women must be fully considered to provide more targeted rehabilitation plans for elderly patients urinary and defecation dysfunction, ultimately enhancing quality of life and functional recovery.

While this computational evidence provides mechanistic insights, validating the proposed muscle prioritization strategies in real-world settings through controlled clinical trials is essential. Shear wave elastography (SWE), an ultrasound-based elastography technique that measures tissue elasticity modulus by generating shear waves within the tissue via external or internal forces and analyzing their propagation speed (Li, 2017). With a penetration depth exceeding 8 cm (Frulio and Trillaud, 2013), SWE provides critical technical support for identifying muscle fibrosis, damage, or excessive laxity, especially in deep pelvic structures. It is non-invasive, repeatable, and has shown potential in evaluating muscle stiffness and contractility in the levator ani (Xie et al., 2015), puborectalis (Wang et al., 2017), and other pelvic floor muscles (Okada et al., 2001; Tang et al., 2020). Studies suggest that SWE can guide early pelvic floor muscle training and monitor rehabilitation outcomes (Zhou M. et al., 2020; Jiang et al., 2019; Bai et al., 2022; Niu et al., 2018). Future research should employ SWE to quantitatively assess the elastic modulus of pelvic floor-related muscles in patients with urinary and fecal dysfunction. Building on the muscle prioritization trends observed in our model, future research may explore constructing personalized rehabilitation protocols that consider individual muscle characteristics, sex differences, and dysfunction types. Subsequently, the effectiveness of these protocols will be evaluated using functional outcomes such as reduction in urinary and fecal dysfunction symptoms, ARA, RVA, and quality of life. This two-stage validation—biomechanical characterization followed by targeted intervention—will help bridge the gap between simulation results and clinical application.

Striking an appropriate balance between model fidelity and practical feasibility remains a critical and ongoing challenge (Moseley et al., 2025). It is important to acknowledge certain limitations of this study. First, in this study, muscle elastic modulus was linearly scaled to represent rehabilitation-induced functional improvement. This approach, while a common simplification in finite element modeling, primarily reflects changes in passive stiffness rather than improvements in active contraction capacity or neuromuscular coordination. As such, our method more closely simulates muscle dynamic alterations such as fibrosis than genuine training-induced strengthening.

In reality, rehabilitation induces complex bioadaptive processes that include neuromuscular control, motor unit recruitment, muscle fiber type conversion, and myofascial coordination, which cannot be fully represented by a single parameter such as passive stiffness. Therefore, our finite element results should be interpreted as reflecting the structural potential of muscle strengthening, rather than capturing the entire spectrum of functional adaptations. Future studies combining finite element modeling with electrophysiological measurements and clinical outcomes are needed to provide a more comprehensive understanding.

Second, the boundary conditions were idealized, with bones being modeled as completely fixed, rigid bodies and intra-abdominal pressure assumed to be uniformly distributed. Compared with soft tissues, the elastic modulus of bone is much higher, and thus its deformation can be considered negligible relative to that of soft tissues. Therefore, bone was simplified as a rigid body in this study. Furthermore, our dynamic MRI measurements indicated that bone displacement during rehabilitation training was minimal, and accordingly, the bones were simplified as fully fixed. In vivo, however, pelvic floor mechanics involve micro-articulations, pressure vessel effects, and holistic load transmission. In future work, we plan to incorporate such dynamic interactions to improve the accuracy of the computational model.

Third, although the constructed model provides valuable biomechanical insights, the modeling was based on a single elderly female participant, which inevitably limits the generalizability of the findings. BMI, individual anatomical differences, and tissue properties may alter biomechanical outcomes. To improve generalizability and clinical applicability, future studies should involve multidisciplinary collaboration to develop multi-subject models incorporating diverse anatomical and clinical characteristics.

Fourth, material properties were varied linearly from 0.05 to 2.0 times baseline to represent functional impairment and enhancement. The analysis primarily contrasted sub-baseline (<1.0) and supra-baseline (>1.0) conditions to describe directional trends, rather than drawing predictions from the extreme ends of the range. Nevertheless, the assumption of linear extrapolation across such a wide interval does not fully reflect biological reality, since muscle performance usually operates within an optimal functional range. Therefore, results at extreme values should be regarded as theoretical boundaries.

The findings of this study are primarily theoretical recommendations derived from computational simulations without clinical validation, direct clinical implementation of its findings risks premature translation. Future research should involve designing studies to evaluate the feasibility and efficacy of these muscle prioritization in real-world elderly population. We are currently conducting a larger-scale SWE-based clinical validation to bridge simulation with real-world applications. Despite these limitations, the present model provides novel mechanistic insights into the interplay of pelvic and extra-pelvic muscles in continence control. This study represents the first attempt to integrate pelvic floor, abdominal, hip, and back muscles into a unified model for elderly women, providing valuable biomechanical insights and a foundation for precision rehabilitation and AI-driven prescription systemsin pelvic floor care.

The study developed a 3D finite element model that is highly consistent with the actual pelvic floor conditions of an elderly woman and validated its effectiveness using dynamic MRI data. By employing FEA, this study delved into the mechanisms underlying the enhancement of urinary and defecation control in the elderly women and identified the differing muscle prioritization for rehabilitation training in elderly men and women. These findings facilitate the personalized recommendation of pelvic floor rehabilitation plans tailored to the muscle condition and dysfunction type of elderly women. The current simplified model was developed from a single elderly female subject, which limits its generalizability. Therefore, it is imperative that future work should focus on developing multi-subject and multi-sample models that account for anatomical and physiological variability. Additionally, clinical validation trials should be undertaken to ensure the broader applicability of rehabilitation strategies.