The aging process of skeletal muscles is multifaceted, with several factors contributing to a decrease in muscle mass, strength, and overall performance.1 The reduction in muscle mass is primarily attributed to the shrinking of muscle fibers and the loss of motor units. While this decline is a natural part of aging, it can lead to reduced mobility, increased vulnerability to falls, disability, decreased self-reliance, and ultimately, a lower quality of life.2 Postmenopausal women are particularly affected by these changes as they experience a decline in bone density due to aging and hormonal fluctuations.3 This deterioration of the musculoskeletal system in this population especially in osteoporotic women, results in a significant decline in daily functional activities and a loss of personal capabilities, increasing the likelihood of falls.4 In a state of quiet standing, the human body naturally sways which the integrating postural sway parameters provides valid fall risk prediction and a holistic analysis of postural stability.5 Certainly, changes in the center of pressure sway (CoP) in the mediolateral direction is a reliable indicator of falling in osteoporotic women,6 however, due to age-related decline in ankle strategy, it becomes vital to prioritize the observation of CoP sway in the sagittal plane.7 The role of the gastrocnemius muscle through ankle strategy, activated in the sagittal plane to maintain balance, has been highlighted in postural sway.8 Although the correlation between decreased muscle strength and power with aging and poor balance has been established, there is limited existing research on the interrelationship between muscle morphology, including cross-sectional area (CSA), muscle thickness (MT), fascicle length (FL), pennation angle (PA), and muscle quality measured through echo intensity (EI), with postural sway parameters and muscle mass in osteoporotic women. Measuring muscle quantity alone is insufficient to detect age-related muscle degradation as muscle mass in itself has no linear relationship with either strength or function.9 Ultrasonography architectural parameters assesses both muscle quantity and quality with good accuracy and thus has the benefit of repeated measurements.10,11 Studies have illustrated excellent intra-and inter-rater consistency in the geriatric population as well as in the younger population.12

Narici et al. conducted a study using ultrasonography to measure the FL and MT of the vastus lateralis muscle in older men and women. They introduced the ultrasound sarcopenic index, which was found to be associated with sarcopenia.13 In sarcopenia, the MT and CSA of the rectus femoris or gastrocnemius muscles have been identified as suitable ultrasound parameters for predicting muscle mass.14 Older adults tend to have lower gastrocnemius thickness and higher muscle echogenicity.15 The mechanical output of the gastrocnemius muscle plays a crucial role in walking performance and it is well-documented that a reduced mechanical output from the triceps sura is likely due to multiple factors, and the functional consequences of sarcopenia and distal leg muscle weakness.16 Hill et al. have investigated the association between knee extensor and ankle plantar flexor MT and EI during rest status with postural sway, mobility and physical function in older adults, but not specifically osteoporotic group.17 Palmer et al. also found significant positive relationship in the older men between the sway index with the eyes closed and muscle EI (r=0.474); however, no such relationship was observed between the sway index and muscle CSA (r=0.021).18 Given the decline in muscle quantity and contractile function in the aging gastrocnemius muscle19, along with the documented correlation between muscle activity and changes in muscle geometry for the medial gastrocnemius20 exploring its ultrasonography parameters may be linked to postural sway and balance behavior in osteoporotic women. In addition, because most architectural parameters change during muscle contraction,20 investigating the sonographic characteristics of MG muscle in two states of rest and activation may provide valuable insights into the relationship between muscle characteristics and balance function in osteoporotic individuals. Additionally, information on normal muscle dynamics during contraction is important for comparison with abnormal findings to aid physical therapy intervention for muscle strengthening. Furthermore, the increased risk of falls in individuals with osteoporosis, is crucial to investigate and monitor muscle quality and quantity, as well as their relationship with balance, to develop effective preventive programs. Therefore, the objective of this study was to examine the association between ultrasonography MG muscle architecture during rest and activation with postural sway parameters, appendicular skeletal muscle mass index (ASMI), and fall efficacy scale (FES) in osteoporotic women. The first hypothesis posited that EI during rest and muscle activation would have the association with postural sway in osteoporotic women. The second hypothesis assumed a significant relationship between EI and other quantitative characteristics of the medial gastrocnemius during rest and muscle activation with ASMI, FES, and T-score in osteoporotic women.