# Empirical Validation of a Streamlined Three-Repetition Sit-to-Stand Protocol Using MAI Motion
## 1 Introduction  
The integration of artificial intelligence (AI) in motion capture has gained significant attention across multiple fields, especially in healthcare and rehabilitation. However, AI-based motion-tracking systems employing markerless technology are emerging as viable alternatives to traditional methods that rely on physical markers and controlled laboratory environments. These advancements are particularly relevant in clinical settings, where the use of markers may be impractical due to the time, expertise, and facilities required to employ such systems [1, 2].  
These AI-based investigative tools facilitate motion capture in diverse clinical domains, including orthopaedic clinics, physiotherapy and rehabilitation, without the need for extensive physical setups [3]. The shift towards markerless motion capture is further supported by its lower cost, greater accessibility, and ability to perform whole-body kinematics tracking [4].  
MAI Motion is a markerless digital platform that allows for the analysis of everyday movements, including the transition from a seated to standing position, known as thesit-to-stand (STS) test [1; 2]. Moving from a seated to standing position involves a multifaceted process that requires a complex interplay of kinematic phases and biomechanical adjustments across four distinct phases: flexion momentum, momentum transfer, extension, and stabilisation [5; 6], also known as the pelvic tilt, trunk extension, hip flexion, and knee extension. Each phase is characterised by specific kinematic actions that facilitate the successful transfer of the centre of mass (COM) from a stable base of support (the seat) to a less stable position (the feet) [7]. The STS test is primarily conducted for its ability to effectively indicate lower limb strength, functional capacity, and risk of falls, which are critical for maintaining independence in daily activities [7].  
The STS test is a widely utilised and validated tool in clinical and research settings for assessing lower limb function, balance, and physical performance, particularly among older adults and individuals with musculoskeletal (MSK) impairments [8; 9]. While various STS protocols exist, the five-repetition format (5x STS) is commonly adopted due to its balance between practicality and sensitivity in detecting functional deficits [10; 11].  
However, in clinical practice, a significant proportion of individuals with chronic MSK conditions are unable to complete five full repetitions due to pain, fatigue, or movement limitations. For example, 18% (13 of 71) of participants who were one year post total knee arthroplasty failed to complete the 5x STS test [12] whilst 6% (6 of 96) of males aged 51-72 years were unable to complete the test in a study investigating the impact of ageing and MSK pain [13]. This constraint presents a potential barrier to inclusive assessment and data acquirement, particularly when advanced kinematic analysis tools such as MAI Motion are employed.  
To address this challenge, we hypothesised that three repetitions (3x) of the STS test may yield data of comparable reliability to the conventional five-repetition protocol. The aim of this study, therefore, was to systematically evaluate whether kinematic parameters captured via 3D MAI Motion during 3x STS trials could reliably replicate those obtained from 5x trials. By directly comparing both protocols across a range of key biomechanical metrics, we sought to establish whether the shorter protocol offers a valid alternative, thereby enhancing accessibility without compromising the integrity of the diagnostic assessment.