## 6 ACLR Diagnosis and RTW  
### ACLR Diagnosis  
Gait variability, defined as the fluctuation in gait features from one step to the next, is an important indicator of impaired mobility in ACLR patients [36; 54; 55]. In this study, eight important features, including step width, swing time (s), stance time (s), double support time (s), single support time (s), double support time (%GC), gait speed (m/s), and stride speed, were selected by utilising OAs and S/N ratios. These essential features will be discussed in detail.  
The human body's degree of stability can be predicted from step width [56]. In many studies, the step width is often associated with local and orbital stability [54; 57]. Stability is defined as the capability of a body to respond to perturbations [58]. During human walking, increasing the step width changes both the anterior-posterior and mediolateral stability along with stability [59].  
Stance time was identified as a significant variable in clinical gait analysis. According to [PERSON] and [PERSON], modifications in stance time may result in increased mechanical function and, therefore, greater energy consumption during walking [58]. For example, a decline in a single limb's stance time may result in a heightened contralateral limb swing speed, which will decrease the effect produced by initial contact [2]. Hence, this strategy raises energy expenditure [2]. Therefore, the analysis of stance time is essential as an indicator of subjects' energy expenditure.  
The time from toe-off to the next heel strike, indicating as a passive movement with little metabolic cost, is known as swing time [60]. Current estimates for leg swing costs are equivocal, covering a range of between 10% and 33% of the net cost of walking [60]. Most of the changes were observed during the longer swing period of ACLR patients. As this speed rises, there is a general increase in activity that leads to an increase in muscle strength generation and ground reaction force [31]. Additionally, the increase in speed will decrease stance time, limiting the amount of time that the body needs to explore the various ranges of movement required to accomplish a task [61].  
Gait speed has been recognised as a significant factor in normal health condition [62]. Under normal conditions, a comfortable gait speed relates to the speed where the energy cost per unit of distance is minimised [63]. Achieving energy efficiency is dependent on joint mobility and muscle activity [64]. ACLR leads to increased energy use, which is usually accompanied by a compensatory drop in the gait speed. In addition, the motions may cause the centre of gravity to shift, which increases energy expenditure [65].  
Stride speed is another important indicator of knee joint health, as well as overall health [56; 66]. Slower stride speeds have been associated with poorer results on psychomotor tests, verbal fluency tests, and decreased cardiovascular health [67]. Stride speed has been used as a measure to predict survival rates in an elderly community [68; 69]. Stride speed is a good measure of energy, motor control, endurance, muscle function, and health status.  
The ACLR knees demonstrated prolonged double support phases and reduced single support phases in comparison to the healthy knees. This reflects the increased time that the ACLR knees tended to remain in flexion prior maximum extension in order to achieve a less sudden weight change. This adaptation mechanism has been also shown by an electromyography experiment, which demonstrated that ACLR patients had prolonged firing of biceps, femoris, and vastus medialis during the stance phase in comparison to the HG [70].