## 1 Introduction  
Manual wheelchair users (MWUs) with spinal cord injury (SCI) are dependent on their upper extremity for almost all activities of daily living (ADL). This includes shoulder loading activities, like wheelchair ambulation, pressure relief to avoid pressure injuries, transferring in or out of the wheelchair into a bed or onto a toilet, and transfer of the wheelchair into or out of a car, for instance [1, 2, 3]. Due to the high dependency on the upper extremity, there is a risk of shoulder overuse. MWUs with SCI frequently report shoulder problems like pain or pathology. In this population, the prevalence of musculoskeletal behavior pain ranges from 39% to 71% [4, 5]. MWUs with SCI have a six-times-higher likelihood to suffer from shoulder pain and a three-times-higher likelihood of developing rotator cuff tendon pathology than able-bodied controls [6, 7]. As most MWUs are dependent on their upper extremities for many aspects of daily life, shoulder health is also highly correlated with independence, social participation, and quality of life [8, 9, 10, 11]. Considering shoulder load from ADL, from these activities, only the magnitude of load on the shoulders is known, but neither the frequency nor duration of these activities is known in real life. If the magnitude, frequency, duration, or a combination thereof is too high, there is no room for rest or recovery, which could lead to fatigue and/or overload [12]. Wheelchair propulsion is a dominating activity in the life of a MWU, but numbers on actual daily wheelchair use are hardly available. Despite the increased availability of, and growing interest in, inertial measurement units (IMUs) to quantify wheelchair use in the past decades, methods for quantifying the daily use of a wheelchair are scarce, and comprehensive data on the MWUs' exposure to risk factors for shoulder problems are hardly available. Methods that quantify wheelchair use with IMUs can be grossly classified into the following domains:  
Activity: activity levels, activity classification, arm use [13; 14; 15; 16; 17; 18; 19];  
Energy expenditure [20; 21; 22; 23; 24; 25];  
Temporal wheelchair propulsion characteristics [26];  
Performance in sports settings [27; 28; 29; 30]. Single variables of wheelchair use include distance covered [31; 32; 33; 34; 35; 36], speed [31; 32; 35], number of pushes [13; 14; 26; 37; 38], turns [39], or duration of movement [35; 40; 41]. Furthermore, as turning a wheelchair can double the shoulder load [42], the number of turns and magnitude thereof are also an essential variable to consider when trying to estimate daily wheelchair ambulation, with respect to shoulder load. The same accounts for uphill wheelchair propulsion, which increases the load on the upper extremity [43; 44; 45]. This study focusses on wheelchair mobility metrics (WCMMs) from the perspective of shoulder problems. Measuring WCMMs can serve as a proxy for shoulder load for the most dominant activity in the daily life of MWUs. While measuring activity levels or energy expenditure can provide information about general activity, it cannot provide the same detailed and long-term information as WCMMs. The method, as described by [PERSON] et al. [46], is comprehensive in quantifying wheelchair mobility in the above-mentioned aspects, on metrics that relate to shoulder load. It is based on two IMUs, one on the wheelchair frame and one on the wheelchair wheel, from which a range of relevant mobility metrics can be derived from one measurement. Furthermore, combinations of variables appeared to be of particular clinical interest, as emerged during focus group discussions [47], like distance covered over a certain incline or average distance per bout of mobility (BoM) [41], to obtain more detailed insight into the daily mobility behavior of an individual. In the following examples, detailed WCMMs could be especially helpful.