## 1 Introduction  
The anterior cruciate ligament (ACL) is a key stabilising ligament of the knee joint. It is vulnerable to injury amongst athletes involved in high-intensity sport activities, even though most injuries occur due to noncontact activities [1; 2]. The treatment options include ACL reconstruction (ACL-R) followed by intensive rehabilitation. The process of returning to activity after surgery is multifaceted, requiring individual rehabilitation protocols while considering recovery milestones. The main goal of current rehabilitation concepts is not only to restore function but also to provide specific training interventions to avoid functional valgus knee movement [3], which has been shown to be a relevant risk factor for reinjury [4; 5; 6; 7]. There are various approaches to evaluate and monitor knee function after injury. Traditionally, clinical tests such as ligament laxity measurements, subjective data acquisition using standardised patient-reported outcome measures (PROMs) [8] and a static timeframe of 9-12 months have been used as monitoring tools during rehabilitation. In order to better assess the biomechanical integrity, neuromuscular capacity and resilience of the knee joint, functional testing has become central to current rehabilitation protocols [9; 10; 11]. Several functional tests have been designed and introduced in recent years to identify deficits and asymmetries in movement patterns, joint function and body control after ACL-R with high validity [12]. However, there is still debate about how these tests should be combined in different patient cohorts, when they should be executed in the course of rehabilitation, and how they should be interpreted when making return-to-play decisions for athletes. These tests may include a single-leg vertical hop, a single-leg horizontal hop for distance [13; 14] and various modifications such as the triple jump test and the triple jump crossover test [9; 15; 16]. Each test targets specific functional domains and varies in complexity. Therefore, different environments, personnel and equipment may be required. The single-leg vertical hop test (SLVHT) has been recommended as a measure of functional performance due to its safety and feasibility and the possibility of implementation during early rehabilitation [17; 18]. The performance in vertical hop tests is usually quantified by video analysis (elevation of the centre of the body, e.g., pelvic bone) or by contact mats [17; 19; 20]. However, these methods are time-consuming, person-dependent, and material- and cost-intensive [17]. In recent years, wearable inertial measurement units (IMUs) (i.e., digital sensors) have been introduced as an alternative tool to monitor athletic performance [21; 22]. An IMU is a small portable device equipped with sensors that measures and records motion-related data based on movement biomechanics, joint kinetics and performance metrics such as height, width, angular velocity and acceleration. The compact size and wireless capabilities make it a versatile and practical tool in clinical and research settings. Only a few of these have been used in rehabilitation settings following injury or surgery and have been able to measure the hop height, width and sagittal plane angles reliably [23; 24; 25]. One of these IMU systems (OPD Corp., Valley, Germany) was validated and shown to be highly correlated with well-established measurement devices [26]. Using an associated mobile software application (Orthelligent, OPED Corp., Valley, Germany; used in the latest version; [[https://www.o-dhs.com/de/produkte-und-loesungen/orthelligent-pro](https://www.o-dhs.com/de/produkte-und-loesungen/orthelligent-pro)]([https://www.o-dhs.com/de/produkte-und-loesungen/orthelligent-pro](https://www.o-dhs.com/de/produkte-und-loesungen/orthelligent-pro))), data can be derived from the sensor, displayed to the user and stored for later application and comparison.