## 1 Introduction  
According to a 2016/17 season's UEFA Elite Club Injury Study Report, most injuries in professional football occur in the thigh and knee area [1]. The report further shows that the most common type of injury is muscle rupture, strain, or cramps, representing 45% of all injuries in professional football [1]. Muscle strains in hamstrings accounts for 12-16% of all injuries among football players and have high recurrence rates (14 to 62%) [2; 3; 4]. Among professional football clubs, a typical hamstring injury will result in an average of 17 days of absence from training and matches with an average cost of EUR 16.666 per day that the player is unavailable to the team [5].  
Several modifiable risk factors for muscle injuries have been identified, including fatigue, high-speed running loads, overall muscle strength, and inter- and intralimb asymmetry [6].  
Asymmetry in both the inter- and intralimb has been proposed to increase the risk of sustaining a hamstring strain injury [6]. Interlimb asymmetry has typically been assessed using the bilateral Nordic Hamstring Exercise, but according to [PERSON] et al., this exercise is not necessarily the best way to evaluate hamstring strength and interlimb imbalances [7]. The authors stated that in bilateral exercises, a compensatory strategy is typically adopted and can shift between test sessions. One way of minimizing the risk of compensatory strategies is using a unilateral exercise as an alternative. Furthermore, regarding the intralimb asymmetry, the difference between the quadriceps and hamstrings strength (H/Q ratio) has been identified as particularly important [8; 9]. Since the assessment of muscle strength is seen as a key risk factor for preventing injuries in the thigh area, objective measurements with high reliability and validity are essential [10]. Currently, stationary isokinetic dynamometers (ID) have proven to be both reliable and valid and are considered the \"golden standard\" [11]. However, IDs are expensive, time- and space-consuming, and often difficult to operate, warranting further development and investigation of more practically applicable measurement methods.  
Previous investigations have evaluated the reliability and validity to measure peak force in different joints less expensive and with the use of portable measuring equipment, such as field-based tests [12] and handheld dynamometers [10; 13; 14; 15]. Handheld dynamometers are generally known to be a reliable measure of muscle strength; however, a systematic review highlights that concurrent validity for handheld dynamometers varies in terms of which joint is measured [13]. Correlations between handheld dynamometers and ID have been found to be very high (ICC from 0.73 to 0.98) for hip inductors/adductors and hip flexors/extensors [16; 17] and moderate (ICC from 0.37 to 0.91) for ankle plantar/dorsiflexion and knee flexors/extensors [16; 17; 18; 19; 20]. Even though handheld dynamometers are less expensive and portable, they still require a strong and experienced practitioner to obtain reliable and valid measures [21].  
Recently, a novel device called an H-station (FysioMeter, Aalborg, Denmark) has been developed with the purpose of monitoring athletes' quadriceps and hamstrings strength, using two Nintendo Wii balance boards (WBB). The equipment is cheaper, easier to use, and portable compared to the ID and may therefore be more useful for in-season monitoring of players. However, no studies have investigated the between-session reliability and concurrent validity of the H-station compared to the ID and its potential to quantify the H/Q ratio.  
Therefore, the purposes of this study were: (a) to determine between-session reliability of the H-station when measuring isometric quadriceps and hamstrings strength and H/Q ratio, and (b) to determine concurrent validity of these measures when comparing to a golden standard ID (Humac NORM, CSMi, Stoughton, MA, USA).