# Single-Leg Squat Performance and Reported Pain within Youth Softball Players
## 2 Materials and Methods  
Forty-two youth softball athletes (13.0 \(\pm\) 2.0 years; 162.2 \(\pm\) 9.7 cm; 60.8 \(\pm\) 14.3 kg; 5.4 \(\pm\) 0.5 years of experience) from the South East United States volunteered to participate. Participants were active softball players with a minimum of two years playing at a competitive level and all were right-handed. All participants considered softball as their primary sport and had no injuries or surgery within the past six months. Institutional Review Board of the University approved all testing protocols. Testing procedures were verbally outlined and explained, and informed written consent and assent were obtained from each participant and parent/guardian prior to any data collection. Kinematic data were collected at 100 Hz using an electromagnetic tracking system (trackSTARTM, Ascension Technologies, Inc., Burlington, VT, USA) synced with The MotionMonitor (Innovative Sports Training, Chicago, IL, USA). Thirteen sensors were attached to the following landmarks: (1) posterior aspect of the torso at the first thoracic vertebrae (T1) spinous process; (2) posterior aspect of the pelvis at the first sacral vertebrae (S1); (3-4) flat, broad portion of the acrom on the bilateral scapula; (5-6) lateral aspect of bilateral upper arm at the deltoid tuberosity; (7-8) posterior aspect of bilateral distal forearm, centered between the radial and ulnar styloid processes; (9-10) lateral aspect of bilateral upper leg, centered between the greater trochanter and the lateral condy of the knee; (10-11) lateral aspect of bilateral lower leg, centered between the head of the fibula and lateral malleolus; (12) posterior aspect of the dominate hand below the second phalanx; and (13) superior aspect of the foot, midway between the metatarsophalangeal and taloclaneonavicular joint [9,10,14]. A fourteenth, moveable sensor was attached to a plastic stylus and used for the digitization of bony landmarks. Using the digitized joint centers for ankle, knee, hip, shoulder, T12-L1, and C7-T1, a link segment model was developed. Joint centers were determined by digitizing the medial and lateral aspect of a joint then calculating the midpoint between those two points [19]. The ankle and knee joints were defined as the midpoint between the digitized medial and lateral malleoli and medial and lateral femoral condyles, respectively, whereas the spinal column was defined as the digitized space between C7-T1 and T12-L1. A rotation method, validated as capable of providing accurate positional data, was utilized to estimate the hip joint centers [20]. Hip joint centers were calculated from the rotation of the femur relative to the pelvis. Raw data regarding sensor position and orientation were transformed to locally based coordinate systems. For the world axis, the Y-axis represented the vertical direction; anterior to the Y-axis and in the direction of movement was the positive X-axis; and orthogonal and to the right of X and Y was the positive Z-axis. After sensor attachment and digitization, an investigator verbally explained the SLS and instructed the participant to \"squat down as far as you can and stand back up.\" The investigator then demonstrated an acceptable and unacceptable trial for each participant. A trial was deemed acceptable if the participant squatted down as far as possible and then rose to their initial position without the raised leg contacting the ground or stance leg [9,10,14]. Prior to testing, participants were allowed to practice the SLS. Following practice, participants performed the SLS once on each leg. Following completion, the SLS participants completed a health history questionnaire and were asked \"Do you currently have any pain/discomfort?\" to which they answered either \"yes\" (n = 13; 13.5 \(\pm\) 2.2 years; 163.0 \(\pm\) 11.7 cm; 62.5 \(\pm\) 17.3 kg; 5.6 \(\pm\) 2.2 years of experience) or \"no\" (n = 29; 12.8 \(\pm\) 1.9 years; 161.8 \(\pm\) 8.7 cm; 60.0 \(\pm\) 12.6 kg; 5.3 \(\pm\) 1.4 years of experience).