Date of Award

August 2023

Degree Type


Degree Name

Master of Science



First Advisor

Kristian M O'Connor

Committee Members

Stephen C Cobb, Kevin G Keenan


ACL Prevention, Athlete Performance, Injury Risk, Joint Coordination Variability, Knee Mechanics, Motor Variability


Injury to the anterior cruciate ligament (ACL) is a very common and debilitating injury suffered by athletes of all ages, genders, and abilities. Practitioners attempt to minimize risks by implementing ACL prevention programs designed to physically prepare athletes for the demands of sport, however, the success of these programs is very inconsistent. The majority of past ACL prevention programs prioritize constrained run and cut activities, the removal of motor variability - attempting to “idealize” mechanics - and limited task complexity. Due to the inherent complexities that exist within sport, it is possible that more task complexity and motor variability is necessary for the transference of training, preparation of athletes, and minimization of ACL injury risk. Therefore, the purpose of this study was to investigate the effects of a lower limb dual motor task and reactivity on joint coordination variability, knee joint mechanics, and knee joint variability during a side-step cutting task. 15 soccer athletes (7 males, 8 females; age 21.3 ± 1.8 years; mass: 70.1 ±14.4 kg; height: 1.7 ± .1 m), high school or higher level, were recruited to complete a run-to-cut task in three different conditions (CUT, KICK, and RXN). The CUT condition required subjects to perform a simple 45 degree run-to-cut. The KICK condition required subjects to do the same, but immediately pass a stationary soccer ball into a goal following the cut. The RXN condition required participants to pass a moving soccer ball into a goal. The soccer ball would be passed or faked to the subjects on random trials within this third condition. Three-dimensional kinematics were collected during the cutting stride. Initial contact (IC) angles and deceleration range of motions (ROM) were reported in all three planes at the hip, knee, and ankle. Vector coding was utilized to measure joint coordination variability. Repeated measure ANOVAs were run for all variables of interest to determine significant differences across conditions (p < .05). The CUT condition caused significantly different IC angles, relative to the KICK and RXN conditions, including greater hip flexion and greater hip internal rotation. Further, there was significantly greater knee sagittal plane and ankle frontal plane ROM during the deceleration phase. The KICK and RXN conditions produced significantly greater joint coordination variability, relative to the CUT condition, in two of the seven joint couplings tested. These two included 1) sagittal plane hip with frontal plane knee and 2) transverse plane hip with frontal plane knee. In general, motor behaviors emerged in the KICK and RXN conditions that were more in-line with those seen as mechanistic for ACL injury. Greater joint coordination variability in the KICK and RXN conditions could be due to the heightened complexity of the task, the external focus of attention the task elicited, and/or the greater perceptual-action demands. In conclusion, our findings indicate that motor behaviors become more consistent with ACL stress, and greater joint coordination variability is created within certain joint couplings, when shifting attentional focus externally. The addition of reactivity to an external focus of attention, as well as the lower limb dual task relative to past studies exploring upper limb dual tasks, did not change motor behaviors.

Included in

Biomechanics Commons