Mechanical, neural, and hormonal influences on neural regulation, plasticity, and precision in movement control
Creators: name: Johnson, Kristin A.
Publisher: name: University of Iowa
Year: 2022
Subjects: subject: Electrical Stimulation
subject: Female
subject: Motor Control
subject: Perturbation
subject: Trunk
subject: public abstract
subject: Hormones
subject: Physiology
subject: FOS: Biological sciences

description: The ability to generate and control movement involves the coordination of multiple body systems. Consequently, there are several factors that may contribute to an injury caused by poor movement control. These types of injuries are common during sport due to the increased potential for tissue overload and the unpredictable environment. One of the most detrimental motor control injuries to an athlete's career and health is an anterior cruciate ligament (ACL) rupture. Most ACL ruptures are non-contact injuries meaning they do not arise from external forces directly contacting the knee. This implicates the athlete’s own movement strategy in the cause of rupture. At the time of ACL injuries athletes are commonly decelerating on a single leg (landing, cutting) with their trunk in an upright position, the knee minimally flexed, and the foot flat on the ground. The effect of these biomechanical positions on joint forces have been the focus of many research investigations and injury prevention programs have shown some success in mitigating injury by training athletes to avoid such positions. However, ACL injury rates continue to rise across several sports perhaps due to little knowledge about another common condition at the time of non-contact ACL rupture, perturbed performance. Accordingly, the central theme of this dissertation is to contribute new knowledge about the influence of mechanical (trunk position), neural (e-sim, operant conditioning), and hormonal (operant conditioning, menstrual cycle) factors on motor control strategies during perturbed performance.
description: Trunk positioning and unexpected perturbations are high-risk conditions at the time of anterior cruciate ligament injury. The influence of trunk positioning on motor control responses to perturbation during dynamic performance is not known. Therefore, we tested the influence of trunk position on feedforward and feedback control during unexpected perturbations while performing a novel single-limb squatting task. We also assessed the degree that feedforward control was predictive of feedback responses. In the flexed trunk condition, there were increased quadriceps and gluteus medius long-latency reflexes and greater quadriceps-to-hamstrings co-contraction during feedforward and feedback time bins. Soleus long-latency reflexes increased more than 100% from feedforward muscle activity regardless of trunk condition. Feedforward muscle activity differentially predicted long-latency reflex responses depending on the muscle. These findings support the concept that trunk positioning influences motor control responses to perturbation and that feedback responses may be invariant to the feedforward control strategy.
description: FMRI studies support that neuromuscular electrical stimulation can modulate the excitability of the somatosensory cortex. Here, we studied whether training and electrical stimulation of the quadriceps would enhance learning of a perturbed weight-bearing task. We showed that a healthy, mixed sex cohort improved precision knee control during perturbed performance by training. Furthermore, we demonstrated that electrical stimulation triggered additional learning, beyond training, during the time of the trans-cortical reflex response to the perturbation.
description: It is consistently shown that females suffer higher rates of ACL injuries than males. Females also suffer more ACL injuries in the follicular phase compared to the luteal phase of the menstrual cycle. These findings support a hormonal contribution to ACL injury yet hormonal etiology is unknown. When females are in a static position and a passive state, sex hormones are shown to alter central nervous system excitability. The question remains as to whether sex hormones influence females’ capacity to actively regulate CNS pathways inherent to movement control. Accordingly, we used an operant conditioning protocol to study females’ capacity to upregulate the soleus H-reflex. We showed that most females learned to increase the H-reflex during operant conditioning and successful learners had lower levels of estradiol than those who were unsuccessful. These findings support hormonal characteristics are distinct among females capable of rapidly upregulating the H-reflex.
description: The menstrual cycle has been shown to influence female athlete exercise performance and injury rates. Recent work among non-athletic females supports that the menstrual cycle influences motor learning. However, the influence of the menstrual cycle on female athletes’ ability to learn to respond to perturbed performance is not known. In this work, we demonstrate that female collegiate athletes’ ability to learn to improve precision knee control during perturbed performance did not depend on their menstrual cycle phase. We also showed that athletes in the mid-luteal phase performed better and activated quadriceps to a greater extent than those in the early follicular phase.
description: Collectively, this research provides new insights and physiological grounding for future longitudinal work to determine the influence of several factors on motor control injuries.