Biomarkers for rapid H-reflex operant conditioning among females.

Operant conditioning of a spinal monosynaptic pathway using the Hoffman reflex (H-reflex) is well established in animal and human studies. There is a subset within the human population (∼20% nonresponders) who are unable to up train this pathway suggesting some distinct or unique identifying characteristics. Importantly, females, who have a nine times higher rate of injury during human performance activities than men, have been understudied in areas of CNS neuroplasticity. Our long-term goal is to understand if innate ability to rapidly up train the H-reflex is predictive of future performance-based injury among females. In this study, we primarily determined whether healthy, young females could rapidly increase the H-reflex within a single session of operant conditioning and secondarily determined if electro-physiological, humoral, cognitive, anthropometric, or anxiety biomarkers distinguished the responders from nonresponders. Eighteen females (mean age: 24) participated in the study. Overall, females showed a group main effect for up training the H-reflex (P < 0.05). Of the cohort, 10 of 18 females met the criteria for up training the H-reflex (responders). The responders showed lower levels of estradiol (P < 0.05). A multivariate stepwise regression model supported that extracellular to intracellular water ratio (ECW/ICW) and H-max/M-max ratio explained 60% of the variation in up training among females. These findings support that females can acutely upregulate the H-reflex with training and that electro-physiological and hormonal factors may be associated with the up training.NEW & NOTEWORTHY Young females who acutely increase their H-reflexes with operant conditioning had lower levels of estradiol. However, the best predictors of those who could up-train the H-reflex were baseline H-reflex excitability (H-max/M-max) and extracellular to intracellular water ratio (ECW/ICW). Future studies are warranted to understand the complex relationship between operant conditioning, human performance, and injury among active young females.

Influence of the Menstrual Cycle and Training on the Performance of a Perturbed Single-Leg Squatting Task in Female Collegiate Athletes.

Background: Anterior cruciate ligament (ACL) injuries often occur when an athlete experiences an unexpected disruption, or perturbation, during sports. ACL injury rates may also be influenced by the menstrual cycle. Purpose: To determine whether training adaptations to knee control and muscle activity during a perturbed single-leg squatting (SLS) task depend on menstrual cycle phase in female athletes. Study Design: Controlled laboratory study. Methods: A total of 21 healthy female collegiate athletes (current or former [<3 years]) who competed in 9 different sports performed an SLS task in which they attempted to match their knee position (user signal) to a target signal. The protocol consisted of a 9-condition pretest, 5 sets of 3 training trials, and a 9-condition posttest. One perturbation was delivered in each condition by altering the resistance of the device. Sagittal knee control (absolute error between the target signal and user signal) was assessed using a potentiometer. Muscle activity during perturbed squat cycles was normalized to maximal activation and to corresponding muscle activity during unperturbed squat cycles (%unperturbed) within the same test condition. Athletes performed the protocol during a distinct menstrual cycle phase (early follicular [EF], late follicular [LF], midluteal [ML]). Two-way mixed analysis of variance was used to determine the effects of the menstrual cycle and training on knee control and muscle activity during task performance. Venous blood was collected for hormonal analysis, and a series of health questionnaires and anthropometric measures were also assessed to determine differences among the menstrual cycle groups. Results: After training, athletes demonstrated better knee control during the perturbed squat cycles (lower absolute error, P < .001) and greater soleus feedback responses to the perturbation (%unperturbed, P = .035). Better knee control was demonstrated in the ML phase versus the EF phase during unperturbed and perturbed squat cycles (P < .039 for both). Quadriceps activation was greater in the ML phase compared with the EF and LF phases, both immediately before and after the perturbation (P < .001 for all). Conclusion: Athletes learned to improve knee control during the perturbed performance regardless of menstrual cycle phase. The best knee control and greatest quadriceps activation during the perturbed squatting task was found in the ML phase. Clinical Relevance: These findings may correspond to a lower incidence of ACL injury in the luteal phase and alterations in exercise performance across the menstrual cycle.

Low-frequency electrically induced exercise after spinal cord injury: Physiologic challenge to skeletal muscle and feasibility for long-term use.

CONTEXT: Skeletal muscle has traditionally been considered a "force generator": necessary for purposes of locomotion, but expendable for non-ambulators who use wheelchairs, such as people with a spinal cord injury (SCI). Active skeletal muscle plays an indispensable role in regulating systemic metabolic functions, even in people with paralysis, but because of severe osteoporosis, high tetanic muscle forces induced with high frequency electrical stimulation may be risky for some individuals. The purpose of this study was to compare the physiologic muscle properties incurred by two low force/low frequency repetitive stimulation protocols (1 and 3 Hz); and, to assess the acceptability of each protocol among people with SCI. METHODS: Ten individuals with chronic SCI (12.9 years) and 11 individuals without SCI (NonSCI) participated in the study. Participants received either 1 or 3 Hz stimulation to the quadriceps muscle on Day 1, then the converse on Day 2. Each session consisted of 1000 stimulus pulses. RESULTS: The initial and maximum forces were similar for the 1 and 3 Hz frequencies. The fatigue index (FI) for SCI and NonSCI groups were lower (P < 0.007) for 3 Hz than for 1 Hz (0.34 ± 0.17 versus 0.65 ± 0.16 and 0.72 ± 0.14 versus 0.87 ± 0.07, respectively). CONCLUSION: The 3 Hz stimulation offered the greatest physiological challenge and was perceived as more acceptable for long term use among people with SCI.

Exercise Prescription Principles among Physicians and Physical Therapists for Patients with Impaired Glucose Control: A Cross-Sectional Study.

Exercise confers a multitude of benefits with limited adverse side effects, making it a powerful "medication" for a plethora of diseases. In people living with uncontrolled glucose levels, exercise can be an effective "medication" to assist in the management of hyperglycemia. We sought to survey healthcare providers (physicians and physical therapists) to determine the current state of exercise recommendation for people with glucose control issues. Healthcare providers were surveyed from six academic medical centers in the Midwest to determine the recommended exercise parameters (type, frequency, duration, intensity, and timing) for patients with glucose control issues. Data from 209 practitioners who completed the survey were used for analysis. Chi-square tests were used to determine differences in exercise recommendations between physical therapists (PTs) and physicians (MD/DOs). PTs and MD/DOs recommended similar exercise parameters. Of all respondents, 78.9% recommended exercise to patients with glucose control issues. Respondents who considered themselves to be active exercisers were more likely to recommend exercise than those who were not exercisers. Only 6.1% of all respondents recommended post-meal exercise. Healthcare providers overwhelmingly recommended exercise for people with glucose control issues, but the "timing" is not congruent with best practice recommendations.

Impaired Glucose Tolerance and Visceral Adipose Tissue Thickness among Lean and Non-Lean People with and without Spinal Cord Injury.

After spinal cord injury (SCI), multiple adaptations occur that influence metabolic health and life quality. Prolonged sitting and inactivity predispose people with SCI to body composition changes, such as increased visceral adipose tissue (VAT) thickness, which is often associated with impaired glucose tolerance. Our goal is to understand whether VAT is an index of leanness, and, secondarily, whether mobility methods influence glucose tolerance for people living with SCI. A total of 15 people with SCI and 20 people without SCI had fasting oral glucose tolerance tests (OGTT) and VAT thickness (leanness) measured during a single session. Glucose was 51% and 67% greater for individuals with SCI relative to those without SCI after 60 and 120 min of an OGTT (p < 0.001). Glucose area under the curve (AUC) was 28%, 34%, and 60% higher for non-lean people with SCI than lean people with SCI and non-lean and lean people without SCI, respectively (p = 0.05, p = 0.009, p < 0.001). VAT was associated with glucose AUC (R2 = 0.23, p = 0.004). Taken together, these findings suggest that leanness, as estimated from VAT, may be an important consideration when developing rehabilitation programs to influence metabolism among people with SCI.

The Accuracy of Ankle Eccentric Torque Control Explains Dynamic Postural Control During the Y-Balance Test.

Background: The Y-Balance Test (YBT), especially the posteromedial (PM) reach direction (PM-YBT), is able to identify dynamic postural control deficits in those who have ankle instability. However, there still exists a need to understand how sensorimotor function at the ankle explains the performance during the PM-YBT. Hypothesis/Purpose: The purpose of this study was to determine whether the ability to accurately control eccentric ankle torque explained PM-YBT performance. It was hypothesized that eccentric dorsiflexion/plantarflexion torque control would be positively related to the maximum reach distance (MRD) of PM-YBT. Study Design: Cross-sectional study. Methods: Twelve healthy subjects performed the PM-YBT, maximum voluntary isometric contractions (MVIC) for both dorsiflexion and plantarflexion muscle strength, and then the torque control testing of the ankle. The torque control testing provided a target torque level on a screen in front of the subject and passive rotations of the ankle joint in the sagittal plane at 10 deg/sec between plantarflexion to dorsiflexion. Subjects were then instructed to eccentrically contract the dorsiflexors and plantar flexors to generate torque while the ankle joint rotated. The accuracy of torque control during eccentric dorsiflexion and plantarflexion by calculating absolute errors, the area between the target torque and the produced torque were evaluated. Tibialis anterior and soleus muscle activities were simultaneously recorded during testing. A step-wise linear regression model was used to determine the best model predicted the MRD of the PM-YBT (PM-MRD). Results: A step-wise linear regression developed a model explaining only eccentric dorsiflexion torque control predicted higher PM-MRD score (R2 = 44%, F1,10 = 7.94, ß = -0.67, p = 0.02). Conclusion: The accuracy of torque control during eccentric dorsiflexion predicts better performance in the PM-YBT. Level of Evidence: 3b.

Trunk Angle Modulates Feedforward and Feedback Control during Single-Limb Squatting.

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. 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 (p < 0.026) and gluteus medius long-latency reflexes (p < 0.001) and greater quadriceps-to-hamstrings co-contraction during feedforward (p = 0.017) and feedback (p = 0.007) 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 (R2: 0.47-0.97). 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.

Disrupted somatosensory input alters postural control strategies during the Star Excursion Balance Test (SEBT) in healthy people.

BACKGROUND: Chronic adaptations, including persistent sensorimotor deficits, remain in individuals with a history of ankle instability, resulting in altered postural control strategies during functional tasks such as gait, running, or landing. However, we do not know the contribution of the altered somatosensory input on postural control strategies during a dynamic balance task such as the Star Excursion Balance Test (SEBT). RESEARCH QUESTION: The purpose of this study was to characterize postural control strategies with and without disrupted somatosensory input during a dynamic balance task in people without chronic ankle sprain. METHODS: This study was a crossover study design. Twenty healthy young adults (10 men, 10 women; age = 23.9 ± 3.0 years, height = 174.2 ± 7.4 cm, mass = 71.2 ± 16.7 kg) performed the posteromedial reach test during the SEBT while standing on the ground and on foam. We measured the maximum reach distance (MRD); joint angles of the ankle, knee, hip, and trunk in the sagittal, frontal, and transverse planes; and position and displacement of the center of mass (COM) and center of pressure (COP) during the posteromedial reach task. RESULTS: The MRD was shorter when standing on the foam than on the ground (p < 0.001). There was a condition by phase interaction for ankle dorsiflexion; tibia internal rotation; and trunk flexion (p < 0.001; p = 0.03; p = 0.01, respectively). The COM and COP were positioned more laterally on the foam (p < 0.001). The COM and COP anterior-posterior displacements were more anterior during the foam condition (p = 0.017). SIGNIFICANCE: By using a foam pad to disrupt somatosensory information, participants demonstrated altered strategies to control the joint kinematics, COM, and COP, as a function of posteromedial distance. Ankle and trunk movement strategies may influence the posteromedial reach distance. This model may simulate changes that occur with chronic ankle instability.

Sex-dependent effects of genetic upregulation of activated protein C on delayed effects of acute radiation exposure in the mouse heart, small intestine, and skin.

Accidental exposure to ionizing radiation may lead to delayed effects of acute radiation exposure (DEARE) in many organ systems. Activated protein C (APC) is a known mitigator of the acute radiation syndrome. To examine the role of APC in DEARE, we used a transgenic mouse model with 2- to 3-fold increased plasma levels of APC (high in APC, APCHi). Male and female APCHi mice and wild-type littermates were exposed to 9.5 Gy γ-rays with their hind-legs (bone marrow) shielded from radiation to allow long-term survival. At 3 and 6 months after irradiation, cardiac function was measured with ultrasonography. At 3 months, radiation increased cardiac dimensions in APCHi males, while decreases were seen in wild-type females. At this early time point, APCHi mice of both sexes were more susceptible to radiation-induced changes in systolic function compared to wild-types. At 6 months, a decrease in systolic function was mainly seen in male mice of both genotypes. At 6 months, specimens of heart, small intestine and dorsal skin were collected for tissue analysis. Female APCHi mice showed the most severe radiation-induced deposition of cardiac collagens but were protected against a radiation-induced loss of microvascular density. Both male and female APCHi mice were protected against a radiation induced upregulation of toll-like receptor 4 in the heart, but this did not translate into a clear protection against immune cell infiltration. In the small intestine, the APCHi genotype had no effect on an increase in the number of myeloperoxidase positive cells (seen mostly in females) or an increase in the expression of T-cell marker CD2 (males). Lastly, both male and female APCHi mice were protected against radiation-induced epidermal thickening and increase in 3-nitrotyrosine positive keratinocytes. In conclusion, prolonged high levels of APC in a transgenic mouse model had little effects on indicators of DEARE in the heart, small intestine and skin, with some differential effects in male compared to female mice.