Motor unit modes in the calf muscles during a submaximal isometric contraction are changed by brief stretches.

The purpose of our study was to investigate the influence of a stretch intervention on the common modulation of discharge rate among motor units in the calf muscles during a submaximal isometric contraction. The current report comprises a computational analysis of a motor unit dataset that we published previously (Mazzo et al., 2021). Motor unit activity was recorded from the three main plantar flexor muscles while participants performed an isometric contraction at 10% of the maximal voluntary contraction force before and after each of two interventions. The interventions were a control task (standing balance) and static stretching of the plantar flexor muscles. A factorization analysis on the smoothed discharge rates of the motor units from all three muscles yielded three modes that were independent of the individual muscles. The composition of the modes was not changed by the standing-balance task, whereas the stretching exercise reduced the average correlation in the second mode and increased it in the third mode. A centroid analysis on the correlation values showed that most motor units were associated with two or three modes, which were presumed to indicate shared synaptic inputs. The percentage of motor units adjacent to the seven centroids changed after both interventions: Control intervention, mode 1 decreased and the shared mode 1 + 2 increased; stretch intervention, shared modes either decreased (1 + 2) or increased (1 + 3). These findings indicate that the neuromuscular adjustments during both interventions were sufficient to change the motor unit modes when the same task was performed after each intervention. KEY POINTS: Based on covariation of the discharge rates of motor units in the calf muscles during a submaximal isometric contraction, factor analysis was used to assign the correlated discharge trains to three motor unit modes. The motor unit modes were determined from the combined set of all identified motor units across the three muscles before and after each participant performed a control and a stretch intervention. The composition of the motor unit modes changed after the stretching exercise, but not after the control task (standing balance). A centroid analysis on the distribution of correlation values found that most motor units were associated with a shared centroid and this distribution, presumably reflecting shared synaptic input, changed after both interventions. Our results demonstrate how the distribution of multiple common synaptic inputs to the motor neurons innervating the plantar flexor muscles changes after a brief series of stretches.

Proton-Coupled Electron Transfer Mechanisms for CO2 Reduction to Methanol Catalyzed by Surface-Immobilized Cobalt Phthalocyanine.

Immobilized cobalt phthalocyanine (CoPc) is a highly promising architecture for the six-proton, six-electron reduction of CO2 to methanol. This electroreduction process relies on proton-coupled electron transfer (PCET) reactions that can occur by sequential or concerted mechanisms. Immobilization on a conductive support such as carbon nanotubes or graphitic flakes can fundamentally alter the PCET mechanisms. We use density functional theory (DFT) calculations of CoPc adsorbed on an explicit graphitic surface model to investigate intermediates in the electroreduction of CO2 to methanol. Our calculations show that the alignment of the CoPc and graphitic electronic states influences the reductive chemistry. These calculations also distinguish between charging the graphitic surface and reducing the CoPc and adsorbed intermediates as electrons are added to the system. This analysis allows us to identify the chemical transformations that are likely to be concerted PCET, defined for these systems as the mechanism in which protonation of a CO2 reduction intermediate is accompanied by electron abstraction from the graphitic surface to the adsorbate without thermodynamically stable intermediates. This work establishes a mechanistic pathway for methanol production that is consistent with experimental observations and provides fundamental insight into how immobilization of the CoPc impacts its CO2 reduction chemistry.

Progress toward the Total Synthesis of Nogalamycin Using a Benzyne Cycloaddition Strategy.

Nogalamycin (NOG) is a member of the anthracycline glycoside natural products; no total syntheses have yet been reported, and there is minimal understanding of how the aglycone substitution pattern and identities of the A- and D-ring sugars impact the anticancer activity and toxicity. This paper reports progress toward a modular approach to NOG that could enable systematic structure-activity relationship studies. Key steps include a regioselective benzyne cycloaddition and reductive ring-opening to assemble a versatile AB core for analogue synthesis.

The Proteasome and Ageing.

The proteasome is a multi-subunit proteolytic complex that functions to degrade normal proteins for physiological regulation and to eliminate abnormal proteins for cellular protection. Generally, the proteasome targets substrate proteins that are marked by attachment of multiple ubiquitin molecules. In various types of cells in an organism, damage to proteins occurs both from internal sources such as reactive oxygen species and from external ones such as UV radiation from the sun. The proteasome functions to protect the cells by degrading damaged proteins. With ageing, however, the capacity of the proteasome to degrade damaged proteins is reduced as indicated by evidence gathered by many studies. Studies on ageing in muscle, skin, and brain show that with age catalytic activity of the proteasome is decreased and the expression of proteasome subunits is altered. Age-related accumulation of damaged or misfolded proteins causes further reduction of proteasome activity. Abnormal proteins also accumulate as a result of age-related neurodegenerative diseases. Deficits in proteasome activity might be responsible for accumulation of protein aggregates and thus contribute to the pathology. Results from several studies suggest a link between the proteasome and longevity. This chapter reviews the various ways in which the proteasome is associated with the ageing process and examines evidence gathered from investigations on cultured cells, model organisms, and humans.

Staged Bilateral Total Knee Arthroplasty: Differing Results Comparing the First and Second Knees.

BACKGROUND: Patients often prefer one knee over the other following staged bilateral total knee arthroplasty (BTKA). Our study compared patient-reported outcomes scores of each knee following BTKA and identified factors that may contribute to the identified discrepancies. METHODS: All patients who underwent staged BTKA between July 2014 and August 2022 were identified. The patient-reported outcomes were collected preoperatively and at 2 weeks, 6 weeks, 1 year, and 2 years postoperatively. Each knee's results were compared using paired t-tests and McNemar tests. Preoperative Kellgren-Lawrence Grade (KLG), postoperative range of motion (ROM), reoperation rates, and manipulations under anesthesia (MUAs) were collected. Results were stratified based on time between TKAs (< 3 months, 3 to 12 months, 1 to 2 years, and > 2 years). RESULTS: There were 911 patients who underwent staged BTKA, with a mean 4.1-year follow-up. The ROM, patient satisfaction, MUAs, and reoperations were not significantly different between knees. Comparing the KLG of the first and second knees, 71% had the same KLG for both knees, 21% had a lower KLG, and 7% of the second knees had a higher KLG. The first knee had greater pain reduction (-10.6 at 2 weeks, -27.4 at 6 weeks) compared to the second (9.3 at 2 weeks, -8.1 at 6 weeks) (P < .0001) and better improvement in Knee Injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS JR) score (8.5 at 2 weeks, 16.9 at 6 weeks) compared to the second (-5.8 at 2 weeks, 5.0 at 6 weeks) (P < .0001). The 1-year outcomes between first and second knees, or recovery curves, were not different when stratifying by time between TKAs. CONCLUSIONS: The second knee in a staged BTKA has less delta improvement in KOOS JR and pain scores at early follow-up, likely due to higher starting KOOS JR and Patient-Reported Outcomes Measurement Information System scores, despite similar final patient satisfaction and clinical outcome measures. Lower KLG in the second total knee arthroplasty (TKA) may contribute to these findings. An MUA after the first TKA is highly predictive of an MUA after the second TKA.

Outpatient Total Joint Arthroplasty at a High-Volume Academic Center: An Analysis of Failure to Launch.

BACKGROUND: Unanticipated failure to discharge home (failure to launch, FTL) following scheduled same-day discharge (SDD) total joint arthroplasty (TJA) is problematic for the surgical facility with respect to staffing, care coordination, and reimbursement concerns. The aim of this study was to review rates, etiologies, and contributing factors for FTL in SDD TJA at an inpatient academic medical center. METHODS: All patients who underwent primary TJA between February 2021 and February 2023 were retrospectively reviewed. Of those scheduled for SDD, risk factors for FTL were compared with successful SDD. Readmission and emergency department (ED) visits were compared with historical cohorts. There were 3,093 consecutive primary joint arthroplasties performed, of which 2,411 (78%) were scheduled for SDD. RESULTS: Overall, SDD was successful in 94.2% (n = 2,272) of patients who had an FTL rate of 5.8%. Specifically, SDD was successful in 91.4% with total hip arthroplasty, 96.0% with total knee arthroplasty, and 98.6% with unicompartmental knee arthroplasty. Factors that significantly increased the risk of FTL included general anesthesia versus spinal anesthesia (P < .0001), later surgery start time (P < .0001), longer surgical time (P = .0043), higher estimated blood loss (P < .0001), women (P = .0102), younger age (P = .0079), and lower preoperative mental health patient-reported outcomes scores (P = .0039). Readmission and ED visit rates were not higher in the SDD group when compared to historical controls (P = .6830). CONCLUSIONS: With a comprehensive multidisciplinary approach dedicated to improving SDDs at an academic medical center, we have seen successful SDD in nearly 80% of primary TJA, with an FTL rate of 5.8%, and no increased risk of readmission or ED visits. Without adding many personnel, hospital recovery units, or other resources, simple interventions to help decrease FTL have included enhanced preoperative education and expectation settings, improved perioperative communications, reallocating personnel from the inpatient to the outpatient setting, the use of short-acting spinal anesthetics, and earlier scheduled surgery times.

Cervical spinal cord susceptibility-weighted MRI at 7T: Application to multiple sclerosis.

BACKGROUND: Susceptibility-weighted imaging (SWI) has been extensively studied in the brain and in diseases of the central nervous system such as multiple sclerosis (MS) providing unique opportunities to visualize cerebral vasculature and disease-related pathology, including the central vein sign (CVS) and paramagnetic rim lesions (PRLs). However, similar studies evaluating SWI in the spinal cord of patients with MS remain severely limited. PURPOSE: Based on our previous findings of enlarged spinal vessels in MS compared to healthy controls (HCs), we developed high-field SWI acquisition and processing methods for the cervical spinal cord with application in people with MS (pwMS) and HCs. Here, we demonstrate the vascular variability between the two cohorts and unique MS lesion features in the cervical cord. METHODS: In this retrospective, exploratory pilot study conducted between March 2021 and March 2022, we scanned 12 HCs and 9 pwMS using an optimized non-contrast 2D T2*-weighted gradient echo sequence at 7 tesla. The overall appearance of the white and gray matter as well as tissue vasculature were compared between the two cohorts and areas of MS pathology in the patient group were assessed using both the magnitude and processed SWI images. RESULTS: We show improved visibility of vessels and more pronounced gray and white matter contrast in the MS group compared to HCs, hypointensities surrounding the cord in the MS cohort, and identify signal changes indicative of the CVS and paramagnetic rims in 66 % of pwMS with cervical spinal lesions. CONCLUSION: In this first study of SWI at 7T in the human spinal cord, SWI holds promise in advancing our understanding of disease processes in the cervical cord in MS.

Neural correlates and effect of jealousy on cognitive flexibility in the female titi monkey (Plecturocebus cupreus).

Jealousy is a social emotion that manifests as behavioral reactions from an individual toward a threat to a valuable relationship. Monogamous species exhibit jealousy-type behaviors as an adaptive response to preserve the relationship. Jealousy is also a complex, negatively-valenced emotion which may include fear of loss, anxiety, suspiciousness, and anger. Negative emotion may impair cognitive processes such as cognitive flexibility, an ability important for coping with new situations. However, little is known about how complex social emotions influence cognitive flexibility. To understand the interaction between jealousy and cognitive flexibility, we examined the neural, physiological, and behavioral factors involved in jealousy and cognitive flexibility in female titi monkeys. We presented subjects with a jealousy provoking scenario, followed by a reversal learning task and a PET scan with a glucose-analog radiotracer. We found that female titi monkeys reacted to a jealousy provoking scenario with increased locomotor behavior and higher glucose uptake in the cerebellum; however, hormone measures and were not affected. As only two females demonstrated cognitive flexibility, the effects of jealousy were difficult to interpret. Locomotion behavior was also negatively correlated with glucose uptake in brain areas linked with motivation, sociality, and cognitive flexibility. Surprisingly, glucose uptake in the orbitofrontal cortex (OFC) was significantly decreased during jealousy scenarios, while uptake in the anterior cingulate cortex (ACC) was decreased during reversal tasks. Our findings suggest that the presence of an intruder produces less visible behavioral reactions in female titis than in males, while still reducing activity in the OFC.

Initial compatibility during a "Speed-Dating" test predicts postpairing affiliation in titi monkeys (Plecturocebus cupreus).

Behavioral compatibility plays a critical role in shaping how potential mates interact with and evaluate each other and whether they choose to pursue a relationship. Compatibility is especially important for mate choice and relationship quality in pair-bonding species that form long-term attachments between mates. Although this process has been studied in humans and birds, relatively few studies have investigated it in non-human primates. In this study, we investigated whether pairing titi monkeys (Plecturocebus cupreus) based on initial compatibility increased postpairing affiliation between mates. Subjects were 12 unpaired adult titi monkeys (two cohorts of three males and three females). We determined each subject's initial interest in each opposite-sex potential mate in their cohort across a series of six 30-min interaction periods (i.e., "speed-dates"). To determine initial compatibility, we used the Social Relations Model to calculate relationship effects in initial interest (how much each subject uniquely preferred each potential mate beyond their own affiliative disposition and their partner's popularity). We then paired monkeys in a way that maximized net relationship effects between pairs, and measured longitudinal pair affiliation (Proximity, Contact, Tail Twining, and Combined Affiliation) for 6 months postpairing using daily scan-sample observations and monthly home-cage video recordings. Multilevel models showed that, on average, the six speed-dating pairs exhibited higher levels of Tail Twining (determined from scan-sample observations; ß = 0.31) compared to a group of 13 age-matched colony pairs that were determined quasi-randomly without quantifying compatibility. The degree of initial compatibility within speed-dating pairs also predicted higher levels of Combined Affiliation (determined from video recordings) at earlier post-pairing time points, with the association peaking at 2 months postpairing (ß = 0.57). These findings suggest that initial compatibility facilitates pair bonding in titi monkeys. We conclude by discussing how the speed-dating design can be used for colony management to inform pair-housing decisions.

Pilot Collection and Evaluation of Head Kinematics in Stock Car Racing.

The goal of this work was to collect on-track driver head kinematics using instrumented mouthpieces and characterize environmental exposure to accelerations and vibrations. Six NASCAR drivers were instrumented with custom-fit mouthpieces to collect head kinematic data. Devices were deployed at four tracks during practice and testing environments and configured to collect approximately 11 min of linear acceleration and rotational velocity data at 200 Hz. This continuous data collection, combined with film review, allowed extraction of complete laps of data. In addition to typical data processing methods, a moving-point average was calculated and subtracted from the overall signal for both linear acceleration and rotational velocity to determine the environmental component of head motion. The current analysis focuses on 42 full laps of data collected at four data collection events. The number of laps per track ranged from 2 to 23. Linear acceleration magnitudes for all 42 laps ranged from 2.46 to 7.48 g and rotational velocity ranged from 1.25 to 3.35 rad/s. After subtracting the moving average, linear acceleration ranged from 0.92 to 5.45 g and rotational velocity ranged from 0.57 to 2.05 rad/s. This study has established the feasibility of using an instrumented mouthpiece to measure head kinematics in NASCAR and presented a technique for isolating head motion due to cornering acceleration from those due to short-term perturbations experienced by the driver.

Reasons and Risk Factors for Failed Same-Day Discharge After Total Joint Arthroplasty.

BACKGROUND: A shift toward same-day discharge (SDD) in primary elective total knee arthroplasty (TKA) and total hip arthroplasty (THA) has created a need to optimize patient selection and improve same-day recovery pathways. The objectives of this study were (1) to identify our institution's most common causes for failed SDD, and (2) to evaluate risk factors associated with failed SDD. METHODS: A retrospective review of SDD patients undergoing primary TKA or THA from January 2021 to September 2022 was conducted. Reasons for SDD failure were recorded and differences between successful and failed SDD cases were assessed via a multivariate logistic regression. RESULTS: Overall, 85.3% (651 of 753) of patients included were successful SDDs. Failed SDD occurred in 16.8% (74 of 441) of TKA and 11.8% (38 of 322) of THA cases. Primary reasons included failure to clear physical therapy (33.0%, 37 of 112), postoperative hypotension (20.5%, 23 of 112), and urinary retention (16.9%, 19 of 112). Analysis revealed that overall failed SDD cases were more likely to have had prior opioid use and a longer surgical time. Failed TKA SDD cases were more likely to have had a longer surgical time and not have receive a preoperative nerve block, while failed THA SDD cases were more likely to be older. CONCLUSIONS: The SDD selection criteria and pathways continue to evolve, with multiple factors contributing to failed SDD. Improving patient selection algorithms and optimizing post-operative pathways can enhance the ability to successfully choose SDD candidates. LEVEL OF EVIDENCE: III.

Head Impact Exposure in Female Collegiate Soccer by Activity Type.

Soccer, one of the most popular sports in the world, has one of the highest rates of sports-related concussions. Additionally, soccer players are frequently exposed to nonconcussive impacts from intentionally heading the ball, a fundamental component of the sport. There have been many studies on head impact exposure in soccer, but few focus on soccer practices or practice activities. This study aimed to characterize the frequency and magnitude of head impacts in National Collegiate Athletic Association Division I female soccer practice activities using a custom-fit instrumented mouthpiece. Sixteen players were instrumented over the course of 54 practice sessions. Video analysis was performed to verify all mouthpiece-recorded events and classify practice activities. Category groupings of practice activities include technical training, team interaction, set pieces, position-specific, and other. Differences in head impact rates and peak resultant kinematics were observed across activity types and category groupings. Technical training had the highest impact rate compared to other category groupings. Impacts occurring during set piece activities had the highest mean kinematic values. Understanding drill exposure can help inform coaches on training plans aimed to reduce head impact exposure for their athletes.

Characterization of head impact exposure in boys' youth ice hockey.

Ice hockey has one of the highest concussion rates among youth sports. Sensor technology has been implemented in contact and collision sports to inform the frequency and severity of head impacts experienced on-ice. However, existing studies have utilized helmet-mounted sensors with limited accuracy. The objective of this study was to characterize head kinematics of contact events in a sample of youth boys' hockey players using a validated instrumented mouthpiece with improved accuracy. Head kinematics from 892 video-verified events were recorded from 18 athletes across 127 sessions. Median peak resultant linear acceleration, rotational velocity, and rotational acceleration of video-verified events were 7.4 g, 7.7 rad/s, and 576 rad/s2, respectively. Contact events occurred at a higher rate in games (2.48 per game) than practices (1.30 per practice). Scenarios involving head contact had higher peak kinematics than those without head contact. This study improves our understanding of head kinematics in boys' youth hockey.

Pd-Catalyzed Heck-Type Reactions of Allenes for Stereoselective Syntheses of Substituted 1,3-Dienes.

A highly stereoselective Pd-catalyzed Heck-type reaction of allenes in which the stereochemistry of both olefins is set simultaneously was developed. The ligand CyJohnPhos was crucial to achieving stereoselectivity, while minimizing isomerization of the starting material through hydropalladation. The stereodetermining factors were proposed to be A1,3 strain between the catalyst and allene substituent, which influences the σ-π-σ equilibrium of the coupled allene intermediate, as well as eclipsing interactions of R groups in the ß-hydride elimination. Good functional group tolerance and stereoselectivities for formation of the Z,E isomer were demonstrated. The methodology was further expanded to include the regioselective formation of 2,4-dienoates and 2,4-dienamides with a variety of substitution patterns, albeit in reduced stereoselectivities favoring the E,E isomer. A plausible mechanism is proposed to account for the observed selectivities and substituent effects.

Long term effects of chronic intranasal oxytocin on adult pair bonding behavior and brain glucose uptake in titi monkeys (Plecturocebus cupreus).

Intranasal oxytocin (IN OXT) administration has been proposed as a pharmacological treatment for a range of biomedical conditions including neurodevelopmental disorders. However, studies evaluating the potential long-lasting effects of chronic IN OXT during development are still scarce. Here we conducted a follow-up study of a cohort of adult titi monkeys that received intranasal oxytocin 0.8 IU/kg (n = 15) or saline (n = 14) daily for six months during their juvenile period (12 to 18 months of age), with the goal of evaluating the potential long-lasting behavioral and neural effects one year post-treatment. Subjects were paired with an opposite-sex mate at 30 months of age (one year post-treatment). We examined pair affiliative behavior in the home cage during the first four months and tested for behavioral components of pair bonding at one week and four months post-pairing. We assessed long-term changes in brain glucose uptake using 18FDG positron emission tomography (PET) scans. Our results showed that OXT-treated animals were more affiliative across a number of measures, including tail twining, compared to SAL treated subjects (tail twining is considered the "highest" type of affiliation in titi monkeys). Neuroimaging showed no treatment differences in glucose uptake between SAL and OXT-treated animals; however, females showed higher glucose uptake in whole brain at 23 months, and in both the whole brain and the social salience network at 33 months of age compared to males. Our results suggest that chronic IN OXT administration during development can have long-term effects on adult social behavior.

Anatomical self-efficacy of undergraduate students improves during a fully online biology course with at-home dissections.

Student enrollments in online college courses have grown steadily over the past decade, and college administrators expect this trend to continue or accelerate. Despite the growing popularity of online education, one major critique in the sciences is that students are not trained in the hands-on skills they may need for the workforce, graduate school, or professional school. For example, the Association of American Medical Colleges has recommended that medical schools evaluate applicants on their motor skills and observation skills, yet many online biology programs do not offer opportunities for students to develop these skills. In on-campus biology programs, students commonly develop these skills through hands-on animal dissections, but educators have struggled with how to teach dissections in an online environment. We designed a fully online undergraduate biology course that includes at-home, hands-on dissections of eight vertebrate specimens. Over three course offerings, we evaluated changes in four student outcomes: anatomical self-efficacy, confidence in laboratory skills, perceptions of support, and concerns about dissections. Here, we describe how we implemented at-home dissections in the online course and show that students taking the course gained anatomical self-efficacy and confidence in multiple laboratory skills. Based on open-ended responses, the students perceived that their experiences with the at-home dissections facilitated these gains. These results demonstrate that at-home, hands-on laboratories are a viable approach for teaching practical skills to students in fully online courses. We encourage science instructors to introduce at-home laboratories into their online courses, and we provide recommendations for instructors interested in implementing at-home laboratories.

Head Impact Kinematics and Brain Deformation in Paired Opposing Youth Football Players.

Head impact exposure is often quantified using peak resultant kinematics. While kinematics describes the inertial response of the brain to impact, they do not fully capture the dynamic brain response. Strain, a measure of the tissue-level response of the brain, may be a better predictor of injury. In this study, kinematic and strain metrics were compared to contact characteristics in youth football. Players on 2 opposing teams were instrumented with head impact sensors to record impact kinematics. Video was collected to identify contact scenarios involving opposing instrumented players (ie, paired contact scenarios) and code contact characteristics (eg, player role, impact location). A previously validated, high-resolution brain finite element model, the atlas-based brain model, was used to simulate head impacts and calculate strain metrics. Fifty-two paired contact scenarios (n = 105 impacts) were evaluated. Lighter players tended to have greater biomechanical metrics compared to heavier players. Impacts to the top of the helmet were associated with lower strain metrics. Overall, strain was better correlated with rotational kinematics, suggesting these metrics may be better predictors of the tissue-level brain response than linear kinematics. Understanding the effect of contact characteristics on brain strain will inform future efforts to improve sport safety.

Head Kinematics in Youth Ice Hockey by Player Speed and Impact Direction.

Hockey is a fast-paced sport known for body checking, or intentional collisions used to separate opponents from the puck. Exposure to these impacts is concerning, as evidence suggests head impact exposure (HIE), even if noninjurious, can cause long-term brain changes. Currently, there is limited understanding of the effect of impact direction and collision speed on HIE. Video analysis was used to determine speed and direction for 162 collisions from 13 youth athletes. These data were paired with head kinematic data collected with an instrumented mouthpiece. Relationships between peak resultant head kinematics and speeds were evaluated with linear regression. Mean athlete speeds and relative velocity between athletes ranged from 2.05 to 2.76 m/s. Mean peak resultant linear acceleration, rotational velocity, and rotational acceleration were 13.1 g, 10.5 rad/s, and 1112 rad/s2, respectively. Significant relationships between speeds and head kinematics emerged when stratified by contact characteristics. HIE also varied by direction of collision; most collisions occurred in the forward-oblique (ie, offset from center) direction; frontal collisions had the greatest magnitude peak kinematics. These findings indicate that HIE in youth hockey is influenced by speed and direction of impact. This study may inform future strategies to reduce the severity of HIE in hockey.

Characterization of Head Impact Exposure in Women's Collegiate Soccer.

Soccer players are regularly exposed to head impacts by intentionally heading the ball. Evidence suggests repetitive subconcussive head impacts may affect the brain, and females may be more vulnerable to brain injury than males. This study aimed to characterize head impact exposure among National Collegiate Athletic Association women's soccer players using a previously validated mouthpiece-based sensor. Sixteen players were instrumented during 72 practices and 24 games. Head impact rate and rate of risk-weighted cumulative exposure were compared across session type and player position. Head kinematics were compared across session type, impact type, player position, impact location, and ball delivery method. Players experienced a mean (95% confidence interval) head impact rate of 0.468 (0.289 to 0.647) head impacts per hour, and exposure rates varied by session type and player position. Headers accounted for 89% of head impacts and were associated with higher linear accelerations and rotational accelerations than nonheader impacts. Headers in which the ball was delivered by a long kick had greater peak kinematics (all P < .001) than headers in which the ball was delivered by any other method. Results provide increased understanding of head impact frequency and magnitude in women's collegiate soccer and may help inform efforts to prevent brain injury.

Changes in neural drive to calf muscles during steady submaximal contractions after repeated static stretches.

KEY POINTS: Repeated static-stretching interventions consistently increase the range of motion about a joint and decrease total joint stiffness, but findings on the changes in muscle and connective-tissue properties are mixed. The influence of these stretch-induced changes on muscle function at submaximal forces is unknown. To address this gap in knowledge, the changes in neural drive to the plantar flexor muscles after a static-stretch intervention were estimated. Neural drive to the plantar flexor muscles during a low-force contraction increased after repeated static stretches. These findings suggest that adjustments in motor unit activity are necessary at low forces to accommodate reductions in the force-generating and transmission capabilities of the muscle-tendon unit after repeated static stretches of the calf muscles. ABSTRACT: Static stretching decreases stiffness about a joint, but its influence on muscle-tendon unit function and muscle activation is unclear. We investigated the influence of three static stretches on changes in neural drive to the plantar flexor muscles, both after a stretch intervention and after a set of maximal voluntary contractions (MVCs). Estimates of neural drive were obtained during submaximal isometric contractions by decomposing high-density electromyographic signals into the activity of individual motor units from medial gastrocnemius, lateral gastrocnemius and soleus. Motor units were matched across contractions and an estimate of neural drive to the plantar flexors was calculated by normalizing the cumulative spike train to the number of active motor units (normalized neural drive). Mean discharge rate increased after the stretch intervention during the 10% MVC task for all recorded motor units and those matched across conditions (all, P = 0.0046; matched only, P = 0.002), recruitment threshold decreased for motor units matched across contractions (P = 0.022), and discharge rate at recruitment was elevated (P = 0.004). Similarly, the estimate of normalized neural drive was significantly greater after the stretch intervention at 10% MVC torque (P = 0.029), but not at 35% MVC torque. The adjustments in motor unit activity required to complete the 10% MVC task after stretch may have been partially attenuated by a set of plantar flexor MVCs. The increase in neural drive required to produce low plantar-flexion torques after repeated static stretches of the calf muscles suggests stretch-induced changes in muscle and connective tissue properties.