Samuel T. Johnson

Spinal reflex excitability and homosynaptic depression after a bout of whole‐body vibration

Although whole‐body vibration (WBV) affects neuromuscular performance, it remains unclear whether the effects are due to spinal reflex potentiation or inhibition, or differ between muscle groups. This study aimed to identify the effect of WBV on measures of spinal reflex excitability (H‐reflex) and homosynaptic depression (HD) in the soleus (SOL) and medial gastrocnemius (MG) muscles. H‐reflex and HD measurements were made in the SOL and MG muscle of 20 participants before and after a bout of WBV. H‐reflex and HD were measured every 15 seconds for 10 minutes post‐WBV and averaged at 1‐minute increments. H‐reflex amplitude was depressed for the first minute post‐vibration, whereas the effect of HD was reduced for the first 2 minutes post‐vibration. WBV significantly decreases spinal reflex excitability and HD, but it does so transiently and independent of muscle group. Muscle Nerve 43: 259–262, 2011

Spinal and Supraspinal Motor Control Predictors of Rate of Torque Development

During explosive movements and potentially injurious situations, the ability to rapidly generate torque is critical. Previous research has suggested that different phases of rate of torque development (RTD) are differentiately controlled. However, the extent to which supraspinal and spinal mechanisms predict RTD at different time intervals is unknown. RTD of the plantarflexors across various phases of contraction (i.e., 0–25, 0–50, 0–100, 0–150, 0–200, and 0–250 ms) was measured in 37 participants. The following predictor variables were also measured: (a) gain of the resting soleus H‐reflex recruitment curve; (b) gain of the resting homonymous post‐activation depression recruitment curve; (c) gain of the GABAergic presynaptic inhibition recruitment curve; (d) the level of postsynaptic recurrent inhibition at rest; (e) level of supraspinal drive assessed by measuring V waves; and (f) the gain of the resting soleus M wave. Stepwise regression analyses were used to determine which variables significantly predicted allometrically scaled RTD. The analyses indicated that supraspinal drive was the dominant predictor of RTD across all phases. Additionally, recurrent inhibition predicted RTD in all of the time intervals except 0–150 ms. These results demonstrate the importance of supraspinal drive and recurrent inhibition to RTD.

Relationships between explosive and maximal triple extensor muscle performance and vertical jump height.

Abstract Chang, E, Norcross, MF, Johnson, ST, Kitagawa, T, and Hoffman, M. Relationships between explosive and maximal triple extensor muscle performance and vertical jump height. J Strength Cond Res 29(2): 545–551, 2015—The purpose of this study was to examine the relationships between maximum vertical jump height and (a) rate of torque development (RTD) calculated during 2 time intervals, 0–50 milliseconds (RTD50) and 0–200 milliseconds (RTD200) after torque onset and (b) peak torque (PT) for each of the triple extensor muscle groups. Thirty recreationally active individuals performed maximal isometric voluntary contractions (MVIC) of the hip, knee and ankle extensors, and a countermovement vertical jump. Rate of torque development was calculated from 0 to 50 (RTD50) and 0 to 200 (RTD200) milliseconds after the onset of joint torque. Peak torque was identified and defined as the maximum torque value during each MVIC trial. Greater vertical jump height was associated with greater knee and ankle extension RTD50, RTD200, and PT (p ⩽ 0.05). However, hip extension RTD50, RTD200, and PT were not significantly related to maximal vertical jump height (p > 0.05). The results indicate that 47.6 and 32.5% of the variability in vertical jump height was explained by knee and ankle extensor RTD50, respectively. Knee and ankle extensor RTD50 also seemed to be more closely related to vertical jump performance than RTD200 (knee extensor: 28.1% and ankle extensor: 28.1%) and PT (knee extensor: 31.4% and ankle extensor: 13.7%). Overall, these results suggest that training specifically targeted to improve knee and ankle extension RTD, especially during the early phases of muscle contraction, may be effective for increasing maximal vertical jump performance.

Greater Hip Extension but Not Hip Abduction Explosive Strength Is Associated With Lesser Hip Adduction and Knee Valgus Motion During a Single-Leg Jump-Cut

Background: The relationships between hip abductor and extensor strength and frontal plane hip and knee motions that are associated with anterior cruciate ligament injury risk are equivocal. However, previous research on these relationships has evaluated relatively low-level movement tasks and peak torque rather than a time-critical strength measure such as the rate of torque development (RTD). Hypothesis: Females with greater hip abduction and extension RTD would exhibit lesser frontal plane hip and knee motion during a single-leg jump-cutting task. Study Design: Descriptive laboratory study. Methods: Forty recreationally active females performed maximal isometric contractions and single-leg jump-cuts. From recorded torque data, hip extension and abduction RTD was calculated from torque onset to 200 ms after onset. Three-dimensional motion analysis was used to quantify frontal plane hip and knee kinematics during the movement task. For each RTD measure, jump-cut biomechanics were compared between participants in the highest (high) and lowest (low) RTD tertiles. Results: No differences in frontal plane hip and knee kinematics were identified between high and low hip abduction RTD groups. However, those in the high hip extension RTD group exhibited lower hip adduction (high, 3.8° ± 3.0°; low, 6.5° ± 3.0°; P = .019) and knee valgus (high, –2.5° ± 2.3°; low, –4.4° ± 3.2°; P = .046) displacements during the jump-cut. Conclusion: In movements such as cutting that are performed with the hip in a relatively abducted and flexed position, the ability of the gluteus medius to control hip adduction may be compromised. However, the gluteus maximus, functioning as a hip abductor, may take on a pivotal role in controlling hip adduction and knee valgus motion during these types of tasks. Clinical Relevance: Training with a specific emphasis on increasing explosive strength of the hip extensors may be a means through which to improve frontal plane hip and knee control during high-risk maneuvers such as cutting.

Sports-Related Emergency Preparedness in Oregon High Schools

Background: Best practice recommendations for sports-related emergency preparation include implementation of venue-specific emergency action plans (EAPs), access to early defibrillation, and first responders—specifically coaches—trained in cardiopulmonary resuscitation and automated external defibrillator (AED) use. The objective was to determine whether high schools had implemented these 3 recommendations and whether schools with a certified athletic trainer (AT) were more likely to have done so. Hypothesis: Schools with an AT were more likely to have implemented the recommendations. Study Design: Cross-sectional study. Level of Evidence: Level 4. Methods: All Oregon School Activities Association member school athletic directors were invited to complete a survey on sports-related emergency preparedness and AT availability at their school. Chi-square and Fisher exact tests were used to analyze the associations between emergency preparedness and AT availability. Results: In total, 108 respondents (37% response rate) completed the survey. Exactly half reported having an AT available. Only 11% (95% CI, 6%-19%) of the schools had implemented all 3 recommendations, 29% (95% CI, 21%-39%) had implemented 2, 32% (95% CI, 24%-42%) had implemented 1, and 27% (95% CI, 19%-36%) had not implemented any of the recommendations. AT availability was associated with implementation of the recommendations (χ2 = 10.3, P = 0.02), and the proportion of schools with ATs increased with the number of recommendations implemented (χ2 = 9.3, P < 0.01). Schools with an AT were more likely to implement venue-specific EAPs (52% vs 24%, P < 0.01) and have an AED available for early defibrillation (69% vs 44%, P = 0.02) but not more likely to require coach training (33% vs 28%, P = 0.68). Conclusions: Despite best practice recommendations, most schools were inadequately prepared for sports-related emergencies. Schools with an AT were more likely to implement some, but not all, of the recommendations. Policy changes may be needed to improve implementation. Clinical Relevance: Most Oregon high schools need to do more to prepare for sports-related emergencies. The results provide evidence for sports medicine professionals and administrators to inform policy changes that ensure the safety of athletes.

Associations between player age and club soccer coaches’ perceptions of injury risk and lower extremity injury prevention program use:

The use of a lower extremity injury prevention program by female soccer players before the completion of puberty may mitigate movement changes that develop during puberty and contribute to post-pubescent females’ greater anterior cruciate ligament-injury risk. It is unknown whether club soccer coaches are using injury prevention programs with younger athletes and if player age is associated with soccer coaches’ knowledge, attitudes, and behaviors surrounding injury prevention programs. Fifty-four coaches of female soccer teams from Oregon and Washington states (USA) completed a web-based survey. Associations between team age (U9–U14 teams = 57 and U15–U19 teams = 19 teams) and coaches’ (a) attitudes and perceptions; and (b) injury prevention program awareness, adoption, and implementation fidelity were assessed. Coaches of U15–U19 teams perceived injuries to be more of a problem and soccer to present a high risk of injury. Coaches of older teams more strongly agreed that preventive exercises should be performed by their players during training. Injury prevention program awareness among coaches of U15–U19 and U9–U14 age groups was 79% and 60%, respectively. Injury prevention program-aware coaches of U15–U19 teams were more likely to use an injury prevention program than injury prevention program-aware coaches of U9–U14 teams (67% vs. 38%), but they were not more likely to do so with high fidelity (60% vs. 54%). Team age is associated with coaches’ short-term perceptions of injury risk, but not perceptions about the long-term ramifications of injury. Educating coaches about the potential benefits of injury prevention programs for mitigating injury-related maturational changes and the long-term ramifications of lower extremity injuries may facilitate greater use of injury prevention programs by coaches of younger female soccer players.

Athletic training and public health summit

OBJECTIVE To introduce athletic trainers to the benefits of using a population-based approach to injury and illness prevention and to explore opportunities for partnering with public health professionals on these initiatives. BACKGROUND Athletic trainers play leading roles in individual injury and illness prevention but are less familiar with policy development, evaluation, and implementation from a population-level standpoint. The Athletic Training and Public Health Summit was convened to understand, explore, and develop the intersection of athletic training and public health. CONCLUSIONS To further the integration of athletic training within the public health arena, athletic trainers must expand their professional focus beyond the individual to the population level.

Spectral Properties of H-Reflex Recordings After an Acute Bout of Whole-Body Vibration

Abstract Kipp, K, Johnson, ST, and Hoffman, MA. Spectral properties of H-reflex recordings after an acute bout of whole-body vibration. J Strength Cond Res 26(7): 1915–1919, 2012—Although research supports the use of whole-body vibration (WBV) to improve neuromuscular performance, the mechanisms for these improvements remain unclear. The purpose of this study was to identify the effect of WBV on the spectral properties of electrically evoked H-reflex recordings in the soleus (SOL) muscle. The H-reflex recordings were measured in the SOL muscle of 20 participants before and after a bout of WBV. The H-reflexes were evoked every 15 seconds for 150 seconds after WBV. A wavelet procedure was used to extract spectral data, which were then quantified with a principle components analysis. Resultant principle component scores were used for statistical analysis. The analysis extracted 1 principle component associated with the intensity of the myoelectric spectra and 1 principle component associated with the frequency. The scores of the principle component that were related to the myoelectric intensity were smaller at 30 and 60 milliseconds after WBV than before WBV. The WBV transiently decreased the intensity of myoelectric spectra during electrically evoked contractions, but it did not influence the frequency of the spectra. The decrease in intensity likely indicates a smaller electrically evoked muscle twitch response, whereas the lack of change in frequency would indicate a similar recruitment pattern of motor units before and after WBV.

A biomechanical comparison of dominant and non-dominant limbs during a side-step cutting task

Abstract Numerous studies have investigated anterior cruciate ligament (ACL) injury risk by examining gender differences in knee and hip biomechanics during a side-step cutting manoeuvre since it is known that ACL injury often occurs during such a task. Recent investigations have also examined lower extremity (LE) biomechanics during side-step cutting in individuals following ACL reconstruction (ACLR). Common research practice is to compare knee and hip biomechanics of the dominant limb between groups but this can add considerable complexity for clinicians and researchers. At this time, it is not known if there is a difference in LE biomechanics between the dominant and non-dominant limb during side-step cutting. Three-dimensional kinematics and kinetics were collected while 31 healthy participants performed five, side-step cutting manoeuvres with the dominant and non-dominant limbs. Knee and hip variables examined are those commonly investigated in ACL injury literature. There were no differences between limbs in all but one variable (knee internal rotation). These results demonstrate that healthy individuals exhibit little side-to-side differences in certain LE biomechanics when performing a side-step cutting manoeuvre. These findings can be utilised by clinicians when conducting dynamic evaluations of their ACLR patients and when developing injury prevention and rehabilitation programmes.

Presynaptic inhibition decreases when estrogen level rises

The objective was to determine estrogens influence on control of a skeletal muscle through measurements of motorneuron excitability (H:M ratio) and presynaptic inhibition (PI). Estrogen serum concentrations were measured at menses and ovulation of female subjects and compared to male controls. Data were analyzed from 12 women and 13 men reporting no history of knee ligament injury. Women reported regular menstrual cycles and no hormone‐based contraceptive use for the previous year. Women were tested at menses (Time1) and ovulation (Time2). Men were tested twice, approximately 14 days apart. Analysis indicated no difference in the H:M ratio between the sexes at either time point. A significant difference for the sexes was detected in the magnitude of estrogen change (∆EST) between observations. At Time1, the male and female estrogen concentrations were not different; however, they were different at Time2, primarily due to the large rise observed in the women. A significant difference between the sexes was also seen in the magnitude of change for PI (∆PI) between observations. As with EST, the levels of PI between the sexes at Time1 were not different; however, a difference existed at Time 2. Estrogen interacts with GABA at several nervous system locations affecting inhibition of synaptic transmission. This is the first study to investigate changes in PI of a skeletal muscle between times of low and high estrogen. Improving the understanding of estrogens influence on skeletal muscles may provide answers to why noncontact anterior cruciate ligament injuries of the knee occur more frequently in women.