Static stretching does not alter pre and post-landing muscle activation

<p>Abstract</p> <p>Background</p> <p>Static stretching may result in various strength and power deficiencies. Prior research has not determined, however, if static stretching causes a change in muscle activation during a functional task requiring dynamic stability. The purpose of this study was to determine if static stretching has an effect on mean pre and postlanding muscle (vastus medialis VM, vastus lateralis VL, medial hamstring MH, and biceps femoris BF) activity.</p> <p>Methods</p> <p>26 healthy, physically active subjects were recruited, from which 13 completed a 14-day static stretching regimen for the quadriceps and hamstrings. Using the data from the force plate and EMG readings, a mean of EMG amplitude was calculated for 150 msec before and after landing. Each trial was normalized to an isometric reference position. Means were calculated for the VM, VL, MH, and BF from 5 trials in each session. Measures were collected pre, immediately following the 1^ststretching session, and following 2 weeks of stretching.</p> <p>Results</p> <p>A 14-day static stretching regimen resulted in no significant differences in pre or postlanding mean EMG amplitude during a drop landing either acutely or over a 14-day period.</p> <p>Conclusions</p> <p>Static stretching, done acutely or over a 14-day period does not result in measurable differences of mean EMG amplitude during a drop landing. Static stretching may not impede dynamic stability of joints about which stretched muscles cross.</p

Adaptations in antagonist co-activation: Role in the repeated-bout effect

Eccentric exercise results in an adaptation which attenuates muscle damage from subsequent exercise—termed the “repeated-bout effect (RBE).” Purpose: Study examined antagonist co-activation and motor-unit recruitment strategy, assessed via dEMG, concomitant to the RBE. Methods: Nine participants performed 5 sub-maximal isometric trapezoid (ramp-up, hold, ramp-down) contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Surface EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between mean firing rate (MFR) of each motor-unit and its recruitment threshold (RT) was examined using linear regression. Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. Surface EMG of the biceps and triceps were collected during eccentric exercise. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated. Results: Changes in MVC (-32±14% vs -25±10%; p = 0.034), ROM (-11% vs 6%; p = 0.01), and DOMS (31.0±19mm vs 19±12mm; p = 0.015) were attenuated following the second bout of exercise. Triceps EMG was reduced (16.8±9.5% vs. 12.6±7.2%; p = 0.03) during the second bout of eccentric exercise. The slope (-0.60±0.13 vs -0.70±0.18; p = 0.029) and y-intercept (46.5±8.3 vs 53.3±8.8; p = 0.020) of the MFR vs. RT relationship was altered during contractions at 80% of MVC prior to the second bout of eccentric exercise. No changes were observed at 50% of MVC. Conclusion: A reduction in antagonist co-activation during the second bout of eccentric exercise suggests less total force was required to move an identical external load. This finding is supported by the increased negative slope coefficient and an increased y-intercept of the linear relationship between RT and MFR.Funded by University of Oklahoma Graduate College Robberson Grant.Ye

YOUTH ATHLETIC DEVELOPMENT AND MUSCULAR CHANGES AFTER 8-WEEK SUMMER EXERCISE PROGRAM

Ashley A. Herda1*, Rosemary Sisillo1, Makenzie Kerans1, Trent J. Herda2 1University of Kansas-Edwards Campus, Overland Park, KS; 2University of Kansas-Lawrence Campus, Lawrence, KS PURPOSE: The purpose of this study was to determine if a structured exercise program would impact body or muscle composition and athletic development in elementary-aged youth. METHODS: An 8-week summer exercise program was implemented for 10young males (n=6) and females (n=4) 5-11 years old. Participants engaged in 45 minutes of various high-intensity activities two times per week over the summer months. Body mass and composition were measured with a digital scaleand2-site skinfolds (subscapular and triceps; FATSF), respectively, and muscle composition [cross-sectional area (VLCSA), thickness (VLTHICK), and echo intensity (EI)] were measured using ultrasound imaging of the right quadriceps vastus lateralis muscle and subcutaneous fat (VLFAT) of the thigh. Additionally, strength, power, and speed were measured and assessed using dominant hand handgrip (HG), broad jump (BJ), 2-poundmedicine ball throw (MBPOW), and 10meter fly sprint (SPEED10M). Three participants did not complete follow-up testing and were not included in the analyses. Paired-samples t-tests were conducted to identify change in performance and body composition variables after the 8-week intervention with an α ≤ 0.05 considered significant. RESULTS: As expected, all participants gained body mass over the 8-week program (mean diff=1.16±0.32kg, p=0.01) as they are all prepubescent and in growth flux age. Total body FATSF did not change (p=0.11). Additionally, muscle composition was altered with an increase in VLCSA (mean diff=1.04±0.30cm2, p=0.01) and VLFAT (mean diff=0.05±0.02cm, p=0.03). However, VLTHICK and EI did not change (p\u3e0.05). For performance, MBPOW was the only variable to improve significantly (mean diff=43.2±11.54cm, p=0.01). CONCLUSION: These results indicated that 8-weeks of structured summer exercise comprising of dynamic warm-up, calisthenics, and weighted strength and power activities increased body mass and VL muscle cross-sectional area without a transient change in lower-body performance in prepubescent youth. The marginal changes (+3.8%) in body mass may be due to their growth stage. However, FATSF did not change and the 15.1% improvement in upper-body power may imply training impacted athletic development more so than growth stage

MUSCLE COMPOSITION AND PERFORMANCE IN MALE AND FEMALE PREPUBESCENT YOUTH

Rosemary Sisillo1, Makenzie E. Kerans1, Jackson R. Dinsmore2, Trent J. Herda2, and Ashley A. Herda1 1University of Kansas-Edwards Campus, Overland Park, KS; 2University of Kansas-Lawrence Campus, Lawrence, KS PURPOSE: The purpose of this study is to identify the relationships among thigh muscle composition and sprint speed in young males and females. METHODS: Fourteen young (range: 5-11 years) males (n=9) and females (n=5) were measured for height, weight, and seated height to determine leg length and maturation. Skinfold thickness of the right tricep and subscapular regions were measured to estimate body fat percentage. Additionally, an ultrasound image of the right quadriceps vastus lateralis (VL) muscle was recorded and analyzed for VL thickness (VLTHK), cross-sectional area (VLCSA), and subcutaneous fat (VLFAT). Participants performed a 10-meter sprint on a 25m marked turf course. The first 10mwere dedicated to acceleration, timing gates were set at the 10m and 20m marks and the final 5m was used for deceleration. Stepwise linear regression was used to determine prediction of the 10m sprint using anthropometric and muscle composition data. Additionally, Pearson correlation coefficients were determined between sprint speed and muscle composition. RESULTS: The results of the linear regression analysis indicated age, leg length, and VLFAT were the primary predictors of 10-meter sprint time (R2=0.916; p\u3c0.05). Cross-validation with a second group of individuals(5-11 years; n=7) resulted in a strong correlation between the actual 10m sprint time and predicted time (r=0.87, p=0.01) and a paired t-test indicated no difference between the predicted and actual 10m sprint time (mean diff=0.02s, p=0.70). Top speed (m/s) during the 10m sprint speed was correlated to VL thickness (r=0.56, p=0.04) and leg length (r=0.56, p=0.04) whereas the 10m sprint time was correlated with VLCSA (r=0.621, p=0.02). CONCLUSION: Application of anthropometric and muscle composition data to predict performance would be simple and non-exertive during recovery if a young athlete were injured and undergoing a rehabilitation protocol. These measurements also assist in determining sport specificity as they mature and begin to specialize in endurance, speed, or strength-based activities. Caution for this prediction should be exercised as this study used a relatively small sample to estimate the 10m sprint. Body composition should not be the primary focus at such a young age, rather skill development to optimize performance and reduce injury

RELIABILITY OF MOTOR UNIT RECRUITMENT THRESHOLD VERSUS MEAN FIRING RATE RELATIONSHIPS DURING SUBMAXIMAL CONTRACTIONS

Eric M. Mosier1, Michael A. Trevino1 & Trent J. Herda1 1University of Kansas, Lawrence, Kansas PURPOSE: The purpose of this study was to examine the reliability of motor unit (MU) recruitment versus mean firing rate relationships of the vastus lateralis (VL) during 40% and 70% maximal voluntary contractions (MVC). METHODS: Eight healthy men (age=21.63±2.39 yrs) and six healthy women (age=19.67±1.37 yrs) volunteered for this investigation. An electromyography (EMG) sensor was placed over the VL. Participants performed 3 isometric maximal voluntary contractions (MVC) followed by isometric trapezoid muscle actions at 40% and 70% MVC calculated from the highest MVC. These procedures were performed twice separated by 3-5 days. Decomposition techniques were applied to the EMG signals to extract action potentials and firing events of single MUs. The recruitment (REC) thresholds and mean firing rate (MFR) were calculated for each MU. Only MUs that were decomposed with accuracies \u3e90% were included for analysis. Linear regressions were performed on REC vs. MFR relationships on MUs with accuracies \u3e90% and MUs excluding accuracies 90% and again on relationships with the exclusion of MUs with accuracies RESULTS: There were no significant differences between days for the slopes and y-intercepts for either contraction or accuracy condition (including MUs with \u3e90% or excluding MUs with 0.05). The ICCs and SEM (expressed as a percentage of the mean, %) from the 70% MVC that excluded MUs with accuracies 90% were -0.07 and 41.30% and 0.14 and 34.29% for the slopes and 0.56 and 6.86% and 0.38 and 14.39% for the y-intercepts. CONCLUSION: The ICCs and SEMs indicated good reliability for slopes and y-intercepts from the 70% MVC when excluding MUs with accuracies90%, the SEMs were similar to that of the regressions that excluded MUs with accuracies \u3c95%

MECHANOMYOGRAPHIC MEAN POWER FREQUENCY OF THE VASTUS LATERALIS DURING AN ISOMETRIC TRAPEZOID MUSCLE ACTION

Michael A. Trevino1, Eric M. Mosier & Trent J. Herda1 1University of Kansas, Lawrence, Kansas PURPOSE: This study examined the mechanomyographic mean power frequency (MMGMPF)-force relationships for 5 aerobically-trained (AT), 5 resistance-trained (RT), and 5 sedentary (SED) individuals. METHODS: Five AT (age=19.20±0.45 yrs), 5 RT (age=25.00±4.53 yrs), and 5 SED (21.20±2.17 yrs) volunteered for this investigation. The AT ran 61±15 miles∙wk-1, all but one RT subject were capable of a one repetition maximum back squat ≥ twice their body mass, and the SED did not participate in any structured physical activity for 3 years prior. Each participant was seated on a Biodex isokinetic dynamometer and isometric leg extensor strength assessments were performed on the right leg at a 90° flexion with isometric strength measured using the force signal from a load cell fitted to the Biodex. Participants performed three maximal voluntary contractions (MVC) followed by an isometric trapezoid muscle action at 60% MVC calculated from the highest MVC. An MMG sensor was placed over the vastus lateralis (VL) and thigh skinfolds were collected at the electrode site. The MMG and force signals were simultaneously sampled at 2 kHz and were bandpass filtered (fourth-order Butterworth) at 5-100 Hz. For the linearly increasing and decreasing muscle actions, linear regression models were fit to the log-transformed MMGMPF-force relationships and the slope (b term) and anti-log of the y-intercept (a term) were calculated. An average of MMGMPF was calculated for the entire steady force segment. Separate 2-way mixed factorial ANOVAs (training status [AT vs. RT vs. SED] x segment [linear increase vs. linear decrease]) were used to examine the b and a terms during the linearly increasing and decreasing muscle actions. Separate 1-way ANOVAs were used to examine MMGMPF during the steady force segment and skinfold thickness among the AT, RT, and SED. When appropriate, follow-up analyses were performed using paired and independent samples t-test with Bonferroni corrections. An alpha level was set at 0.05 to determine statistical significance. RESULTS: The b and a terms were not different among training statuses (P\u3e0.05) or linearly increasing and decreasing segments (P\u3e0.05). The 95% confidence intervals for the b terms suggested deceleration in MMGMPF throughout the force spectrum for linearly increasing and decreasing muscle actions. MMGMPF during the steady force segment (P=0.287) and skinfold thicknesses (P=0.142) were not different among training status. CONCLUSION: MMGMPF did not distinguish between training statuses or muscle actions. Higher targeted forces with faster contraction rates may be necessary for MMGMPF to distinguish between training statuses and muscle actions

VASTUS LATERALIS HYPERTROPHY EXCEEDS WEIGHT AND HEIGHT INCREASES FOLLOWING SUMMER TRAINING PROGRAM IN PREADOLESCENT BOYS

Jonathan D. Miller1, Mandy E. Wray1, Hannah L. Dimmick1, and Trent J. Herda1 1University of Kansas, Lawrence, KS It is well documented that preadolescent children gain unhealthy body mass during the summer months which is largely attributed to physical inactivity. PURPOSE: To analyze changes in vastus lateralis (VL) muscle cross-sectional area (mCSA) and composition via ultrasonography in relation to growth in height and body mass following an 8-week summer training program for children ages 5-11 years old. METHODS: Nine boys (age=8.2±2.1 years, height=135.1±10.5 cm, body mass=35.7±7.3 kg, BMI range=15.7‒34.5 kg/m2) completed 2, 50-minute training sessions per week for 8 weeks. The training sessions consisted of functional high intensity upper and lower body resistance exercises as well as anaerobic sprint and agility drills. Height, body mass, and ultrasound measurements were collected prior to- and following the training program. Cross-sectional ultrasound scans were taken at 50% of the distance from the anterior superior iliac spine to the superior border of the patella. Subcutaneous fat above the muscle was measured from the bottom of the cutaneous layer to the superficial fascia above the muscle. The borders of the VL were carefully outlined and this cross-sectional area was considered mCSA, while the mean greyscale of the area was considered the muscle echo intensity (mEI). RESULTS: Paired samples t-tests indicated following the 8-week training program there no significant changes in body mass (P=0.242), mEI (P=0.562), or subcutaneous fat (P=0.327). However, there was a significant 0.98% increase in height (P=0.001), and a significant 7.6% increase in mCSA (P=0.002) from pre- to post-training. CONCLUSIONS: The 8-week training program was effective to limit increases in body mass and subcutaneous fat as well as intramuscular fat (as measured by mEI) while increasing VL mCSA. In addition, the 7.6% increase in mCSA is likely not due to normal growth alone as the increase in height (0.98%) and the nonsignificant increase in body mass (1.8%) were proportionally smaller

THE EFFECT OF VERTICAL JUMP FATIGUE ON YANK-TIME CHARACTERISTICS IN RECREATIONALLY TRAINED MALES

Matthew J. Hermes1, Mandy E. Parra2, Andrew C. Fry1, & Trent J. Herda1 1The University of Kansas, Lawrence, Kansas; 2Baker University, Baldwin City, Kansas The vertical jump (VJ) can assess lower body performance through force-time data analysis, but kinetic and kinematic events may not be easily explained from the force-time data alone. Yank, a time-derivative of force, represents instantaneous rate of force development and may be a viable method to understand kinematic and kinetic events during a VJ. How fatigue affects yank is unknown. PURPOSE: To assess the effect of fatigue on VJ yank-time characteristics. METHODS: Recreationally trained males (X±SD; n = 11, age = 22.7±3.3 yrs, height = 182.1±5.2 cm, mass = 86.9±11.8 kg) completed a repeated jump protocol on a force plate consisting of 10 sets of repeated jumps. Sets lasted 15 seconds, with 15 seconds rest between sets. Subjects were instructed to reach 90º knee flexion, jump maximally with no arm swing, and perform as many jumps as possible per set. The first jump of the first and last sets were analyzed. Yank-time data was derived from force-time data using a low-pass Hamming filter with a cutoff frequency of 10 Hz. Countermovement (CM) depth was integrated from force-time data. Yank events were defined as: 1 = jump initiation (yank drops below 0), 2 = first yank curve trough, 3 = lowest force value (yank moves above 0), 4 = force peak one (yank drops below 0), 5 = trough between force peaks (yank moves above 0), 6 = force peak two (yank drops below 0), 7 = flight initiation. Paired samples t-tests compared durations of yank-based VJ phases between nonfatigued and fatigued jumps (p \u3c 0.05). RESULTS: Shorter VJ phase durations and moderate (g = 0.5-0.8), large (g = 0.8-1.2), or very large (g = 1.2-2.0) effect sizes were found in non-fatigued jumps for phases 4-5 (p = 0.004, g = 1.083), 5-6 (p = 0.07, g = 0.986), and 6-7 (p = 0.001, g = 1.369). Phases 1-2, 2-3, and 3-4 were not significantly different (p \u3e 0.05), but phase 1-2 did have a moderate effect size (g = 0.561). Total jump time was significantly faster in non-fatigued jumps (p = 0.04, g = 0.684). Though non-significant (p = 0.112, g = 0.505), CM depth was shallower for fatigued jumps. CONCLUSION: VJ phase durations were shorter for some events in non-fatigued jumps, despite larger CM. This indicates yank is sensitive to fatigue. As yank has been suggested to help explain kinetic and kinematic events, yank-time assessments during fatigue may help to understand how fatigue affects kinetics and kinematics

ACUTE EFFECTS OF VIBRATION ON RECRUITMENT THRESHOLD VERSUS PEAK FIRING RATE RELATIONSHIP DURING AN ISOMETRIC CONTRACTION

Eric M. Mosier1, Michael A. Trevino1, Johnathon D. Miller1, and Trent J. Herda1. 1University of Kansas, Lawrence, KS; e-mail: [email protected] PURPOSE: This study examined the acute effects of vibration (VIB) on motor unit (MU) behavior of the vastus lateralis (VL) during 40% maximal voluntary contraction (MVC). METHODS: Five males (age= 22.0±2.4 yrs) and three women (age=20.7±3.1 yrs) volunteered for this study. An electromyographic (EMG) sensor was placed over the VL. Participants preformed 3 isometric MVCs followed by isometric trapezoid muscle actions at 40% MVC (CON). Procedures were preformed twice separated by 3-5 days. VIB was then applied at 55 Hz. to the patella tendon for 10 min. prior to testing and continued throughout the remainder of testing, including a 40% MVC (VIB) at the same absolute force as the 40% CON. Decomposition techniques were applied to the EMG signals to extract action potentials and firing events of single MU. Only MUs decomposed with accuracies \u3e92% were included for analysis. Linear regressions were performed on the MU recruitment threshold (REC) vs. peak firing rate (PFR) relationships during the first, middle, and final sec of the steady force segment for each subject. Slopes and y-intercepts were calculated for each subject and time point. Two separate 2-way repeated measures ANOVAs (treatment [CON vs. VIB] x time [first vs. middle vs. final sec]) were used to examine possible differences among y-intercepts and slopes. When appropriate, follow-up analyses for the ANOVA models were preformed using paired samples t-test with Bonferroni corrections. The level of significance was set at P ≤ 0.05. RESULTS: The analyses for the y-intercepts indicated that there was no 2-way interaction (P = 0.681) and no main effect for time (P = 0.803), however, there was a main effect for treatment (P = 0.047). The y-intercepts were greater for the VIB (29.49 ± 1.74 pps) than CON (26.49 ± 1.83 pps). The analyses for the slopes indicated no 2-way interaction (P = 0.684) or main effects for time (P = 0.729) and treatment (P = 0.055). CONCLUSION: The firing rates of the motoneuron pool were greater during VIB than the CON at the same absolute force. Thus, supporting the hypothesis that muscle spindles provide inhibitory effects on MU firing rates. When Ia afferent feedback from muscle spindles is removed the firing rates of the motoneuron pool increase. In addition, there was no time-dependent alterations in the REC vs. PFR relationships as function of VIB

RELIABILITY OF MOTOR UNIT FIRING RATE VERSUS RECRUITMENT THRESHOLD RELATIONSHIPS OF THE FIRST DORSAL INTEROSSEOUS

Mandy E. Wray1, Jonathan D. Miller1, Adam J. Sterczala2, Trent J. Herda1 1University of Kansas, Lawrence, KS; 2University of Pittsburg, Pittsburgh, PA Traditionally, motor unit (MU) mean firing rates (MFRs) have been regressed against their recruitment thresholds (RT), to provide insight on changes in MU behavior following intervention. However, the reliability of these parameters via decomposition methods has yet to be assessed. PURPOSE: To examine the reliability of MFR vs. RT relationships of the first dorsal interosseous (FDI) during a low and moderate intensity contraction. METHODS: Twenty-three healthy subjects (male=10 female=13, age=20.26 ± 2.05 years, weight=76.54 ± 20.13 kg, height=174.07 ± 10.30 cm) volunteered for this investigation. Participants visited the laboratory for one familiarization trial and two experimental trials separated by 24 hours. Participants performed isometric muscle actions of the FDI which included 3 maximal voluntary contractions (MVCs). The highest MVC force was used to determine the target force for the subsequent, randomized contractions at 20% and 50% MVC. Surface EMG signals were recoded from the FDI using a 5-pin surface array sensor. Action potentials were extracted into the firing events of single MUs via decomposition. Slopes and y-intercepts were calculated for the linear MFR vs. RT relationships and used for statistical analysis. Intraclass correlation coefficients (ICC) model “2,1”, standard error of the measurements (SEM), and repeated measures ANOVAs were calculated to assess reliability between the first and second experimental trials. P level was set to 0.05 RESULTS: The means ± SDs (collapsed across trials), ICCs, SEMs, and ANOVA P values are presented in Table 1. For the MFR vs. RT relationships there were no differences between trials 1 and 2. The ICC values of the slopes (20% MVC=0.443, 50% MVC=0.343) indicated poor reliability, and the ICC values of the y-intercepts (20% MVC=0.637, 50% MVC=0.494) indicated moderate reliability. CONCLUSIONS: There is a lack of reliability overall between trials when RT range is not controlled for. The lack of reliability between trials may be due to inconsistency in MUs observed between trials. Future studies should determine if controlling for RT range of recorded MUs between trials effects reliability