Positional Differences in Areal Bone Mineral Density and Body Composition in NCAA Division - I Football Athletes

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Association Between DXA and HR-pQCT Measurements of Bone Characteristics in Recreationally Active, Recruit-Aged Men

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Bone Mineral Density and Tibial Microarchitecture Changes in Division I Male and Female Cross - Country Runners

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Structural Differences in the Tibial Metaphysis Between Female NCAA Division I Cross - Country Runners and Gymnasts

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The Effect of 12 - weeks of Concurrent Exercise Training on Body Composition and Bone Microarchitecture

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Sex-Related Differences in Motor Unit Behavior of the Vastus Lateralis During a High Intensity Contraction

Alex Olmos, Michael Trevino, Trent Herda, Adam Sterczala, Jonathan Miller, Mandy Parra, Hannah Dimmick, Jake Broeckel PURPOSE: The purpose of this study was to examine motor unit (MU) firing rates and muscle activation of the vastus lateralis (VL) between sedentary males and females during a 70% maximal voluntary isometric contraction (MVIC). METHODS: Eleven males (mean ± SD; Age: 20.36 ± 1.50 yrs, Height: 179.73 ± 4.13 cm, Body Mass: 78.84 ± 13.29 kg) and ten females (Age: 21.10 ± 6.08 yrs, Height: 166.40 ± 5.70 cm, Body Mass: 64.88 ± 15.03 kg) participated in this study. A surface electromyographic (sEMG) 5-pin sensor was placed over the VL. Participants performed three MVICs of the knee extensors on an isokinetic dynamometer and the greatest value was recorded. Participants then performed an isometric trapezoid muscle action at 70% of the participants greatest MVIC. For the isometric trapezoid muscle action, the torque was increased at a rate of 10% MVIC/s to the desired force level, which was maintained for 12 s followed by a decrease of 10% MVC/s to baseline. Decomposition techniques were applied to the sEMG signals to extract individual action potentials and firing events for single MUs. For each MU, the recruitment threshold (RT) and mean firing rates (MFR) were calculated. Only MUs with firing accuracies \u3e 90% were used for analysis. Linear regressions were performed on the MFR vs. RT relationships to determine y-intercepts (pulses per second [pps]) and slopes (%MVC/pps) for each participant. EMG amplitude (RMS) during the plateau of the trapezoid was normalized to MVC (N-EMGRMS). Three independent-sample t-test were used to examine sex-related differences NEMGRMS and the slopes and y-intercepts from the relationships. RESULTS: For the y-intercepts there were no significant differences (P = 0.184; males = 32.84 ± 5.01 pps, females = 32.60 ± 6.65 pps). For the slopes, there was a significant difference (P = 0.003) between the males (-0.487 ± 0.126 %MVC/pps) and the females (-0.335 ± 0.101 %MVC/pps). For N-EMGRMS, there was a significant difference (P = 0.003) between males (71.74 ± 7.06 %) and females (93.22 ± 15.63 %). CONCLUSION: The results indicate sex-related differences in motor unit firing rate behavior and muscle activation for the VL during high intensity contractions. Research should further investigate if these differences can be elucidated by MU size and the fiber area of the VL. ACKNOWLEDGEMENTS: This study was supported by the Doctoral Research Grant fund from the National Strength and Conditioning Association, Colorado Springs, CO

ALTERATIONS IN MOTOR UNIT POTENTIATION DURING FATIGUING CONTRACTIONS BY SEDENTARY INDIVIDUALS

Colin Miller1, Michael Trevino1, Trent Herda2, Adam Sterczala3, Jonathan Miller2, Mandy Parra4, Hannah Dimmick5 1Oklahoma State University, Stillwater, OK; 2University of Kansas, Lawrence, KS; 3University of Pittsburgh, Pittsburgh, PA; 4Baker University, Baldwin City, KS; 5University of Calgary, Calgary, AB PURPOSE: The purpose of this study was to examine the effects of consecutive long duration contractions on motor unit (MU) derecruitment behavior of the vastus lateralis (VL) for sedentary individuals. METHODS: Thirteen females (20.69 ± 2.75 yrs) and nine males (20.00 ± 1.41 yrs) volunteered for this study. An electromyographic (EMG) sensor was placed over the VL. Each participant completed isometric maximal voluntary contractions (MVC) on an isokinetic dynamometer followed by two consecutive isometric trapezoidal submaximal contractions (40% MVC) of the right knee extensors. For the submaximal contractions, the torque was increased at a rate of 10% MVC/s to the deserved torque level for 45s followed by a decrease of 10% MVC/s to baseline. Ten seconds of rest was given between the submaximal contractions. Decomposition techniques were applied to the EMG signals to extract action potentials and the firing events of single MUs. For each MU, the recruitment (REC; %MVC) and derecruitment (DEREC; %MVC) thresholds were calculated and linear regressions were performed on the DEREC versus REC relationships for each individual to determine the slopes and y-intercepts (y-ints). Separate two-way mixed factorial ANOVAs (sex x repetition) examined the slopes and y-ints. Follow-up analyses included Bonferroni corrections and alpha was 0.05. RESULTS: For the slopes and y-ints, there were no significant (p \u3e 0.05) two-way interactions or main effects for sex. However, there were main effects for repetition (REP). The slopes were greater (p = 0.030) for REP 1 (1.33 ± 0.50) than REP 2 (1.12 ± 0.43), whereas the y-ints were greater (p = 0.024) for REP 2 (1.27 ± 14.57) than REP 1 (-6.50 ± 20.39). CONCLUSION: Caution is warranted when interpreting the findings for the y-ints as half of the subjects exhibited negative values. Indeed, the slopes indicated that males and females derecruited MUs at higher torque values (slopes \u3e 1) than the initial recruited torque levels for both REPS. However, the slopes significantly decreased during the second contraction, likely due to fatigue. Future research should investigate if endurance training can improve fatigue resistance and prevent a decrease in MU potentiation during a second, long duration contraction

β2-adrenergic receptor maladaptations to high power resistance exercise overreaching

The effects of a recovery drink on overreaching induced by high frequency, high power resistance exercise was assessed. Resistance trained men were assigned to a supplemented (SUP, n = 8), placebo (PL, n = 3) or control (CON, n = 6) groups. All groups completed two weeks of familiarization training using the barbell squat. In week three, SUP and PL performed ten sets of five repetitions of speed squats twice daily, for a total of 15 training sessions. CON maintained their prior training schedule. Data were collected before week three (T1), after week three (T2) and after a week of recovery by training cessation (T3). During week three, SUP consumed an amino acid, carbohydrate and creatine monohydrate containing recovery drink immediately after each training bout. PL was provided a drink of similar appearance and taste but containing minimal nutritional value. At T2, both SUP and PL decreased mean squat velocity and power at 70% 1RM. Additionally, SUP and PL decreased muscle β2-adrenergic receptor (β2-AR) expression by 61 and 83%, respectively. Increases in the ratio of nocturnal urinary epinephrine/β2-AR ratio (EPI: β2AR) for SUP and PL suggested impaired sympathetic nervous system sensitivity. SUP demonstrated a smaller decrease in β2-AR expression and a lower EPI: β2AR, suggesting the recovery drink attenuated the detrimental effects of overreaching on the sympathetic activity. In conclusion, high power resistance exercise overreaching can induce performance decrements and impair sympathetic activity, but these effects may be attenuated by supplementation

INFLUENCE OF SEX AND MUSCLE ACTION ON MECHANOMYOGRAPHIC MEAN POWER FREQUENCY OF THE VASTUS LATERALIS

Sergio Perez Jr.1, Stephanie Sontag1, Trent Herda2, Adam Sterczala3, Jonathan Miller2, Mandy Parra4, Hannah Dimmick5, Michael A. Trevino1 1Oklahoma State University, Stillwater, Oklahoma; 2University of Kansas, Lawrence, Kansas; 3University of Pittsburgh, Pittsburgh, Pennsylvania; 4Baker University, Baldwin City, Kansas; 5University of Calgary, Calgary, Alberta PURPOSE: To examine mechanomyographic mean power frequency (MMGMPF)-torque relationships for the vastus lateralis (VL) in sedentary males and females. METHODS: Eleven females (mean ± SD; age = 22.7 ± 5.5 yrs) and eight sedentary males (age=19.4 ± 0.7 yrs) volunteered for this study. An MMG sensor was placed over the VL and subcutaneous fat (sFAT) was measured via ultrasonography at the sensor site. Participants completed 3 isometric maximal voluntary contractions (MVCs) of the knee extensors on an isokinetic dynamometer followed by an isometric trapezoid muscle action at 40% MVC. For linearly increasing and decreasing muscle actions, linear regressions models were fit to the log-transformed MMGMPF-force relationship and the slope (b term) was calculated. MMGMPF was averaged during the steady force segment. A 2way mixed factorial ANOVA (sex [male vs. female] x segment [ramp up vs. ramp down]) examined differences in the b terms during the linearly increasing and decreasing muscle actions. Independent samples t-tests examined sex-related differences in skinfold measurements and MMGMPF during steady torque. Pearson’s product moment correlations were used to determine relationships among sFAT and the b terms and MMGMPF. Alpha was set at 0.05. RESULTS: For the b terms, there was no significant two way interaction (p = 0.714) or main effect for sex (p = 0.259). There was a significant main effect for segment (p = 0.001). The b terms were greater during the linearly increasing (0.154 ± 0.131) than decreasing (0.006 ± 0.113) segment when collapsed across sex. During steady torque, MMGMPF was significantly greater (p = 0.002) for males (26.36 ± 2.57 Hz) than females (20.91 ± 3.64 Hz), while sFAT was significantly greater (p = 0.001) for females (1.59 ± 0.69 cm) than males (0.57 ± 0.32 cm). sFAT was significantly correlated with MPF during the plateau (p \u3c 0.001, r = -0.736), but not the b terms for either segment (p \u3e 0.05). CONCLUSION: MMGMPF distinguished between muscle activation and deactivation strategies, but not between sexes. MMGMPF during steady torque was greater for males; however, this may be due to the influence sFAT low-pass filtering the MMG signal to a greater extent for females. Higher targeted torques may be necessary to distinguish muscle activation and deactivation strategies between sexes

Myosin Heavy Chain Expression Influences Motor Unit Behavior During High- But Not Low-Intensity Isometric Contractions

Stephanie Sontag1, Alex Olmos1, Trent Herda2, Philip Gallagher2, Andrew Fry2, Jake Deckert3, Adam Sterczala4, Jonathan Miller2, Mandy Parra5, Hannah Dimmick6, Michael Trevino1 1Oklahoma State University, Stillwater, OK, 2The University of Kansas, Lawrence, KS, 3Gonzaga University, Spokane, WA, 4The University of Pittsburg, Pittsburg, PA, 5Baker University, Baldwin City, KS, 6The University of Calgary, Calgary, AB PURPOSE: To examine the influence of percent (%) type I myosin heavy chain (MHC) isoform expression on the slope and y-intercept values from the mean firing rate (MFR) versus motor unit (MU) recruitment threshold (RT) relationships of the vastus lateralis (VL) for sedentary individuals during a 40% and 70% maximal voluntary contraction (MVC). METHODS: Seven men (mean ± SD; age: 21.0 ± 1.5 yrs, height: 179.71 ± 5.25 cm, body mass: 81.77 ± 15.59 kg) and seven women (age: 24.0 ± 6.9 yrs, height: 165.14 ± 7.10 cm, body mass: 58.70 ± 15.37 kg) volunteered for this study. An electromyographic (EMG) sensor was placed on the VL. Each participant performed three isometric MVCs of the leg extensors on an isokinetic dynamometer and the highest value was used. Participants then performed separate isometric trapezoidal muscle actions with torque increasing at a rate of 10% MVC/s to the desired torque level, which was maintained for 12 s followed by a decrease of 10% MVC/s to baseline. Action potentials and firing events of single MUs were extracted from EMG signals using decomposition techniques. The MFRs and RTs were calculated for each MU. MUs with accuracies \u3e 90% were used for analysis. Linear regressions were performed to determine the slopes and y-intercepts of the MFR versus RT relationships. Subjects gave a muscle biopsy of the VL after testing. SDS-PAGE was used to determine the % type I MHC isoform expression. Pearson’s product moment correlations were used to determine relationships among % type I MHC isoform expression with the slope and yintercept values from the MFR versus RT relationships. Alpha was set at 0.05. RESULTS: Pearson’s product moment correlations were significant when comparing % type I MHC isoform expression to the slope from the MFR versus RT relationship at 70% MVC (p = 0.022, r = 0.604) but not at 40% MVC (p = 0.066, r = 0.504). There were no significant correlations when comparing % type I MHC isoform expression to the y-intercept from the MFR versus RT relationship at 70% MVC (p = 0.138, r = -0.416) or at 40% MVC (p = -0.179, r = 0.318). CONCLUSION: The slope values from the MFR versus RT relationships were correlated with % type I MHC isoform expression at high- but not low-intensities. Higher contraction intensities may be necessary for the physical properties of the MU pool to influence MU behavior in untrained individuals