Influence of box height on inter-limb asymmetry and box jump performance

Box jumps are often included in training programs as an introductory exercise to novice athletes and untrained individuals and are an efficient option of lower-body explosiveness training. However, it is unclear whether the use of boxes of differing heights affect the inter-limb asymmetry during this exercise. The purpose of this study is to investigate the effect of box height in inter-limb asymmetry during box jumps. Recreationally active young males (n = 14) and females (n = 16) performed three jumps at boxes that corresponded to approximately 0, 20, 40, 60, and 80% of their individual countermovement jumps. The selected performance variables were peak force (PF), peak power (PP), rate of force development (RFD), and time to take-off (TToff). The intraclass correlation coefficients ranged from 0.76 to 0.99, and the coefficient of variation ranged from 4.03 to 16.52%. A series of one-way repeated measures ANOVA tests were used to test for significant differences of the performance variables and inter-limb asymmetries. The females’ PF at 80% was significantly higher from 0% (p < 0.05). No significant differences were observed for inter-limb asymmetry across box heights (p ≥ 0.25). This study shows that the box height does not affect the overall intra-session inter-limb asymmetries in recreationally active individuals

BLOOD FLOW RESTRICTION INFLUENCES ELECTROMYOGRAPHIC BEHAVIOR OF THE BICEPS BRACHII DURING A HIGH-INTENSITY CONTRACTION

Alex Olmos1, Tony Montgomery1, Kylie Sears1, Taylor Dinyer-McNeely1, Shane Hammer1, Haley Bergstrom2, Ethan Hill3, Pasquale Succi2 and Michael Trevino1 1Oklahoma State University, Stillwater, OK 2University of Kentucky, Lexington, KY 3University of Central Florida, Orlando, FL PURPOSE: To determine the effects of blood flow restriction (BFR) on electromyographic amplitude (EMGRMS)-force relationships of the biceps brachii. METHODS: Twelve recreationally active males (mean ± SD; age: 25 ± 4 yrs) and twelve recreationally active females (age: 22 ± 5 yrs) participated in this study. Surface EMG was recorded from the biceps brachii during a control (CON) and BFR visit. For each visit, participants performed 2-3 maximal voluntary isometric contractions (MVC) of the elbow flexors and an isometric trapezoidal muscle action at 70% MVC. For BFR, a cuff was inflated to 60% of the pressure required to completely occlude the brachial artery at rest. Individual b- (slopes) and a-terms (anti-log of the y-intercepts) were calculated from the log-transformed EMGRMS-force relationships during the linearly increasing (LI) and decreasing (LD) segments of the trapezoid. EMGRMS during the steady force segment of the trapezoid was normalized (N-EMGRMS) to MVC EMGRMS. Separate, 3-way mixed factorial ANOVAs (sex x condition x segment) were used to examine potential differences for b- and a-terms. A two-way mixed factorial ANOVA (sex x condition) was used to identify differences in N-EMGRMS. RESULTS: For the b- and a-terms, there were no 3-way interactions; however, there were condition x segment interactions for the b- (p = 0.008) and a-terms (p = 0.037). During the LI, b¬-terms were greater for BFR (1.57 ± 0.38) than CON (1.31 ± 0.30; p \u3c 0.001). For BFR, the b-terms were greater during the LI (1.57 ± 0.38) than LD (1.28 ± 0.29; p \u3c 0.001). The only difference for a-terms was between the LI (0.15 ± 0.29) and LD (0.39 ± 0.78; p = 0.045) during BFR. There was no 2-way interaction for N-EMGRMS (p \u3e 0.05); however, there was a main effect for sex (p = 0.025). Females (92.58 ± 31.92%) exhibited greater N-EMGRMS than males (69.54 ± 18.20%) collapsed across conditions. CONCLUSION: The greater b-terms during the LI for BFR compared to CON and a-terms during the LD than LI for BFR, suggests BFR induced greater muscle excitation for the biceps brachii during motor unit activation and deactivation. In addition, the sex-related differences for N-EMGRMS during the steady force segment of both conditions suggests a greater neural cost was necessary for females compared to males when completing a contraction at the same relative high-intensity

SEX DIFFERENCES IN CONTRACTION-INDUCED BLOOD FLOW LIMITATIONS DURING SMALL MUSCLE MASS EXERCISE IN HUMANS

Kylie Sears1, Tony Montgomery Jr.1, Alex Olmos1, Pasquale Succi2, Ethan Hill3, Haley Bergstrom2, Michael Trevino1, Taylor Dinyer-McNeely1 and Shane Hammer1 1Oklahoma State University, Stillwater, OK 2University of Kentucky, Lexington, KY 3University of Central Florida, Orlando, FL Differences in oxygen delivery across the contraction-relaxation cycle of intermittent contractions may contribute to sex differences in exercise tolerance at matched relative intensities. Specifically, consequences of increased intramuscular pressures during contraction, including reductions in blood flow (BF), may differ between sexes resulting in sex-specific determinants of time to task failure (TTF). PURPOSE: To determine sex differences in absolute- and %-reductions in BF during muscular contractions and relationships between BF reductions and TTF. METHODS: Thirteen men (25 ± 4 yrs) and 13 women (22 ± 5 yrs) completed intermittent isometric trapezoidal forearm flexion at 50% maximal voluntary contraction until task failure. Doppler ultrasound was used to measure brachial artery BF during the 12-s plateau phase and 12-s relaxation phase. Sex-differences in absolute BF between phases at end-exercise were tested using ANOVA (phase × sex). Sex-differences in absolute- and %-reductions in BF at end-exercise were determined by t-tests. Linear regression was performed to identify relationships between BF reductions and performance characteristics. RESULTS: Target torque was less in women compared to men (24 ± 5 vs. 42 ± 7 Nm; p \u3c 0.001); however, TTF was not different between sexes (W: 425 ± 187 vs. M: 401 ± 158 s; p = 0.72). Relaxation-phase BF at end-exercise was less in women compared to men (435 ± 161 vs. 937 ± 281 mL/min; p \u3c 0.001) but contraction-phase BF was not different (127 ± 46 vs. 190 ± 99 mL/min; p = 0.42). Absolute- and %-reductions in BF by contraction were less in women compared to men (309 ± 146 vs. 747 ± 210 mL/min and 69 ± 10 vs. 80 ± 6%, respectively; both p \u3c 0.01) and were associated with target torque independent of sex (r = 0.78 and 0.56, respectively; both p \u3c 0.01). Interestingly, absolute BF reduction per target torque (mL/min/Nm) and TTF were positively associated in men (r = 0.60; p = 0.031) but negatively associated in women (r = -0.61; p = 0.029). CONCLUSION: Reductions in BF during the contraction phase of intermittent isometric contractions at matched relative intensities were sex-dependent and likely a consequence of differences in absolute torque production between men and women. However, the relationship between contraction-induced BF reductions and TTF was inverted in women compared to men