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Nick J. Siekirk1,,2, Jacob Smith3, Kylee West1, Jordan Brown1, Brianna Simmons1, Austen Arnold1, Derick Anglin1, Sarah North1. 1Department of Health Sciences and Kinesiology, Georgia Southern University, Statesboro, GA. 2Center for Rehabilitation and Independent Living, Waters College of Health Professions, Statesboro, GA. 3University of Texas at Austin, Austin, TX.

Recent evidence has questioned the utility of eccentric exercise in the prevention of hamstring (HS) strains. Recruitment strategies between limbs may influence eccentric HS exercise. PURPOSE: The purpose of this investigation is to examine whether the bilateral eccentric HS displays unilateral bias and whether that bias can be attributed to strength asymmetries. METHODS: Thirty college-aged participants (weight: 77.91 ± 12.76 kg; height: 172.82 ± 7.74 cm) with prior resistance training performed dynamic mobility drills targeting the posterior chain prior to maximum voluntary contractions (MVC). Isometric dynamometry determined peak torque for the gluteus maximus and medial gastrocnemius in prone. Seated Isokinetic dynamometry at 60o and 120o per second determined concentric and eccentric peak torque of the quadriceps (Q) and HS. Ipsilateral strength ratios (Qcon60o /HSecc120o) were examined with a paired t-test, α = 0.05. Surface electromyographic (sEMG) was bilaterally recorded from the medial gastrocnemius (MG), semitendinosus (ST), biceps femoris (BF), and gluteus maximus (GMax) and was normalized to peak torque output during MVCs (%MVC). Eccentric HS exercise was normalized to MVCs and reported as mEMG within each variation; Flat Nordic Hamstring Exercise (F-NHE), Razor Curl (RC), NHE at 7o Decline (D-NHE), and Ab-Wheel Assisted NHE (A-NHE) (n = 3 reps each). Heart rate (HR) was obtained prior to each exercise variation in conjunction with verbal confirmation of exercise readiness. Separate 3-way (variation, side, rep) RM-ANOVAs were conducted for mEMG on each muscle and are presented relative to EMG collected at peak force during Isokinetric MVC (ST, BF) and Isometric MVC (Gmax, MG). RESULTS: Left (M = 1.21, SD = 0.45) and right (M = 1.12, SD = 0.50) Qcon60o /HSecc120o did not differ, p = 0.24, d = 0.18. Separate Greenhouse-Geisser corrected RM-ANOVAs did not detect a 3-way interaction between variation, side, and rep on MG mEMG, F(2.53, 70.75) = 1.16, p = 0.329, n2 = 0.040 or BF, F(1.38, 37.34) = 1.55, p = 0.226, n2 = 0.054. However, a main effect of the side was detected for Gmax, F(1, 27) = 22.753, p < 0.001, n2 = 0.46. The pairwise comparison found an 18.54 %MVC (95% CI: 10.57 to 26.52) difference between the right (M = 56.27 %MVC, 95% CI: 44.20 to 68.34) and left (M = 37.73 %MVC, 95% CI: 28.54 to 46.92) sides, p < 0.001. A main effect of the side was also detected for ST, F(1, 28) = 7.46, p < 0.011, n2 = 0.210. Left ST (M = 60.39 %MVC, 95% CI: 45.86 to 74.92) was 15.95 %MVC (95% CI: 3.99 to 27.92) greater the right (M = 44.44 %MVC, 95% CI: 37.26 to 51.62), p < 0.011. CONCLUSIONS: Despite no difference in strength ratios, these data support biased recruitment in the GMax the contralateral ST across eccentric HS variations.

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