1. Acute effects of Nordic hamstring exercise and Romanian deadlift on hamstring shear modulus and flexibilityKlemen Kalc, Amadej Jelen, Masatoshi Nakamura, Konstantin Warneke, Živa Jovanović, Leja Budja, Žiga Kozinc, 2026, original scientific article Abstract: The aim of this study was to investigate the acute effects of Romanian deadlift (RDL) and Nordic hamstring exercise (NHE) on the stiffness and flexibility of hamstring muscles. We hypothesized that a) the RDL would have a greater acute effect on increasing the flexibility, b) the NHE would have a greater acute effect on the increase in stiffness of the biceps femoris (BF) and semitendinosus (ST) in comparison to the semimembranosus (SM), and c) RDL would have a greater acute effect on the increase in stiffness of the SM in comparison to the ST. Twelve young, healthy participants (6 females; 22.8±3.0 years) performed RDL, and 12 participants (8 females; 22.8±3.1 years) performed NHE, with both interventions comprising 5 sets of 10 repetitions. Baseline and post-intervention stiffness was measured using shear– wave elastography, and flexibility using the passive straight leg raise test. There were no significant main effects of time (p=0.744), group (p=0.202), nor the interaction between time and group (p=0.131) for shear modulus. Similarly, no significant main effects of time (p=0.717) and group (p=0.856), nor an interaction between time and group (p=0.444), were observed for flexibility. These findings suggest that neither RDL nor NHE induce immediate measurable changes in hamstring muscle stiffness or flexibility. However, given the small group-specific sample size, the absence of significant effects should be interpreted cautiously and cannot be generalized beyond young, healthy recreationally active individuals.
Keywords: hamstring muscles, stiffness, ultrasound
Published in RUP: 01.06.2026; Views: 149; Downloads: 8
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2. Lower-limb damping characteristics during various repetitive jumping forms : reliability and sensitivity analysisMarko Kapeleti, Marc Elmeua González, Igor Zlatović, David Nikolić, Oskar Cvjetičanin, Nejc Šarabon, 2026, original scientific article Abstract: An essential aspect of jumping performance is the ability to control impact forces. One of the key measures of this ability is the damping ratio, a dimensionless parameter that quantifies the rate at which oscillations decay following a disturbance. This study aims to assess the intra-session and inter-session reliability of damping ratios at different lower-limb anatomical landmarks and evaluate their sensitivity in detecting differences between various repetitive jumping forms, with an emphasis on establishing a specific methodological approach. Twenty-three physically active male and female students performed maximal countermovement jumps (CMJs) and bilateral (BL) and unilateral (UL) hopping (HOP). Ten wireless inertial measurement units recorded inertial data of five bilateral lower-limb anatomical landmarks. Results showed that the damping ratios during CMJ and HOPUL demonstrated good reliability, whereas in HOPBL were moderately reliable. The damping ratios of nearly all the anatomical landmarks differed between the jumping forms. CMJ had the highest values, HOPBL the lowest, and HOPUL values in between, likely due to variations in joint stiffness and amplitude displacement, as well as stretch‒shortening cycle reliance. Methodologically, using 20 repetitions in an intra-session trial and averaging left and right sides for inter-session reliability and inter-jump sensitivity, damping ratios are acceptably reliable and sufficiently sensitive for biomechanical analysis of different repetitive jumping forms. This metric could potentially be a valuable tool in developing theoretical nonlinear multibody mass‒spring‒damper models and could enhance biomechanical analysis with practical applications in sports training and rehabilitation.
Keywords: stretch–shortening cycle, stiffness, impact absorption, oscillation, intraclass, correlation coefficient
Published in RUP: 13.04.2026; Views: 337; Downloads: 10
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3. Myotonometric assessment of achilles tendon and gastrocnemius stiffness in recreationally active young adults : reliability, impact of sex, and links to linear sprintMatic Sašek, Petra Brnelić, Žiga Kozinc, 2026, original scientific article Abstract: This study examined the relationship between passive Achilles tendon (AT) and gastrocnemius (GAS) stiffness, measured by myotonometry, and 40-m sprint performance across acceleration and maximal velocity phases, while accounting for sex differences. Twenty-one student athletes (10 males, 11 females) underwent bilateral passive stiffness assessments of the AT and GAS using MyotonPRO, followed by 40-m sprint testing with 10, 20, 30, and 40 m splits. Reliability was assessed using the intraclass correlation coefficient (ICC) and coefficient of variation (CV). Sex differences were examined with independent t-tests. Partial correlations controlling for sex were used to assess associations between passive stiffness and sprint performance. Passive stiffness and sprint variables demonstrated excellent within-session reliability (ICC = 0.95–0.99; CV < 5%). Males showed significantly greater passive GAS stiffness bilaterally (p = 0.006–0.049) and faster sprint times at 30 m (p = 0.040) and 40 m (p < 0.001), while passive AT stiffness did not differ significantly between sexes. Partial correlations indicated that greater passive AT stiffness in both legs was associated with faster sprint times at 10 m (r = -0.46 and -0.58, p = 0.008 and 0.043) and 20 m (r = -0.49 and -0.58, p = 0.008 and 0.029). No associations were observed at 30 m or 40 m, nor between passive GAS stiffness and sprint performance. Myotonometry provides reliable measures of AT and GAS stiffness. Greater passive AT stiffness is associated with faster acceleration sprint performance independent of sex.
Keywords: stiffness, muscle, tendon, sprint
Published in RUP: 06.04.2026; Views: 324; Downloads: 10
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4. Effects of an uphill marathon on running mechanics and lower-limb muscle fatigueNicola Giovanelli, Paolo Taboga, Enrico Rejc, Boštjan Šimunič, Guglielmo Antonutto, Stefano Lazzer, 2016, original scientific article Keywords: kinematika, maraton, togost, tenziomiografija, postaktivacijska potenciacija, kinematics, marathon, stiffness, tensiomyography, postactivation potentiation
Published in RUP: 08.08.2016; Views: 5574; Downloads: 251
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