Introduction. Monitoring muscle stiffness in athletes can be a good method of assessing fatigue caused by high training loads, and the early detection of fatigue can help prevent the occurrence of micro-trauma in the muscles that can cause contusions. The research carried out by Wilson et al.  confirmed that an optimal level of muscle stiffness is significantly correlated with high muscle loads. The aim of the current study was to determine changes in muscle stiffness of the left and right thighs during six weeks of plyometric training (PT) in volleyball players.
Material and methods. The study involved 16 volleyball players from the second-league Opole University of Technology Club (age = 21.12 ± 1.66 years, height = 191.62 ± 5.73 cm, and weight = 86.25 ± 6.66 kg) with at least five years of competitive experience (7.5 ± 2.44 years). Muscle stiffness was measured during three stages of the plyometric training using a MYOTON PRO device (Estonia).
Results. An RM-ANOVA analysis showed a significant difference in the resting stiffness of the semitendinosus (posterior thigh) muscles of the left and right limbs before the plyometric training began, but no significant differences were found in the stiffness of these muscles in the fourth or sixth weeks of training. The results of the measurement performed for the anterior muscles of the thigh did not reveal a significant difference in the stiffness of the left limb compared to that of the right limb in subsequent weeks of training.
Conclusion. The loads used in plyometric training in volleyball players caused a decrease in the differences in muscle stiffness between the left and right limbs, and in both limbs, adaptation trended towards an increase or a decrease in stiffness.
1. Wilson G.J., Wood G.A., Elliott B.C. (1991). Optimal stiffness of series elastic component in a stretch-shorten cycle activity. Journal of Applied Physiology 70(2), 825-33.
2. Mullix J., Warner M., Stokes M. (2012). Testing muscle tone and mechanical properties of rectus femoris and biceps femoris using a novel hand held MyotonPRO device: Relative ratios and reliability. Working Papers in Health Sciences 1(1), 1-8.
3. Croisier J.L., Ganteaume S., Binet J., Genty M., Ferret J.M. (2008). Strength imbalances and prevention of hamstring injury in professional soccer players: A prospective study. American Journal of Sports Medicine 36(8), 1469-75.
4. Mooney K., Warner M., Stokes M. (2013). Symmetry and within-session reliability of mechanical properties of biceps brachii muscles in healthy young adult males using the MyotonPRO device. Working Papers in Health Sciences 1(3), 1-11.
5. Aird L., Samuel D., Stokes M. (2012). Quadriceps muscle tone, elasticity and stiffness in older males: Reliability and symmetry using the MyotonPRO. Archives of Gerontology and Geriatrics 55(2), e31-9.
6. Masi A.T., Nair K., Evans T., Ghandour Y. (2010). Clinical, biomechanical, and physiological translational interpretations of human resting myofascial tone or tension. International Journal of Therapeutic Massage & Bodywork 3(4), 16-28.
7. Brazier J., Bishop C., Simons C., Antrobus M., Read M.J., Turner A.N. (2014). Lower extremity stiffness: Effects on performance and injury and implications for training. Strength and Conditioning Journal 112(3/5), 103-112.
8. Brughelli M., Cronin J. (2008). Influence of running velocity on vertical, leg and joint stiffness: Modelling and recommendations for future research. Sports Medicine 38(8), 647-57.
9. Gunther M., Blickhan R. (2002). Joint stiffness of the ankle and the knee in running. Journal of Biomechanics 35(11), 1459-74.
10. Arampatzis A., Schade F., Walsh M., Brüggemann G.-P. (2001). Influence of leg stiffness and its effect on myodynamic jumping performance. Journal of Electromyography and Kinesiology 11(5), 355-364.
11. Thalheimer W., Cook S. (2002). How to calculate effect sizes from published research: A simplified methodology. Somerville, MA: Work-Learning Research.
12. Chen J.J., Wu Y.N., Huang S.C., Lee H.M., Wang Y.L. (2005). The use of a portable muscle tone measurement device to measure the effects of botulinum toxin type a on elbow flexor spasticity. Archives of Physical Medicine and Rehabilitation 86(8), 1655-60.
13. Gavronski G., Veraksits A., Vasar E., Maaroos J. (2007). Evaluation of viscoelastic parameters of the skeletal muscles in junior triathletes. Physiological Measurement 28(6), 625-37.
14. Gennisson J.L., Cornu C., Catheline S., Fink M., Portero P. (2005). Human muscle hardness assessment during incremental isometric contraction using transient elastography. Journal of Biomechanics 38(7), 1543-50.
15. Leonard C.T., Brown J.S., Price T.R., Queen S.A., Mikhailenok E.L. (2004). Comparison of surface electromyography and myotonometric measurements during voluntary isometric contractions. Journal of Electromyography & Kinesiology 14(6), 709-14.
16. Tous-Fajardo J., Moras G., Rodriguez-Jimenez S., Usach R., Doutres D.M., Maffiuletti N.A. (2010). Inter-rater reliability of muscle contractile property measurements using non-invasive tensiomyography. Journal of Electromyography & Kinesiology 20(4), 761-6.
17. Pearsall A.W.T., Hollis J.M., Russell G.V.Jr., Scheer Z. (2003). A biomechanical comparison of three lower extremity tendons for ligamentous reconstruction about the knee. Arthroscopy 19(10), 1091-6.
18. Riemann B.L., Lephart S.M. (2002). The Sensorimotor System, Part II: The role of proprioception in motor control and functional joint stability. Journal of Athletic Training 37(1), 80-4.
19. Wright V. (1973). Stiffness: A review of its measurement and physiological importance. Physiotherapy 59(4), 107-11.
20. Klinge K., Magnusson S.P., Simonsen E.B., Aagaard P., Klausen K., Kjaer M. (1997). The effect of strength and flexibility training on skeletal muscle electromyographic activity, stiffness, and viscoelastic stress relaxation response. American Journal of Sports Medicine 25(5), 710-6.
21. Gleim G.W., Stachenfeld N.S., Nicholas J.A. (1990). The influence of flexibility on the economy of walking and jogging. Journal of Orthopaedic Research 8(6), 814-23.
22. Craib M.W., Mitchell V.A., Fields K.B., Cooper T.R., Hopewell R., Morgan D.W. (1996). The association between flexibility and running economy in sub-elite male distance runners. Medicine and Science in Sports and Exercise 28(6), 737-43.