R. Raikova, Z. Ivanova, S. Angelova, An indeterminate problem for an upper limb model with four biarticular muscles and its three modifications–analytical and numerical solution and sensitivity analysis, Series on Biomechanics 36 (2022) 3-17, SJR(2021):0.201. http://dx.doi.org/10.7546/SB.36.2022.02.01 

Z.D. Nedyalkova, T.B. Ivanov, Qualitative analysis of a mathematical model of calcium dynamics inside the muscle cell, Annual of Sofia University "St. Kliment Ohridski", Faculty of Mathematics and Informatics 106 (2019) 127–151. https://www.fmi.uni-sofia.bg/en/node/8889