The study of myosin protein is directly related to the tenderness, growth, development and muscle contraction, and expresses different isoforms, where the way it operates has great influence on meat quality. This study aimed to relate the Nellore meat (Bos indicus) separating 2 contrasting groups through the identification and quantification of heavy chain isoforms of myosin (I, IIa, IIx and IIb). A total of 718 samples of the Longissimus thoracis from animals slaughtered in commercial abattoir were used in the shear force analysis (SF) to determine tenderness. According to SF were selected 2 groups: (1) Group of tender meat (n = 15) and (2) Group of tough meat (n = 15) who underwent extraction, identification and quantification of heavy chain isoforms myosin (MyHC) by electrophoresis (SDS-PAGE) and photo documentation of gels. Statistical analyzes were performed using the GLM procedure, CORR and averages were compared using the Tukey test using Statistical Analysis System. For MyHC was identified and quantified 3 isoforms (I, IIa and IIx), the presence of MyHC IIb isoform has not been identified. Mean values of the optical density index (IOD) in relative percentage of MyHC isoforms were observed in the tender meat group and tough respectively MyHC-I (18.2 ± 3.54 and 16.9 ± 3.14), MyHC-IIa (16.7 ± 3.09 and 14.0 ± 3.38) and MyHC-IIx (64.9 ± 3.79 and 66.3 ± 5.05). It can be seen a greater amount of type II isoform (fast) compared with type I (slow). There were no differences (P > 0.05) between the extreme groups of type I and IIx, unlike IIa (P < 0.05), with the highest relative percentage in tender meat group and reported a significant negative association with SF (rp = −0.34; P < 0.05). It is concluded from the results that the Nellore have good relationship between the hypertrophy of the Longissimus thoracis, as measured by the concentration of MyHC IIa, and meat tenderness, measured by the SF. The study of MyHC isoforms is an appropriate type of muscle fibers identification tool having a good correlation with the meat tenderness feature.