Mitsuyoshi Murayama

Changes in hardness of the human elbow flexor muscles after eccentric exercise

Abstract The purpose of this study was to investigate changes in muscle hardness after eccentric exercise of the elbow flexors muscles that produce muscle shortening and swelling. To assess muscle hardness, a pressure method was used in which the force required to deform the tissue (skin, subcutaneous tissue, muscle) was recorded. Eleven healthy male students performed 24 maximal eccentric actions of the elbow flexor muscles with their non-dominant arms. Muscle hardness, maximal isometric force (MIF), muscle soreness, plasma creatine kinase (CK) activity, relaxed elbow joint angle (RANG), upper-arm circumference (CIR) and B-mode ultrasound transverse images were measured before, immediately after, and 1–5 days after exercise. A long-lasting decrease in MIF, muscle swelling shown by increases in CIR and muscle thickness, large increases in plasma CK activity, and development of muscle soreness indicated that damage occurred to the elbow flexor muscles. The RANG had decreased by approximately 20° at 1–3 days after exercise and showed a gradual recovery thereafter. The CIR increased gradually after exercise and peaked on day 5 post-exercise, the mean amount of increase in CIR being 18 mm. Muscle hardness measured at the relaxed elbow position did not change until 3 days after exercise, but increased significantly (P < 0.01) on days 4 and 5 post-exercise. On the other hand, muscle hardness measured when forcibly extending the shortened elbow joint increased significantly (P < 0.01) with time and peaked at 3 days after exercise. Muscle hardness assessed by the pressure method seems to reflect changes in muscle stiffness and swelling.

Relationship between muscle hardness estimated by the indentation method and muscle contractile level

The purpose of this study was to investigate the relationship between muscle contractile level and hardness by the indentation method. Eleven healthy male subjects were involved in this study. The subjects put their arms horizontally on a table. The subjects were instructed to keep the isometric contractile force constant at three elbow angles. The indentation depth and reaction force of the biceps brachii muscle were measured, and electromyogram (EMG) were done. First, EMGs before and during the indentation were compared. The EMGs showed no significant increase during the indentation. The indentation did not affect muscle activities. Next, the force indentation depth curves were approximated with the non-linear Voigt model by the least square method, and the indices of the elasticity and the viscosity were estimated. Then the relationship between the indices and the contractile levels were investigated. The contractile level was the force normalized by that in the maximum voluntary contraction. The elastic indices increased almost linearly as the contractile levels increased. The relationships at different elbow angles did not show significant differences. However, the characteristics of the viscous indices varied depending on the subjects.

Elastic- and viscous-like properties of the upper arm estimated by the indentation method

The purpose of this study is to estimate the elastic-and viscous-like properties of the human upper arm using the indentation method. Healthy male subjects put their upper arms horizontally on a table. An indenter was placed 1-2 mm over the upper arm. When the indenter was moved vertically with a robot arm, the displacement of the indenter and the force were measured. First, the relationships between the displacement and the force were modeled using several variations on the non-linear Voigt model. The model based on Herzs theory did not provide a good approximation of the indentation responses, but the proposed model did. Next, the indentation point dependency of the upper arm was investigated. The muscle hardness (elasticity and viscosity) was invariant in a relatively large area of the upper arm. Finally, the elastic and viscous coefficients of the upper arm were estimated when the subject was holding a weight. The heavier the weight was, the larger the elastic and viscous coefficients were. The results suggest that the muscle hardness can be evaluated quantitatively with the elastic and viscous coefficients estimated by the proposed model.

Relationship between Muscle Tension and Hardness in Isolated Frog Muscle with Electrical Stimulation

Muscle hardness increases as the contractile level increases. This increase is caused by changes in structure of the muscle fiber and blood flow; however, the mechanism of increasing hardness has not been clearly demonstrated. The objective of this study was to investigate the relationship between isolated frog muscle tension and hardness. Gastrocnemius muscles were mounted horizontally in a chamber. The femur was fixed, and the Achilles tendon was attached to a stretching device. The muscle tension and hardness were measured during various muscle stretches and with and without electrical stimulation. We applied two protocols. In the first, the muscle was stimulated and then stretched, whereas, in the second, it was stretched and then stimulated. The muscle hardness was proportional to the muscle tension at each amount of stretching in both protocols. There were no significant differences between protocols 1 and 2, although the stretch enhancement of the muscle force was expected in protocol 1. In our experiments, the muscle length corresponds to the ascending limb of the length-tension curves of a sarcomere. The results of this study suggest that the relationship between muscle tension and hardness was not affected by the stretch enhancement in the ascending limb of the length-tension curve. The slope of the regression line between the muscle tension and hardness decreased as the amount of the stretch increased. The decrease of the slope might be caused by structural changes in the filaments.