Relationships between muscle size and hardness of the medial gastrocnemius at different ankle joint angles in young men

Background: Measurement of muscle hardness is important for objective evaluation of the muscle condition. Muscle hardness may be related to muscle size, but this relationship has not yet been examined. Purpose: To examine the relationship between the size and hardness of the medial gastrocnemius muscle (MG) at different ankle joint angles. Material and Methods: Twelve young male subjects comprised the present study. Muscle thickness of the right MG (MT _MG) was measured by ultrasonography as an index of muscle size. The MT _MG was determined at 30% of the lower leg length from the popliteal crease to the lateral malleolus. At the same site, muscle hardness of the right MG (MH _MG) was measured with ultrasound real-time tissue elastography. Both measurements were taken while each subject was instructed to lie in a prone position on a test bench. The right ankle was secured to a handmade apparatus by a non-elastic belt, with the hip and knee joints fully extended and the ankle positioned at +30°, 0° and -20° (0°: neutral position, positive values indicating plantar flexion). Results: There were no significant differences in the MT _MG among the ankle joint angles whereas the MH _MG significantly increased as the ankle was moved from plantar flexion to dorsiflexion. As a result, there was a difference in the MT _MG-MH _MG relationship between the angles. The MT _MG significantly correlated with the MH _MG only at +30° (r = 0.635, P < 0.05) where passive torque around the ankle has been shown to be near zero in previous studies. Conclusion: The hardness of the relaxed MG with no passive tension appears to depend on its size.