The amplitude and distribution of both strain and stress in human Achilles tendon during jumping were investigated by non-linear finite element analysis with a 5 parameter Mooney-Rivlin model. A hexahedral constitutive mesh model of Achilles tendon was constructed based on cross-sectional MR images. Anatomical insertion of tendon was fully fixed, also a 4000 N lengthening force was applied to the origin part to simulate jumping. A large deformation non-linear finite element analysis was used for computation. The first principal strain over 0.2 was shown in the part with small cross sectional area and in the upper part of the calcaneus, also the maximal first principal strain exceeded 0.33. The first principal stress in the similar parts was over 100 MPa (maximum 293 MPa). These parts are approximately where Achilles tendon's rupture and/or inflammation frequently occur. For sub-maximal increasing load level, these parts showed concentrated larger strain and stress earlier than other part, which could be related with the non-linear material property of Achilles tendon tissue. These results suggest that these parts frequently experience micro damage from this strain concentration during dynamic movements, thus become high risk part of ruptures and inflammation in tendon tissues. Therefore, the focused observation and caution of these parts would be important while investigating on injury risk of Achilles tendon.

Purpose: This study aims to clarify the biomechanical changes in double-poling performance in a sprint race of cross-country skiing caused by fatigue. Methods: Five cross-country skiers including those representing Japan national team performed double poling for 3 min in a flat ski circuit (320 m long), and their movement and force at around 30 m and 670 m were compared. Results: Because of fatigue, (1) the velocity significantly decreased because of a decrease in the cycle rate during 1 cycle, (2) the cycle rate decreased because of the slowly change in the upper body movement, (3) while body became to erect standing position and (4) the leg motion supported the upper body motion, and it was controlled together with upper body motion. These results show that in order to maintain the velocity of double poling in the second half of a race, it is important to maintain the movement of the upper body.

The aim of this study was to examine the effects of pelvic tilt angles on the maximal voluntary knee extension torque (MVC_KE torque), in relation with electromyographic activities of agonist and antagonist muscles. For nine healthy men, MVC_KE torque was determined in extended (40°, full extension=0°), intermediate (70°), and flexed (100°) trunk-thigh positions that were performed with anterior pelvic tilt (AP) and neutral pelvic tilt (NP) positions at the knee angle of 70° flexion. The pelvis-thigh angles were determined from the video images taken from the lateral side of the subject, and surface electromyographic (EMG) signals were recorded from the rectus femoris, vastus lateralis, vastus medialis, and biceps femoris muscles. The pelvis-thigh angles in AP were significantly smaller (pelvis was more anteriorly tilted) than those in NP for all trunk-thigh positions. The MVC_KE torque was significantly greater in NP than in AP in the intermediate (12 ± 6%) and flexed (15 ± 10%) trunk-thigh positions, while it was comparable for AP and NP in the extended trunk-thigh position. There were no significant differences in the EMG amplitudes of any muscles between positions. in addition, the MVC_KE torque was identical for the positions with similar pelvis-thigh angles in different trunk-thigh positions. it was concluded that the effect of pelvic tilt angles on the maximal voluntary knee extension torque depends on the positions of the trunk and thigh.