Lateral force transmission between human tendon fascicles Bjarki T. Haraldsson a , Per Aagaard e , Klaus Qvortrup c , Jens Bojsen-Moller a , Michael Krogsgaard d , Satu Koskinen a , Michael Kjaer a , S. Peter Magnusson a,b, ⁎ a Institute of Sports Medicine Copenhagen, Bispebjerg Hospital, Copenhagen, Denmark b Department of Physiotherapy, Bispebjerg Hospital, Copenhagen, Denmark c Department of Biomedical Sciences, The Panum Institute, University of Copenhagen, Denmark d Department of Orthopedic Surgery, Bispebjerg Hospital, Copenhagen, Denmark e Institute of Sports Exercise and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark Received 2 November 2006; received in revised form 30 August 2007; accepted 12 September 2007 Abstract Whether adjacent collagen fascicles transmit force in parallel is unknown. The purpose of the present study was to examine the magnitude of lateral force transmission between adjacent collagen fascicles from the human patellar and Achilles tendon. From each sample two adjacent strands of fascicles (ø 300–530 μm) enclosed in a fascicular membrane were dissected. The specimen was deformed to ∼ 3% strain in three independent load–displacement cycles in a small-scale tensile testing device. Cycle 1: the fascicles and the fascicular membrane were intact. Cycle 2: one fascicle was transversally cut while the other fascicle and the fascicular membrane were kept intact. Cycle 3: both fascicles were cut in opposite ends while the fascicular membrane was left intact. A decline in peak force of 45% and 55% from cycle 1 to cycle 2, and 93% and 92% from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively. A decline in stiffness of 39% and 60% from cycle 1 to cycle 2, and of 93% and 100% from cycle 2 to cycle 3 was observed in the patellar and Achilles tendon fascicles, respectively. The present data demonstrate that lateral force transmission between adjacent collagen fascicles in human tendons is small or negligible, suggesting that tendon fascicles largely act as independent structures and that force transmission principally takes place within the individual fascicles. © 2007 Elsevier B.V./International Society of Matrix Biology. All rights reserved. Keywords: Lateral force transmission; Collagen fascicles; DIC; Transmission electron microscopy 1. Introduction It is intuitive that contractile force generated within skeletal muscle is transmitted in series to the aponeurosis, tendon and bone to produce movement. There is some understanding of force transmission in skeletal muscle, but relatively little is known about force transmission in tendon. Skeletal muscle fibers do not always extend from one tendon plate to the other, which requires that contractile force can be transmitted laterally to adjacent fibers to initiate movement (Ounjian et al., 1991), which confirmed the elegant study by Street (1983). Force is transmitted from the muscle fibers to the aponeurosis via the myotendinous junction, and can also be transmitted to parallel adjacent structures via the aponeurosis (Huijing et al., 1998; Huijing and Jaspers, 2005). The aponeurosis, which has different mechanical properties than free tendon (Finni et al., 2003; Lieber et al., 2000; Magnusson et al., 2003a), may be loaded heterogeneously during skeletal muscle contraction (Finni et al., 2003), although how this affects tendon force transmission and to what extent lateral force transmission exists in human free tendon remains unknown. The tendon is a hierarchical structure (Kastelic et al., 1978), and in order to understand how force is transmitted within the structure it is necessary to examine the mechanical properties at various levels. A disparity in magnitude of strain at different hierarchical levels has been observed, and while the exact mechanism for such a hierarchy of strain distribution within the tendon structure remains unknown, it suggests that ‘gliding’ Available online at www.sciencedirect.com Matrix Biology 27 (2008) 86 – 95 www.elsevier.com/locate/matbio ⁎ Corresponding author. Institute of Sports Medicine Copenhagen, Bispebjerg Hospital, Bispebjerg Bakke 23, 2400 Copenhagen NV, Copenhagen, Denmark. E-mail address: P.Magnusson@mfi.ku.dk (S.P. Magnusson). 0945-053X/$ - see front matter © 2007 Elsevier B.V./International Society of Matrix Biology. All rights reserved. doi:10.1016/j.matbio.2007.09.001