## 2 Materials and Methods  
Human flexor tendons of the forearm and hand (flexor digitorum profundus and superficialis) were harvested bilaterally from cadavers from the Department of Anatomy, University of Otago, New Zealand, in a Crosado-embalmed condition [17]. Ethical approval was obtained from the University of Otago Ethics Committee (H17/20). The tissues were retrieved from the central part of the tendons, i.e., areas remote from the origin of insertion sites and also distant from the transition between muscle fibers and tendon. The tendons were sectioned perpendicular to the main fiber direction in a condition without applying prestrain and then freed from surrounding tissues. Following tissue retrieval, the tissues were stored in sealed plastic bags filled with 1.3 volume% phenoxyethanol under light exclusion conditions. Immediately prior to the tests, the tendons were rinsed with isotonic (9 g/L NaCl) saline for 24 hours to remove the reversible effects of the ethanol embalming related to the denaturation of the extracellular matrix tertiary structures before the fatigue tests began [13]. All manipulations of the tendons were carried out in a moist environment to minimize drying [18]. Fifty samples from six different human donors were tested at 20 \({}^{\circ}\)C under ambient room conditions. Seven samples were only used for the combined analysis, as the number of samples per donor was too low for subject-dependent analysis (n < 6). The tendons were mounted individually in two 3D-printed clamps printed of commercial type polylactic acid (PLA) using an Ultimaker 3 Extended printer (Ultimaker B.V., Geldermalsen, The Netherlands). One clamp was used for each side. Deviating from previous interlocking two-part designs [18; 19; 20; 21], this new setup involved a single squeezing part with a central opening. The clamps consisted of a pattern of small 4-sided pyramids (design with flat pyramids, pyramid height = 0.75 mm, diagonal base = 2.12 mm). The pyramid patterns of the opposing sides interlock during clamping and thus create an additional form fit. Further details on the clamps and the setup can be found elsewhere [14; 15]. The inner aspect of the clamp was designed to have flattened pyramid structures, customized for the cyclic tensile testing. Holder arms that can be attached to the clamps ensure a consistent clamping length and facilitate the mounting. Figure 1 shows the described parts assembled. For the mechanical tests, an MTS Landmark Model 370.02 Servo Hydraulic Test System (MTS Systems Corporation, Eden Prairie, MN, USA) with a maximum applicable load of 25 kN and a standard load cell was used under cyclic tensile fatigue loading. Before the cyclic loading began, the axial displacement of the machine was set to 0 mm. The tissue clamping distance was standardized to 40 mm, and a parallel alignment of the tissues within the clamps as well as between the two clamps for each specimen was ensured. To this end, 3D-printed holder arms of thermoplastic polyurethane (TPU) were attached to the clamps as shown in Figure 2A. The clamps were then placed in the machine's jaws, and the pressure of the surrounding hydraulic clamps was standardized to 6 MPa (Figure 2B). Prior to the fatigue testing, an axial pre-load of 10 N on the specimen allowed for stabilized conditions to determine the cross-section area. A digital caliper (Mitutoyo Corporation, Sakado, Kawasaki, Kanagawa, Japan; accuracy \(\pm\) 0.02 mm) was used to measure the tendons' widths and thicknesses. These measures were obtained from three  
Figure 1: Computer-aided design model of the assembled clamps and holder arms (including the schematic representation of a sample). (**A**) 3D view, (**B**) cross-sectional view. Figure 2: Specim mounting in the testing machine (**A**) using clamps on each side and flexible holder arms to facilitate accurate placement with a consistent testing length and (**B**) setup ready for mechanical testing with 3D-printed clamps and a clamped tendon placed between the closed external jaws.