Human locomotion during daily activities and athletic performance is accompanied by the repetitive lengthening and subsequent shortening of the muscle-tendon unit (MTU) (Biewener et al. 2009). During such stretch-shortening cycle movements, energy is stored in the elastic parts of the MTU during the lengthening phase and then regained during the shortening phase (Biewener et al. 2009; Obst et al. 2013). The interaction of the muscle and tendon during such stretch-shortening cycles is complex and could be better understood by separately monitoring the relative length changes of the contractile and elastic parts of the muscle (Lichtwark et al. 2007). In addition to the properties of the muscle (e.g., force-velocity relation (Hill, 1938) and force-length relation (Gordon et al., 1966)), the properties and behavior of the tendons in human movement are of great importance for locomotion. The mechanical property that describes the relation between the force exerted on the tendon and its change in the length is the tendon stiffness (Kubo et al., 1999), which greatly affects the kinematics of the tendon during movement. The mechanical characteristics of tendons and their response to exercise have been examined in previous studies. An increase of elastic modulus, the material property of the tendon, was reported by several authors as a response to repeated stress (Buchanan and Marsh, 2001) or exercise at high strain magnitude (Arampatzis et al., 2007b; 2010). Several authors (Buchanan and Marsh, 2001; Michna and Hartmann, 1989; Woo et al., 1980; Vilarta and Vidal, 1989) have reported an increase in ultimate tensile force and hypertrophy of tendons as an adaptation to exercise, while immobilization has been reported to reduce these properties (Kubo et al., 2004; Woo et al., 1982; Yamamoto et al., 1999).
Such properties are also closely related to performance. Kim (2013) and Alexander and Vernon (2009) reported that the recoil of elastic energy caused by stretching of the muscle-tendon complex is reutilized to enhance the performance for the following shortening during a counter movement jump (CMJ) or hopping.
Lichtwark et al. (2007) showed that Achilles tendon compliance is optimized for muscle efficiency during prolonged running. Similarly, Arampatzis et al. (2007a) and Fletcher et al. (2010) reported that stiffer Achilles tendons are positively related to running economy. Besides efficiency, peak performance is also affected by tendon properties. Lai et al. (2014) found that during human ankle plantar flexion, storage and recovery of the tendon's elastic strain energy supports muscle performance and increases maximum sprinting speed. While efficiency is important for prolonged running, a short GCT is important in sprint running and jumping, and could be realized by direct transfer of force via a stiff tendon.
Bobbert and Casius (2011) and Farley et al. (1999) reported that the stiffness of the leg spring, including both tendon and muscle properties, is regulated according to the hopping height or frequency, and therefore affects GCT.
The ability to move with a short GCT and achieve a high jump height is needed in various sports (Phillips and Flanagan, 2015). During jumping and other types of sport movements with short GCT, muscle force should be transmitted directly to the bone and subsequently to the movement surface. Hence, Proske and Morgan (1987) suggested that a stiff tendon, which directly transmits force, is advantageous. This was confirmed by Bojsen-Moller et al. (2005), who reported a correlation between tendon (aponeurosis) stiffness and jumping height in SJ, CMJ, respectively. Regarding the whole MTU, previous studies have already shown a negative correlation between leg/ankle stiffness and GCT (Arampatzis et al., 2001; 2004; Morin et al., 2007). This phenomenon is related to both the muscle (by its activation status) and the tendon properties. However, to the best of our knowledge, no studies to date have investigated the relationship between tendon stiffness and GCT. Understanding the tendon-specific mechanisms may explain the negative relationship between ankle/leg stiffness and GCT.
Therefore, in this study, we investigated the relationship between Achilles tendon stiffness (ATS) and GCT. A secondary aim was to relate ATS with jumping ability during squat jumps (SJs) and CMJs, respectively. We hypothesized that ATS would be negatively related to GCT (i.e., that stiffer tendons would be related to shorter GCT). We further hypothesized that ATS will be positively related to the jump height of SJs and CMJs.
Nineteen physically active healthy males (mean + SD: 26.7 [+ or -] 3.9 years, 1.77 [+ or -] 0.07 m, 76.5 [+ or -] 6.7 kg, 7.2 [+ or -] 2.6 h training per week) participated in this study. The subjects were informed about the test procedure, and they each gave written consent to participate in the study. Participants with a history of lower-leg injuries were excluded. The study was approved by the Ethical Committee of the University of Graz.
We performed an empirical experimental cross-sectional study in laboratory conditions. After a warm-up of 5 min cycling at a moderate intensity, subjects performed CMJs and SJs, from which we determined jump height. In addition, drop jumps were performed on a force plate, from which we determined GCT. Subsequently, we asked the volunteers to perform isometric maximal voluntary plantar flexions on a dynamometer, while ultrasound measurements were also taken, from which we determined isometric maximum voluntary contraction (MVIC) torque and ATS.
Participants were asked to perform SJs and CMJs (Kistler Quattro Jump Bosco Protocol, version 126.96.36.199). During SJ tests subjects initiated the jump from a squatted position with a knee angle of 90[degrees]. During CMJ tests, subjects started from an erect position and jumped by flexing and subsequently extending their joints of the lower limbs. Both types of jump were executed with the hands on the hips with the aim to reach maximum height. Each subject performed three trials of both SJs and CMJs, respectively. The highest jump height was used for the further analysis. The participants were then instructed to perform three drop jump trials from a 40-cm box on a force plate (Kistler[R] force platform (1000 Hz)), with rest periods of at least 1 min between the measurements to avoid any fatigue. Participants were asked to perform with maximum effort, i.e. keep the GCT as short as possible and jump as high as possible (Taube et al., 2012). Jumps that did not appear to...