Exploring the In Vivo Stretch-Shortening Cycle Effect and the Role of Muscle-Tendon Behavior in the Human Triceps Surae Muscle Exploring the In Vivo Stretch-Shortening Cycle Effect and the Role of Muscle-Tendon Behavior in the Human Triceps Surae Muscle Denis Holzer1*, Daniel Hahn2,3, Ansgar Schwirtz1, Tobias Siebert4, Wolfgang Seiberl1,5 1Technical University of Munich, 2Ruhr University Bochum, 3University of Queensland, Australia, 4University of Stuttgart, 5Universität der Bundeswehr München Keywords: muscle-tendon dynamics, decoupling, architectural gearing, in vivo Introduction During the shortening phase of stretch-shortening cycles (SSC), muscle force and work are increased compared with pure shortening muscle action (SHO). This is referred to as the SSC-effect and has been demonstrated in various experiments on a variety of structural levels (Seiberl et al., 2015). However, the underlying mechanisms and their interactions are still controversially discussed (Seiberl et al., 2021). The aim of this study was to identify how architectural muscle-tendon unit (MTU) gearing and MTU decoupling contributes to the SSC-effect during in vivo contractions. Methods Eleven participants (age: 30±5 years, height: 179±5cm, body mass: 79±7kg) performed sub- maximal fixed-end, SSC and SHO plantarflexion contractions on a dynamometer (IsoMed2000, D&R Ferstl GmbH) elicited by electrical nerve stimulation of the tibialis nerve. Motion capture (Vicon Peak) was used to assess lower leg kinematics. Ultrasound (Echo Blaster 128 CEXT LV7.5/60/128Z-2, UAB Telemed) measures were used to determine fas- cicle and tendon behavior of the gastrocnemius medialis MTU. A repeated measure analysis of variance was performed to identify significant differences during active muscle shortening across different contraction conditions. Results are presented as means ± standard devia- tion, the level of significance is set at α=0.05 and the effect size (η²) of .01, .06, and .14 refers to small, medium, and large effects, respectively. Results In SSCs, ankle joint torque and work, MTU force and work, and fascicle force were signifi- cantly increased during shortening compared with the SHO condition, (joint torque: 19±12%, p<0.001 [η²=0.58], MTU force: 22±14%, p<0.001 [η²=0.43], fascicle force: 22±17%, p<0.001 [η²=0.37]; joint work: 12±13%, p<0.001 [η²=0.83], MTU work: 17±18%, p<0.001 [η²=0.74]). In contrast, fascicle work was decreased by 35±41% during SSC shortening compared with SHO (p<0.001 [η²=0.24]). During shortening of SSC, fascicle rotation (44±12%, p=0.006 [η²=0.14]), fascicle length change (58±72%, p=0.009 [η²=0.13]) and shortening velocity (32±46%, p=0.039 [η²=0.07]) were significantly reduced compared with SHO condition. Achilles tendon (AT) length at the beginning of shortening, AT tendon shortening velocity and AT length change throughout the shortening phase were unaffected by the contraction type (p=0.457 [η²=0.01], p=0.131 [η²<0.01], p=0.127 [η²<0.01], respectively). Discussion Our results indicate that architectural muscle gearing leads to a reduction in fascicle short- ening velocity and fascicle rotation, thereby increasing the muscle’s force production capac- ity during shortening of SSC. Findings demonstrate how architectural gearing contributes to the SSC-effect on all structural levels and underline the compromised transferability of joint work measurements to estimated fascicle work. Literature Seiberl, W., Hahn, D., Power, G. A., Fletcher, J. R. & Siebert, T. 2021 Editorial: The Stretch-Shortening Cycle of Active Muscle and Muscle-Tendon Complex: What, Why and How It Increases Muscle Perfor- mance? Frontiers in physiology 12, 693141. Seiberl, W., Power, G. A., Herzog, W. & Hahn, D. 2015 The stretch-shortening cycle (SSC) revisited: residual force enhancement contributes to increased performance during fast SSCs of human m. adductor pollicis. Physiological Reports 3. Denis Holzer1*, Daniel Hahn2,3, Ansgar Schwirtz1, Tobias Siebert4, Wolfgang Seiberl1,5 1Technical University of Munich, 2Ruhr University Bochum, 3University of Queensland, Australia, 4University of Stuttgart, 5Universität der Bundeswehr München Keywords: muscle-tendon dynamics, decoupling, architectural gearing, in vivo Introduction During the shortening phase of stretch-shortening cycles (SSC), muscle force and work are increased compared with pure shortening muscle action (SHO). This is referred to as the SSC-effect and has been demonstrated in various experiments on a variety of structural levels (Seiberl et al., 2015). However, the underlying mechanisms and their interactions are still controversially discussed (Seiberl et al., 2021). The aim of this study was to identify how architectural muscle-tendon unit (MTU) gearing and MTU decoupling contributes to the SSC-effect during in vivo contractions. Methods Eleven participants (age: 30±5 years, height: 179±5cm, body mass: 79±7kg) performed sub- maximal fixed-end, SSC and SHO plantarflexion contractions on a dynamometer (IsoMed2000, D&R Ferstl GmbH) elicited by electrical nerve stimulation of the tibialis nerve. Motion capture (Vicon Peak) was used to assess lower leg kinematics. Ultrasound (Echo Blaster 128 CEXT LV7.5/60/128Z-2, UAB Telemed) measures were used to determine fas- cicle and tendon behavior of the gastrocnemius medialis MTU. A repeated measure analysis of variance was performed to identify significant differences during active muscle shortening across different contraction conditions. Results are presented as means ± standard devia- tion, the level of significance is set at α=0.05 and the effect size (η²) of .01, .06, and .14 refers to small, medium, and large effects, respectively. Results In SSCs, ankle joint torque and work, MTU force and work, and fascicle force were signifi- cantly increased during shortening compared with the SHO condition, (joint torque: 19±12%, p<0.001 [η²=0.58], MTU force: 22±14%, p<0.001 [η²=0.43], fascicle force: 22±17%, p<0.001 [η²=0.37]; joint work: 12±13%, p<0.001 [η²=0.83], MTU work: 17±18%, p<0.001 [η²=0.74]). In contrast, fascicle work was decreased by 35±41% during SSC shortening compared with SHO (p<0.001 [η²=0.24]). During shortening of SSC, fascicle rotation (44±12%, p=0.006 [η²=0.14]), fascicle length change (58±72%, p=0.009 [η²=0.13]) and shortening velocity (32±46%, p=0.039 [η²=0.07]) were significantly reduced compared with SHO condition. Achilles tendon (AT) length at the beginning of shortening, AT tendon shortening velocity and AT length change throughout the shortening phase were unaffected by the contraction type (p=0.457 [η²=0.01], p=0.131 [η²<0.01], p=0.127 [η²<0.01], respectively).