@article{SchoenrockMuckeltHastermannetal.2024,
  author    = {Schoenrock, Britt and Muckelt, Paul E. and Hastermann, Maria and Albracht, Kirsten and MacGregor, Robert and Martin, David and Gunga, Hans-Christian and Salanova, Michele and Stokes, Maria J. and Warner, Martin B. and Blottner, Dieter},
  title     = {Muscle stiffness indicating mission crew health in space},
  series = {Scientific Reports},
  volume    = {14},
  journal   = {Scientific Reports},
  number    = {Article number: 4196},
  publisher = {Springer Nature},
  address   = {London},
  issn      = {2045-2322},
  doi       = {10.1038/s41598-024-54759-6},
  pages     = {13 Seiten},
  year      = {2024},
  abstract  = {Muscle function is compromised by gravitational unloading in space affecting overall musculoskeletal health. Astronauts perform daily exercise programmes to mitigate these effects but knowing which muscles to target would optimise effectiveness. Accurate inflight assessment to inform exercise programmes is critical due to lack of technologies suitable for spaceflight. Changes in mechanical properties indicate muscle health status and can be measured rapidly and non-invasively using novel technology. A hand-held MyotonPRO device enabled monitoring of muscle health for the first time in spaceflight (> 180 days). Greater/maintained stiffness indicated countermeasures were effective. Tissue stiffness was preserved in the majority of muscles (neck, shoulder, back, thigh) but Tibialis Anterior (foot lever muscle) stiffness decreased inflight vs. preflight (p < 0.0001; mean difference 149 N/m) in all 12 crewmembers. The calf muscles showed opposing effects, Gastrocnemius increasing in stiffness Soleus decreasing. Selective stiffness decrements indicate lack of preservation despite daily inflight countermeasures. This calls for more targeted exercises for lower leg muscles with vital roles as ankle joint stabilizers and in gait. Muscle stiffness is a digital biomarker for risk monitoring during future planetary explorations (Moon, Mars), for healthcare management in challenging environments or clinical disorders in people on Earth, to enable effective tailored exercise programmes.},
  language  = {en}
}
@article{LiphardtFernandezGonzaloAlbrachtetal.2023,
  author    = {Liphardt, Anna-Maria and Fernandez-Gonzalo, Rodrigo and Albracht, Kirsten and Rittweger, J{\"o}rn and Vico, Laurence},
  title     = {Musculoskeletal research in human space flight - unmet needs for the success of crewed deep space exploration},
  series = {npj Microgravity},
  volume    = {9},
  journal   = {npj Microgravity},
  number    = {Article number: 9},
  publisher = {Springer Nature},
  issn      = {2373-8065},
  doi       = {10.1038/s41526-023-00258-3},
  pages     = {1 -- 9},
  year      = {2023},
  abstract  = {Based on the European Space Agency (ESA) Science in Space Environment (SciSpacE) community White Paper "Human Physiology - Musculoskeletal system", this perspective highlights unmet needs and suggests new avenues for future studies in musculoskeletal research to enable crewed exploration missions. The musculoskeletal system is essential for sustaining physical function and energy metabolism, and the maintenance of health during exploration missions, and consequently mission success, will be tightly linked to musculoskeletal function. Data collection from current space missions from pre-, during-, and post-flight periods would provide important information to understand and ultimately offset musculoskeletal alterations during long-term spaceflight. In addition, understanding the kinetics of the different components of the musculoskeletal system in parallel with a detailed description of the molecular mechanisms driving these alterations appears to be the best approach to address potential musculoskeletal problems that future exploratory-mission crew will face. These research efforts should be accompanied by technical advances in molecular and phenotypic monitoring tools to provide in-flight real-time feedback.},
  language  = {en}
}
@article{HeieisBoeckerD'Angeloetal.2023,
  author    = {Heieis, Jule and B{\"o}cker, Jonas and D'Angelo, Olfa and Mittag, Uwe and Albracht, Kirsten and Sch{\"o}nau, Eckhard and Meyer, Andreas and Voigtmann, Thomas and Rittweger, J{\"o}rn},
  title     = {Curvature of gastrocnemius muscle fascicles as function of muscle-tendon complex length and contraction in humans},
  series = {Physiological Reports},
  volume    = {11},
  journal   = {Physiological Reports},
  number    = {11},
  publisher = {Wiley},
  issn      = {2051-817X},
  doi       = {10.14814/phy2.15739},
  pages     = {e15739, Seite 1-11},
  year      = {2023},
  abstract  = {It has been shown that muscle fascicle curvature increases with increasing contraction level and decreasing muscle-tendon complex length. The analyses were done with limited examination windows concerning contraction level, muscle-tendon complex length, and/or intramuscular position of ultrasound imaging. With this study we aimed to investigate the correlation between fascicle arching and contraction, muscle-tendon complex length and their associated architectural parameters in gastrocnemius muscles to develop hypotheses concerning the fundamental mechanism of fascicle curving. Twelve participants were tested in five different positions (90°/105°*, 90°/90°*, 135°/90°*, 170°/90°*, and 170°/75°*; *knee/ankle angle). They performed isometric contractions at four different contraction levels (5\%, 25\%, 50\%, and 75\% of maximum voluntary contraction) in each position. Panoramic ultrasound images of gastrocnemius muscles were collected at rest and during constant contraction. Aponeuroses and fascicles were tracked in all ultrasound images and the parameters fascicle curvature, muscle-tendon complex strain, contraction level, pennation angle, fascicle length, fascicle strain, intramuscular position, sex and age group were analyzed by linear mixed effect models. Mean fascicle curvature of the medial gastrocnemius increased with contraction level (+5 m-1 from 0\% to 100\%; p = 0.006). Muscle-tendon complex length had no significant impact on mean fascicle curvature. Mean pennation angle (2.2 m-1 per 10°; p < 0.001), inverse mean fascicle length (20 m-1 per cm-1; p = 0.003), and mean fascicle strain (-0.07 m-1 per +10\%; p = 0.004) correlated with mean fascicle curvature. Evidence has also been found for intermuscular, intramuscular, and sex-specific intramuscular differences of fascicle curving. Pennation angle and the inverse fascicle length show the highest predictive capacities for fascicle curving. Due to the strong correlations between pennation angle and fascicle curvature and the intramuscular pattern of curving we suggest for future studies to examine correlations between fascicle curvature and intramuscular fluid pressure.},
  language  = {en}
}
@article{WaldvogelFreylerHelmetal.2023,
  author    = {Waldvogel, Janice and Freyler, Kathrin and Helm, Michael and Monti, Elena and St{\"a}udle, Benjamin and Gollhofer, Albert and Narici, Marco V. and Ritzmann, Ramona and Albracht, Kirsten},
  title     = {Changes in gravity affect neuromuscular control, biomechanics, and muscle-tendon mechanics in energy storage and dissipation tasks},
  series = {Journal of Applied Physiology},
  volume    = {134},
  journal   = {Journal of Applied Physiology},
  number    = {1},
  publisher = {American Physiological Society},
  address   = {Bethesda, Md.},
  issn      = {1522-1601 (Onlineausgabe)},
  doi       = {10.1152/japplphysiol.00279.2022},
  pages     = {190 -- 202},
  year      = {2023},
  abstract  = {This study evaluates neuromechanical control and muscle-tendon interaction during energy storage and dissipation tasks in hypergravity. During parabolic flights, while 17 subjects performed drop jumps (DJs) and drop landings (DLs), electromyography (EMG) of the lower limb muscles was combined with in vivo fascicle dynamics of the gastrocnemius medialis, two-dimensional (2D) kinematics, and kinetics to measure and analyze changes in energy management. Comparisons were made between movement modalities executed in hypergravity (1.8 G) and gravity on ground (1 G). In 1.8 G, ankle dorsiflexion, knee joint flexion, and vertical center of mass (COM) displacement are lower in DJs than in DLs; within each movement modality, joint flexion amplitudes and COM displacement demonstrate higher values in 1.8 G than in 1 G. Concomitantly, negative peak ankle joint power, vertical ground reaction forces, and leg stiffness are similar between both movement modalities (1.8 G). In DJs, EMG activity in 1.8 G is lower during the COM deceleration phase than in 1 G, thus impairing quasi-isometric fascicle behavior. In DLs, EMG activity before and during the COM deceleration phase is higher, and fascicles are stretched less in 1.8 G than in 1 G. Compared with the situation in 1 G, highly task-specific neuromuscular activity is diminished in 1.8 G, resulting in fascicle lengthening in both movement modalities. Specifically, in DJs, a high magnitude of neuromuscular activity is impaired, resulting in altered energy storage. In contrast, in DLs, linear stiffening of the system due to higher neuromuscular activity combined with lower fascicle stretch enhances the buffering function of the tendon, and thus the capacity to safely dissipate energy.},
  language  = {en}
}
@article{StaeudleSeynnesLapsetal.2022,
  author    = {St{\"a}udle, Benjamin and Seynnes, Olivier and Laps, Guido and Br{\"u}ggemann, Gert-Peter and Albracht, Kirsten},
  title     = {Altered gastrocnemius contractile behavior in former achilles tendon rupture patients during walking},
  series = {Frontiers in Physiology},
  volume    = {13},
  journal   = {Frontiers in Physiology},
  publisher = {Frontiers Research Foundation},
  address   = {Lausanne},
  issn      = {1664-042X},
  doi       = {10.3389/fphys.2022.792576},
  pages     = {12 Seiten},
  year      = {2022},
  abstract  = {Achilles tendon rupture (ATR) remains associated with functional limitations years after injury. Architectural remodeling of the gastrocnemius medialis (GM) muscle is typically observed in the affected leg and may compensate force deficits caused by a longer tendon. Yet patients seem to retain functional limitations during—low-force—walking gait. To explore the potential limits imposed by the remodeled GM muscle-tendon unit (MTU) on walking gait, we examined the contractile behavior of muscle fascicles during the stance phase. In a cross-sectional design, we studied nine former patients (males; age: 45 ± 9 years; height: 180 ± 7 cm; weight: 83 ± 6 kg) with a history of complete unilateral ATR, approximately 4 years post-surgery. Using ultrasonography, GM tendon morphology, muscle architecture at rest, and fascicular behavior were assessed during walking at 1.5 m⋅s-1 on a treadmill. Walking patterns were recorded with a motion capture system. The unaffected leg served as control. Lower limbs kinematics were largely similar between legs during walking. Typical features of ATR-related MTU remodeling were observed during the stance sub-phases corresponding to series elastic element (SEE) lengthening (energy storage) and SEE shortening (energy release), with shorter GM fascicles (36 and 36\%, respectively) and greater pennation angles (8° and 12°, respectively). However, relative to the optimal fascicle length for force production, fascicles operated at comparable length in both legs. Similarly, when expressed relative to optimal fascicle length, fascicle contraction velocity was not different between sides, except at the time-point of peak series elastic element (SEE) length, where it was 39 ± 49\% lower in the affected leg. Concomitantly, fascicles rotation during contraction was greater in the affected leg during the whole stance-phase, and architectural gear ratios (AGR) was larger during SEE lengthening. Under the present testing conditions, former ATR patients had recovered a relatively symmetrical walking gait pattern. Differences in seen AGR seem to accommodate the profound changes in MTU architecture, limiting the required fascicle shortening velocity. Overall, the contractile behavior of the GM fascicles does not restrict length- or velocity-dependent force potentials during this locomotor task.},
  language  = {en}
}
@article{QuittmannAbelAlbrachtetal.2022,
  author    = {Quittmann, Oliver J. and Abel, Thomas and Albracht, Kirsten and Str{\"u}der, Heiko K.},
  title     = {Biomechanics of all-out handcycling exercise: kinetics, kinematics and muscular activity of a 15-s sprint test in able-bodied participants},
  series = {Sports Biomechanics},
  volume    = {21},
  journal   = {Sports Biomechanics},
  number    = {10},
  publisher = {Taylor \& Francis},
  address   = {London},
  issn      = {1752-6116 (Onlineausgabe)},
  doi       = {10.1080/14763141.2020.1745266},
  pages     = {1200 -- 1223},
  year      = {2022},
  abstract  = {This study aims to quantify the kinematics, kinetics and muscular activity of all-out handcycling exercise and examine their alterations during the course of a 15-s sprint test. Twelve able-bodied competitive triathletes performed a 15-s all-out sprint test in a recumbent racing handcycle that was attached to an ergometer. During the sprint test, tangential crank kinetics, 3D joint kinematics and muscular activity of 10 muscles of the upper extremity and trunk were examined using a power metre, motion capturing and surface electromyography (sEMG), respectively. Parameters were compared between revolution one (R1), revolution two (R2), the average of revolution 3 to 13 (R3) and the average of the remaining revolutions (R4). Shoulder abduction and internal-rotation increased, whereas maximal shoulder retroversion decreased during the sprint. Except for the wrist angles, angular velocity increased for every joint of the upper extremity. Several muscles demonstrated an increase in muscular activation, an earlier onset of muscular activation in crank cycle and an increased range of activation. During the course of a 15-s all-out sprint test in handcycling, the shoulder muscles and the muscles associated to the push phase demonstrate indications for short-duration fatigue. These findings are helpful to prevent injuries and improve performance in all-out handcycling.},
  language  = {en}
}
@article{HerssensCowburnAlbrachtetal.2022,
  author    = {Herssens, Nolan and Cowburn, James and Albracht, Kirsten and Braunstein, Bjoern and Cazzola, Dario and Colyer, Steffi and Minetti, Alberto E. and Pavei, Gaspare and Rittweger, J{\"o}rn and Weber, Tobias and Green, David A.},
  title     = {Movement in low gravity environments (MoLo) programme - the MoLo-L.O.O.P. study protocol},
  series = {PLOS ONE / Public Library of Science},
  volume    = {17},
  journal   = {PLOS ONE / Public Library of Science},
  number    = {11},
  editor    = {Cattaneo, Luigi},
  publisher = {Plos},
  address   = {San Francisco},
  issn      = {1932-6203},
  doi       = {10.1371/journal.pone.0278051},
  pages     = {e0278051},
  year      = {2022},
  abstract  = {Exposure to prolonged periods in microgravity is associated with deconditioning of the musculoskeletal system due to chronic changes in mechanical stimulation. Given astronauts will operate on the Lunar surface for extended periods of time, it is critical to quantify both external (e.g., ground reaction forces) and internal (e.g., joint reaction forces) loads of relevant movements performed during Lunar missions. Such knowledge is key to predict musculoskeletal deconditioning and determine appropriate exercise countermeasures associated with extended exposure to hypogravity.},
  language  = {en}
}
@article{HeinkeKnickerAlbracht2021,
  author    = {Heinke, Lars N. and Knicker, Axel J. and Albracht, Kirsten},
  title     = {Test-retest reliability of the internal shoulder rotator muscles' stretch reflex in healthy men},
  series = {Journal of Electromyography and Kinesiology},
  volume    = {62},
  journal   = {Journal of Electromyography and Kinesiology},
  number    = {Article 102611},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1050-6411},
  doi       = {10.1016/j.jelekin.2021.102611},
  year      = {2021},
  abstract  = {Until now the reproducibility of the short latency stretch reflex of the internal rotator muscles of the glenohumeral joint has not been identified. Twenty-three healthy male participants performed three sets of external shoulder rotation stretches with various pre-activation levels on two different dates of measurement to assess test-retest reliability. All stretches were applied with a dynamometer acceleration of 104°/s2 and a velocity of 150°/s. Electromyographical response was measured via surface EMG. Reflex latencies showed a pre-activation effect (ƞ2 = 0,355). ICC ranged from 0,735 to 0,909 indicating an overall "good" relative reliability. SRD 95\% lay between ±7,0 to ±12,3 ms.. The reflex gain showed overall poor test-retest reproducibility. The chosen methodological approach presented a suitable test protocol for shoulder muscles stretch reflex latency evaluation. A proof-of-concept study to validate the presented methodical approach in shoulder involvement including subjects with clinically relevant conditions is recommended.},
  language  = {en}
}
@article{WaldvogelRitzmannFreyleretal.2021,
  author    = {Waldvogel, Janice and Ritzmann, Ramona and Freyler, Kathrin and Helm, Michael and Monti, Elena and Albracht, Kirsten and St{\"a}udle, Benjamin and Gollhofer, Albert and Narici, Marco},
  title     = {The Anticipation of Gravity in Human Ballistic Movement},
  series = {Frontiers in Physiology},
  journal   = {Frontiers in Physiology},
  publisher = {Frontiers},
  address   = {Lausanne},
  issn      = {1664-042X},
  doi       = {10.3389/fphys.2021.614060},
  year      = {2021},
  abstract  = {Stretch-shortening type actions are characterized by lengthening of the pre-activated muscle-tendon unit (MTU) in the eccentric phase immediately followed by muscle shortening. Under 1 g, pre-activity before and muscle activity after ground contact, scale muscle stiffness, which is crucial for the recoil properties of the MTU in the subsequent push-off. This study aimed to examine the neuro-mechanical coupling of the stretch-shortening cycle in response to gravity levels ranging from 0.1 to 2 g. During parabolic flights, 17 subjects performed drop jumps while electromyography (EMG) of the lower limb muscles was combined with ultrasound images of the gastrocnemius medialis, 2D kinematics and kinetics to depict changes in energy management and performance. Neuro-mechanical coupling in 1 g was characterized by high magnitudes of pre-activity and eccentric muscle activity allowing an isometric muscle behavior during ground contact. EMG during pre-activity and the concentric phase systematically increased from 0.1 to 1 g. Below 1 g the EMG in the eccentric phase was diminished, leading to muscle lengthening and reduced MTU stretches. Kinetic energy at take-off and performance were decreased compared to 1 g. Above 1 g, reduced EMG in the eccentric phase was accompanied by large MTU and muscle stretch, increased joint flexion amplitudes, energy loss and reduced performance. The energy outcome function established by linear mixed model reveals that the central nervous system regulates the extensor muscles phase- and load-specifically. In conclusion, neuro-mechanical coupling appears to be optimized in 1 g. Below 1 g, the energy outcome is compromised by reduced muscle stiffness. Above 1 g, loading progressively induces muscle lengthening, thus facilitating energy dissipation.},
  language  = {en}
}
@article{RichterBraunsteinStaeudleetal.2021,
  author    = {Richter, Charlotte and Braunstein, Bjoern and Staeudle, Benjamin and Attias, Julia and Suess, Alexander and Weber, Tobias and Mileva, Katya N. and Rittweger, Joern and Green, David A. and Albracht, Kirsten},
  title     = {Contractile behavior of the gastrocnemius medialis muscle during running in simulated hypogravity},
  series = {npj Microgravity},
  volume    = {7},
  journal   = {npj Microgravity},
  number    = {Article number: 32},
  publisher = {Springer Nature},
  address   = {New York},
  issn      = {2373-8065},
  doi       = {10.1038/s41526-021-00155-7},
  pages     = {7 Seiten},
  year      = {2021},
  abstract  = {Vigorous exercise countermeasures in microgravity can largely attenuate muscular degeneration, albeit the extent of applied loading is key for the extent of muscle wasting. Running on the International Space Station is usually performed with maximum loads of 70\% body weight (0.7 g). However, it has not been investigated how the reduced musculoskeletal loading affects muscle and series elastic element dynamics, and thereby force and power generation. Therefore, this study examined the effects of running on the vertical treadmill facility, a ground-based analog, at simulated 0.7 g on gastrocnemius medialis contractile behavior. The results reveal that fascicle-series elastic element behavior differs between simulated hypogravity and 1 g running. Whilst shorter peak series elastic element lengths at simulated 0.7 g appear to be the result of lower muscular and gravitational forces acting on it, increased fascicle lengths and decreased velocities could not be anticipated, but may inform the development of optimized running training in hypogravity. However, whether the alterations in contractile behavior precipitate musculoskeletal degeneration warrants further study.},
  language  = {en}
}
@article{RichterBraunsteinStaeudleetal.2021,
  author    = {Richter, Charlotte and Braunstein, Bjoern and St{\"a}udle, Benjamin and Attias, Julia and Suess, Alexander and Weber, Tobias and Mileva, Katja N. and Rittweger, Joern and Green, David A. and Albracht, Kirsten},
  title     = {Gastrocnemius medialis contractile behavior is preserved during 30\% body weight supported gait training},
  series = {Frontiers in Sports and Active Living},
  volume    = {2021},
  journal   = {Frontiers in Sports and Active Living},
  number    = {2},
  publisher = {Frontiers},
  address   = {Lausanne},
  issn      = {2624-9367},
  doi       = {10.3389/fspor.2020.614559},
  pages     = {Artikel 614559},
  year      = {2021},
  abstract  = {Rehabilitative body weight supported gait training aims at restoring walking function as a key element in activities of daily living. Studies demonstrated reductions in muscle and joint forces, while kinematic gait patterns appear to be preserved with up to 30\% weight support. However, the influence of body weight support on muscle architecture, with respect to fascicle and series elastic element behavior is unknown, despite this having potential clinical implications for gait retraining. Eight males (31.9 ± 4.7 years) walked at 75\% of the speed at which they typically transition to running, with 0\% and 30\% body weight support on a lower-body positive pressure treadmill. Gastrocnemius medialis fascicle lengths and pennation angles were measured via ultrasonography. Additionally, joint kinematics were analyzed to determine gastrocnemius medialis muscle-tendon unit lengths, consisting of the muscle's contractile and series elastic elements. Series elastic element length was assessed using a muscle-tendon unit model. Depending on whether data were normally distributed, a paired t-test or Wilcoxon signed rank test was performed to determine if body weight supported walking had any effects on joint kinematics and fascicle-series elastic element behavior. Walking with 30\% body weight support had no statistically significant effect on joint kinematics and peak series elastic element length. Furthermore, at the time when peak series elastic element length was achieved, and on average across the entire stance phase, muscle-tendon unit length, fascicle length, pennation angle, and fascicle velocity were unchanged with respect to body weight support. In accordance with unchanged gait kinematics, preservation of fascicle-series elastic element behavior was observed during walking with 30\% body weight support, which suggests transferability of gait patterns to subsequent unsupported walking.},
  language  = {en}
}
@article{RichterBraunsteinStaeudleetal.2021,
  author    = {Richter, Charlotte and Braunstein, Bj{\"o}rn and St{\"a}udle, Benjamin and Attias, Julia and S{\"u}ss, Alexander and Weber, Tobias and Mileva, Katya N. and Rittweger, J{\"o}rn and Green, David A. and Albracht, Kirsten},
  title     = {Gastrocnemius medialis contractile behavior during running differs between simulated Lunar and Martian gravities},
  series = {Scientific reports},
  volume    = {11},
  journal   = {Scientific reports},
  number    = {Article number: 22555},
  publisher = {Springer Nature},
  address   = {London},
  issn      = {2045-2322},
  doi       = {10.1038/s41598-021-00527-9},
  pages     = {13 Seiten},
  year      = {2021},
  abstract  = {The international partnership of space agencies has agreed to proceed forward to the Moon sustainably. Activities on the Lunar surface (0.16 g) will allow crewmembers to advance the exploration skills needed when expanding human presence to Mars (0.38 g). Whilst data from actual hypogravity activities are limited to the Apollo missions, simulation studies have indicated that ground reaction forces, mechanical work, muscle activation, and joint angles decrease with declining gravity level. However, these alterations in locomotion biomechanics do not necessarily scale to the gravity level, the reduction in gastrocnemius medialis activation even appears to level off around 0.2 g, while muscle activation pattern remains similar. Thus, it is difficult to predict whether gastrocnemius medialis contractile behavior during running on Moon will basically be the same as on Mars. Therefore, this study investigated lower limb joint kinematics and gastrocnemius medialis behavior during running at 1 g, simulated Martian gravity, and simulated Lunar gravity on the vertical treadmill facility. The results indicate that hypogravity-induced alterations in joint kinematics and contractile behavior still persist between simulated running on the Moon and Mars. This contrasts with the concept of a ceiling effect and should be carefully considered when evaluating exercise prescriptions and the transferability of locomotion practiced in Lunar gravity to Martian gravity.},
  language  = {en}
}
@article{WerkhausenWillwacherAlbracht2021,
  author    = {Werkhausen, Amelie and Willwacher, Steffen and Albracht, Kirsten},
  title     = {Medial gastrocnemius muscle fascicles shorten throughout stance during sprint acceleration},
  series = {Scandinavian Journal of Medicine \& Science in Sports},
  volume    = {31},
  journal   = {Scandinavian Journal of Medicine \& Science in Sports},
  number    = {7},
  publisher = {Wiley-Blackwell},
  address   = {Oxford},
  issn      = {0905-7188 (Druckausgabe)},
  doi       = {10.1111/sms.13956},
  pages     = {1471 -- 1480},
  year      = {2021},
  abstract  = {The compliant nature of distal limb muscle-tendon units is traditionally considered suboptimal in explosive movements when positive joint work is required. However, during accelerative running, ankle joint net mechanical work is positive. Therefore, this study aims to investigate how plantar flexor muscle-tendon behavior is modulated during fast accelerations. Eleven female sprinters performed maximum sprint accelerations from starting blocks, while gastrocnemius muscle fascicle lengths were estimated using ultrasonography. We combined motion analysis and ground reaction force measurements to assess lower limb joint kinematics and kinetics, and to estimate gastrocnemius muscle-tendon unit length during the first two acceleration steps. Outcome variables were resampled to the stance phase and averaged across three to five trials. Relevant scalars were extracted and analyzed using one-sample and two-sample t-tests, and vector trajectories were compared using statistical parametric mapping. We found that an uncoupling of muscle fascicle behavior from muscle-tendon unit behavior is effectively used to produce net positive mechanical work at the joint during maximum sprint acceleration. Muscle fascicles shortened throughout the first and second steps, while shortening occurred earlier during the first step, where negative joint work was lower compared with the second step. Elastic strain energy may be stored during dorsiflexion after touchdown since fascicles did not lengthen at the same time to dissipate energy. Thus, net positive work generation is accommodated by the reuse of elastic strain energy along with positive gastrocnemius fascicle work. Our results show a mechanism of how muscles with high in-series compliance can contribute to net positive joint work.},
  language  = {en}
}
@article{MontiWaldvogelRitzmannetal.2021,
  author    = {Monti, Elena and Waldvogel, Janice and Ritzmann, Ramona and Freyler, Kathrin and Albracht, Kirsten and Helm, Michael and De Cesare, Niccol{\`o} and Pavan, Piero and Reggiani, Carlo and Gollhofer, Albert and Narici, Marco Vincenzo},
  title     = {Muscle in variable gravity: "I do not know where I am, but I know what to do"},
  series = {Frontiers in Physiology},
  volume    = {12},
  journal   = {Frontiers in Physiology},
  publisher = {Frontiers Research Foundation},
  address   = {Lausanne},
  issn      = {1664-042X},
  doi       = {10.3389/fphys.2021.714655},
  pages     = {19 Seiten},
  year      = {2021},
  abstract  = {Performing tasks, such as running and jumping, requires activation of the agonist and antagonist muscles before (motor unit pre-activation) and during movement performance (Santello and Mcdonagh, 1998). A well-timed and regulated muscle activation elicits a stretch-shortening cycle (SSC) response, naturally occurring in bouncing movements (Ishikawa and Komi, 2004; Taube et al., 2012). By definition, the SSC describes the stretching of a pre-activated muscle-tendon complex immediately followed by a muscle shortening in the concentric push-off phase (Komi, 1984). Given the importance of SSC actions for human movement, it is not surprising that many studies investigated the biomechanics of this phenomenon; in particular, drop jumps (DJs) represent a good paradigm to study muscle fascicle and tendon behavior in ballistic movements involving the SSC. Within a DJ, three main phases [pre-activation, braking, and push-off (PO; Komi, 2000)] have been recognized and extensively studied in common and challenging conditions, such as changes in load, falling height, or simulated hypo-gravity (Avela et al., 1994; Arampatzis et al., 2001; Fukashiro et al., 2005; Ishikawa et al., 2005; Sousa et al., 2007; Ritzmann et al., 2016; Helm et al., 2020). These studies show that the timing and amount of triceps-surae muscle-tendon unit pre-activation in DJs are differentially regulated based on the load applied to the muscle, being optimal in normal "Earth" gravity conditions (Avela et al., 1994), but decreased in simulated hypo-gravity, hyper-gravity (Avela et al., 1994; Ritzmann et al., 2016), or unknown conditions (i.e., unknown falling heights; Helm et al., 2020). Some authors indicated that, when falling from heights different from the optimal one [defined as the drop height giving a maximum DJ performance indicated as peak ground reaction force (GRF) or jump high], electromyographic (EMG) activity of the plantar flexors increases from lower than optimal to higher than optimal heights (Ishikawa and Komi, 2004; Sousa et al., 2007). These findings highlight the ability of the central nervous system to regulate the timing and amount of pre-activation according to different jumping conditions, thus regulating muscle fascicle length, tendon and joint stiffness as well as position, in order to safely land on the ground and quickly re-bounce. Similarly, to pre-activation, also in the braking phase, the plantar flexors are differentially regulated. In optimal height (i.e., load) jumping conditions, gastrocnemius medialis (GM) fascicles shorten at early ground contact (possibly due to the intervention of the stretch reflex; Gollhofer et al., 1992) and behave quasi-isometrically in the late braking phase, enabling tendon elongation, and storage of elastic energy (Gollhofer et al., 1992; Fukashiro et al., 2005; Sousa et al., 2007). When increasing the falling height (augmenting the impact GRF), the quasi-isometric behavior of fascicles disappears, and fast fascicle lengthening occurs (Ishikawa et al., 2005; Sousa et al., 2007). In the third and last PO phase, fascicles shorten and the tendon releases the elastic energy previously stored. Bobbert et al. (1987) reported no influence of jumping height on the work done and on the net vertical impulse assessed during PO; this observation suggests that, despite an optimal DJ performance might be achieved only in specific conditions (falling heights, loads), the central nervous system seems to be able to regulate muscle behavior in order to effectively perform the required task also in challenging situations. Although the regulation of triceps-surae muscle-tendon unit in DJs has been extensively investigated, very few studies focused on sarcomeres behavior during the performance of this SSC movement (Kurokawa et al., 2003; Fukashiro et al., 2005, 2006). Sarcomeres represent muscle contractile units and are known to express different amounts of force depending on their length (Gordon et al., 1966; Walker and Schrodt, 1974); thus, understanding the time course of their responses during DJs is fundamental to gain further insights into muscle force-generating capacity. In vivo measurement of sarcomere length in humans has been so far been performed only in static positions and under highly controlled experimental conditions (Llewellyn et al., 2008; Sanchez et al., 2015). Instead, human sarcomere length estimation (achieved by dividing GM measured fascicle length for a fixed sarcomere number) in dynamic contractions provided an indirect measure of sarcomere operating range during squat jump, countermovement jump, and DJ (Fukashiro et al., 2005, 2006; Kurokawa et al., 2003). The results of these studies showed that sarcomeres operate in the ascending limb of their length-tension (L-T) relationship in all types of jumps, and particularly so in DJ. However, most of the available observations on sarcomere and muscle fascicle behavior were made in condition of constant gravity. Thus, in order to understand how sarcomere and muscle fascicle length are regulated in variable gravity conditions, we performed experiments in a parabolic flight, involving variable gravity levels, ranging from about zero-g to about double the Earth's gravity (1 g; Waldvogel et al., 2021). Specifically, the aims of the present study were as follows: 1. To investigate the ability of the neuromuscular system in regulating fascicle length in response to conditions of variable gravity. 2. To estimate sarcomere operative length in the different DJ phases, in order to calculate its theoretical force production and its possible modulation in conditions of variable gravity. We hypothesized that muscle fascicles would be differentially regulated in different gravity conditions compared to 1 g, particularly in anticipation of landing and re-bouncing in unknown gravity levels. In addition, we hypothesized that sarcomeres would operate in the upper part of the ascending limb of their L-T relationship, possibly lengthening during the braking phase (especially in hyper-gravity) while operating quasi-isometrically in 1 g.},
  language  = {en}
}
@article{StaeudleSeynnesLapsetal.2021,
  author    = {St{\"a}udle, Benjamin and Seynnes, Olivier and Laps, Guido and G{\"o}ll, Fabian and Br{\"u}ggemann, Gert-Peter and Albracht, Kirsten},
  title     = {Recovery from achilles tendon repair: a combination of Postsurgery Outcomes and Insufficient remodeling of muscle and tendon},
  series = {Medicine \& Science in Sports \& Exercise},
  volume    = {53},
  journal   = {Medicine \& Science in Sports \& Exercise},
  number    = {7},
  publisher = {American College of Sports Medicine},
  address   = {Philadelphia, Pa.},
  issn      = {1530-0315},
  doi       = {10.1249/MSS.0000000000002592},
  pages     = {1356 -- 1366},
  year      = {2021},
  abstract  = {Achilles tendon rupture (ATR) patients have persistent functional deficits in the triceps surae muscle-tendon unit (MTU). The complex remodeling of the MTU accompanying these deficits remains poorly understood. The purpose of the present study was to associate in vivo and in silico data to investigate the relations between changes inMTU properties and strength deficits inATR patients. Methods: Elevenmale subjects who had undergone surgical repair of complete unilateral ATR were examined 4.6 ± 2.0 (mean ± SD) yr after rupture. Gastrocnemius medialis (GM) tendon stiffness, morphology, and muscle architecture were determined using ultrasonography. The force-length relation of the plantar flexor muscles was assessed at five ankle joint angles. In addition, simulations (OpenSim) of the GM MTU force-length properties were performed with various iterations of MTU properties found between the unaffected and the affected side. Results: The affected side of the patients displayed a longer, larger, and stiffer GM tendon (13\% ± 10\%, 105\% ± 28\%, and 54\% ± 24\%, respectively) compared with the unaffected side. The GM muscle fascicles of the affected side were shorter (32\% ± 12\%) and with greater pennation angles (31\% ± 26\%). A mean deficit in plantarflexion moment of 31\% ± 10\% was measured. Simulations indicate that pairing an intact muscle with a longer tendon shifts the optimal angular range of peak force outside physiological angular ranges, whereas the shorter muscle fascicles and tendon stiffening seen in the affected side decrease this shift, albeit incompletely. Conclusions: These results suggest that the substantial changes in MTU properties found in ATR patients may partly result from compensatory remodeling, although this process appears insufficient to fully restore muscle function.},
  language  = {en}
}
@techreport{GoellBraunsteinAlbracht2021,
  author    = {G{\"o}ll, Fabian and Braunstein, Bj{\"o}rn and Albracht, Kirsten},
  title     = {Lernende roboterassistierte Systeme f{\"u}r das neuromuskul{\"a}re Training - RoSylerNT; Teilvorhaben: Entwicklung eines neuromuskuloskelettalen Modells als Basis f{\"u}r die Interaktionsf{\"a}higkeiten autonomer Assistenzsysteme},
  publisher = {Deutsche Sporthochschule K{\"o}ln},
  address   = {K{\"o}ln},
  doi       = {10.2314/KXP:1855318741},
  pages     = {14 Seiten},
  year      = {2021},
  language  = {de}
}
@article{HeinkeKnickerAlbracht2020,
  author    = {Heinke, Lars N. and Knicker, Axel J. and Albracht, Kirsten},
  title     = {Increased shoulder muscle stretch reflex elicitability in supine subject posture},
  series = {Isokinetics and Exercise Science},
  volume    = {28},
  journal   = {Isokinetics and Exercise Science},
  number    = {2},
  publisher = {IOS Press},
  address   = {Amsterdam},
  issn      = {1878-5913},
  doi       = {10.3233/IES-192219},
  pages     = {139 -- 146},
  year      = {2020},
  abstract  = {BACKGROUND: Muscle stretch reflexes are widely used to examine neural muscle function. The knowledge of reflex response in muscles crossing the shoulder is limited. OBJECTIVE: To quantify reflex modulation according to various subject postures and different procedures of muscle pre-activation steering. METHODS: Thirteen healthy male participants performed two sets of external shoulder rotation stretches in various positions and with different procedures of muscle pre-activation steering on an isokinetic dynamometer over a range of two different pre-activation levels. All stretches were applied with a dynamometer acceleration of 104∘/s2 and a velocity of 150∘/s. Electromyographical response was measured via sEMG. RESULTS: Consistent reflexive response was observed in all tested muscles in all experimental conditions. The reflex elicitation rate revealed a significant muscle main effect (F (5,288) = 2.358, ρ= 0.040; η2= 0.039; f= 0.637) and a significant test condition main effect (F (1,288) = 5.884, ρ= 0.016; η2= 0.020; f= 0.143). Reflex latency revealed a significant muscle pre-activation level main effect (F (1,274) = 5.008, ρ= 0.026; η2= 0.018; f= 0.469). CONCLUSION: Muscular reflexive response was more consistent in the primary internal rotators of the shoulder. Supine posture in combination with visual feedback of muscle pre-activation level enhanced the reflex elicitation rate.},
  language  = {en}
}
@article{QuittmannMeskemperAlbrachtetal.2020,
  author    = {Quittmann, Oliver J. and Meskemper, Joshua and Albracht, Kirsten and Abel, Thomas and Foitschik, Tina and Str{\"u}der, Heiko K.},
  title     = {Normalising surface EMG of ten upper-extremity muscles in handcycling: Manual resistance vs. sport-specific MVICs},
  series = {Journal of Electromyography and Kinesiology},
  volume    = {51},
  journal   = {Journal of Electromyography and Kinesiology},
  number    = {Article 102402},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1050-6411},
  doi       = {10.1016/j.jelekin.2020.102402},
  year      = {2020},
  abstract  = {Muscular activity in terms of surface electromyography (sEMG) is usually normalised to maximal voluntary isometric contractions (MVICs). This study aims to compare two different MVIC-modes in handcycling and examine the effect of moving average window-size. Twelve able-bodied male competitive triathletes performed ten MVICs against manual resistance and four sport-specific trials against fixed cranks. sEMG of ten muscles [M. trapezius (TD); M. pectoralis major (PM); M. deltoideus, Pars clavicularis (DA); M. deltoideus, Pars spinalis (DP); M. biceps brachii (BB); M. triceps brachii (TB); forearm flexors (FC); forearm extensors (EC); M. latissimus dorsi (LD) and M. rectus abdominis (RA)] was recorded and filtered using moving average window-sizes of 150, 200, 250 and 300 ms. Sport-specific MVICs were higher compared to manual resistance for TB, DA, DP and LD, whereas FC, TD, BB and RA demonstrated lower values. PM and EC demonstrated no significant difference between MVIC-modes. Moving average window-size had no effect on MVIC outcomes. MVIC-mode should be taken into account when normalised sEMG data are illustrated in handcycling. Sport-specific MVICs seem to be suitable for some muscles (TB, DA, DP and LD), but should be augmented by MVICs against manual/mechanical resistance for FC, TD, BB and RA.},
  language  = {en}
}
@article{QuittmannAbelAlbrachtetal.2020,
  author    = {Quittmann, Oliver J. and Abel, Thomas and Albracht, Kirsten and Meskemper, Joshua and Foitschik, Tina and Str{\"u}der, Heiko K.},
  title     = {Biomechanics of handcycling propulsion in a 30-min continuous load test at lactate threshold: Kinetics, kinematics, and muscular activity in able-bodied participants},
  series = {European Journal of Applied Physiology},
  journal   = {European Journal of Applied Physiology},
  number    = {120},
  publisher = {Springer},
  address   = {Heidelberg},
  issn      = {1439-6327},
  doi       = {10.1007/s00421-020-04373-x},
  pages     = {1403 -- 1415},
  year      = {2020},
  abstract  = {Purpose This study aims to investigate the biomechanics of handcycling during a continuous load trial (CLT) to assess the mechanisms underlying fatigue in upper body exercise. Methods Twelve able-bodied triathletes performed a 30-min CLT at a power output corresponding to lactate threshold in a racing recumbent handcycle mounted on a stationary ergometer. During the CLT, ratings of perceived exertion (RPE), tangential crank kinetics, 3D joint kinematics, and muscular activity of ten muscles of the upper extremity and trunk were examined using motion capturing and surface electromyography. Results During the CLT, spontaneously chosen cadence and RPE increased, whereas crank torque decreased. Rotational work was higher during the pull phase. Peripheral RPE was higher compared to central RPE. Joint range of motion decreased for elbow-flexion and radial-duction. Integrated EMG (iEMG) increased in the forearm flexors, forearm extensors, and M. deltoideus (Pars spinalis). An earlier onset of activation was found for M. deltoideus (Pars clavicularis), M. pectoralis major, M. rectus abdominis, M. biceps brachii, and the forearm flexors. Conclusion Fatigue-related alterations seem to apply analogously in handcycling and cycling. The most distal muscles are responsible for force transmission on the cranks and might thus suffer most from neuromuscular fatigue. The findings indicate that peripheral fatigue (at similar lactate values) is higher in handcycling compared to leg cycling, at least for inexperienced participants. An increase in cadence might delay peripheral fatigue by a reduced vascular occlusion. We assume that the gap between peripheral and central fatigue can be reduced by sport-specific endurance training.},
  language  = {en}
}
@inproceedings{PohleFroehlichDalitzRichteretal.2020,
  author    = {Pohle-Fr{\"o}hlich, Regina and Dalitz, Christoph and Richter, Charlotte and Hahnen, Tobias and St{\"a}udle, Benjamin and Albracht, Kirsten},
  title     = {Estimation of muscle fascicle orientation in ultrasonic images},
  series = {VISIGRAPP 2020 - Proceedings of the 15th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications, 5},
  booktitle = {VISIGRAPP 2020 - Proceedings of the 15th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications, 5},
  pages     = {79 -- 86},
  year      = {2020},
  language  = {en}
}