Browsing by Author "von Tscharner, Vinzenz"

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    Gait Entrainment in Coupled Oscillator Systems: Clarifying the Role of Energy Optimization in Human Walking
    (2020-01-13) Schroeder, Ryan T.; Bertram, John E. A.; Croft, James L.; Sawicki, Gregory S.; von Tscharner, Vinzenz; Shrive, Nigel; Cluff, Tyler; Rubenson, Jonas
    Empirical evidence suggests that parameters of human gait (e.g. step frequency, step length) tend to minimize energy expenditure. However, it is unclear if individuals can adapt to dynamic environments in real time, i.e. continuously optimize energy expenditure, and to what extent. Two coupled oscillator systems were used to test the learned interactions of individuals within dynamic environments: (1) experienced farmworkers carrying oscillating loads on a flexible bamboo pole and (2) individuals walking on a treadmill while strapped to a mechatronics oscillator system providing periodic forces to the body. Reductionist trajectory optimization models predicted energy-minimizing gait interactions within the coupled oscillator systems and were compared to experimental data assessed with linear mixed models. On average, pole carriers significantly adjusted step frequency by 3.3% (0.067 Hz, p=0.014) to accommodate the bamboo pole – consistent with model predictions of energy savings. Novice subjects entrained (i.e. synchronized) their step frequency with machine oscillations up to ±10% of preferred step frequency and at amplitudes as low as 5% body weight (or ~33 N). Still, some subjects rarely entrained at all, and many exhibited transient entrainment, i.e. they drifted in and out of step frequencies matching the machine oscillations. Overall, subject entrainment was more robust and consistent with lower frequencies and higher amplitudes (20-30% of body weight). Although no systematic difference was found between the metabolic consumption of subjects during and not during entrainment, the net mechanical work done on subjects by the force oscillations had a strong effect on metabolic output (p<0.0001). Net work was largely determined by the alignment of oscillation forces within the gait cycle. Both the optimization model and subjects aligned force oscillations with their body velocity to increase positive power. All in all, it seems that subjects prefer entrainment with environmental oscillations under certain circumstances. However, entrainment does not appear to be motivated by energetic cost, at least not directly and not as a first priority. It is possible that individuals stabilize interactions with the environment (e.g. entrainment) as a prerequisite for effective feedforward and/or feedback gait control.
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    Neuromuscular adaptations and sensorimotor integration following a unilateral transfemoral amputation
    (2019-09-14) Claret, Claudia R; Herget, Georg W; Kouba, Lukas; Wiest, Daniel; Adler, Jochen; von Tscharner, Vinzenz; Stieglitz, Thomas; Pasluosta, Cristian
    Abstract Background Following an amputation, the human postural control system develops neuromuscular adaptations to regain an effective postural control. We investigated the compensatory mechanisms behind these adaptations and how sensorimotor integration is affected after a lower-limb transfemoral amputation. Methods Center of pressure (CoP) data of 12 unilateral transfemoral amputees and 12 age-matched able-bodied subjects were recorded during quiet standing with eyes open (EO) and closed (EC). CoP adjustments under each leg were recorded to study their contribution to posture control. The spatial structure of the CoP displacements was characterized by measuring the mean distance, the mean velocity of the CoP adjustments, and the sway area. The Entropic Half-Life (EnHL) quantifies the temporal structure of the CoP adjustments and was used to infer disrupted sensory feedback loops in amputees. We expanded the analysis with measures of weight-bearing imbalance and asymmetry, and with two standardized balance assessments, the Berg Balance Scale (BBS) and Timed Up-and-Go (TUG). Results There was no difference in the EnHL values of amputees and controls when combining the contributions of both limbs (p = 0.754). However, amputees presented significant differences between the EnHL values of the intact and prosthetic limb (p <  0.001). Suppressing vision reduced the EnHL values of the intact (p = 0.001) and both legs (p = 0.028), but not in controls. Vision feedback in amputees also had a significant effect (increase) on the mean CoP distance (p <  0.001), CoP velocity (p <  0.001) and sway area (p = 0.007). Amputees presented an asymmetrical stance. The EnHL values of the intact limb in amputees were positively correlated to the BBS scores (EO: ρ = 0.43, EC: ρ = 0.44) and negatively correlated to the TUG times (EO: ρ = − 0.59, EC: ρ = − 0.69). Conclusion These results suggest that besides the asymmetry in load distribution, there exist neuromuscular adaptations after an amputation, possibly related to the loss of sensory feedback and an altered sensorimotor integration. The EnHL values suggest that the somatosensory system predominates in the control of the intact leg. Further, suppressing the visual system caused instability in amputees, but had a minimal impact on the CoP dynamics of controls. These findings points toward the importance of providing somatosensory feedback in lower-limb prosthesis to reestablish a normal postural control. Trial registration DRKS00015254 , registered on September 20th, 2018.
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    Shoe Bending Stiffness and Muscle-Tendon Unit Function in Running
    (2021-01-08) Čigoja, Saša; Nigg, Benno Maurus; Edwards, William Brent; Stefanyshyn, Darren John; von Tscharner, Vinzenz; Boyd, Steven K.; Hoogkamer, Wouter
    The latest records set during long-distance running competitions have been attributed to recent footwear midsole innovations. One of these midsole innovations that has been claimed to have large effects on biomechanical, physiological, and performance variables is the use of a carbon fibre plate to increase the longitudinal bending stiffness of a shoe. Several mechanisms were proposed to be associated with performance improvements when running in footwear with carbon fibre plates. One of these mechanisms, the principle of optimising muscle function is currently not well understood. Therefore, this thesis aimed to investigate the effects of midsole bending stiffness of athletic footwear on muscle and muscle-tendon unit function in running. The first part of this thesis showed that running in stiff footwear resulted in a redistribution of positive work from proximal to distal lower limb joints. Specifically, it was found that a fatigue-induced redistribution of joint work from distal to proximal joints can be delayed when running in stiff footwear. The second part of this thesis dealt with the shortening velocities of muscle-tendon units. Estimated shank muscle-tendon unit shortening velocities were reduced when running in stiffer shoes. Experimental results using ultrasound imaging of the gastrocnemius medialis muscle revealed that the muscle shortened to a lesser extent and with lower average velocities in stiff running footwear. These findings could have implications for long-distance running performance. Positive work generation at more distal joints (i.e., ankle) may result in lower active muscle volume, which has been shown to be the main determinant of changes in the energetic cost of running. Slower shortening velocities of the gastrocnemius medialis could allow the muscle to operate on a more favourable position on its force-velocity relationship. This could allow for more economical force generation for a longer period during long-distance running. Altered muscle function could be a source of improved performance when running in stiff shoes.