Browsing by Author "Mohr, Maurice"

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    Lower Extremity Muscle Activation Following a Previous Knee Injury: Implications for Post-Traumatic Knee Osteoarthritis
    (2018-12-05) Mohr, Maurice; Nigg, Benno Maurus; Emery, Carolyn A.; Von Tscharner, Vinzenz; Herzog, Walter
    Individuals who sustain an intra-articular knee injury are at a high risk of developing post-traumatic knee osteoarthritis (PTOA) 10-20 years later. Compensatory activation patterns of knee muscles in response to the trauma may persist past the acute injury phase and result in abnormal mechanical loading and subsequent osteoarthritis of the knee. This dissertation aimed to use surface electromyography (sEMG) to explore abnormal leg muscle activation patterns and their possible involvement in PTOA development in individuals who suffered a previous knee injury 3-12 years ago. The first part of this thesis presents methodological investigations related to two sEMG-based assessments of knee muscle activation strategies. It was shown that 1) sEMG amplitude-based co-contraction indices during gait exhibit poor between-day reliability and 2) the magnitude of intermuscular coherence strongly depends on the configuration and alignment of the sEMG electrodes. Building on the methodological findings, the second part of this thesis investigated the association between a previous knee injury and leg muscle activation during walking and squatting while considering the influence of sex. Sex-specific abnormalities in quadricep and hamstring muscle activation patterns were present for the affected leg in individuals more than three years after a previous knee injury. Altered quadricep and hamstring muscle activation may result in abnormal movement and loading of the knee joint and thus be linked to mechanical risk factors for PTOA development. This dissertation could not provide evidence, however, that altered thigh muscle activation was associated with more self-reported knee pain or symptoms indicative of PTOA development. From a methodological perspective, the poor reliability of current sEMG-based markers for abnormal muscle activation may lower the sensitivity to detect associations with risk factors for PTOA. From a conceptual perspective, the development of PTOA is not solely based on joint mechanics but depends on the interplay between mechanical, biological, and structural abnormalities of the joint following a knee injury. Therefore, the pathway to PTOA is likely unique to each individual such that a consistent association between abnormal muscle activation following a knee injury and PTOA risk may not exist.
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    Task-Dependent Intermuscular Motor Unit Synchronization between Medial and Lateral Vastii Muscles during Dynamic and Isometric Squats
    (PLoS ONE, 2015-11-03) Mohr, Maurice; Nann, Marius; von Tschamer, Vinzenz; Eskofier, Bjoern; Nigg, Benno Maurus
    Purpose Motor unit activity is coordinated between many synergistic muscle pairs but the functional role of this coordination for the motor output is unclear. The purpose of this study was to investigate the short-term modality of coordinated motor unit activity–the synchronized discharge of individual motor units across muscles within time intervals of 5ms–for the Vastus Medialis (VM) and Lateralis (VL). Furthermore, we studied the task-dependency of intermuscular motor unit synchronization between VM and VL during static and dynamic squatting tasks to provide insight into its functional role. Methods Sixteen healthy male and female participants completed four tasks: Bipedal squats, single-leg squats, an isometric squat, and single-leg balance. Monopolar surface electromyography (EMG) was used to record motor unit activity of VM and VL. For each task, intermuscular motor unit synchronization was determined using a coherence analysis between the raw EMG signals of VM and VL and compared to a reference coherence calculated from two desynchronized EMG signals. The time shift between VM and VL EMG signals was estimated according to the slope of the coherence phase angle spectrum. Results For all tasks, except for singe-leg balance, coherence between 15–80Hz significantly exceeded the reference. The corresponding time shift between VM and VL was estimated as 4ms. Coherence between 30–60Hz was highest for the bipedal squat, followed by the single-leg squat and the isometric squat. Conclusion There is substantial short-term motor unit synchronization between VM and VL. Intermuscular motor unit synchronization is enhanced for contractions during dynamic activities, possibly to facilitate a more accurate control of the joint torque, and reduced during single-leg tasks that require balance control and thus, a more independent muscle function. It is proposed that the central nervous system scales the degree of intermuscular motor unit synchronization according to the requirements of the movement task at hand.