EFFECT OF FATIGUE FROM REPEATED SPRINTS ON HAMSTRING MUSCLE ACTIVATION PATTERNS DURING RUNNING

Hamstring injury has been associated with fatigue-induced reductions in activation levels during running. This study examined neuromuscular changes of the hamstring muscles as a result of fatigue following sprinting in a group of nine team sport athletes. Hamstring muscle activation, lower-limb kinematics and isokinetic eccentric hamstring strength were assessed to examine the effects of fatigue during running at five different sub-maximal speeds. As expected, there were significant increases in both Biceps Femoris (BF) and Semitendinosus (ST) activations with running speed (P \u3c 0.001). After fatigue, BF activation during late swing significantly decreased by an average of 11% (P=0.002). There was evidence in some subjects that ST activity was increased with fatigue but the increase (4%) was non-significant for the group. There was also a tendency for reduced BF activity with fatigue to be more evident at the faster speeds of running. These findings support other evidence in the literature that the lateral hamstrings (BF) are more susceptible to fatigue. In addition, there were signs of compensatory increased ST activation levels in some subjects. These effects lend support to the potential benefit of this neuromuscular assessment of the hamstrings as a useful measure of both performance and recovery

MUSCLE INERTIA DURING RUNNING: A MASSIVE CHANGE OF MOMENTS?

Skeletal muscles have substantial inertia that cause inertial forces working around joints. These inertial forces are not typically considered in musculoskeletal models used for sport biomechanics research, which can lead to considerable errors in estimated joint kinetics. How large these errors are in common sports movements is yet unclear. We therefore examined the role of shank muscle inertia on ankle joint moments during the swing phase of running at different speeds. Ankle moments were considerably affected when muscles were modelled as separate masses, with a general shift towards reduced dorsiflexion and higher plantarflexion moments. These results show that ignoring inertial muscle forces in musculoskeletal simulations can lead to under- or overestimations of structure-specific loads and possibly erroneous conclusions. We therefore encourage sport biomechanics researchers to consider the impact of muscle inertia on inverse dynamics calculations

Adjustments with running speed reveal neuromuscular adaptations during landing associated with high mileage running training.

It remains to be determined whether running training influences the amplitude of lower limb muscle activations before and during the first half of stance and whether such changes are associated with joint stiffness regulation and usage of stored energy from tendons. Therefore, the aim of this study was to investigate neuromuscular and movement adaptations before and during landing in response to running training across a range of speeds. Two groups of high mileage (HM; >45 km/wk, n = 13) and low mileage (LM; <15 km/wk, n = 13) runners ran at four speeds (2.5-5.5 m/s) while lower limb mechanics and electromyography of the thigh muscles were collected. There were few differences in prelanding activation levels, but HM runners displayed lower activations of the rectus femoris, vastus medialis, and semitendinosus muscles postlanding, and these differences increased with running speed. HM runners also demonstrated higher initial knee stiffness during the impact phase compared with LM runners, which was associated with an earlier peak knee flexion velocity, and both were relatively unchanged by running speed. In contrast, LM runners had higher knee stiffness during the slightly later weight acceptance phase and the disparity was amplified with increases in speed. It was concluded that initial knee joint stiffness might predominantly be governed by tendon stiffness rather than muscular activations before landing. Estimated elastic work about the ankle was found to be higher in the HM runners, which might play a role in reducing weight acceptance phase muscle activation levels and improve muscle activation efficiency with running training.NEW & NOTEWORTHY Although neuromuscular factors play a key role during running, the influence of high mileage training on neuromuscular function has been poorly studied, especially in relation to running speed. This study is the first to demonstrate changes in neuromuscular conditioning with high mileage training, mainly characterized by lower thigh muscle activation after touch down, higher initial knee stiffness, and greater estimates of energy return, with adaptations being increasingly evident at faster running speeds

Identifying generalised segmental acceleration patterns that contribute to ground reaction force features across different running tasks

Objective: To support future developments of field-based biomechanical load monitoring tools, this study aimed to identify generalised segmental acceleration patterns and their contribution to ground reaction forces (GRFs) across different running tasks. Design: Exploratory experimental design. Methods: A multivariate principal component analysis (PCA) was applied to a combination of segmental acceleration data from all body segments for fifteen team-sport athletes performing accelerated, decelerated and constant low-, moderate- and high-speed running, and 90° cutting trials. Segmental acceleration profiles were then reconstructed from each principal component (PC) and used to calculate their specific GRF contributions. Results: The first PC explained 48.57% of the acceleration variability for all body segments and was primarily related to the between-task differences in the overall magnitude of the GRF impulse. Magnitude and timing of high-frequency acceleration and GRF features (i.e. impact related characteristics) were primarily explained by the second PC (12.43%) and also revealed important between-task differences. The most important GRF characteristics were explained by the first five PCs, while PCs beyond that primarily contained small contributions to the overall GRF impulse. Conclusions: These findings show that a multivariate PCA approach can reveal generalised acceleration patterns and specific segmental contributions to GRF features, but their relative importance for different running activities are task dependent. Using segmental acceleration to assess whole-body biomechanical loading generically across various movements may thus require task identification algorithms and/or advanced sensor or data fusion approaches

J21 Using inertial measurement units to estimate ground reaction force and knee angular velocity during decelerations

Despite undergoing anterior cruciate ligament (ACL) reconstruction, athletes continue to present altered ground reaction force (GRF) and knee angular velocities (AV) for 6-24 months as they return to highly demanding tasks such as rapid decelerations (Schmitt et al., 2015, Med Sci Sports Ex, 47, 1426). In-field identification of GRF and knee AV during such deceleration tasks is challenging due to the laboratory restrictions of force platforms and 3D motion capture. Therefore, the use of inertial measurement units (IMU) has been suggested as an appropriate alternative to estimate such variables in the field (Pratt &amp; Sigward, 2018. Sensors, 3460). The study aimed to establish whether IMU-derived measures can be used to estimate GRF and knee AV that are indicative of ACL injury risk during decelerations. Following ethical approval from the Cardiff Metropolitan Ethics Committee, ten male team-sport athletes performed five maximal decelerations at three approach speeds (100, 85, and 70% relative to maximal attainable speed), for a total of fifteen trials per athlete. GRF and kinematics were measured using four Kistler force platforms and Theia3D markerless motion capture. Pearson’s correlation coefficient was used to determine the relationship between IMeasureU Blue Trident IMUs mounted at the thigh or shank and vertical GRF, horizontal GRF, and knee AV during the first step of deceleration. A measure was considered field-viable if a very large significant correlation (r ≥ 0.7; P ≤ 0.05) was observed. At approach speeds of 85 and 70%, shank AV had a very large significant correlation (r = 0.79, 0.76; P = 0.001, 0.01, respectively) to vertical GRF, a moderate to large significant correlation to horizontal GRF (r = 0.46, 0.51; P = 0.04, 0.02, respectively), and a moderate significant correlation to knee AV (r =0.46, 0.40; P = 0.004, 0.02, respectively). At 100% approach speeds, shank AV had no significant correlation to GRF (r &lt;0.3; P&gt;0.05) and a moderate correlation to knee AV (r = 0.40; P = 0.02). This study was the first to find a strong association between shank AV and vertical GRF for whole-body decelerations at 85 and 70% approach speeds. This supports the use of IMUs in sport-specific settings (e.g., football pitches) to better quantify vertical GRF deficits during decelerations. This can further enhance in-field monitoring of GRF deficits and better assess athletes' readiness to return to sport following ACL reconstruction. However, alternative methods are required to accurately estimate horizontal GRFs and knee AV in the field