ECSS Paris 2023: OP-BM10
INTRODUCTION: Single leg hops (SLH) are a vital measure for evaluating the readiness of athletes to return to sports post-injury. The efficacy of SLH is based on its indication of adequate lower limb movement and stability, which is essential for a variety of sports activities. This study investigated the differences in muscle synergies in trunk and lower limbs during SLH at 30% (SLH30) and 100% (SLH100) of maximum distance to understand the neuromechanical mechanisms underlying the high performance of SLH. METHODS: Ten healthy males were recruited for the study. During the SLH30 and SLH100 tests, unilateral surface EMG data were collected from 15 muscle groups in the trunk and lower limbs. Non-negative matrix factorization (NMF) was employed to extract the muscle synergies. RESULTS: The number of muscle synergies observed in SLH100 was significantly higher than in SLH30 (p = 0.0078, effect size = 1.28). Median values were 4.0 (3.0 – 5.0) for SLH30 and 5.0 (4.0 – 6.0) for SLH100. The cluster analysis identified four muscle synergies shared between SLH30 and SLH100, with a distinct non-knee related synergy emerging in SLH100. CONCLUSION: Interventions targeting SLH performance should consider muscles linked to the synergy specific to SLH100, especially in individuals with sports-related injuries. Shared synergies between SLH30 and SLH100 represent a foundational neuromuscular control strategy, while a distinct synergy exclusive to SLH100 plays a role in facilitating longer single-leg hops. The synergy specific to SLH100 represents the neuromechanical output for extended forward jumps. However, the presence of a distinct synergy in SLH100, associated with non-knee muscles, suggests potential limitations of SLH in evaluating knee-specific functions.
Read CV Hiroki SaitoECSS Paris 2023: OP-BM10
INTRODUCTION: Conventional gravity-based strength training methods have been used for centuries. Barbell (Bb) back squat is considered as the golden standard for lower extremity strength training. A disadvantage of conventional strength training is that the resistance stays constant throughout the movement. Flywheel (FW) training is a strength training method which utilizes moment of inertia as a way of inducing resistance, which varies the external load throughout the movement based on applied muscle force (1). The purpose of this study was to compare biomechanical differences in one repetition maximum (1RM) done with Bb exercise with 1RM done with FW squat. METHODS: 14 male athletes with previous strength training background participated in the study. Subjects performed 1RM of Bb and FW squat. Forces were measured with a force plate in Bb and with a pulling force transducer in FW. A 2D motion analysis was done on a highspeed camera for knee and hip joint angles and angular velocities. Muscle activities were measured with surface EMG electrodes from rectus femoris (RF), vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF) and gluteus maximus (GM) muscles. Squat movement was divided into eccentric and concentric phases with 5 subsections each (ECC 1-5 and CON 1-5). Pre- and post-isometric leg press maximal voluntary contractions were also measured. RESULTS: Greater eccentric forces were seen throughout the eccentric subsections in Bb, however significantly greater forces were seen in FW squat in the last two sections of the concentric phase (19,6 % - 42.7 %, p < 0.05 – 0.001). No significant differences were seen in knee and hip joint angles. However, corresponding angular velocities in concentric subsections showed some significant differences between the two squat types: lower velocities in FW in CON 1 and CON 5, but greater velocities in FW from CON 2 to 4 (p < 0.05 – 0.001). Muscle activities were significantly greater in FW squats in RF (14.8 % - 101.8 %), VL (4.6 % - 45.6 %), VM (2.5 % - 54.4 %), and BF (16.2 % - 48.4 %) in eccentric sections, with RF showing also significantly greater activity in concentric sections (21.3 % - 54.8 %) (p < 0.05). GM activity was significantly lower in FW in eccentric (14.3 % - 40.0 %) and concentric (11.9 % - 58.6 %) sections (p < 0.05–0.001). CONCLUSION: Interestingly FW squats showed less force, but greater muscle activities compared to Bb squats. Thus, resistance type (moment of inertia vs. gravity) might not be the only factor that influences the results. Position of the center of gravity differs between the squat types according to the location of the load. For example, back needs to be more upright position in Bb which is not the case in FW due to the harness used. This can already create differences in operative muscle lengths and thus muscle activities. REFERENCES: 1) Norrbrand et al., Aviation, Space, and Environmental Medicine, 2011
Read CV Janne AvelaECSS Paris 2023: OP-BM10
INTRODUCTION: The human foot arch could have a function to utilize elastic energy from a muscle and tendon complex during bipedal locomotion. During running and jumping, foot arches store and release elastic energy when they compress and reform. This stretch-shortening process increases mechanical energy and improve physical performance. Also, the force generating capacity of the foot increases when the foot arch height decreases due to loading. We suggest that the force amplification mechanism is mechanically regulated by the dynamic function of the foot arch in conjunction with the stretching of a muscle and tendon complex of the foot. These functions of the foot could play an important role in enhancing the initiation of jump performance. Indeed, we showed that toe flexor strength was required for the force reacting on the ground during the jump performance while the foot arch function might help to potentiate energy in a countermovement jump (Yamauchi and Koyama, 2020). However, there have been no studies addressing how the dynamic function of the foot arch affects the muscles that generate force during the ground contact phase in the jump performance. The aim of this study was to use kinetic and kinematic motion analysis to examine how changes in the foot arch with increased vertical force affect the jump performance in drop jumping. METHODS: Twenty-six subjects performed drop jumping from a box of 45-cm hight under the barefoot condition. Vertical ground reaction force (GRF) was measured on a force plate on a right foot during the contact phase of a drop jump, and GRF valuables were calculated. Three-dimensional position data of retroreflective markers and vertical ground reaction force data were synchronously collected with an eight-camera three-dimensional optical motion capture system and a force plate, respectively. The medial longitudinal arch was represented as three retroreflective markers, which were placed on the skin: the navicular tuberosity, the medial border of the first metatarsal head and the medial tubercle of the calcaneus. The foot arch dynamics was analyzed from these kinematic data, and was quantified as the amount of changes in foot arch height and angle at the landing phase and the take-off phase. RESULTS: The jump height of the drop jumping was 27.6 ± 5.2 cm. The foot arch height and angle on the landing phase were 1.82 ± 0.28 cm and 18.2 ± 4.4 degrees, and those on the take-off phase were 1.78 ± 0.38 cm and 19.4 ± 4.2 degrees. There was no significant relationship between drop jump heights and foot arch variables on the landing phase; however, a significant relationship between drop jump heights and foot arch variables on the take-off phase. CONCLUSION: The results of this study suggests that the mechanical contribution of the foot arch dynamics could help to enhancing human jump performance. The foot arch has an ability to integrate and generate force in the take-off phase of human countermovement jumping.
Read CV Keiji KoyamaECSS Paris 2023: OP-BM10