Abstract
Without high impact forces, it is not clear how humans can utilize tendon elasticity during low-impact activities. The purpose of the present study was to examine the muscle-tendon behavior together with the electromyographic (EMG) activities of the vastus lateralis (VL) muscle during the human dolphin-kicking. In a swimming pool, each subject (n = 11) swam the 25 m dolphin-kicking at two different speeds (NORMAL and FAST). Surface EMGs were recorded from the VL and biceps femoris (BF) muscles. Simultaneous recordings of the knee joint angle by electro-goniometer and of the VL fascicle length by ultrasonography were used to calculate the muscle-tendon unit and tendinous length of VL (LMTU and LTT, respectively). In the dolphin-kicking, the stretching and shortening amplitudes of VL LMTU did not differ significantly between the two kicking speed conditions. However, both stretching and shortening amplitudes of the VL fascicle length were lower at FAST than at NORMAL speed whereas the opposite was found for the VL LTT values. At FAST, the contribution of the VL tendinous length to the entire VLMTU length changes increased. The EMG analysis revealed at FAST higher agonist VL activation from the late up-beat (MTU stretching) to the early down-beat phases as well as increased muscle co-activation of VL and BF muscles from the late down-beat to early up-beat phases of dolphin-kicking. These results suggest that at increasing kicking speeds, the VL fascicles and tendinous tissues during aquatic movements can utilize tendon elasticity in a similar way than in terrestrial forms of locomotion. However, these activation profiles of VL and BF muscles may differ from their activation pattern in terrestrial locomotion.
Highlights
In terrestrial locomotion, the actively stretched skeletal muscle can enhance the power and/or efficiency, following the stretch-shortening cycle (SSC) concept
The stretching and shortening amplitudes of the vastus lateralis (VL) LMTU did not show any significant differences between NORMAL and FAST (Figure 2B)
The relative stretching and shortening amplitudes of the VL fascicle length were smaller at FAST than at NORMAL ( VL fascicle stretching: p < 0.01, VL fascicle shortening: p < 0.05, respectively) (Figure 2C)
Summary
The actively stretched skeletal muscle can enhance the power and/or efficiency, following the stretch-shortening cycle (SSC) concept (cf. Komi, 1992, 2003). This is likely not the case given the low impact load and slow movement conditions due to hydrodynamics, viscosity, Dolphin-Kick Muscle-Tendon Interaction and buoyancy as compared to the movements on land (Grimston and Zernicke, 1993; Enoka, 2015). It remains questionable whether humans can utilize elastic strain energy caused by muscle-tendon interactions to improve performance during relatively low impact movements such as in swimming. This information is important when considering the training strategies of elite swimmers
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