In this study, we demonstrated that skin deformation was affected differently by kinesio taping and taping methods. The total length (L0) and segment lengths (L1, L6, and L11) in the NT group were longer than those in the RPT, RNT, SPT, and SNT groups, regardless of right knee flexion or extension, indicating that Kinesio application resulted in shorter skin distances. In addition, during right knee flexion and extension, L0, L1, and L11 lengths were longer in the stretching position than in the rest position. However, there seemed to be no similar patterns in the length changes obtained following negative or positive taping. This suggests that limb position may affect the effects of Kinesio tape on dynamic skin deformation, while the change in taping direction does not seem to have a significant effect.
According to the traditional theory of Kinesio taping, stress and tangential forces are two different directions of action of the patch, i.e., downward or horizontal forces [21, 22]. When combined with the taping position, the patch can also produce an upward grip on the skin [23]. The type of force that plays a role depends on the tension of the taping to some extent [24]. Usually, the greater the local tissue stress induced by the taping, the greater the vertical effect exerted on the target tissue, and the greater the pressure and support of the target tissue, such as mechanical correction of ligament taping, mainly through the downward force generated by the large stress [25]. When the vertical stress of the patch is small, the horizontal tangential force is easily generated to drive the fascia and subcutaneous tissues, the skin is easily grabbed and produces folds, and the interstitial fluid and circulation metabolism are accelerated [26]. Commonly used lymphatic taping and neuromuscular taping play roles through stress-induced horizontal and upward grasping forces [27]. Natural tension I-tape was used in our study. The results show that this form of taping shortens the skin distance between the two ends of the taping which is consistent with the theory that when the vertical stress of the tape is small, taping can cause the skin to be pulled up and produce folds. In addition, our study showed that taping under different limb positions had a greater impact on the change of dynamic skin deformation. While under the same positioning, different taping directions did not seem to have a significant effect on skin deformation. This shows that kinesio taping does have an impact on the biomechanical changes of the skin, but it does not support the concept of taping direction in the traditional taping theory. In the 1990s, Europe and the United States and Taiwan, China did a lot of research on the concepts of "anchor", "tail" and taping direction [20]. Some researchers believe that when the tape is applied from the starting point of the muscle to the end point (the retraction direction of the tape is the same as that of the muscle contraction), it can promote muscle contraction. On the contrary, when the tape is applied from the end point of the muscle to the starting point (the direction of retraction of the tape is opposite to the direction of muscle contraction), it will have a relaxing effect, which can inhibit the excessive tension caused by excessive use of the muscle, and shows a protective effect [28, 29]. However, in recent years, several studies have shown that the taping direction has no significant effect on the treatment outcome. A study by Choi et al. [30] to determine the effect of taping orientation on quadriceps muscle strength under fatigue showed that regardless of taping orientation, muscle strength under fatigued conditions could be improved. Another study has shown that taping in different directions does not adjust the reflex excitability of muscles, nor does it have different effects on muscle strength and joint mobility [31]. A study done in China showed that there was no obvious correlation between the taping direction and the change of the subcutaneous space, which also confirmed this point of view [32]. This is also in line with some monographs emphasizing placement, tension, and weakening the direction of taping.
We found that during right knee flexion, the deformation of L1 segment skin was not significantly different between the 10 subjects, which may be related to the fact that the L1 segment was the observed section furthest from the knee. In addition, it has been found that skin stretching changes in each region of the leg during cycling, mainly in the outer thigh region and especially in the knee region [33]. The target movements selected for this study were knee flexion and extension. The skin deformation of the L1 segment was small because of far distance from the active part of the joint; therefore, it is possible that the effect of Kinesio taping on its deformation was not significant. Additionally, the skin deformation of the L6 segment seemed to be unrelated to patch placement. The causative factor may be that L6 is the middle of the active area. We conjecture that the effect of limb position during taping may be mainly manifested at the two ends of the taping area, which needs to be verified in further studies. Moreover, the small number of subjects in this study is a possible reason for the above situation.
In this study, the effects of Kinesio taping on skin deformation are actually the result of a change in the stress–strain of the skin, suggesting that Kinesio taping may play a pre-stressing role in the dynamic deformation of the skin. The skin, consisting of the epidermis and dermis, can be regarded as being composed of an elastic gel and collagen fibers [34]. At low stress, collagen fibers are in a relaxed state [35]. As the stress increases, some fibers start to become taut and stressed, and the elastic modulus starts to increase [36]. At high stress, all fibers are stressed and exhibit a high stiffness and elastic modulus [36, 37]. Not all collagen fibers are stressed at the same time during the stretching process, and the fiber distribution has obvious directionality; therefore, the overall skin stress shows non-linearity and anisotropy [37]. In our study, Kinesio taping was introduced; some collagen fibers in the dermis contract under the action of ligation, stretching from a flexed and relaxed state to an extended state, and start to bear certain tensions. Hence, the effect of Kinesio taping is equivalent to applying pre-stress to the fibers, stressing the collagen fibers in advance and releasing some of their strength reserves. Taping is like applying an extra layer of protection to the skin, which explains that compared with the non-applied group, all types of taping shorten the overall length and the spacing of each section. It has been pointed out that the shape of the skin stress–strain curve is not the same depending on whether the experiment is performed in vivo or in vitro, and that this difference is related to the initial stress in the skin. Similarly, the positioning before taping in this study is equivalent to changing the initial stress of the skin, and this initial stress interacts with the "pre-stress" generated by kinesio taping, which shows an added-on effect of skin deformation during exercise.
Many limitations are found in this study, including the state of the instrumentation and number and age of the subjects. Further expanding the sample size and obtaining a reasonable range of data is needed in future studys. A better use of the motion capture system to further investigate the effect of Kinesio taping on skin deformation to build up on the mechanisms of taping is also a must.