Study on the Influence of Elastic Compression Pants Elasticity and Movement Speed on Human Joint Protection

A review of the relevant literature^21,22,23 shows that more research has been done on the changes in exercise parameters in males caused by compression pants, while less has been done on females. Besides that, there are individual differences between males and females. Hence, this paper took females as the research subject. Three females aged 22 with an average body size were chosen as the subjects, whose details are shown in Table 1. All subjects reported no history of orthopedic injury of the lower extremity joints. Before testing, informed written consent was obtained from each subject in accordance with the University's Ethics Committee. The data obtained by each experimenter are multiple cycles, so that the trend of the human joint protective effect can be obtained.

According to the market research results, one pair of tight pants (CP1) and two pairs of elastic compression pants (CP2 & CP3) with a higher spandex content and gradient compression effect (MZP-X, China) were selected for the experimental clothing. All of these garments were ergonomic and met the needs of women in sports condition. In addition, one pair of cotton sport pants (CS) was added to compare the influence of fabric parameters on the protection effect of the human knee joint. The basic parameters are shown in Table 2.

A treadmill (U20, provided by Zhejiang U-may technology Co. Ltd.) and a Qualisys-Oqus500+ dynamic capture instrument (provided by Sweden Qualisys Co. Ltd.) were adopted in the experiments. The dynamic capture instrument, surrounded by 9 cameras, was able to capture the whole laboratory and motion capture software as well as data export software were used to process the three-dimensional coordinates of the point after optically capturing the marker ball on the experimental object. The instrument is shown in Fig. 1.

Fig. 1

The movement cycle of the lower limb during our running process is shown in Fig. 2. From Fig 2 we can see that the running cycle consists of front suspension, support & back suspension periods and both legs have nearly the same movement characteristics. When the left leg is in the position of the support period, the right leg is in the position of the suspension period. Furthermore, the included angle between the thigh & calf, i.e. the angle of the knee joint, is changing constantly and the angle of the knee joint for the leg in the support period is larger than that of the leg in the suspension period.

Fig. 2

Since both legs have the same movements, one was necessary for further investigation and the left leg was taken as an example to be the dynamic capture object of the test. With the movement characteristics and the marking point definition rules recommended by the instrument taken into consideration, three, two and one marker points were selected and stuck on the thigh, calf and hip parts of the pants, respectively, shown in Fig. 3 & Table 3.

Fig. 3

As shown in Fig. 4, in the process of periodic gait movement, the knee angle is the included angle between the thigh and calf, hence the angle of the knee joint (AKJ) is selected for evaluation. In order to verify the influence of compression pants on human joint protection performance, three experimental subjects were required to wear pants with different elasticity, shown in Table 2, and run on the treadmill at a speed of 5km/h, 8km/h and 11km/h. The three speeds can represent low, medium and high speed during daily running.

Fig. 4

After the space vector coordinates of each marker point were obtained, the knee joint midpoint (KMP) and ankle joint midpoint (JMP) were calculated using the intermediate coordinates of KL & KI, AL & AI. Then the knee angle β was acquired according to the formula of the space vector included angle and the anti-formulas of trigonometric functions calculated, shown in Eq. (1) and Eq. (2). (1) α=(xKMP−xHP)(xKMP−xJMP)+(yKMP−yHP)(yKMP−yJMP)+(zKMP−zHP)(zKMP−zJMP)(xKMP−xHP)2+(yKMP−yHP)2+(zKMP−zHP)2(xKMP−xKMP)2+(yKMP−yKMP)2+(zKMP−zJMP)2 \alpha = {{\left( {{x_{KMP}} - {x_{HP}}} \right)\left( {{x_{KMP}} - {x_{JMP}}} \right) + \left( {{y_{KMP}} - {y_{HP}}} \right)\left( {{y_{KMP}} - {y_{JMP}}} \right) + \left( {{z_{KMP}} - {z_{HP}}} \right)\left( {{z_{KMP}} - {z_{JMP}}} \right)} \over {\sqrt {{{\left( {{x_{KMP}} - {x_{HP}}} \right)}^2} + {{\left( {{y_{KMP}} - {y_{HP}}} \right)}^2} + {{\left( {{z_{KMP}} - {z_{HP}}} \right)}^2}} \sqrt {{{\left( {{x_{KMP}} - {x_{KMP}}} \right)}^2} + {{\left( {{y_{KMP}} - {y_{KMP}}} \right)}^2} + {{\left( {{z_{KMP}} - {z_{JMP}}} \right)}^2}} }} (2) β=arcos(α) \beta = {arcos}(\alpha ) Where α is the cosine of the joint angle, and β is the angle of the knee joint (AKJ).

Since there are differences among the body shape parameters of experimental subjects, in order to verify whether there is any difference in the protective performance when different subjects wear the same pants at the same running speed, the AKJ values of different experimental subjects were plotted. As we found that there are similar trends for CP1 to CP3, they could be grouped as the elasticity category, in comparison with CS, the non-elasticity group. Fig. 5 and Fig. 6 show the results of three subjects wearing CP2 and CS at three different speeds, representing the elasticity and non-elasticity groups, respectively. From Fig. 5 and Fig. 6, it can be seen that except 5km/h, the overall AKJ change trends of the different subjects in different states are basically similar and the curve changes show a wave shape, with concaves and peaks in different degrees.

Fig. 5

Fig. 6

From Fig. 5 and Fig. 6, we can also see that when wearing elastic pants, the lower the movement speed, the more obvious the difference of AKJ among the individual subjects, whereas with the increase of speed, the difference among individuals gradually decreases and tends to be stable. However, when wearing non-elasticity pants, the effect is opposite. That is, the faster the speed, the greater the influence on the gap of AKJ among individuals and the greater the curve fluctuation of AKJ with the increase of speed. Generally speaking, different people will have different changes in AKJ when wearing the same elastic or non-elastic pants and running at the same running speed.

In order to further analyse the influence of pants elasticity on the human AKJ on the whole, the AKJ of the three subjects and ten gait periods were averaged. The results of different pants at different speed are shown Fig. 7. As mentioned above, when running. the two legs of the human change constantly to complete gait periods. Although the AKJ of over ten gait periods were obtained, it is unnecessary and there is not sufficient space to list all the results in the paper, therefore they are averaged to present the overall trend. In the following analysis, detailed data of the AKJ were acquired from ten gait periods.

Fig. 7

As shown in Fig. 7(a), at q speed of 5 km/h, the AKJ of CP1, CP2 and CP3 shows little difference (≤8°) in the front suspension period of 0~25 frames, and the AKJ of CP1, CP2, CP3 and CS are 168.15°±0.61°, 164.03°±0.63°, 160.90°±1.79° and 172.82°±0.36°, respectively, in the support period. Moreover, the AKJ of CP2 and CP3 (belonging to the compression pants) during the whole process, whether for the front support, the suspension or the back support period, are almost the same. By analysing the whole trend in Fig. 7(a), we can see that the frame numbers as well as the AKJ of CP1, CP2 and CP3 (grouped into the elastic pants) are smaller than CS, which means the elastic force may not only reduce a single gait cycle but also change the mobility of human lower limbs. The average gait cycle of CS is about 110 frames, while that of CP1 (96 frames), CP2 (83 frames) and CP3 (83 frames) are less. In short, compared with non-elastic pants (CS), elastic pants can not only shorten the gait cycle but also decrease the change amplitude of AKJ significantly, playing an obvious role in protecting the human knee. Especially, the two compression pants nearly have the same trend.

It can be seen from Fig. 7(b) that at a speed of 8 km/h, the AKJ of CP1, CP2 and CP3 show a small difference (≤3.12°) during the front suspension period. Not only that, the peaks of elastic pants first appear in the supporting period (about 21 frames), and the overall gait cycle decreases to within 80 frames along with the speed. However, the peak of CS appears later (about 27 frames), and the overall gait cycle is shortened from 110 frames to about 83. The overall trend of the AKJ of non-elastic pants is larger than that of elastic pants, and the change amplitudes of AKJ from big to small are CS (60.15°±0.21°), CP1 (48.84°±0.16°), CP2 (45.73°±0.98°) and CP3 (44.25°±0.47°) after the support period, because CP2 and CP3 have higher spandex content than CP1, which proves that fabric elasticity can effectively reduce the range of knee joint movement and flexion movement ²⁴.

Fig. 7(c) shows that the AKJ gap between different pants is larger, and the AKJ of elastic pants is more than that of non-elastic pants in the suspension period, which is consistent with the change rule in Fig. 7(a). In addition, at a speed of 11km/h, the gait cycles of CS is reduced to fewer than 80 frames, and that of elastic pants is also smaller than at 8km/h. During the support period, the depressions of four curves appear alternately. And the change in elastic pants in the suspension period is still significantly smaller than that of CS.

To sum up, elasticity has a great influence on the AKJ. The change in the AKJ is more obvious in the suspension period than in the support period. The reason may be that the legs are supported by the ground during the support period, which weakens the covering force of pants elasticity on joints to some extent. However, the legs hang in the air during the suspension period, and the elasticity of pants has an obvious binding force on the joints, which reduces the movement range of the knee joint. That is, elastic pants can play a more positive protective role on the joints than non-elastic pants. Compared with non-elastic pants (CS), elastic pants can not only shorten the gait cycle, but also decrease the amplitude of the AKJ significantly,

The AKJ of the same pants at different speeds is shown in Fig. 8, from which a common rule can be seen that the influence of the movement speed on the back suspension period is greater than that of the front suspension period, as three curves of the back suspension period fall apart from each other. On top of that, the movement speed also has an influence on the gait cycle, that is, with the increase of the movement speed, the gait cycle gradually shortens.

Fig. 8

As shown in Fig. 8(a)–(c), at the initial stage, the AKJ at 11km/h is larger than at 5 km/h and 8 km/h. However, during the support period, the maximum AKJ, from large to small, are at 5, 11 and 8km/h in turn. In addition, the angle curve of AKJ appears concave with the increase of speed, such as at 8 km/h and 11 km/h. And the maximum value shows a trend of first decreasing and then increasing. Moreover, for CP2 and CP3, the greater the speed, the more obvious the fluctuation and the deeper the concave the curve has. With the change of speed, the number of frames gradually decreases, with CP1 dropping from 96 to 73 frames, CP2 s from 83 to 66 frames, and CP3 from 83 to 62 frames, respectively.

Fig. 8(d) shows that there is little difference in the front suspension period at 8 km/h and 11 km/h, and the minimum value of the AKJ in the front and back suspension periods from large to small are at 5, 8 and 11km/h. Moreover, the minimum value decreases by 4.82±0.60° ~ 7.43±0.76° every 3 km/h with an increase in the process from 5 km/h to 11 km/h. And the maximum value also decreases with 8.81±0.15° ~ 13.42±0.81°. The trends of non-elastic pants are completely different from those of elastic pants, while wearing elastic pants can change the rule of non-elastic pants. Especially, the compression pants (CP2 and CP3) have the most obvious change compared with CP1, whose cycle is about 110 frames at 5 km/h, 80 frames at 8 km/h and 75 frames at 11 km/h.

Generally speaking, different speeds have different effects on the human knee joint angle. For CS, with the increase of the speed, the overall trend shows a gradual decline. However, when the lower limb is wrapped by elastic fabric, the trend of AKJ changes, and that during 8km/h and 11km/h tends to be similar. Not only that, whether the pants are elastic or not, the single gait cycle will gradually decrease with the increase of speed.

In order to further quantify the AKJ difference between elastic pants and non-elastic pants, the maximum and minimum values of the three speeds were extracted and averaged. Finally, the maximum AKJ and minimum AKJ were obtained, as shown in Fig. 9.

Fig. 9

It can be seen that for CP1, CP2 and CP3, their maximum AKJ is less by about 4°~13° than that of CS at the speed of 5 km/h and 8km/h. And at the speed of 11 km/h, it is about 2°~5° larger than the value of CS. However, when people wear non-elastic pants, the maximum and minimum AKJ gradually decrease with the increase of speed, which is consistent with the result of change in Fig.8(c). In addition, the minimum AKJ of compression pants appears at 8 km/h, and the maximum AKJs at the three speeds are relatively close. According to the difference in AKJ, for the speed of 5km/h, the motion amplitudes of the knee joint (subtract the max-value from the min-value) from CP1 to CS are 56.46°, 46.22°, 41.65° and 66.02° respectively. For 8km/h, they are 46.54°, 47.73°, 46.67°and 61.22°, respectively, and for 11km/h, they are 50.96°, 43.79°, 44.20° and 57.55°, respectively. In other words, the pants with elasticity at high speeds can effectively reduce the range of motion of the knee joint and play a more positive role in protective performance than non-elastic pants. Generally speaking, in order to avoid knee joint injury, it is necessary to wear compression pants.