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The present study compared the isometric strength of LE extensors developed in CKC between the KEs in female and male students. Findings revealed significant differences in the MS/BM values between the six KEs (80°, 70°, 60°, 50°, 40° and 30°). Hence, the decrease in KE by 10° significantly increased the strength values from 13.7 N kg− 1 (females) and 14.9 N kg− 1 (males) for 80° to 30.9 N kg− 1 (females) and 33.5 N kg− 1 (males) for 30°. Although, it was not expected that a change in angle by 10° would cause a significant improvement in strength. Therefore, these results may play an important role in selecting the appropriate knee angle when evaluating the maximum strength of the LE extensors.
Previous studies also evaluated variation in the LE extensors strength developed in CKC [7, 8]. For example, Wojtkowiak et al. [7] reported the influence of LEs joint angles on the isometric strength of LE extensors and the greatest values of the isometric strength of LE extensors at the KE in the range 40°-50° and HE in the range 70°-80° in women and men. Moreover, Urbanik et al. [8] found an increase in the ground reaction force with the decrease in the KE from 75°to 30° and HE from 130° to 90° for both LEs of students. However, it was only this study that compared the isometric strength of LE between the KEs and determined the influence of gender on the strength differences between these angles.
In addition to the MS/BM comparisons between the angles, the influence of gender on the change in strength with increasing angle was also determined. The analysis showed a non-significant main effect, i.e., non-significant differences between women and men as well as significantly higher MS/BM values in males than females only at 50° and 70° angles. Importantly, a non-significant interaction between the angle and gender factors was demonstrated; the gender factor did not influence the variation of muscle strength from the KE. Thus, for the isometric test of LE extensors strength in CKC, change in MS/BM values from the KE does not depend on gender. In comparison, Kong and Burns [3] also examined the effect of gender on the variation in quadriceps peak torque with the change of the KE. In contrast to these findings, Kong and Burns [3] demonstrated significantly higher values of the overall quadriceps torque in males than females and a significant angle-gender interaction. However, these researchers evaluated muscle strength in OKC.
Several studies compared the isometric quadriceps strength developed in OKC between the different KE for both females and males [3, 4]. The highest values of the knee extensors peak torque were found at 80° [3] and at 60° [4]. In turn, the present study revealed the highest MS/BM values of the LEs extensors at the KE of 30°. Thus, maximum muscle strength of the knee extensors and hip extensors in CKC is developed at the smaller KE than for the quadriceps peak torque measured in OKC, i.e., when muscles are less stretched. This difference results from a different sitting position of the subject (different position of the thigh and lower leg) during the test in CKC and the production of muscle strength by both the knee and hip extensors.
This study also compared the values of bilateral and unilateral muscle strength developed by the LEs extensors at the optimal KEs of 30° between females and males. Findings showed a significant between-subject main effect and significantly higher MS/BM values in men than women, as well as a non-significant interaction effect between the condition and gender factors. Hence, the increase in LEs extensors strength for unilateral conditions compared to bilateral conditions does not depend on gender.
Considering the comparisons between the bilateral conditions and unilateral conditions, it was observed significantly a lower summed muscle strength developed by extensors of both LEs simultaneously than of both LEs independently, i.e., BD of approximately 11% in females and approximately 8% in males. Similarly, other studies demonstrated significant BD of knee extensors strength in recreationally active young women [20] and healthy young men [23]. In contrast, Bulzing et al. [24] reported BD values close to zero in volunteers; however, this was most likely caused by intrinsic random error of measurement.
For the isometric contraction of LEs muscles, some authors suggested possible causes of this deficit, such as the neural inhibition mechanism leading to the decrease in muscle strength produced bilaterally [23, 25, 26], difference in antagonist muscle coactivation between the bilateral contraction and unilateral contraction [27], and the reduction in motor neurons’ excitability during the bilateral conditions [13, 16, 17]. In addition, other authors indicate the body adjustments and mechanical configuration of the dynamometer as factors of BD [28]. The magnitude of BD can be increased by using a strength training program incorporating mainly unilateral knee extension exercises [20]. For example, Botton et al. [20] showed a greater increase in unilateral isometric strength in females after unilateral training than in females after bilateral training, thus higher BD for the unilateral group compared to the bilateral group.
Assessment of the maximum strength of the LE extensors in CKC is an important part of strength capabilities control in active population athletes. Based on these data, the highest strength was developed by the subjects at the KE of 30°. Moreover, the MS/BM value was significantly higher compared to results of this variable for the lower angles. Thus, measurements in CKC should first include setting of the KE at 30° (e.g., using a goniometer) as the optimal angle for assessing the maximum isometric strength of the LEs extensors.
This study has limitations. First, due to the safety of the subjects, measurements were not performed for KEs lower than 30°. Developing maximum strength at angles close to 0° can lead to hyperextension in the knee joint and increase the injury risk of the joint structures. Second, it is not possible to extrapolate these findings to other muscle groups tested in OKC because only the LEs extensors strength in CKC was examined.
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