## 4 Discussion  
The present data show limited association among CMJ performance and knee joint stabilization; however, we propose an important muscle activation strategy during landing on unstable surface. The main finding is that participants with greater hip abduction force performed greater CMJ impulse, and activated the BF less when landed on unstable surface. Furthermore, participants with smaller \(\mathrm{M_{f}}/\mathrm{M_{e}}\) ratio increased BF/VM activation ratio at landing. The lack of task by muscle interaction suggests that muscle activation strategies are similar in CMJ and landing, probably because both tasks require the same joint kinematics. This is confirmed by the moderate correlation in VL, VM, and GM activation between the two tasks. Regardless of task, BF was less activated than the vasti or the GM muscles, suggesting that its role is limited to knee joint stabilization [21]. In contrast, the vasti muscles, being primary movers (either eccentrically during landing or stretch-shortening  
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like during CMJ) were highly activated. The role of GM has been investigated both during depth vertical jump [22] and landing [9], using EMG, musculoskeletal models and EMG-informed simulation techniques. GM functions as hip abductor, and the high GM activation indicates the importance of proper femur or/and pelvis positioning during both CMJ and landing on instable surface. We found that CMJ impulse correlated with both knee extensor and hamstring torque. The contribution of knee extensor strength to vertical jump performance is well documented [23]. The role of hamstring in jump movements is rather limited to knee joint stabilization [21], but hamstring strengthening intervention can improve jump performance [24]. It is a novel finding in the present investigation that hip abduction force correlated with CMJ impulse. This confirms previous data showing positive relationship between hip abduction moment and vertical ground reaction force measured during drop vertical jump [22]. Hip abductor mal-function has been shown to contribute to dynamic knee valgus and pelvic tilt [25] and, therefore, the improper femur (valgus collapse) and pelvis positioning could be biomechanically disadvantageous in the direction of force production during CMJ. This is confirmed by studies showing that GM fatigue alters ankle joint kinematics [20], impairs postural control [26], and increases medial-lateral center of pressure displacement [27], contributing to uncertain balancing and reduced vertical ground-reaction force during unilateral CMJ. Countermovement jump impulse in the present study correlated with only one joint stabilization marker measured at landing: the BF activation (negative correlation), partly supporting our hypothesis. In addition, participants with less M\({}_{\text{f}}\)/M\({}_{\text{e}}\) ratio increased the BF/VM activation ratio at landing, and participants with less quadriceps torque activated the VM more at landing. Biceps femoris co-activation has been suggested to play an important role in most tasks that require strong knee joint stabilization [28]. Moreover, individuals with greater hamstring stiffness demonstrated smaller peak valgus during a controlled joint perturbation task, suggesting less ACL loading [29]. Based on our aforementioned findings and the fact that quadriceps and hamstring torque correlated with CMJ impulse (as shown in our results) in our participants, we suggest that individuals with smaller quadriceps and hamstring torque levels produce smaller jump performance, and that they compensate with higher VM activation and BF co-activation at landing on unstable surface to maintain knee joint stability. It seems realistic to assume that individuals with better jump performance feel more confident at landing and require less activation of the stabilizing muscles.