# Changes in Joint Power and Energetics during a Sport-Specific Jumping Fatigue Protocol
## 4 Discussion
There are differences in the methodologies employed in the previous study and our own, with the previous study measuring kinematics and kinetics following a continuous jumping fatiguing protocol to 70% of the jump height of the unfatigued condition. With a substantially greater reduction in jumping performance and significantly shortened and more intense fatiguing protocol compared to the current study, it may be that the other joints of the lower extremities were more affected by that protocol compared to the hip joint. At the hip joint, during a dynamic sport-specific jump fatigue protocol, a reduction in peak hip power was observed to be due to reduced hip angular velocity and not due decreased hip joint moment. Conversely, at the knee joint, the athletes compensated for the decreased ankle and hip joint power by increasing their knee joint power, which occurred as early as at 21-40% of the fatiguing protocol. The increased knee joint power was due to the athletes increasing their knee joint moment, which offset the subtle decrease that occurred in knee joint angular velocity. This resulted in a greater knee joint power during the final three fatigued jumps than during the start of the jump-based fatiguing protocol. These data suggest a general trend towards athletes having the capacity to increase their knee joint moment during prolonged periods of repeated jumping, which is facilitated by an increase in knee joint power to compensate for decreased ankle and hip joint power. This result is in direct contrast to previous studies, which indicated a reduction in knee joint peak power following a fatiguing protocol [8; 10; 11; 15]. However, those previous studies performed more rapid fatiguing protocols that are substantially different from those of the current study, with some directly targeting the knee extensors. Our study indicated that during a game-specific fatigue protocol, the knee joint can maintain peak power and athletes can even increase their knee moment as they become fatigued. Regarding joint energetics, the hip joint was the greatest contributor of energy at the beginning of the fatigue protocol. However, during the last phase (81-100%), as well for the final fatigued jumps, the knee joint generated a greater amount of net energy. At the end of the fatigue protocol, decreases in the net joint energy at the ankle and hip were present, which were due to decreases in the amount of energy generated at each joint and not to an increase in energy absorption. Comparing the magnitudes of net energy from the start to the end of the jump protocol, the ankle joint had a decrease of 6 J, the knee joint had an increase in net energy of 6 J, and the hip joint had a decrease of 20 J. Overall, the knee joint was able to compensate and offset the decreased net energy of the ankle joint but could not compensate for the greater energy decrease seen at the hip joint as well. A decrease in the total net energy of the lower extremities of 3.6% occurred at the 81-100% phase, but it did not result in consistent decreases in jumping performance (<88% of initial jump height) during that phase of the protocol, with participants being able to recover in subsequent jumps. However, during the final three fatigued jumps, the total net energy was reduced by 5.9%. Many questions arise from the results of this study. For example, why did the participants not utilize their maximal knee joint moment when jumping to achieve maximal height during the early phases of the protocol? The exact reasoning behind this is currently unknown; however, the required coordination of the joints of the lower extremities during jumping [24] or the many bi-articular muscles of the knee joint may provide some insight. Specifically, the main ankle plantarflexor muscle, the gastrocnemius, also contributes to knee flexion, while one of the main knee extensors, the rectus femoris, has been labelled as a hip flexor muscle [25].