A strength and strength conditioning has two primary objectives when preparing athletes for the demands of sport. One : to increase the required performance qualities of an athletes chosen sport, and two : to reduce (where possible) the chance of an athlete gaining an injury within training or competitive play. The second objective involves the identification of any specific injuries associated with the athletes chosen sport, and the implementation of preventive measures to reduce the chance of that injury occurring.
Such ‘injury prevention’ based training is often associated with mobility drills, muscle imbalances, stretching programs, and other corrective exercise-based strategies. However, what is often overlooked is the vital link between improving the required performance qualities within athletes (e.g. force and torque capabilities, tendon stiffness within the musculotendinous unit, etc.) and an athlete’s overall robustness to injury. Eccentric strength capabilities are one such performance quality that is of great importance in regards to injury prevention.
Co-contraction of Muscle
The biomechanical demands of landing (such as when performing plyometric-based training) places excessive forces on the skeletal structures, including the joints within the lower extremities. Such joint forces can exceed many multiplications of an athlete’s equivalent bodyweight (measured in units of Newtons) due to the effect of local gravitational force. The greater the level of muscular activity and tendon stiffness around the joint upon landing, the greater the reduction in forces experienced on the actual skeletal structures. This is due to the local muscularity being able to efficiently transmit force throughout the muscle, therefore reducing the magnitude of force experienced at the joint itself. This is achieved via the co-contraction of muscles around a joint upon landing. In particular, the ability of a muscle to contract eccentrically.
Eccentric Muscle Action
When landing from an elevated height, the quadricep muscularity undergo a rapid eccentric contraction, therefore reducing the forces experienced at the knee joint ligaments. Hence, an increase in eccentric force capabilities may reduce the chance of a knee joint ligament injury occurring within athletes and general populations alike. However, previous research by Amiridis et al (1996) clearly highlighted the difference in eccentric force capabilities between eccentrically trained athletes (high jumpers) and sedentary individuals.
The authors investigated the difference in eccentric quadricep torque performance between both groups at varying angular velocities, and found that the high jump group produced significantly higher eccentric torque values when compared to the sedentary group at all recorded angular velocities. The plyometric nature of high jump-based training doesn’t make such results so surprising. However, the results also demonstrate the dangers in novice athletes and sedentary individuals attempting highly demanding plyometrics (such as box jumps from a considerable height) without adequate eccentric strength capabilities.
Interestingly, the authors also demonstrated that the difference seen in eccentric force capabilities between both groups is trainable. The authors had the same participants perform the same eccentric maximal contractions at the same angular velocities, but with an added superimposed contraction. This involved providing an additional electrical stimulus via electrodes placed on the participants quadriceps at the point at which each participant had reached via maximal eccentric contraction torque value. The theory being that if no more motor units were available, then no change in eccentric torque would occur as the additional electrical stimulus was applied, as all the available motor units were already being recruited. The difference between the voluntary and superimposed eccentric contractions in the high jump group showed no significant difference in torque values, meaning the voluntary maximal contractions being performed were of a true ‘maximal nature’. However, a significant increase in eccentric torque performance was evident between the voluntary and superimposed contractions within the sedentary group.
The increase in eccentric torque capabilities seen during the superimposed contractions (within the untrained group) demonstrate that the ability to generate high eccentric torques is a trainable adaptation. The research findings also highlight the inherent dangers of individuals attempting accentuated eccentric landings from a considerable height without any form of plyometric or strength training background, and how a progressive plyometric training program should be considered not only as a form of power-based training, but also as a means of injury prevention.
Amiridis, I, G. Martin, A. Morlon, B. Martin, L. Cometti, G. Pousson, M. Van Hoecke, J. (1996). Co-activation and tension-regulating phenomena during isokinetic knee extension in sedentary and highly skilled humans. European Journal of Applied Physiology and Occupational Physiology. 73, pp. 149–156.
