Most baseball players train to express force and train rate of force development, yet many don't actively train the ability to accept and distribute forces about the body. Ballistic trampoline rebounders and popular wrist weight exercises are two good (and well known) methods to train force acceptance.
Force Development vs Force Acceptance
Most athletic training is focused on concentric (muscle shortening) acceleration and power production which is called rate of force development (RFD). RFD is simply how well an athlete can express the greatest amount of force in the smallest window of time. Some examples would be throwing, jumping and sprinting. Force development training is worthy of multiple articles but since we are focusing on force acceptance today, this will suffice.
Rate of Force Acceptance (RFA) varies greatly from RFD due to most force acceptance movements being eccentric in nature. There are three primary functions that muscles use to deal with force (different than types of contractions): eccentric absorption, isometric redirection and concentric application. The latter is associated with force development whereas eccentric absorption and isometric redirection work under the umbrella of force acceptance. RFA is simply how well an athlete can handle the stresses and loads placed on muscle systems, ligamentous structures and other joint complexes to reduce the risk of injury. In order to prepare the body for the rigors and stresses of maximal effort throwing in games and intense training, certain stresses need to be applied to ignite the adaptation process.
Force Acceptance Rebounders
Force acceptance and rebounding training has been around for many years and has been employed and studied by various research institutes, including NASA. The theory behind this method of force acceptance training is that the rebounding ball accelerates upwards after the point of contact with the trampoline and, then, into the hand causing a more of a collision than a catch. The ball possesses an added G-Force off the rebound, which produces excess weight (consider weight as a force and the gravitational pull of an object is its weight) of the ball as it travels back toward the hand. At the point of contact between the ball and the hand, using Newton’s second law we can gauge that the forces acting upon the ball and hand is rather substantial. Rebounders do a great job of reinforcing stability through multiple joints and around the rotator cuff. Rebounders allow the body to figure out a way to accept and deliver forces through ballistic means and protocols. When the ball is caught off the rebound, forces begin to travel up the arm (distal to proximal) and are allocated throughout the surrounding tissues, systems and complexes of the arm and shoulder. Catching the rebounds also adds a great proprioceptive value to the movement.
Wrist weights not only offer great value in motor recruitment and patterning but they serve as practice to accept force. Wrist weights have been used by ball players for many decades, beginning with Dr. Mike Marshall in the late 20th century but Kyle and the Driveline Team have done a great job of selling high quality weight weights and furthering their use in today's market. You may be wondering why wrist weight drills, like the ones below, are classified as force acceptance movements even though they seem to be high effort movements and similar to throwing. The answer is because throwing a 5oz baseball requires elite arm speed whereas the stresses from throwing a 10 lb wrist weight are much more mild due to decreased angular velocities of the arm. The more limited speed of the throws and less concentric based movements, such as rapid elbow extension as you would experience with throwing, allows more a safe movement while patterning an efficient arm path and eccentrically working on the deceleration phase of throwing. Not to mention, the differing arc of deceleration with wrist weights, provides acute stress levels (which can act as game stress preparation) since there is no ball release.