The final principle is unique in nature as it only applies to objects which are airborne or on surfaces which have extremely low coefficients of friction. An excellent example of this principle is figure skaters. Have you ever watched a figure skating competition and wondered how the athletes are able to complete a stationary spin on ice while increasing and decreasing their rate of spin. This motion is made possible due principle #7 and the conservation of angular momentum.

Angular momentum is the product of inertia (a body’s resistance to changes in motion) and that body’s angular velocity. As a result, in order to maintain a constant angular momentum, the figure skater can manipulate their body in order to increase its inertia, while subsequently decreasing its angular velocity or visca versa. In the figure skating example, the figure skaters will utilize their arms in order to decrease their inertia (bring the arms closer to their body) which in turn causes them to increase their rate of spin or angular velocity. As they near the end of the spin, they will move their arms further away from their body (increasing their inertia) which in turn will cause a decrease in their angular velocity and slow down their rate of spin.

This principle also plays a vital role in the success of elite level divers. When the diver moves from a layout position to a tuck position, they are ultimately manipulating their moment of inertia in order to adjust their angular velocity. If the divers timing is off, they will not make a clean entrance into the pool. This is part of the reason why you will see divers break form (ie. flex the knees) near the completion of their jump in order to increase their velocity in hopes of making a cleaner entrance into the pool. Elite level divers are extremely gifted in knowing where they should be when coming out of a dive in order to make a clean entrance into the water. This is where repetition and quality feedback from the coach and/or Biomechanist can help to improve the diver’s performance and overall score on a dive.

Brian Shackel, MSc

When looking at the principle of angular motion, it is important to have a solid understanding of several other biomechanics terms.
• Angular Velocity = change in angular position (or displacement) / change in time
• Torque = Force X Distance (perpendicular)

In order to generate angular motion, a force must be applied onto an object acting a certain distance away from the object. As a result, in order to increase the torque, an athlete can either increase the force which they are applying to an object or apply this force further away from their axis of rotation.

A classic example of angular motion is the throwing arm of a baseball pitcher. Professional baseball players are able to generate extremely high angular velocities in their throwing shoulder (specifically internal rotation of the pitching arm) which increases the linear velocity of the ball coming out of the hand. If you consider the baseball pitcher as an example and use the pitchers spine as their axis of rotation and the perpendicular distance from the ball to the axis of rotation as their distance, you will be able to determine ways to increase the linear velocity of the ball at release. A pitcher can produce a larger torque by applying a larger force or by moving this ball further away from the axis of rotation. Unfortunately, this example is not a simple as we just described it, but it does give you a valuable reference point. For a pitcher to truly increase the velocity of the ball at release, they need to factor in nearly every biomechanical principle we have talked about to date. Specifically, the pitcher needs to emphasize the principles which help to produce maximum effort or force (principles #2 and #3) as well as the principles which apply to linear motion (principles #4 and #5). Throwing a baseball requires full body mechanics and factors in linear and angular movements as well as the importance of proper sequencing of these movements.

Thanks,
Brian Shackel, MSc

What is impulse (I)?

Impulse is the product of force (F) and the time (t) over which the force acts (I=Ft). With that being said, when a force is applied to an object, the resulting motion of the body is dependent not only on the magnitude of the force but also on the duration of the force application. Therefore, an object can be set in motion by applying a large force over a relatively short period of time or a small force over a relatively long period of time. Both of these impulses can create motion in an object, but what is more effective at producing motion in sports.

One of the most common examples of impulse in sports is the volleyball player who jumps up to spike a ball during a match. Theoretically, the volleyball player has two options. Option number one, the volleyball player could apply a relatively small force over a longer time frame to produce an impulse. Option number two, the same volleyball player could apply a large force over a relatively short period of time to produce the exact same impulse. However, in theory this makes sense, but when it is put into practice, the volleyball player who choses option two will generate more vertical velocity and has a much higher jump height. In sports or activities which require more “explosive movements” a large force applied over a short time frame will produce a much better result. Another example of this is a hockey player attempting to take slap shot. The hockey player can produce a much harder shot if the force is applied to the puck over a very short period of time (ie. the less time the puck is on the stick, the harder the shot).

With that being said, there are several examples in sports where it is important to produce less impulse. For example, a hockey player receiving a pass or baseball player receiving a ground ball will want to allow the time of force application to be increased in order to decrease the force with which the object is received. This is done by cushioning the pass or grounder. A hockey player is taught to have soft hands and allow the puck to hit the stick and absorb the puck. This motion allows the force to be absorbed over a longer period of time, thus decreasing the overall impulse. Have you ever heard you coach say that receiving a pass should be “quiet”? In the baseball example, the fielder will absorb the force of the ball by bringing the hands into the body and transitioning it into a throw. The longer time spend absorbing the force, the less the impulse which will be applied to the glove and hand. This principle can help ease the pain of the baseball player who catches the ball in the palm of the glove without allowing time to absorb the ball.

Check back next week for more in Linear Motion and Principle #5!

Thanks,
Brian Shackel, MSc

The production of maximum velocity requires the use of joints in order – from the largest to the smallest. Now that you have a greater understanding of using all of the joints possible to produce maximum force, it is important to gain a greater understanding of the sequential timing of these movements.

A classic example of using your joints from largest to smallest is found in almost all rotational skills (ie. throwing a shot, hitting and throwing a baseball, hitting a golf ball, etc). During these sports skills, it is important to utilize “segmental rotation” where the rotational velocity of one joint is transferred into the next joint. For example, if you look at a golf professional hitting a ball, you will notice a very distinct sequence of events……

1. During the takeaway, the golfer will turn the shoulders back prior to turning the hips back which will create “shoulder hip separation” at the top of the backswing as well as allow the golfers weight to be transferred to the back foot.
2. The downswing will be initiated with the hips rotating counter clockwise (for a right handed golfer) while the shoulders “lag” behind and start their movement slightly after the hips. This allows the hips (the larger joint) to move prior to the shoulders (the smaller joint) and start the process of segmental rotation.
3. The shoulders will then begin the downswing in a counter clockwise direction while the hand elbow and hand “lag” behind. The elbow and hand will begin to move in the counter clockwise direction leaving the head of the golf club behind.
4. Finally the club head will begin the downswing and continue to accelerate through impact.

By going through this distinctive series of events, the angular velocity of the hip rotation (the initial movement) is able to be transferred into the shoulders, the shoulders into the elbow, elbow into the wrist and wrist into the club head. This allows the club head to be travelling at its maximum velocity leading up to ball contact. If one of these motions is done out of order (ie. the shoulder and hips rotate together) there is a significant loss in club head velocity and thus ball speed and overall length of the shot. This series of events allows players with a shorter, more compact swing to hit the ball as far as or further than someone with a longer swing, because they are going through the proper sequencing to generate peak club head velocity at impact.

A similar thought process can also be applied to a soccer player striking a soccer ball. If a soccer player performs the proper sequence, with the kicking hip moving forwards and leaving the knee behind, followed by the knee coming forwards and the foot “lagging” behind and finally the foot coming through at contact they will be able to increase their ball speed. If everything moves towards the ball at the same time, then there is a significant loss of power in the shot. This relationship holds true due to the summation of the joints!!!

Thanks and check back next week for Principle #4: The greater the applied impulse, the greater the increase in velocity.

Thanks,
Brian Shackel, MSc