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14 Jan 2009

 


 

HOW CAN BIOMECHANICAL PRINCIPLES HELP US IN OUR ANALYSIS OF SKILLS?

By Doug Haw and Carol Rossignol

(This is the first in a series of articles on biomechanical principles)
 

 Analyzing skills is a challenging and important task for you as a coach. The first step in analyzing skills is observing them. The second step understands how skills should be performed


 One of the best aids in this area is biomechanics, which describes and analyzes the mechanical aspects of athletic performance. It is very helpful to have a working knowledge of this science. 

As a coach you need to have some basic information on how the body moves and a working knowledge of biomechanics.

Movement of the body is produced by the rotation of body segments around joints. Bones form the framework of the body and they are attached to each other at the joints by ligaments, muscles are attached to bones by tendons.

Movement at joints occurs when muscles shorten and change the angle between the bones at a joint. This rotation occurs around a fixed axis of rotation, which runs through the center of the joint.

Forces produced by muscles pulling on bones inside the body are called internal forces. Forces acting on the body such as, air resistance or friction are called external forces. When a skater applies internal forces against the ice, the ice reacts by applying external forces against the body.

There are seven biomechanical principles* that can help us analyze skating skills:

#1. The lower the center of gravity, the larger the base of support, the closer the line of gravity to the center of the base of support and the greater the mass, the more stability increases.

#2. The production of maximum force requires the use of all the joints that can be used.

#3. The production of maximum impulse requires the use of joints in order from the largest to the smallest (hip, knee, ankle, toes).

#4. The greater the applied impulse, the greater the increase in velocity (speed and direction).

#5. Movement usually occurs in the direction opposite to that of the applied force.

#6. Angular motion is produced by the application of force acting at some distance from an axis that is by a torque.

#7. Angular momentum is constant when an athlete or object is free in the air.


To help us understand the first principle let’s look at the basic concept of stability:

Principle #1: The lower the center of gravity, the larger the base of support, the closer the line of gravity to the center of the base of support and the greater the mass, the more stability increases.
 Gravity is a force that exerts a downward pull on people and objects.

The center of gravity is a balance point - the imaginary point at which a person’s mass may be thought of as being concentrated. The location of the center of gravity changes as the skater’s arms and legs move and so does balance. Generally males have a higher center of gravity than females.

The line of gravity is an imaginary line passing straight down through the center of gravity to the ground or ice.


Mass is a measure of resistance to linear motion and it is usually measured in weight (pounds or kilograms). A greater mass will have a greater resistance to motion, i.e. a 200 lb. athlete has much more resistance to linear motion than one weighing 100 lb.

There are three basic states of motion: a motionless state, linear motion and angular motion (rotation).

Motionless states occur when a body or object is balance and there is no significant movement (balancing on one foot in a stork stand can be a motionless state).

Linear motion is movement in a straight line. The motion of a skater doing a spiral or a bobsled sliding down a track is an example of linear motion. The force is applied directly through the center of gravity.

Angular motion refers to rotation or circular motion about an axis. Rotating jumps in figure skating and somersaults in diving are examples of angular motion. Angular motion is produced when forces are NOT applied directly through the center of gravity (an off-center force).

How can we apply these biomechanical principles to skating skills?


If a jump lacks height check:

  •  Complete utilization of power at take-off
  •  Correct placement of toe (force must be applied directly through the center of gravity)
  •  Curvature of take-off edge
  •  Speed
  •  Angle of take-off
  •  Balance point on the blade on take-off
  •  Head position
  •  Arms and free leg complimenting the skating leg thrust

 If a jump lacks length check:

  • Speed
  •  Angle of take-off
  •  Curvature of edge
  •  Arm and free leg motion

 If there are problems with balance check:
 

  • Off-center forces during entry phase
  •  Transfer of momentum
  •  Balance point on the blade Strength and flexibility of the skater
  •  Support line during the spin (line of gravity)
  •  The trunk of the body as it should be straight when spinning and jumping
  •  Bending the hip, knee and ankle on the landing of a jump will lower the center of gravity

 If a spin has insufficient speed check:

  • Impulse
  •  The amount of angular motion produced by the entry edge
  •  Contact point of blade with the ice throughout the spin
  •  Use of arms and legs in decreasing the moment of inertial
  •  Curvature of spiraling edge into spin
  •  Skater absorbs energy in the body rather than stretching for control

 Biomechanical Considerations for Skating:

  • The ice is a low friction surface
  •  The faster you move across the ice the less pressure there is on the ice. This gives less feedback to the skater. The skater must use faster quicker forces the faster they move.
  •  The ice temperature can constantly change; therefore you must adapt your technique and timing to the conditions
  •  Every move in skating is executed on a curve with lean
  •  Off-center forces are transferred through the ankle, knee and hip hence there are significant differences between male and female
  •  Differing body structure, muscle types, reaction times and sensitivity to movement dictate variations in technique
  •  The blade is the interface between the skater and the medium which is responsible for all off-center forces. It constantly changes with degrees of sharpness.

 * Coaching Theory Level 2 NCCP by CAC, 1989.

Copyright PSA 2009. This article is reprinted with permission from the Professional Skater’s Association

Doug Haw and Carol Rossignal, PSA