# The Mechanics of Jumping in Ice Skating

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Without question, the most spectacular tricks in free skating are the jumps. There are many factors that go into successfully completing a high, fast, multi-rotation jump. In this article we take a look at what these factors are and how they affect the difficulty of a jump.

### Horizontal speed of the takeoff (speed across the ice)

Most jumps can be executed from a standing takeoff, both on and off the ice. Skating, however is about movement over the ice, and jumps on ice should takeoff and land with speed. The greater the speed of the takeoff, the more precise the timing and coordination required to takeoff without losing any of that speed. Also, the more complex the footwork and movements preceding the takeoff, the greater the coordination and control required to complete the jump. Jumps should land and flow out with nearly the same speed with which they took off. Jumps that are landed correctly will not lose significant horizontal speed in the act of landing. Jumps which come to a screeching halt at the takeoff or impale the ice on the landing suffer from serious errors in technique that cannot be ignored in judging them no matter how high the jump or how many rotations are executed.

While the speed at the takeoff of a jump can impact the effectiveness with which a skater executes the takeoff - and thus the height of the jump - in ice skating very little of a skater's horizontal speed actually is converted into vertical speed to increase the height of a jump. For this reason the analogy to pole vaulting used in describing tap jumps is not particularly accurate. In pole vaulting the energy of the horizontal motion of the vaulter is briefly converted to potential energy by bending the pole nearly in half. When released this energy is converted to vertical speed to attain height in the vault. The degree to which blades, boots, and the skater's body can "bend" to store potential energy, however, is limited, and thus limits the extent to which the skater gets a vaulting action in a tap jump.

### Vertical speed of the takeoff (speed up into the air)

For both edge jumps and tap jumps, and regardless of the edge of the takeoff, the purpose of the takeoff is to achieve the vertical speed that determines the height of the jump. The greater the vertical speed at the instant of takeoff, the higher the jump and the greater the time in the air. Once a skater leaves the ice there is nothing they can do to increase the height of the jump.

Using the laws of mechanics it is trivial to calculate from the height of a jump the time in the air and the vertical speed needed to achieve the height. At an upper limit of four feet a skater spends one second in the air. At two feet the time in the air is 0.7 second, and at one foot it is 0.5 second.

The vertical speed of the takeoff can be used to determine the energy expended in the takeoff, and the force exerted by the skater to execute the jump. The vertical speed needed to achieve a given height is the same for all skaters. The force to attain that speed, however, depends on the weight of the skater and the time interval over which the force is applied during the takeoff. The heavier the skater the greater the force required to attain a given takeoff speed. Most of this force is developed in the muscles of the thighs and buttocks in flexing the knee and hip joints, while the muscles of the lower leg contribute to a lesser extent in the action of flexing the ankle. Different jumps use the muscles of the leg in somewhat different ways; but in general, big jumpers have well developed thighs and firm butts to get their body's up into the air and to absorb the shock of the landing. In addition, since the force applied in jumping is an "explosive" force of brief duration, bodies with a higher fraction of fast twitch muscles have an advantage in jumping skill.

The vertical speed at the moment of landing a jump is identical to the vertical speed at the moment of takeoff. This speed it eliminated in the landing contact of the blade (we hope) with the ice and in the action of the landing blade, boot, and leg acting as a shock absorber.

The greater the height of a jump, the greater strength it demonstrates, and the more time in the air it gives the skater to complete the rotations of the jump. On the aesthetic side, the vertical and horizontal speeds of a jump should be matched so that the jump has a "pleasing" arc to it. This arc is a parabola whose exact shape is determined by the horizontal and vertical speeds at the moment of takeoff, and the force of gravity acting on the skater. When the vertical and horizontal speeds of a jump are equal the arc of the jump will leave and return to the ice at a 45 degree angle. Jumps steeper than about 60 degrees (spikes) give the appearance of impaling the ice and usually result from taking off with too little horizontal speed in the entry, or from the sudden loss of horizontal speed due to a technical error in the takeoff. Jumps flatter that about 30 degrees (scooters) give the appearance of skimming the ice with too little height and usually result from taking off with too little vertical speed to achieve significant height.

### Angular momentum (rotating motion of the skater)

The main purpose of the takeoff is to the get the skater in the air. A second, but also important part of the takeoff, however, is to prepare the skater for rotating in the air. Entering the takeoff, the skater is already rotating slowly and has an initial amount of angular momentum. In general, however, a skater has only 0.5 to 1.0 second to complete the rotations of a jump in the air (and they must be completed in the air), and the initial slow rotational motion of the skater is inadequate to do this in the time available. Something must be done to get the skater turning faster. That something is pulling in on the jump.

In the air, the angular momentum of the skater is a constant, but the angular speed of the skater can be controlled by the distribution of body parts relative to the rotation axis of the skater. The tighter the arms and legs of the skater are pulled in towards the rotation axis the more the rotational speed of the skater will increase.

At the start of the takeoff the arms and legs of the skater are relatively extended and the skater is rotating slowly. Pulling the arms and legs in towards the rotation axis increases the speed of rotation at the beginning of the jump. Extending them again at the end of the jump (the checkout) slows the rotation down in preparation for the landing. If the skater did not slow the rotation prior to the landing the skater would end up spinning into the ice, preventing the skater from flowing out on a clean exit edge, and also putting a severe - likely damaging - strain on the landing knee.

Usually skaters pull in at the start of the jump and rapid rotation begins immediately. In some cases, however, they delay pulling in to execute a "delayed" jump. In a delayed jump, the jump rotates slowly at first with the initial slow angular motion of the takeoff. Near the peak of the jump the skater then pulls in and quickly finishes the rotation in the second half of the jump. The most common example of this is the delayed Axel, but there are others.

In developing jumping skills from singles to doubles to triples, each step up requires greater skill in controlling angular motion. To fit more rotations into a jump skaters can draw on three tools: increase the height of the jump to allow more time in the air, increase the initial angular momentum of the jump to allow greater rotational speed. or pull in tighter to get more rotational speed out of the angular momentum available. Different skaters probably draw on these tools to different extents for different jumps, but quantitative studies have only been done for the triple Axel.

The initial amount of angular motion at the takeoff and the entry edge of the jump have a significant impact on the difficulty of performing a jump. A right handed jump rotates counter-clockwise when viewed from above the skater. When skaters enter a jump, a true entry edge caries them on either a clockwise or counter-clockwise circle. This motion is also a form of angular momentum and angular motion. For a right handed skater in a normal rotation jump, the entry edge carries the skater on a counter-clockwise circle with the same sense of rotation the skater will have in the jump. In a counter-rotation jump the entry edge caries a right handed skater on a clockwise circle, opposite the sense of rotation in the air. Thus, in a counter-rotation jump the angular motion of the entry edge fights the angular motion of the jump. This makes it more difficult to achieve the initial amount of angular momentum needed to complete a counter-rotation jump compared to the corresponding normal rotation jump, and makes the counter-rotation jump more difficult to perform.

A flip jump, for example, is a normal rotation jump. The corresponding counter-rotation jump is the Lutz. The most common error in a Lutz is to flop over onto the inside edge before the takeoff - turning it into a flip - in order to remove the obstructing effect of the clockwise entry edge (a flutz in skating jargon). When learning the Lutz jump after first learning the flip, there is an obviously unpleasant sensation that "something" is in the way impeding the rotation of the jump. What is in the way is the angular motion of the entry edge, and the easiest way to remove this unpleasant sensation is to change edge to a flip. Some skaters will ride a long outside edge prior to a Lutz only to change edge several feet before the takeoff, others might change the edge a few inches before the takeoff. If you cannot see the actual trace on the ice, one way to decide if a jump is really a Lutz is to watch the motion of the shoulders in relation to the takeoff. In general, if the shoulders completely release (initiate the rotation of the jump) before the skater leaves the ice, the jump changed edge before the takeoff - and it ain't really a Lutz!

Pulling in and checking out in a jump requires strength of its own beyond that needed to takeoff and land. Twirl any non-rigid inanimate object with "arms" or "legs" (do not try this with your puppy dog) between your palms and it is obvious that when set into rotation the arms and legs will move away from the rotation axis. During the rotation of a jump the skater is fighting this tendency for the arms and legs to move outwards from the rotation axis. The muscles of the arms, shoulders, and chest are used to pull in the arms, hold them in place close to the chest, and then release then in a controlled way during the checkout. The muscles of the leg and buttocks are used to pull in the legs, hold them in place during the rotation, and then release them in a controlled manner for the checkout and in assuming the landing position.

The faster the rotation and the more mass in the arms and legs, the more strength that is required in this part of the jump. In triples and quads considerable strength is required, and even small skaters with microscopic jumps who spin in a blur to complete the rotation in the microsecond available require significant strength to remain pulled in. The importance of upper body strength in jumps should not be overlooked, and its lack is sometimes the basic problem in some jumps that go wrong.

Because control of rotation in a jump is determined by the relative distribution of body parts in relation to the rotation axis of the jump, certain body shapes lend themselves to more effective use of the skater's angular momentum. Specifically, narrow shoulders and hips are advantageous for getting the most bang for the buck in controlling angular momentum.

### In the air its just you and gravity

The main goal in skating is to "stay vertical", and nowhere is that more true than in jumping. If a skater takes off vertically they will return to the ice in a vertical position, and have the best chance of landing the jump. If the takeoff position is cock-eyed, however, so will the landing position, and if additional extraneous rotational motions or body positions are present landing becomes a challenge. Carelessly flinging body parts in the wrong direction, for example, can result in undesirable reaction motions in other body parts that can spell disaster. While in the air it is impossible to move one body part without another reacting in some way. In some cases this results in a movement or position that is desirable, in others not. The "Boitano" variation on the Lutz, for example, works because the arm is moved and oriented along the rotation axis of the jump and does not disrupt the overall body position when moved that way.

If a skater is out of position at the landing they may have to attempt to save the jump in several ways (if the jump is too far gone, of course, they will just fall in a lump). These include putting one or both hands down, breaking at the waist, or putting a foot down, etc. If the skater executes the checkout late or incorrectly and cannot control the angular motion at the landing they may have to put a foot down, step out of the jump, or turn out of the landing on the exiting edge. All of these actions are errors that affect the first mark, and many of them result in required deductions in the short program.