In the context of muscle physiology, the phase between concentric (shortening) and eccentric (lengthening) phases of movements is often described by the Amortization Phase, a brief isometric (same length) hold where tension is generated without much external movement, acting as a crucial transition during direction changes in skiing.
While “amortization” is the biomechanical term, instructors and coaches relate different terms to describe this specific point in a turn such as “transition”, “neutral point” or “finishiation”.
This phase is the crucial, fleeting moment in the shortening-stretch cycle (SSC) where stored elastic energy is transferred, and muscles briefly contract with a shorter phase, generally leading to more power. It’s the transition where we oscillate our CoM towards the next direction, and minimizing this delay is key for an efficient direction change performance.
It is the momentary transition where muscles switch from absorbing force with the old outer leg to generating force with the new outer leg.
This transition refers to the phase where we release one turn and initiate the next, often characterized by a feeling of “floating” or weightlessness. The neutral point applies to the moment during a transition when our weight is centered, for a very brief moment, over our feet and our skis are flat on the snow.
In this phase, we utilize the elastic recoil of tendons and muscles of the downhill leg for an explosive start of the turn. It is a “buffering” time where the muscles transition from absorbing force to generating force. A shorter amortization phase, that is, a faster transition, indicates better neuromuscular coordination and power, allowing for quicker movements. It also prevents energy from being lost, resulting in a more efficient edge change.
This critical moment between the end of one turn, where muscles resist external forces, and the start of the next, where force is applied to engage the new edges, presents the following aspects:
- Efficiency: if the transition is quick and fluid, the elastic energy stored in the supporting ski and the muscles is harnessed to ‘launch’ the skier into the new direction.
- Ineffiency: if the skier gets ‘stuck’ or takes too long to shift their weight, the energy is lost and the next turn requires much more muscular effort, leading to premature fatigue and a loss of rhythm.
In other words, if the transition is too long, the stored elastic energy dissipates, and the stretch reflex of the uphill leg fails to activate, making the subsequent movement less powerful. An efficient amortization phase shouldn’t be longer than a ski length.
In linked turns, this phase occurs when we have finished absorbing the reaction force of the present turn and are about to push off into the next one.
When skiing over moguls, this phase represents the key moment where we stop absorbing pressure on the crest, and begin to generate it toward the next turn toward the trough.
In this scenario, the amortization phase is constant and extreme, presenting the following aspects:
- Absorption: when reaching the mogul, the legs flex to absorb the impact.
- Extension: immediately after the crest, the legs must extend to maintain contact with the snow.
- The key: a minimal amortization phase allows this transition from ‘shrinking’ to ‘stretching’ to be instantaneous, maintaining control and speed without being launched off the mogul.
Ski boots act as transmitters for this phase.
- Stiff boots mechanically reduce the amortization time, transmitting muscle force to the skis almost instantly for a more precise response.
- Soft boots allow for a longer and more forgiving amortization phase, ideal for beginners or for better absorbing irregularities in off-piste snow.
The benefits of a short amortization phase are:
- Faster and more explosive edge change.
- Better permanent contact between skis and snow.
- Reduced fatigue by taking advantage of the elastic energy form the skis and tendons.
Energy management during the Amortization Phase
The amortization phase is the critical bridge where energy is transformed so it doesn’t dissipate or cause injury.
It’s the brief instant where we stop descending our CoM and prepare to rise or change direction. It is the point of “minimum potential energy” because is at the lowest point relative to our skis, that is “maximum accumulated energy” in muscles, and skis’ flex.
When flexing during turns, we carry a large amount of kinetic energy. Our quadriceps stretch under tension to withstand the pressure. Here, we “amortize” the force of gravity and inertia. If amortization is efficient and short, that energy is not lost as heat (fatigue); it is stored in the tendons and the skis’ core.
When skiing in bumps, upon hitting the visible face of the bump, we perform a legs’ retraction. This is the active amortization phase where we transform the impact (vertical kinetic energy) into a controlled flexion. Our goal for our CoM is to follow a straight, fluid line ‘swallowing’ the bump with our legs so the terrain’s potential energy doesn’t launch us into the air.
The oblique transition, as an exit from amortization, is executed once we have amortized the pressure from the previous turn. We use that ‘rebound’ to initiate the oblique extension, moving from the amortization phase (being low and compressed) to the acceleration phase toward the new turn. If amortization is too long, we get ‘stuck’ to the snow and lose rhythm. If it is too short or rigid, the ski kicks us and we lose balance.
Summary: amortization is the moment where we ‘tame’ the potential energy coming from external forces to convert it into useful edging pressure, preventing the forces from collapsing our legs.
