Question: What kind of force do gymnasts have to overcome with muscle force?
About the article "Gymnastics", section "Landing".

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In an aerial routine, or any form of tumbling, landing is the final phase, following take off and flight This is a critical skill in terms of execution in competition scores, general performance, and injury occurrence.
Without the necessary magnitude of energy dissipation during impact, the risk of sustaining injuries during somersaulting increases.
These injuries commonly occur at the lower extremities and include; cartilage lesions, ligament tears, and bone bruises/fractures.
To avoid such injuries, and to receive a high performance score, proper technique must be used by the gymnast.
"The subsequent ground contact or impact landing phase must be achieved using a safe, aesthetic and well-executed double foot landing."
A successful landing in gymnastics is classified as soft, meaning the knee and hip joints are at greater than 63 degrees of flexion.
A higher flight phase results in a higher vertical ground reaction force.
Vertical ground reaction force represents external force which the gymnasts have to overcome with their muscle force and has an impact on the gymnasts linear and angular momentum.
Another important variable that affects linear and angular momentum is time the landing takes Gymnasts can alter the shape of the area by increasing the time taken to perform the landing.
Gymnasts can achieve this by increasing hip, knee and ankle amplitude.
With the increase of height, the amplitude in ankles knees and hips rise the bars.
Skeletal muscle contractions can be broadly separated into twitch and tetanic contractions.
In a twitch contraction, a short burst of stimulation causes the muscle to contract, but the duration is so short that the muscle begins relaxing even before reaching peak force.
The shape of the graph of force vs time in a twitch contraction can give information about the relative rates of calcium release and re-uptake from the sarcoplasmic reticulum.
If the stimulation is long enough, the muscle reaches peak force and plateaus at this level, resulting in a tetanic contraction.
If the stimulation is not intense enough, force will oscillate during the plateau and be submaximal, but with sufficient stimulation, there will be a constant force level until stimulation stops.
Voluntary muscular contractions can be further classified according to either length changes or force levels.
In spite of the fact that the muscle actually shortens only in concentric contractions, all activations are typically referred to as "contractions".
In "concentric" contraction, the force generated is sufficient to overcome the resistance, and the muscle shortens as it contracts.
This is what most people think of as a muscle contraction.
In "eccentric" contraction, the force generated is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract.
An eccentric contraction is used as a means of decelerating a body part or object, or lowering a load gently rather than letting it drop.
In "isometric" contraction, the muscle remains the same length.
An example would be holding an object up without moving it; the muscular force precisely matches the load, and no movement results.
In "isotonic" contraction, the tension in the muscle remains constant despite a change in muscle length.
This can occur only when a muscle's maximal force of contraction exceeds the total load on the muscle.
In "isovelocity" contraction (sometimes called "isokinetic"), the muscle contraction velocity remains constant, while force is allowed to vary.
True isovelocity contractions are rare in the body, and are primarily an analysis method used in experiments on isolated muscles that have been dissected out of the organism.
In reality, muscles rarely perform under any sort of constant force, velocity, or speed, but these contractions are useful for understanding overall muscle properties present in more complex contractions that occur in vivo.
Cyclic in vivo contractions can be modeled using work loops. 
Isometric exercises have some differences in training effect as compared to dynamic exercises.
While isometric training increases strength at the specific joint angles of the exercises performed and additional joint angles to a lesser extent, dynamic exercises increase strength throughout the full range of motion.
Generally speaking however, people who train isometrically don't train through a full range of motion as the strength gained at the training joint angle is where they require it.
While dynamic exercises are slightly better than isometric exercises at enhancing the twitch force of a muscle, isometrics are significantly better than dynamic exercises at increasing maximal strength at the joint angle.
Flexibility may be increased when isometrics are performed at joint range of motion extremes.
These isometric contractions recruit muscle fibers that are often neglected in some dynamic exercises.
For example, gymnasts are extremely strong at great ranges of motion through the practice of isometric holds. 
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Fast twitch muscle units and slow twitch muscle units differ in their ability to produce force and resist fatigue.
Fast twitch muscle units have the ability to produce great amounts of force but they do not resist fatigue for long periods of time whereas slow twitch muscle units do not produce great amounts of force but can resist fatigue for very long periods of time.
Fast twitch muscles include large muscle groups such as the upper thigh and upper arm muscles whereas slow twitch muscles include high endurance muscles such as those used for posture.
However, despite their drastic differences in structure and function, studies have shown that these types of muscle show the same trends in plasticity as a result of training and aging.