The fundamental variables of physiologic stress are the intensity, duration, and frequency of training.

The intensity of effort it a qualitative component and in ultra-endurance activities where distance and time are factors, absolute intensity can be recorded as speed.

Relative intensity, on the other hand, can be quantified as a proportion of an athlete's maximum speed, or by a physiologic variable such as percentage of maximum heart rate or percent heart rate reserve.

Intensity is generally considered to be the most critical factor of training. Within the training process, the correct balance of low-, medium-, and high-intensity training is critical to the adaptation process and if too much moderate- or high-intensity training is undertaken, there is a significant risk of fatigue which may lead to over-reaching or overtraining.

The duration of training is a quantitative component referring to the length in time of a training session. Training frequency refers to the number of training sessions within a given time frame, such as a day or a week. Athletes and coaches often refer to combinations of these variables.

Training volume refers to the product of duration and frequency of training (usually in a week) and training load refers to the product of all three fundamental components, frequency, duration, and intensity.

The correct sequencing of changes in volume and load throughout a training year is critical to the adaptation process.
Quantification of Training Load
The concept of training impulse (TRIMP), developed by Bannister et al. is determined as the product of training duration and intensity where the average heart-rate of a workout is multiplied by a nonlinear metabolic adjustment factor based on the classically described blood-lactate curve.

The intensity of training is calculated as a delta heart rate ratio, which equals exercise heart rate minus resting heart rate divided by maximum heart rate minus resting heart rate.

Although a drawback to the TRIMP method is that only aerobic intensities eliciting heart rate response up to maximum heart rate can be determined, for the ultra-endurance athlete this is a minor limitation.

An alternative method to the TRIMP system is the rating of perceived exertion (RPE) originally proposed by Gunnar Borg in 1970.

The Fitness-Category scale includes a wide range of intensities ranges (6 to 20) from "no exertion at all" (6) to "maximal exertion" (20). Foster et al. used the modified Category-Ratio scale from 0 to 10 as a representation of a training session RPE.

Training load was defined as the product of the rating of the global intensity of an entire training session and the duration of the session. Thus, load equals session RPE multiplied by duration. The session RPE was found to be related to the average percent heart rate reserve during an exercise session. These two methods of quantifying training load (TRIMP and session RPE) allow an athlete to track his or her training in order to evaluate the effectiveness of different training sequences.
Periodization of Training
The sequence in which training loads or volumes are applied, as a series of training blocks, is critical to the final outcome for an ultra-endurance athlete.

Periodization, as popularized by Bompa [10], is a training concept in which the year is divided into large, medium and small training blocks referred to as macro-, meso-, and microcycles.

A microcycle refers to a structure of separate training sessions or small grouping of several sessions; mesocycles (3-5 weeks) are a grouping of several microcycles with a predetermined training objective or performance goal, and macrocycles (12-16 weeks) reflect groupings of mesocycles within a semiannual or annual plan. Periodization provides a framework which allows a coach to formulate a specific program to achieve improvements in physiologic, technical, or psychologic components of performance.

When the sequencing of training is correctly applied through a periodized plan, athletes can achieve a high state of competition readiness and during the months of hard training, avoid the overtraining syndrome.

The underlying concept of periodization is that training should progress from general to specific with the intention of peaking at competition time. Pyne listed common features of periodized training programs as follows:

1. The long-term performance goal for the season forms the basis for designing the training program.

2. There is a progressive and cyclical increase in training loads.

3. There is a logical sequence to the order of the training phases.

4. The training process is supported by a structured program of scientific monitoring in the areas of physiology, biomechanics, psychology, and physiotherapy.

5. There is intensive use of recovery or regenerative techniques throughout the training program.

6. Emphasis on skill development and refinement is maintained throughout the training program.

7. There is an underlying component of the improvement and maintenance of general athletic abilities.

8. Each part of the training program builds on the preceding phase.

The success of a periodized plan is thus determined by how the training stress is applied within the comprehensive plan while taking into consideration certain principles of training.
Principles of Training
Principles of training have been described by various authors, including Harre and Siff and Verkhoshansky. For an ultra-endurance athlete, the following principles may be considered critical to success:

1. All-around development: this principle suggests the need for an underlying general athletic ability that is supported by a strong psychological platform and technical ability in the various activities in which an athlete engages. Within the training process, overcoming training and competition stresses promotes willpower, self-confidence, and tolerance for higher training and competition demands.

2. Overload: this principle addresses the concept of progressively increasing the training load and volume of physical work so that after a recovery period, an over-compensation and improved fitness is achieved. Thus, an athlete will be able to compete or train at an improved absolute intensity.

3. Specificity: this principle emphasizes the need for practice under similar conditions to those of competition and for the ultra-endurance athlete is fundamental to success. It recognizes that specific exercises and skills are required to compete efficiently and effectively in an event.

4. Individualization: it is recognized that individual athletes will react and adapt differently when presented with identical training regimes. On a continuum, there are two broad categories of athletes. At one end are those who are genetically talented and at the other end are those with a highly developed work ethic, with a system guiding their effort. Thus, there is a need for individualized training programs with monitoring systems available to evaluate individual responses to a training load.

5. Reversibility: this principle highlights the requirement for consistent training. As suggested by the fitness-fatigue model of Bannister et al. fitness and fatigue are never constant and interruptions to training caused by injury, illness, or social needs, break the consistency of training that is required to achieve improvements. Loss of fitness can occur through inconsistent training and fatigue may occur through nontraining stress factors and inadequate recovery.

6. Structural tolerance: the body needs time to adapt to a training load. Structural tolerance is the ability to withstand weeks or months of high-volume training without the incidence of injury, illness, or fatigue that may lead to overreaching or overtraining. Through years of general and specific training, structural tolerance can be greatly improved.