Have you ever tried to learn a new skill? You may have practiced it repeatedly and over time, you likely noticed improvement. Conversely, have you ever taken an extended break from an activity due to a life event, illness, or injury, only to find that it felt more difficult when you returned to it?

If so, you have already experienced one of the most fundamental concepts in physical training: Specific Adaptation to Imposed Demands, or the SAID principle [1]. Simply put, the body becomes better at the things it does regularly and less capable at the things it does not. In the context of physical fitness, this distinction is often described as the difference between training appropriately and under-training.

The SAID principle was first explored in the late-19th century when Julius Wolff observed changes in bone structure related to the specific ways that they were loaded over time [2]. This concept has since been extensively studied, and we now understand that nearly all body tissues adapt in a similar manner to the stresses placed upon them. Bones, muscles, tendons, as well as the heart and lungs, all require consistent exposure to stress in order to perform well. When that exposure is present, performance is maintained or improved. When it is absent, capacity declines.

As an example, hard, protective callouses may develop on the hands after several weeks of performing manual labor. By contrast, muscles, hearts, and lungs weaken with disuse in those adopting an increasingly sedentary lifestyle. The body must constantly calibrate its resource allocation to match its needs. It relies on prior experience to determine which tissues require investment and which do not. Systems that are used frequently are reinforced, while those that are not are gradually deprioritized.

Importantly, more stress does not always equate to better outcomes. Body tissues can also experience harm when loads exceed their capacity to adapt. Instead of a callous, a sudden bout of manual labor may result in a blister on an unprepared hand. Similarly, an abrupt sprint to cross the road may lead to a muscle strain. In these cases, the resulting “maladaptation” is excessive tissue damage.

In alignment with the SAID principle, Physical Stress Theory (PST) describes an optimal range of wherein enough stimulus is applied to tissues over time to promote maintenance or improvement without causing harm.3 A balance exists between under-training and over-training. Between the two is where effective training occurs.

The challenge lies in identifying the optimal stimulus boundaries for a given individual and their specific activities. Human beings are exceptionally dynamic, and failure to account for this variability often results in maladaptation, poor exercise program adherence, and/or suboptimal outcomes. Physiology-based training frameworks aim to address this challenge. While individuals differ widely, much is known about the similarities in how tissues respond to stress. By grounding training systems in these shared biological principles, we can better account for variation both between and within individuals.

Though straightforward in concept, this principle warrants explicit attention. It forms the foundation upon which all physical training concepts are built and provides the context needed to interpret more complex ideas that follow. By presenting it clearly and deliberately, we aim to equip readers with a durable understanding and ability to apply our frameworks with greater precision.

References
1. The Team Physician and Conditioning of Athletes for Sports: A Consensus Statement. Med Sci Sports Exerc. 2001;33(10):1789–1793.
2. Frost HM. A 2003 update of bone physiology and Wolff’s law for clinicians. Angle Orthod. 2004;74(1):3–15.
3. Mueller MJ, Maluf KS. Tissue adaptation to physical stress: a proposed “physical stress theory” to guide physical therapist practice, education, and research. Phys Ther. 2002;82(4):383–403.

The Applied Physiologics Team
Lead Author: Michael Wahlig PT, DPT, OCS, COMT, CSCS
Published: 4/22/2026

This content is intended for educational purposes only and does not constitute medical, physical therapy, or individualized exercise advice.