Which training concept focuses on the need to alter various training factors to ensure continual progress?

The focus of the training concept you're inquiring about is on the necessity to change different training factors to ensure continual progress. This is specifically what the principle of variability addresses. It emphasizes that in order to keep making progress and to avoid plateaus in training, it is essential to regularly change aspects such as the exercises performed, the intensity levels, the volumes, and the modalities used. This continual variation in training stimuli allows the body to adapt and improve over time.

In contrast, while other concepts like the SAID principle and the principle of specificity also play important roles in training adaptation, they do not primarily emphasize alterations in training variables. The SAID principle (Specific Adaptations to Imposed Demands) focuses on how the body specifically adapts to the stresses placed upon it, while the principle of specificity highlights the importance of training in a manner that closely relates to the intended goal or sport. Muscle confusion, although widely promoted in popular fitness, is a term that lacks formal definition in exercise science and does not represent a well-established training principle.

Thus, the principle of variability is the most accurate answer as it directly relates to the need for altering training factors to promote continual progress.