The 3-layer system design of the clothing is important element of keeping body warm. Within 3-layer system, moisture management layer is the most important element.
1) Moisture management is the first consideration to keep the body
warm during high-energy activities. Clothing should transport moisture away from the skin, to the outer surface of the fabric where it can evaporate. Fortunately companies such as Duofold are now using new ultra-light fabrics like CoolMax and Thermastat in their long underwear. These materials wick moisture from the skin and move it away, so start with this as an inner layer.
2) Next layer should be a lightweight stretchy insulator such as a breathable fleece sweater or vest. ; While we might not need it once we keep warmed up, wearer might not need it.
3) The final layer should be a lightweight and versatile shell fabric that will function for highly aerobic as well as less strenuous activities, depending on what layer under it. With the creation of The North Face, Gore-Tex and other innovative fabrics, today's shells are ultra lightweight, while remaining waterproof, windproof, and breathable.