#2 Reason To Do Winter Training: You Want To Be More Efficient At Lower Heart Rates
While I have discussed numerous personal stories with a variety of our regular Winter Training participants, there is substantial independent, university based research to support the need to know both your LOW threshold (often referred to as aerobic threshold) as well as your HIGH threshold (previously considered the only threshold). The graphic below shows how the research has evolved over time.
Also one of the many studies on training below T1 has proven the expansion of capillaries which is essentially enlarging your aerobic engine: “Indices of capillarization increased 30%–40% in group “A” and 20%–30% in group “B” and were elevated at post training as well compared with pre- and midtraining in both groups.

J Gerontol A Biol Sci Med Sci. 2011 Sep;66(9):957-64. doi: 10.1093/gerona/glr096. Epub 2011 Jun 29. Adaptations in capillarization and citrate synthase activity in response to endurance training in older and young men.