Why should you be running easy? What does easy actually mean?
There are many benefits of easy running and many reasons why easy running holds a firm place throughout training cycles. Let's break it down in a simple and concise manner.
We are made up of two types of skeletal muscle fibres - slow-twitch and fast-twitch. Genetic disposition accounts for an individual's ratio of slow to fast, however, training can, and does make a difference. Easy running recruits slow-twitch fibres as they have a lower contraction (firing rate) threshold than fast-twitch fibres. The more you use them, the more they develop. Simple right?!
While slow-twitch fibres are more fatigue resistant, they are also more efficient in using fat for energy. At low intensity, the body burns approximately 70% fat and 30% carbohydrate. As your pace increases, so to does the rate at which carbohydrates are burned. Easy running serves as a tool to coach your body to utilise slow-twitch muscle fibres, while at the same time teaching your body to burn fat instead of carbohydrates. Still with me?
As the need for fat burning increases during exercise, the mitochondria that is housed within slow-twitch muscle fibres grow in size and are dispersed throughout the muscles. More mitochondria = efficient fat burning.
Easy running also stimulates the enzymes responsible for burning fat. Enzymes are 'switched on' by aerobic activity (running) and facilitate the process of fats entering the bloodstream to be used by the muscles for fuel. Stimulated enzymes = longer duration of fat burning.
Easy running develops capillaries. Capillaries supply muscle fibres with oxygen. Dense capillaries = greater / more efficient supply of oxygen. That one speaks for itself.
Easy running increases hemoglobin, responsible for transporting more oxygen and clearing waste products that occur as a result of running. More hemoglobin and more plasma volume = greater delivery of oxygen. For those cycling fans, the famous blood doping cases exposed during Operation Puerto refer in depth to hemoglobin levels and their relationship to hematocrit (ratio of red blood cells to blood volume).
How could we forget the heart? The heart is your 'pump' and like muscle, gets stronger with exercise. The heart's left ventricle increases in thickness and size, creating a bigger chamber to pump blood outwards to the arterial system. A bigger left ventricle means less heart beats. Less heart beats = less stress on the heart.
Progressive overload of tendons and joints is also an important factor related to easy running. Have you ever done too much, too soon and ended up sore and on the verge of injury? Easy running serves to strengthen the tendons and joints responsible for facilitating a strong gait. We also recover quicker from easy running than we do from hard workouts, making easy running a more sustainable output for long term aerobic development.
Lastly, easy running is an integral part in building, maintaining and developing fitness across a range of systems. Think of your fitness as a piece of string. When you pull one part of the string up (through specific workouts targeting specific systems), the rest of the string moves as a result. As such, easy running also elicits responses to lactate threshold and V02 max. More on this later though!