"I need to get in shape, so I'm going to start running more."
As a coach and personal trainer, I hear this a lot. Both from sport coaches talking about what they're going to do for conditioning work with their teams, and from individuals who are setting big goals for the year. Now, a lot of these sentiments are usually attached to something emotional like a big loss by a team due to lack of fitness, a doctor's appointment where health choices and habits are brought up, or via peer pressure. In any regard, it seems like any time there's a fitness or health issue, everyone jumps to "I/We need to run more." However, if we're trying to make substantial gains in cardiovascular fitness, we need to consider the changes we're really trying to make before we go all "gung-ho" on pounding the pavement.
Here's the thing we need to understand about increasing conditioning (for the sake of this article, I'll use conditioning as the blanket term for all cardiovascular goals) via running:
Running is a far trickier tool to wield in the development of baseline conditioning due to the number of factors which cause quick changes in energy system demand.
When we look at developing baseline levels of conditioning, most people are referring to cardiac output and one's ability to recover between different activities, or what is known as recoverability. One of the key drivers of this, is being able to have a large amount of blood flow per heartbeat. Stroke volume is really hard to measure, but we can get a good grip on what's happening physiologically based on Heart Rate (the number of beats per minute; bpm). The higher the heart rate, the higher the amount of demand for oxygen is going to be at different muscle tissues in order to make ATP or metabolize excess lactate. Lactate is a byproduct of more powerful movements that happen faster than the body can use oxygen for ATP (energy) production. The lower the heart rate, the more our bodies can utilize oxygen for the creation of ATP.
This is where running becomes tricky for increasing baseline conditioning due to environmental factors, namely the following two: hills, and temperature.
Everyone knows what it's like to get out of breath walking up hill or up stairs. The reason being, your body has to create more powerful strides to move you against gravity. This causes the Heart Rate to spike as it tries to deliver oxygen to the muscle and clear out any metabolic waste. If you're not able to control elevation changes to the running route, then you're bound to start switching in and out of the necessary ranges of BPM's needed to build up baseline conditioning. Especially as distance accrues, the Heart Rate will be much harder to control as fatigue sets in. (The photos to the right show the impact of elevation on HR, as the HR constantly fluctuates as elevation changes in the bottom sequence.)
The other environmental factor is temperature or weather. It seems like there's a sweet spot which most people have in regards to the temperature as how their internal physiology interacts with the environment. For example, my ideal temperature for running is between 45-60 degrees Fahrenheit. That range is where most of my best running times have come from, and where I'm able to control my heart rate much more easily. Drop the temperature below 32 degrees, and my heart rate will spike significantly in order to get blood flow to the extremities to keep them warm. Go up above 60 degrees, and my heart rate will spike as a result of trying to get blood to cool down via convection at the skin. Start changing the environment, and you may change the demands your body is experiencing, thus making it harder to manage specific adaptations.
Now, you may be wondering, what should I do instead? Well, there's a bunch of great options you can do to help this. First, you can eliminate the environmental factors by using a treadmill. Most people hate this option, but if you're deadset on running, then you can use the treadmill. The other option is low impact exercises such as stationary bikes, rowers, airbikes, etc. These allow a much easier control of energy system and control all environmental factors. The the third option is full body circuit/interval training which incorporates low impact plyometrics (skips, jump ropes, side shuffles, etc) and low power movements like crawling patterns in order to work on building up that aerobic base.
But don't get me wrong, there are definitely times to use running for conditioning. I merely want to state how it's not a great way to "get in shape" for those starting out or who want to build up an aerobic base of conditioning. For those who have the base already, running can actually be the perfect tool for developing the lactic and alactic energy pathways (making ATP with blood sugar and creatine). Short bursts of running such as distance repeats, shuttles, and hill sprints can all be useful tools for developing those energy pathways. But if we want to build a really robust system, then we should aim for other means than running with more control over the variables involved.