In the simplest sense, spectrums are just gradual accelerations. The distance can be as short as around 200m to as long as a couple of kilometers and the precise distance is made up as I go. The top speed is always determined at the moment and is not held for long at all. Constantly and gradually changing the pace is the theme here, so we should be continuing to accelerate or stop the spectrum as soon as anything tells us it's a good idea to slow down.
It should also be noted that there is a big mental aspect to these as well. It's important to feel strong, cautious, and inspired while doing these. Each spectrum is an opportunity to capture some form of greatness and carry it forward to an ever increasing pace. Your greatness could be in focus, relaxation, smoothness, or something else that presents itself at the moment.
I have been getting great results from this type of training and here is my take on some of the reasons:
- Changing the pace continually activates a changing 'spectrum' of muscle fibers as we go. It's mind boggling to think how our brain determines which muscle fibers are activated, but constantly changing the pace seems to give a very comprehensive workout to as many fibers as possible.
- By constantly changing which fibers are used, the fiber workload is more distributed, making muscle recovery easier. However, my last two marathon experiences have convinced me that sustained race pace work is also required for the longer races to give those fibers the proper training to handle the constant activation during the race.
- By mentally thinking "strong" and "cautious", perhaps more fibers can be activated at a given time as we increase the pace. It is my belief that this is only possible while things are feeling optimal and unstrained, as our brain will activate the protection mechanisms and shut down some fibers if potential injury to the connective tissue is detected.
- Biomechanics are enhanced at the faster paces as various forms of optimization are carried forward from the slower paces.
- By thinking "inspired" as well as "cautious", some very fast paces can sometimes be realized at the end of the spectrums. The proper way to have this happen is to let it happen when it happens. As stated before, the top end speed is not predetermined and the spectrum is stopped when any signal says it's over. As long as we have 'green lights across the board', we continue to increase the pace without a predetermined limit.
In summary, spectrums are just gradual increases in the pace. The length, top speed, and rate of pace increase are variable and made up as we go.