When it comes to sleep, we all know individuals who require different amounts. Some can get by on just five hours, while the next person is a zombie after seven. If we know that sleep needs are individualized, why would training volume not be the same?
There are a great number of things that affect how much training volume a person can absorb in a given time period, including individual physiology, stress, lifestyle, nutrition, age, recovery technique, sleep amount and quality, and of course practicality.
If you are following a plan that requires a set number of weekly or monthly training hours, this may or may not align itself with how many hours you can actually profit from. More importantly, the right amount of training volume may be a moving target, changing weekly or even daily.
Training volume is a component of three things: frequency, intensity and duration. If your target is to complete 10 hours of training in a week, this really only addresses one facet: duration. You conceivably could complete all your training volume for the week in just two days—but this would not be an effective course of action.
If your frequency goal is two workouts per day, one of which cuts several hours of needed sleep out of your life, you may benefit more from just one quality workout per day. Intensity is an overlooked element, but you may notice acute muscles soreness after a 5K race that took less than 25 minutes to complete. A one-hour time trial near lactate threshold may require 48 hours or more of recovery before any "build" workouts can be resumed. All three components of volume are equally important, and equal emphasis must be placed upon balancing them.
Very few people are able to train consistently without some sort of minor or major upset. Work, family or a sudden illness are just a few things that can interject themselves into a training program. Mental stress levels can affect workout quality to a high degree, and one of the most noted physiological responses to mental stress (rising cortisol levels) is the same as physical stress.
Working multiple 10-hour days to complete a project may leave you as physically exhausted as a tough workout and unable to summon the energy to complete your training for the day—even though you have been sedentary at your desk.
Dr. Timothy Noakes has proposed the central governor theory to explain fatigue. In essence, the body has a central governor that gradually imposes itself during training and racing to shut the body down as physical stress load increases. Perhaps this central governor is active under high mental stress loads as well, or the brain perceives stress as simply stress no matter what the source. At any rate, failing to adapt training volume to rising stress levels may just be the kiss of death for your next race.
More: Can You Control Fatigue?
For this reason, systems have been created to help the athlete subjectively quantify stress so that adjustments can be made proactively to training volume. Training Peaks software has a daily interface to indicate such metrics as sleep quality and amount, fatigue, muscle soreness, stress and overall health.
A banking system or "CNS Score" designed by coach Rick Crawford allows the athlete to apply a scoring system to stress (physical, emotional, mental) and recovery (sleep, rest/recovery, therapy) to help determine when training stress has become imbalanced with recovery. There are also objective algorithms that assign numeric value to training volume (i.e. distance and intensity) and allow comparison of ongoing training volume to stress/recovery balance.