Recent Runner's World [February 2021] magazine mentions a study, which suggests that the number of miles you can handle in training may depend on whether you have more slow-twitch or fast-twitch muscles fibres.
Here is the comparison of the two types of fibres.
(Fast twitch muscles are further split into fast fatigue (arm muscles) and fatigue resistant (leg muscles in sprinters).
FAST TWITCH MUSCLE FIBRES SLOW TWITCH MUSCLE FIBRES
Not resistant to fatigue Very resistant to fatigue
High frequency firing rate Slow frequency firing rate
Pale (small amounts of myoglobin) Red (large amount of myoglobin)
Strong, fast, ballistic movement Fine controlled movement
Degrade quickly without load Rely on proprioception rather than load
Fast contractions Slow, sustained contraction
High perception of effort Low perception of effort, especially when
trained
Most muscles have all 3 types of fibres in different proportions, but generally the big muscles like quads or biceps have more of the fast twitch and the deep stabilising muscles have predominantly slow twitch fibres. (e.g. postural muscles work all the time but fatigue slowly like neck muscles holding your head up). Different motor units in a muscle are recruited in a specific order, depending on the need. If a weak contraction suffice to perform a task, only slow twitch motor units are activated. Low load training optimises slow motor unit recruitment. If more force is needed, fast twitch muscle fibres are also recruited (first the fatigue resistant, then the fatigable ones).
The study put 24 middle distance runners through 3 weeks of ‘overload training’, increasing their mileage to the point of overtraining. The runners who handled this best without getting injured were found to have a higher proportion of slow-twitch fibres.
While it is difficult to determine how many slow twitch of fast twitch muscle fibres you have without being tested in the lab, by observing your running performance you may get an inkling. You may respond better to low intensity and high mileage, or you may thrive on faster, shorter runs.
Are we therefore limited by the way our muscles are built and forever stuck in one of these training frameworks? Absolutely not and the right type of cross training can assist with speed or endurance.
The key to successfully increasing mileage without getting injured is to stimulate your muscles to “grow” more fatigue resistant fibres. How? Pilates is a perfect answer to that.
In Pilates we target the fast fatigue resistant and slow twitch muscle fibres as they are mostly present in muscles responsible for postural hold and fine motor control. Majority of the slow twitch muscle fibres are found in the deep stabilising muscles which help to improve posture and movement patterns. By effectively training your fine motor control, you boost the slow twitch fibres and increase the fatigue resistant fibres in the fast twitch motor units, which can help our bodies to become better at handling longer mileage.
Conversely, if we want to add a bit more speed and intensity to our runs, we need to probably look at more traditional strength training using higher load. Building up fast twitch fibre muscles will result in more explosive power, useful for those fast tempo and speedwork training sessions.
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