Those familiar with the HIIT Science material, know the issue all too well. There is the training load an athlete encounters, which we have some level of control over. But then there is the training load response — a completely different beast. This issue, and the problem it can cause, has frustrated athletes, teams and coaches for years.
Why on earth is the response to training so variable?
Recently, we had the opportunity to collaborate with a large group of colleagues on some projects aiming to understand more about the individual responses to training, in the hope of learning how we can individualize training prescription more effectively, particularly for athletes. To be clear, if two athletes perform the same type of training session or training block, it is unlikely that the athletes will have the same acute response or adaptation, nor will they perceive the training stimulus in exactly the same way. There are a multitude of reasons for these divergent responses that we find very intriguing.
One factor that may explain some degree of these variable responses is the variability in the ratio of type I (slow-twitch) and II (fast-twitch) muscle fibres (i.e., muscle fibre typology). Type I muscle fibres produce force relatively slowly (6) but have superior fatigue resistance compared to type II fibres (10, 27). As illustrated simplistically, Type II fibres can produce more power (6), but are thought to have greater fatigability than type I fibres (29).
Predominant muscle fibre type in an athlete affects their ability to produce speed/power.
It is conceivable that variation in the proportion of these fibres may dictate how an athlete responds to training from an acute recovery time course perspective, as well as the long-term chronic adaptations to a given training block.
Is muscle fibre type typology important for sports performance?
Classical studies from the 1970’s and 1980’s (8, 9, 13, 14, 17, 25) showed that muscle fibre typology differentiated between athletes specializing in different sport disciplines, as well as the competitive level of athletes within these disciplines. For example, international level distance runners possessed a significantly greater percentage of type I fibres (mean; range: 69; 63-74%) in the gastrocnemius compared to middle-distance (61; 44-73%) and sprint-distance runners (27; 27-28%) (9).
Other studies show that athletes with a higher proportion of type II fibres typically have superior sprint performance, but have greater fatigability during and a longer time course of recovery following high-intensity exercise compared to those athletes with a higher proportion of type I fibres (15, 16, 19). As such, variation in muscle fibre typology might be important for specializing in different sports and for individualizing training and recovery cycles.
Classifying muscle fibre types
Muscle fibres have traditionally been classified by analyses of their myosin heavy chain (MHC) isoforms and can be identified as pure (i.e., type I, IIa, IIx) or hybrid fibres that co-express two or more MHC isoforms (i.e., I/IIa, I/IIa/IIx, IIa/IIx, I/IIx) (6, 30).
Normalized power was 2-, 6-, and 12-fold greater in MHC I/IIa, MHC IIa, and MHC IIa/IIx fibres, respectively, when compared with MHC I fibres. (Galpin et al. (12)).
One of the major reasons that muscle fibre typology is not routinely measured to guide sport and discipline specialisation and training advice for athletes, is that the muscle biopsy technique is typically met with resistance from athletes given that it is highly invasive and disruptive to training. While muscle biopsies allow for the direct determination of MHC composition, the disadvantage is the low test-retest reliability (5, 26, 28), which may result from alterations in the depth of the muscle biopsy (20, 22), the yield of fibres that are obtained (24) and/or other sources of technical error (28). As such, a valid and reliable non-invasive technique to estimate muscle fibre typology would circumvent some of these limitations and allow us to apply the information that we’ve learned from classical muscle biopsy studies for talent identification and training prescription for athletes.
The Muscle Talent Scan
Nearly ten years ago, Professor Wim Derave’s lab of Ghent University developed a non-invasive method to estimate muscle fibre typology, based on the measurement of muscle carnosine by proton magnetic resonance spectroscopy (1H-MRS) (1).
Carnosine is a stable, dipeptide that is present in two-fold higher concentrations in type II fibres compared to type I fibres (7, 18) and has a strong positive association with the muscle biopsy determined percentage of total area occupied by type II fibres in males (r = 0.714, P = 0.009, n = 12 males) (1). 1H-MRS allows for the measurement of an absolute carnosine concentration and this value is expressed as a Z-score relative to a non-athlete control population that have previously been scanned (1, 2).
We do this so that both men and women can be integrated into the same analysis given that women consistently show lower levels of muscle carnosine (11, 23), which may be mutually exclusive from any potential sex-based difference in muscle fibre typology. Studies that we have conducted at Ghent University (Belgium) and at Griffith University (Gold Coast, Australia) have shown extremely high levels of reliability between inter-day 1H-MRS measurements of muscle carnosine (coefficient of variation: 3 – 8%).
While the 1H-MRS measurement of muscle carnosine is an estimation of muscle fibre typology, this technique has the advantage of sampling a substantially larger volume of muscle compared to the muscle biopsy technique (1), which better accounts for the variation in muscle fibre typology in the superficial and deep parts of a given muscle (20, 22).
Furthermore, the major advantage of the 1H-MRS measurement of muscle fibre typology is the non-invasive nature, making this much more palatable for elite athletes.
Absolute carnosine concentration is expressed as a Z-score relative to a non-athlete control population that have previously been scanned. The carnosine Z-score provides an estimation of the muscle fibre typology of an athlete. (Original figure, Prof Derave’s Lab).
Our first studies showed that the estimation of muscle fibre typology with 1H-MRS provides a systematic distinction in the estimated muscle fibre typology between sprint- and endurance-type athletes both within and between different sports (1, 3, 4).
Acute responses to training
In the first of our experimental studies (21), we aimed to determine whether recreationally active athletes with a higher estimated proportion of type II fibres (carnosine Z-score >0.5) had greater fatigue during a sprint-interval training session as well as delayed recovery in the subsequent 5 h, compared to those subjects with a higher estimated proportion of type I fibres (carnosine Z-score <-0.5).
The thought behind this study came from our observations of training sessions with groups of athletes who, despite giving the same perceived effort, recovered more quickly between repetitions, while others required longer recovery times to run the same time or produce the same power output as the previous repetition.
In this study, both groups produced the same mean power output across the sprint-interval training session; however, the drop in power output throughout the sprint intervals was higher in the group of subjects with more type II fibres (-61%) compared to the group with more type I fibres (-41%). The torque at maximal voluntary contraction had fully recovered in the latter group after 20 min, while the subjects with more type II fibres had not yet recovered 5 h into recovery.
We interpret these findings to suggest that only 90 seconds of all-out sprint exercise can induce long-lasting fatigue and impairments in the contractile function of the muscles of subjects with more type II fibres and resulted in incomplete recovery up to 5 h post sprint training.
The left figure shows the power output during the 3 Wingates, the group with a higher estimated proportion of type II fibres displayed a larger power drop when compared to the group with a higher estimated proportion of type I fibres. The middle figure shows that the total work done during the sprints was the same in both groups. The right figure shows that the group with a higher estimated proportion of type I fibres was able to recover maximal force from the sprint-interval session in 20 min, while the group with a higher estimated proportion of type II fibres was not yet recovered after 5 hours. (Lievens et al. (21)).
- The 1H-MRS measurement of muscle fibre typology can be used to categorise athletes into sports and/or disciplines that are fast and explosive or slow and endurance-based.
- While coaches can use various performance measures to estimate the muscle fibre typology in their athletes, most of these estimation methods are responsive to training. Carnosine is a stable intramuscular metabolite that is largely independent of extrinsic factors, such as diet (except for beta-alanine supplementation) and training, and its concentration is markedly different between type I and II fibres. This makes it a good candidate for the estimation of muscle fibre typology.
- The acute recovery time following a sprint interval training session is dependent upon an individual’s estimated muscle fibre typology, which has implications for how training sessions are distributed across a weekly program.
- Athletes with a greater estimated proportion of type II fibres had a 20% higher drop in power output during subsequent sprints within a HIIT session, which has implications for session design given that the work interval intensity is a key training prescription variable.
- The distribution of hard interval sessions within a given training week could be a major consideration for athletes with a greater estimated proportion of type II fibres given that their recovery between sessions is considerably slower compared to athletes with a greater estimated proportion of type I fibres.
We found these findings intriguing, and with such polarized responses in the acute recovery time course following a single sprint-interval training session, we wondered whether we would also see divergent responses across more chronic training periods. In the follow-up blog, we will discuss the findings from these projects.Phil Bellinger & Eline Lievens
About the authors
Eline Lievens is a PhD student at the Department of Movement and Sports Sciences Ghent University (Ghent, Belgium)
Dr Phil Bellinger is a lecturer at Griffith University (Gold Coast, Australia)
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