CrossFit – Basic Exercise Physiology and Nutrition

This is going to be the first thousand-mile-high look at what we know and what we can hypothesise about the exercise physiology and nutrition requirements of a CrossFitter. It is a short look at the exercise physiology of CrossFit, and the high-level macronutrient requirements that might support that. It acknowledges the potential importance of nutrient timing. Much more detail to follow in later posts!

CrossFit … what and why!

CrossFit® is physically and metabolically demanding, as athletes train to improve strength, power, speed, endurance, agility and flexibility. The exercise physiology required to achieve these goals is described by the graphic in figure 1: the elite CrossFitter needs a high energetic capacity (aerobic and, particularly, anaerobic capacity), combined with high force generating capacity (muscular strength) and neuromuscular coordination (for movement efficiency, gymnastics etc). Importantly, force generating capacity must be considered as a function of bodyweight – as a CrossFitter must maximise strength : bodyweight so as to be able to both lift heavy weights and move their bodyweight with ease through sprints, box jumps, gymnastics and the like!

To translate this to something more relatable, take a workout like DT. This hero workout comprises 5 rounds of 12 deadlifts, 9 hang cleans and 6 push jerks at a moderate-heavy weight. Performance in this workout is dependent on the athlete’s power-endurance. In other words, by the athlete’s muscle strength (force generating capacity), the ability of this muscle to maintain high power output over time (energetic capacity across aerobic and anaerobic domains) and continued co-ordination of muscular contraction (neuromuscular coordination) under fatigue (Burke, 2007, p145). A heavy one rep max is dependent on strength-power performance, which is determined by muscle strength (force generating capacity), and the rate at which the muscle can produce the energy (anaerobic energetic capacity) required to power the explosive movement (Maughan and Gleeson, 2010, p15).

To achieve this exercise physiology and the goals of strength, power, speed, endurance, agility and flexibility, CrossFit® incorporates gymnastics, sprint, middle distance, resistance and strongman training in various combinations, repetitions, and load, over varying timeframes (CrossFit® Mainsite, 2002; Bellar et al., 2015; Butcher et al., 2015). Whereas one or a limited number of these may be the focus in any one training cycle, a CrossFit® athlete will typically continue a degree of concurrent training across all disciplines.

In simplistic terms, the resistance elements of CrossFit® training exert mechanical load stress on the body to potentiate adaptations that increase muscle strength and bioenergetic capacity (MacDougall et al., 1977; Kadi et al., 2004; Andersen et al., 2005). Muscle size is a key determinant of muscle strength, and increases in muscle mass are driven by increases in the amount of contractile protein within muscle (Cribb et al., 2007; Maughan and Gleeson, 2010, p15). As essential amino acids cannot be synthesised by the body, the addition of muscle mass requires dietary protein (McArdle, Katch and Katch, 2015, p30). Nutritional guidelines for resistance training athletes recommend a daily protein intake incorporating a range from 1.2-2.0g/kg bodyweight per day (American Dietetic Association, 2009; Kerksick et al., 2018). Individual requirements will vary depending on training regime, and body composition. Protein synthesis is also energetically demanding, as the formation of each peptide bond requires 4 adenosine triphosphate (ATP) molecules (McArdle, Katch and Katch, 2015, p14).

And, also in simplistic terms, the conditioning elements of CrossFit® training includes elements designed to exert metabolic stress to enhance both anaerobic and aerobic capacity (Seip, 2008, p85). Many of these are performed at high intensity (>60% VO2 Max), meaning that carbohydrates are expected to provide the primary fuel for both the glycolytic and aerobic pathways (Jensen and Richter, 2012). Similarly, resistance training sessions rely primarily on the glycolytic pathway, and therefore carbohydrates, when the phosphagen pathway is exhausted (MacDougall et al., 1977; Tesch et al., 1986; Robergs et al., 1991). Dietary carbohydrate consumption should reflect this; whilst the typical intensity and duration of a CrossFit® training session is not expected to exhaust replete muscle glycogen stores, they are expected to deplete them. Similar resistance training sessions have been shown to reduce muscle glycogen stores by up to 40% (MacDougall et al., 1977; Tesch et al., 1986; Robergs et al., 1991). In the absence of sufficient carbohydrate intake after a training session an individual will have lower levels to support subsequent training sessions, particularly where multiple training sessions are undertaken within a 24 hour period (Burke et al., 2011). Sufficient carbohydrate intake enables an athlete to exercise at a higher intensity for a longer duration and feel less fatigued following a training session, supporting training adaptations and performance enhancement (Jensen and Richter, 2012). Current guidelines recommend that individuals participating in 1-2 hours of intense training per day, should consume 5-7g/kg bodyweight of carbohydrates per day, and this rises to over 10g/kg bodyweight per day if more than 4 hours of such training are undertaken each day (Burke et al., 2011).

Nutrient timing can be significant to recovery and training adaptations (Kerksick et al., 2017). There is evidence that regular intake of 20-40g complete protein may support optimal recovery and training adaptations (Moore et al., 2012; Areta et al., 2013; MacNaughton et al 2016). There is also evidence that ingestion of carbohydrates within two hours of a glycogen depleting training session supports the fastest rate of glycogen replenishment, and this is likely to be of importance to individuals training multiple times within 24 hours (Ivy et al., 1998; Jentjens et al., 2003).


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