Capillarisation and The Importance To Rock Climbing Performance
Capillaries are an important physical feature of the body – more on that shortly – that climbers, especially route climbers, inadvertently obsess about. When I say inadvertently, I mean that they’re constantly thinking about how they get fitter by way of better endurance and recovery, yet they hardly ever realise that what they’re specifically thinking about is the role our capillaries play in our muscular endurance and how this relates to performance. The reason why capillaries are key to muscle function and exercise capacity, is that they are central to the delivery of oxygen and nutrients and removal of metabolites and carbon dioxide.
Outside of climbing, research has found that 98% of the variance in critical power (a great benchmark for a muscle’s level of aerobic ‘fitness’) can be explained by the capillary to muscle fibre (C:F) ratio in relevant exercising muscles. The C:F ratio describes the amount of contact points that the capillaries have in relation to the number of muscle fibres. The higher it is, the better the aerobic performance of the muscle. The reason for this is that capillaries are the ‘end point’ of delivery of oxygenated blood to the muscles. Oxygen is the terminal electron acceptor that makes up part of the lengthy process that is the aerobic pathway, and as such the greater the supply of oxygen the greater the output of ATP via aerobic metabolism.
Before looking at what happens when capillarisation (often referred to as angiogenesis in the literature) occurs and how we can affect it through training and exercise, let’s have a look at the basic physiology.
Capillaries are a single layer of simple epithelium cells, which do not have the muscular or elastic nature of other blood vessels (arteries, veins etc). They’re smaller and have thinner membranes, allowing fast gaseous exchange and transmission of water, fats and proteins. They are essential for the exchange of gases (delivery of oxygen and removal of carbon dioxide), nutrients and metabolites to the muscle, which means that they have a significant impact on exercise capacity.
The presence of higher density muscle capillaries means a larger blood-to-muscle exchange surface area, which in turns allows for faster exchange of exercise-limiting factors such as oxygen and carbon dioxide transmission. The growth of these capillaries is termed ‘angiogenesis’ and as such, any factors increasing this are known as ‘angiogenic factors’ and those which prevent it are known as ‘angiostatic factors.’ During exercise endothelial growth factor (VEGF) is secreted, which is the primary driver of angiogenesis and at present it is believed that this remains the most effective factor involved in increasing C:F ratios in the muscle.
How Does Exercise Affect Our Capillaries?
The effect of exercise on angiogenesis is still an area of research requiring much more work, but at present it is understood to involve both metabolic (change in tissue oxygen and nutrient levels) and hemodynamic signals (shear stress and passive stretch). The mechanical signalling of shear stress (literally the friction of the red blood cells rubbing against the capillary walls) and stretching (the capillary being stretched as red blood cells pass through it) causes new capillary growth via either longitudinal splitting and sprouting. This process ultimately increases C:F ratios. In untrained individuals increases in capillarisation of 10-30% are observed after 6-8 weeks of training and elite athletes with years of consistent endurance training demonstrate a C:F ratio two times greater than that of untrained subjects.
Oxygen uptake from a resting muscle is 1 ml O2/min per 100g of tissue and may increase up to 30-fold during high intensity exercise, which will be benefited by higher levels of capillarisation. This extreme range and change in oxygen transmission is supported by the low diffusion distance and large diffusion area of the capillaries in the muscle.
In terms of the effects that varying training modalities have on capillarisation, the research at present points practitioners in two key directions when it comes to the performance levels of their athletes. Firstly, in untrained athletes or those of low training status exercise has a positive (if somewhat limited) angiogenic effect via a broad range of submaximal and supra-maximal resistance training, classical endurance training and even passive leg extensions. However, there is a notably limited effect with these modalities in well trained athletes. When it comes to the elite, athletes will likely respond best to a specific regimen of low intensity, high volume training. One early study observed a 28% increase in C:F ratio following a 24 week low intensity/high volume endurance training program and subsequent studies have confirmed the strong effect of this approach.
Despite this clear pattern of positive response to high volume, low intensity training, when it comes to angiogenesis, we should not ‘throw out’ other forms of higher intensity endurance work such as interval training. The improvements in enzyme function, mitochondrial biogenesis and cardiac remodelling are still positive ‘forces for improvement’ for athletes interested in fitness and localised aerobic output. It is critical to understand that both forms of exercise modality (low intensity/long duration and interval) have their pros and cons but if a climber wishes to specifically work on capillarisation it will be most effective under the conditions provided by long duration, low intensity training.
Finally, as an interesting footnote to this section, some researchers have noted that the angiogenic response is not entirely limited to active exercise. More than one study has observed VEGF secretion and consequently significant increases in blood flow (2.8x in one study) with 90 minutes of passive leg movement. Unfortunately, the effect appears somewhat transient and without chronic stimulus the sport-specific benefits may be limited. However it does demonstrate that even a very low intensity stimulus can be beneficial in terms of increased capillarisation.
How Should Climbers Work On Capillarisation?
Like many areas of current research, we still lack climbing-based studies which means that some assumptions have to be made and little is certain. Where we can have some confidence in our approaches, is in the combined observation and understanding of the literature, our own findings in athlete response to training methods and experience as evidence-based sports practitioners. In other words, it’s a combination of theory, research and experience!
1. Capillarisation matters: We have a high confidence that Critical Power (CP) is a key determinant in route climbing performance (see our work on Digital Testing). We know that research places a very high statistical explanation (98%) of CP on the capillary-to-muscle fibre contact ratio. A higher CP results in better aerobic ATP production, intermittent recovery during exercise efforts and slower ‘burn’ through anaerobic work capacity at a given exercise/route intensity due to the greater ability of the aerobic system to be able to contribute at higher exercise intensities.
2. Duration matters: The angiogenic factor of shear stress appears to correlate strongly with duration of blood flow at a lower intensity. Climbers should likely concentrate on completing endurance training in intervals (and total session lengths) sufficient in duration to maximise the shear stress effect. However, some attention should be given to the potential negative effect on further training or recovery if duration is excessive. As research suggests that the stimulus for increased capillarisation does not increase with intensity, but instead scales with duration, a practical solution is to perform endurance exercise at a very low intensity to maximise duration while not inducing excessive fatigue. This ensures that the endurance training sessions do not compromise the high intensity training sessions which are key for other key elements of performance.
3. Intensity matters: Whilst research has found increases in capillary growth as the result of passive exercise, resistance training and supra-maximal strength training, the effects are either limited in duration or only provide a weak stimulus for growth in well trained athletes. Climbers should complete endurance training well below critical power and at a level that increases blood flow to promote shear stress. Our current guidelines would recommend this being at around 6 grades under your current onsight level. In periods of training focused on capillarisation, high intensity training should be limited/tightly controlled due to release of anti-angiogenic factors when this form of training is completed.
In summary, what we class as ARC or continuity training is likely to be the most effective form of training for capillarisation. Training strategies should aim to include this form of training into periodised cycles of ‘blocked’ work that allow for maintenance of strength training and adequate recovery from sequential training days. This is not necessarily a ‘more is always better’ form of exercise and as such a balanced approach should be applied depending on the overall needs and training history of the athlete.
1. Invited review: activity induced angiogenesis, Eggington (2009)
2. Capillary growth in human skeletal muscle: physiological factors and the balance between pro-angiogenic and angiostatic factors, Hellsten & Hoier (2014)
3. Intense intermittent exercise provides weak stimulus for vascular endothelial growth factor secretion and capillary growth in skeletal muscle, Hoier et al
4. Growth of capillaries in skeletal and cardiac muscle, Hudlická (1982)
5. VEGF, shear stress and muscle angiogenesis: a complicated triangle, Banfi & Gianni-Barrera (2015)
6. Effect of high intensity training on capillarisation and presence of angiogenic factors in human skeletal muscle, Jensen et al (2004) 7. 10-20-30 training increases performance and lowers blood pressure and VEGF in runners, Gliemann et al (2015)