Research in ecological dynamics has shown that movement system variability is not necessarily noise that is detrimental to performance [45–47], or a deviation from a putative expert performance model, which should be corrected in beginners [7]. Instead, variability may be functional to support adaptive behaviours [4]. Consideration of the functional role of movement variability leads to an exploration of appropriate adaptive behaviour. Adaptability relates to an appropriate relationship between stability (i.e. persistent behaviours) and flexibility (i.e. variable behaviours) during performance [43, 48, 49]. Skilled climbers are able to exhibit stable patterns of behaviour when needed, but can vary actions depending on dynamic performance conditions [50]. Although human movement systems have a tendency to become stable and more economical with experience and practice [51], stability and flexibility are not opposing characteristics of performance. Notably, flexibility is not a loss of stability but conversely is a sign of adaptability [43, 49] and is essential for skilled performance in extreme sports such as climbing. Even if movement patterns showed regularities and similarities within their structural components, an individual is not fixed into performing a rigidly stable solution, but can adapt an emergent movement pattern in order to maintain behavioural functionality. When a gap existing between a stable movement pattern repertoire of an individual and the demands of a task is small, and/or when the tasks constraints are weak, movement variability will likely emerge. The capacity for an individual to adapt to environmental changes exploited through different coordination patterns reflects neurobiological system degeneracy. Edelman and Gally [52] originally defined degeneracy as ‘the ability of elements that are structurally different to perform the same function or yield the same output’. Degeneracy allows an individual to vary motor behaviour (structurally) without compromising function, revealing the adaptive and functional role of coordination pattern variability at different levels of organisation (i.e. within and between individuals), in order to satisfy task, environmental and organism constraints. Mason [53] recently outlined important sources of degeneracy in movement systems, including (i) structural duplication (e.g. using either hand to perform different functions), (ii) structurally different parts of the movement performing the same function (e.g. using a hand or foot to make contact with a surface), (iii) the ability of different parts of the movement to come together in different ways to achieve the same function (e.g. components of a coordination pattern being more functional as environmental conditions change) and (iv) for the same part of a movement system to achieve the same function in different ways (e.g. a hand being used to grip a surface feature in different ways depending on its orientation). System degeneracy is a platform for complex systems to dissipate energy coming into the system that might otherwise perturb it and is hence intimately linked with explaining how neurobiological systems are able to self-organise into functional patterns [52]. Degeneracy is a central principle in an ecological dynamics framework for explaining the sources of within- and between-individual variability that arise during learning and performance.

A good example of degeneracy in rock climbing is the large range of hand grasping patterns and body positions regularly used to achieve a specific hold (e.g. crimp, gaston, jug, mono, pinch, pocket, sloper and undercling grasping patterns; bridge, campus, crossover, deadpoint, flag, heel hook, knee bar and mantle body positions; see also [1] exhibiting several individual climbing profiles). Similarly, in ice climbing, recent studies have revealed that climbers exhibit several stable patterns of motor coordination (e.g. horizontal, diagonal, vertical and crossed located angular positions of the ice tools and crampons) to achieve specific task goals [33, 34, 53]. These multistable patterns of coordination reflect a functional adaptation to dynamic environmental properties. In particular the location of the anchorage and the type of actions used to anchor the ice tools and crampons were selected to protect the icefall structure. Ice tools are usually separated from each other by 20 cm to protect icefall surface structure, which might be fragile in some parts. Similarly, different types of actions could be realised by individuals (i.e. swinging, kicking or hooking), depending on the icefall shape. When the ice is dense without any holes, climbers usually swing their ice tools and kick their crampons. Conversely, when the ice is hollow, climbers hook holes with their ice tools and crampons. Thus, the functional ability of each climber to vary the types of action used to engage with the dynamic properties of each specific icefall has been quite easy to assess by analysing the types of actions undertaken by the climber. Thus, the multistability of coordination patterns has been revealed by observing the efficient coupling of a skilled climber with properties of a performance environment, likely predicated on inherent neurobiological system degeneracy [52, 54]. From there, one key challenge is to effectively perceive information, which is meaningful in terms of action opportunities (affordances).

Johnson [62] defined expertise as the combination of speed, accuracy, form and adaptability. In fact, Johnson [62] documented an interesting fable that captures this characterisation of expertise by comparing Swedish and Finnish woodchoppers. In the fable called ‘The Woodchoppers’ Ball’, both Swedish and Finnish lumberjacks were able to chop ten cords of wood at the same speed, with the same accuracy levels in splitting matches and straws, hitting pencil marks and bird shot. Since form was related to effort and economy (e.g. the minimal amount of energy expenditure was expected), both Swedes and Finns chopped the same amount of wood in 2 h. However, when a novel task was sought to compare the performance of the two sets of woodchoppers, the issue of movement adaptability was raised. Adaptive skill implied that performance remained proficient under varying and even unpredictable environmental constraints. The Swedish woodchopper was the only one to chop wood of various heights and to chop with various types of axes [62]. By highlighting the importance of adaptability in a comprehensive definition of expertise, Johnson [62] was implicitly raising questions on the role of movement variability. As suggested previously, an ecological dynamics model of expertise articulates the roles of stability and flexibility: experts and non-experts each have their stable states and sometimes share the same coordination patterns. However, a particularity of expert performance is the capacity for adaptability, i.e. to produce behaviour, which is stable when needed and variable when needed. Expert behaviour is characterised by stable and reproducible movement patterns, which are consistent over time, resistant to perturbations and reproducible in that a similar movement pattern may recur under different task and environmental constraints. However, it is not stereotyped and rigid but flexible and adaptive. As stated previously by Johnson [62], expertise is a function of the combination of different characteristics.

In rock, ice and mixed climbing, and particularly in extreme mountaineering, adaptability of perceptual–motor skills enables a rapid ascent up a vertical surface. Speed is a criterion of success and survival because the faster one climbs, the shorter the length of exposure to danger, especially in places like the high-altitude summits of the Himalayas where weather conditions can alter rapidly. The professional Swiss mountaineer, Ueli Steck, considered as the fastest climber in the world, exemplifies this rule. In his book, ‘Speed’, Ueli Steck [63] explains how he broke the speed record when he performed a soloing ascent of three famous North faces in the Alps: the Eiger (in 2 h 47′ by the Heckmair route in 2008), the Grandes Jorasses (in 2 h21 by the Colton-MacIntyre route in on-sight climbing in 2008) and the Cervin (in 1 h 56′ by the Schmid route in on-sight climbing in 2009). Ueli Steck has also applied speed climbing in the Himalayas on summits up to 8000 m, with the South face ascent of the Shishapangma (8013 m in 10 h 30′ in 2011) and the soloing ascent of the South face of the Annapurna (8091 m in 28 h in 2013). Notably, the first summiteers of these mountains took four days to climb the North face in 1938.

Therefore, speed in body displacement may reveal functional movement variability because less time is spent exploring the functionality of a movement pattern. However, more than speed (revealing the performance outcome), climbing ‘fluency’ could be a good indicator of efficiency and adaptability to constraints. Climbing fluency usually involves the spatial–temporal assessment of a climber’s centre of gravity or hip motion. For instance, climbers can exhibit saccades (variations in speed) and/or different trajectories of hip displacement when they explore hold grasping [34, 60] as well as longer pauses dedicated to the tasks of active resting [64], route finding [35, 36] and postural regulation [27, 30].

In the following sections, we present the contributions of studies from the perspective of the ecological dynamics rational for understanding how expert climbers exhibit higher adaptability of their motor skills and better perception of affordances than novices and, how in return, these skills impact on climbing fluency.

A prominent characteristic of the adaptability of experts relates to their capacity to demonstrate functional variability in coordination patterns. Indeed, due to their extensive experience in different performance contexts, experts exploit to the fullest their individual abilities to satisfy task and environmental constraints. Moreover, since environmental constraints are neither predictable nor controllable, climbing requires experts to use numerous types of actions and patterns of inter-limb coordination during performance by exploiting system properties of degeneracy [7, 33]. In fact, our evidence revealed that, to interact as they did with key environmental information constraints, expert climbers tend to alternate their exploitation of horizontal, diagonal, vertical and crossed angular limb positions on an icefall surface, exploiting the functionality of intra-individual coordination pattern variability. Indeed, to achieve that level of performance, expert climbers sometimes moved their right and left limbs across the vertical midline of their bodies to exploit surface properties and hook existing holes in an icefall [33]. Indeed, ice climbers tended to either swing their ice tools to create their own holes or hook an existing hole when the icefall structure is soft or ventilated, supporting the functional role of intra-individual variability. Conversely, beginners showed a more constricted range of movement and coordination patterns as they tended to adopt a basic quadruped climbing pattern that resembles climbing a ladder. In particular, the limb anchorages employed by beginners remained the same with both arms (or legs) extended (or flexed), corresponding to simultaneous muscular activation of arms (or legs) [34]. This strategy involved freezing available motor system degrees of freedom which is quite understandable, given that beginners in ice climbing tended to prioritise stability and security of posture rather than taking risks to climb quickly, with insecurities about their support (anchorage of ice tools and crampons anchorage).

An important characteristic of expert climbers is their perceptual attunement to relevant informational variables, revealing that experts are better at perceiving climbing affordances than beginners. In ice climbing, [33] showed that beginners seemed mostly attuned to visual characteristics of the icefall, as they focused on the size and depth of holes and steps. Beginners exhibited a global perception of icefall shape for which big and deep holes in icefall were synonymous with deep and confident anchorages. This unique perceptual approach to perceiving functional icefall properties resulted in them not varying their limb coordination patterns enough. In particular they tended to display a static ‘X’ or spider-like body position that allowed them to maintain equilibrium, with respect to gravity. However, these positions do not provide them with sufficient mobility on the ice surface, because once one limb is moved from the ice fall, beginners quickly perceive a lack of stability. Therefore, they infrequently varied the types of actions they used, mostly corresponding to swinging their ice tools, an activity likened to ‘hammering’. Moreover, they tended to over-repeat the same actions in order to create a deep hole anchorage, which is a mark of confidence. Conversely, expert ice climbers exhibited greater perceptual attunement to visual, acoustic and haptic sources of action specifying information, which allowed climbers to detect ‘use-ability’ of holes in an icefall. Attunement to icefall properties (e.g. thickness, density, shape and steepness) that a climber might come across during an ascent may specify hole properties revealing how a climber might interact with surface structure signified by various actions such as ice tool swinging and hooking [33]. Climbing skill level and past experiences seem to influence the nature of specific environment–performer couplings and the manner of perceiving affordances, since only experts could vary their motor behaviours to save energy, balance their body and maintain constant climbing fluency and speed of ascent. Indeed, experts exhibited functional multistability of their coordination patterns and types of action (e.g. ice tool and crampons swinging and hooking).