of NMF of the lower limbs
The human neuromuscular system encompasses the nervous system and all the muscles of the body. Assessment of the mechanical capabilities of the lower limb muscles allows the mechanical limits of the neuromuscular system to be characterized and has been previously assessed during ballistic movements in both animals (James et al., 2007) and humans (Cormie et al., 2011; Samozino et al., 2012). These mechanical limits include the maximal amount of force that can be produced, the highest velocity at which the limbs can move, the highest level of maximal power output and the optimal velocity it corresponds to. The assessment of NMF, particularly maximal power and torque generation is of importance for a multitude of purposes including the assessment of individual performance, the efficacy of training and rehabilitation programs and talent identification (Abernethy et al., 1995). The assessment of maximal power and torque is standard practice in athletic populations but is also important for older populations, those suffering from movement disorders which degenerate over time and normally healthy individuals recovering from injury to the lower limbs. Traditionally, an understanding of NMF was provided by values of maximal torque and power produced by a given muscle group during strength testing protocols using isometric and isokinetic exercises (Wilson & Murphy, 1996). However, given that most functional movement tasks are characterized by the rapid, forceful actions of many muscle groups simultaneously (e.g. running, jumping, rising from a chair, ascending stairs ), the importance of
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ballistic exercises to assess NMF is emerging in the literature (Hoffrén et al., 2007; Millet & Lepers, 2004; Sarre & Lepers, 2005). With this in mind, in both sport science and clinical settings there is a need to assess NMF using exercises (e.g. cycling) that encompass the muscles largely used in functional tasks.
2.2.1 Limits of lower limb NMF in sport science
The ability to produce a high level of power is considered to be fundamental in a successful sporting performance (Martin et al., 2007; Morin et al., 2002; Vandewalle et al., 1987), with many studies showing that high force and power outputs are well correlated with athletic performance (Baker, 2001; Kraemer & Newton, 2000; Sleivert & Taingahue, 2004). With regards to sprint cycling, a high maximal power output and the ability to maintain a high level of power output over a wide range of cadences is favorable to a successful sporting performance, especially as the velocity of the movement is continually changing over the duration of an event (e.g. a flying 200- m sprint) (Gardner et al., 2007; Martin et al., 2007; Morin et al., 2002; Vandewalle et al., 1987). Indeed, Dorel and colleagues (2005), found that when corrected for frontal area, maximal power was found to be a significant predictor of 200-m sprint performance in their cohort of world class athletes. Similarly, in other ballistic exercises maximal power has been positively correlated with jump height (Vandewalle et al., 1987) and sprint running speed (Morin et al., 2002). Further, during sprint cycling events that require a stationary start (e.g. 1000-m time trial, 500-m time trial, team sprint) a high torque generating capability is required at the start of the event to get the bike into motion as fast as possible, to allow the cyclist to reach velocities that maximise their power output.
The assessment of lower limb NMF can be used to define the level and training status of an athlete, via the reporting of maximal torque (i.e. strength) and velocity (i.e. speed) generating capabilities of an individual’s neuromuscular system. Previously, Samozino and colleagues (2012) reported that both maximal power output and force-velocity profiles provided information regarding the NMF of the lower limbs. In particular, they suggested that an optimal force-velocity profile exists for each individual, for which performance is maximized. Quantifying these limits of NMF can also be used for the programming of athletic training, assessment of training program efficacy (Cormie et al., 2011; Cronin & Sleivert, 2005) and has implication for the identification and development of talent (Tofari et al., 2016).
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2.2.2 Limits of lower limb NMF in clinical exercise science
An adequate level of NMF is required by all humans to perform activities of daily living. Muscle power has been strongly linked to the performance of activities of daily living (e.g. sit to stand, climbing stairs), with a reduction in muscle power leading to an inability to perform these activities (Bassey et al., 1992; Clark et al., 2006; Ferretti et al., 1994; Foldvari et al., 2000; Martin et al., 2000c). The maintenance of NMF over the life span improves the ability of an individual to move without assistance which is necessary for maintaining independent functioning and is of great importance to lessen the burden on public health systems. With these findings in mind it appears essential to have testing procedures that can be implemented with older and frail individuals, those recovering from injury and for those with motor impairment disorders (e.g. stroke, cerebral palsy) to monitor their limits of NMF.
Often, lower limb functionality is assessed using single-joint exercises (e.g. knee extension and flexion), evaluating the force and power producing capabilities of a small number of muscles during isometric contractions (Bassey et al., 1992; Clark et al., 2010). However, the results from isometric exercise tests have been previously shown to correlate poorly with dynamic performances (Baker et al., 1994). Although single-joint and isometric exercises are often deemed to be ‘safer’ for clinical populations to perform, they do not appear to provide an ecological evaluation of the power and torque producing capabilities of the lower limb muscles, therefore do not represent the requirements of the tasks and activities performed on a daily basis.
2.2.3 Assessing the limits of lower limb NMF on a stationary cycle ergometer
As maximal cycling is a ballistic, dynamic, multi-joint movement requiring the production of power from the lower limb muscles (the largest muscle mass of the body) it is well suited to provide an overall assessment of NMF. Like other ballistic running and jumping exercises, most of the external force and power is produced by the lower limb muscles during cycling (Nagano et al., 2005; van Ingen Schenau, 1989; Zajac, 2002). Further, as cycling involves repetitive alternating flexion and extension of the lower limb joints and alternating contraction of agonist and antagonist muscles similar to exercises such as running, it is ideal to evaluate the limits of lower limb NMF in a range of different populations and sports.
Indeed, all-out cycling has been used largely in previous literature to evaluate the power and force producing capabilities of the lower limb muscles (Arsac et al., 1996; Dorel et al., 2005; Driss & Vandewalle, 2013; Hintzy et al., 1999; Sargeant et al., 1981). Although cycling is a complex movement requiring the successful coordination of three joints and more than 20 muscles by the CNS, it is a simple exercise task to implement, requiring little more than a commercial stationary cycle ergometer. Due to the accessibility of stationary cycle ergometers in most
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exercise testing laboratories, community gyms and clubs, the ease and affordability of performing a maximal cycling test on an ergometer is high. Furthermore, due to its closed kinetic chain nature and ability for individuals to be seated during the movement it is a relatively safe exercise, with the ergometer modifiable (e.g. upright or dropped hand positioning, flat or clipless pedals, addition of a back rest to improve stability) to suit the population tested (e.g. athletes, elderly, the injured and those with movement disorders) (Janssen & Pringle, 2008). Indeed, several studies have been conducted whereby the stationary cycle ergometer was modified to suit the requirements of the research aim (Lopes et al., 2014; Reiser Ii et al., 2002; Sidhu et al., 2012). Also, unlike other ballistic movements such as jumping and sprint running the risks of falling and injury are very low in stationary cycle ergometry, even for those who are not accustomed to the movement.