Maintaining and improving NMF is necessary for sustaining healthy movement across the lifespan (Martin et al., 2000c). Therefore, the improvement of the limits of lower limb NMF (i.e. maximal power, maximal force, maximal velocity and optimal cadence) is often a major focus in training programs for a wide range of populations from athletes and healthy individuals (Cormie et al., 2011; Cronin & Sleivert, 2005) to the elderly, the injured and those with movement disorders (Fielding et al., 2002; Marsh et al., 2009). Traditional resistance training programmes (e.g. squat, leg press) are often used to improve the amount of force and power that can be produced (Cormie et al., 2007; McBride et al., 2002). However, ballistic training (e.g. squat jump) is commonly recommended in favour of more traditional resistance training exercises when improvements in power are sought, due to their specificity to many sports, allowing better transfer of adaptations to performance (Cady et al., 1989; Cronin et al., 2001; Kraemer & Newton, 2000; Kyröläinen et al., 2005; Newton et al., 1996). Although not viewed as a traditional form of ballistic exercise training, sprints performed on a stationary cycle ergometer also requires individuals to maximally activate muscles over a larger part of the movement, facilitating greater adaptations and thus may be beneficial for improving the limits of NMF. Further, the external resistance at which the exercise is performed can be easily and safely manipulated on a stationary cycle ergometer, making it an ideal exercise for interventions aimed at improving the power producing capacities of the lower limb muscles.
It is well known that improvements in power can occur as little as three weeks into an exercise program. The gains in power are attributable to neural adaptations such as increased neural drive and more optimal inter-muscular coordination of the trained muscles (Enoka, 1997; Hakkinen et al., 1985; Hvid et al., 2016; Kyröläinen et al., 2005; Moritani & DeVries, 1979). Indeed, neural adaptations have been suggested to be behind the improvements in power observed after just two days of maximal cycling practice in untrained cyclists (Martin et al., 2000a), and after longer interventions of between 4 to 8 weeks (Creer et al., 2004; Linossier et al., 1993). Although these studies are useful for quantifying the overall efficacy of training, these authors did not analyse the changes in the limits of the NMF, only changes in Pmax or power produced over a sprint.
It is well known that cadence affects the amount of torque and power that can be produced during maximal cycling, as illustrated by the torque-cadence and power-cadence relationships. The production of a high level of power at a given cadence requires optimal coordination of the lower limb muscles and joints to produce high levels of power (Raasch et al., 1997). In particular, co-
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activation of proximal-distal muscle pairs has been suggested as essential for effective force/power transfer to the crank (Kautz & Neptune, 2002; Van Ingen Schenau et al., 1995). However, our ability to produce power on the left side of the T-C and P-C relationships (i.e. low cadences and high resistances) may be affected by different physiological mechanisms such as neural inhibitions and muscle potentiation (Babault et al., 2002; Perrine & Edgerton, 1978; Robbins, 2005; Westing et al., 1991; Yamauchi et al., 2007) compared to those playing a role on the right side of these relationships (i.e. at high cadences) which include activation-deactivation dynamics and altered motor control strategies (McDaniel et al., 2014; van Soest & Casius, 2000). Further, there is an abundance of motor solutions offered within the human body to produce power using different movement strategies (Bernstein, 1967; Latash, 2012). Training appears to reduce the variability in execution variables (Muller & Sternad, 2009) (i.e. joint kinematic, EMG, co-activation and crank torque profiles) optimizing joint motion and inter-muscular coordination (Chapman et al., 2008b; Hug et al., 2008; Wilson et al., 2008). Indeed, less variability is well accepted as an indicator of motor learning and movement control, exhibited by those well-trained in a task (Hug et al., 2008; Muller & Sternad, 2009). Despite these findings, the key adaptations occurring with intervention- specific training on a stationary cycle ergometer have not been previously examined, nor have the adaptations been linked to changes in the limits of lower limb NMF.
In light of the previous literature, the first aim of this study was to investigate if the adaptations of the limits of NMF would be specific to the training intervention selected. To investigate this power produced between 60-90 rpm and 160-190 rpm and key variables calculated from T-C and P-C relationships (i.e. Pmax, Copt, T0 and C0) were assessed before and after the training. Extending upon principles of training specificity, it was assumed that training against high resistances would alter the limits of NMF on the left side of the P-C relationship (i.e. increase T0 and the power generating capacity at low to moderate cadences), while training at high cadence would alter the limits of NMF on the right side of the P-C relationship (i.e. increase the power generating capacity at moderate to high cadences and C0). The second aim of this study was to investigate if different motor control adaptations would accompany the changes in the limits of NMF. Also, due to the short duration of the intervention (only four weeks), it was assumed that any changes in the limits of the NMF would be due to neural adaptations. To investigate this aim, changes in the amplitudes of crank torque and joint angle profiles and average co-activation of muscle pairs were assessed before and after training, as well as inter-cycle and inter-participant variance ratios were calculated for crank torque, hip, knee and ankle joint, EMG and co-activation profiles. It was assumed that the variability of crank torque, kinematic and EMG profiles would be reduced after training for the same cycling condition (i.e. at low to moderate cadences for those training against high resistances and at moderate to high cadences for those training at high
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velocities). Further, modifications to torque applied to the crank, inter-joint and inter-muscular coordination after training could also explain the potential change in the limits of NMF.
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