BACKGROUND
Explosive leg power is an essential characteristic for improved sports performance. High-intensity actions such as sprinting, jumping, throwing, striking and changes of direction are commonplace in most individual and team sports. All of these actions require highly developed muscular power, allowing athletes to out-sprint or out-jump an opponent. In talent identification settings, vertical jump height, acceleration, sprint speed, and agility are often used as assessments of an athlete’s physical ability. Indeed, these abilities often translates into career progression, with AFL footballers shown to score better on jump, sprint and agility tests compared to their non-selected counterparts [1]. Maximising an athlete’s capacity to perform explosive muscular actions is therefore essential for any coach or trainer. Power can be calculated as a product of the force and the velocity of a movement. The key to power development is to train athletes to produce high levels of force as fast as possible. However, force and velocity share an inverse relationship (i.e. as the velocity of concentric muscle action is increased, less force is capable of being generated during that contraction). Therefore, both the intensity and load of training must be carefully considered when aiming to improve muscular power.
Kaneko et al. [2] examined the effect of various loading conditions on muscular power, with loads of 0, 30, 60 and 100% (expressed as a percentage of maximum isometric strength). After 12 weeks of training the elbow flexors, it was reported that 0% was the most efficient load for improving maximal velocity, 100% was the most effective load for improving maximum force output, whereas the ‘optimal load’ for maximising power output was 30%. In a follow-up investigation, it was found that using combined loads of 30 and 100% was more effective at maximising power output than training with a combination of 30 and 0% loads [3]. It was therefore theorised that a combined training method (strength-power training) may elicit the greatest improvement in muscular power.

STRETCH-SHORTEN CYCLE

Muscular function in athletic movements rarely requires the use of eccentric or concentric muscle actions in isolation. Rather, explosive movements often involve a stretch-shorten cycle (SSC), where a muscle fibre is activated, stretched, then immediately shorted, producing greater muscle force and power than a concentric contraction alone. Theory suggests that the eccentric phase enhances performance by: a) allowing time for the working muscles to stiffen and develop force for the concentric contraction, b) enhancing the potentiation of contractile elements, c) utilising elastic energy harnessed during the stretching phase, and d) exploiting the stretch reflex, initiated upon the eccentric stretching of the muscle [4]. In order to take full advantage of the SSC, the eccentric phase must be short and fast, and there must be an immediate transition between eccentric and concentric phases. Therefore, any training intervention aimed at improving muscular power should address the need to improve absolute concentric power, while maximising eccentric utilisation during the SSC. Vertical jump testing is a common method used by coaches to assess muscular power and jump height. Alternatively, performance of the SSC may be measured by adding a pre-stretch to a movement, such as comparing a counter-movement jump to a squat jump performance. A comparison of the two jumps indicates an athlete’s ability to utilise the eccentric phase to maximise concentric performance. This is commonly referred to as the eccentric utilisation ratio [5]. Maximal strength training plays a crucial role in power development, as an individual is unable to possess a high level of power without first being relatively strong. Integrating forms of maximal strength training (e.g. traditional resistance training) is therefore essential for long-term power development, especially in the early phases of training [6]. For example, strength training will result in considerable gains in muscular power for relatively untrained athletes, while stronger, more experienced athletes require a variety of more specific power training modalities [7].
Ballistic exercises, such as a jump squat or bench throw, require athletes to accelerate through an entire range of motion, resulting in high levels of concentric velocity, force, power and muscle activation [8]. This allows for enhanced maximal power in sports-specific movements, improving the ability to generate more force in shorter periods of time. Loads ranging from 0 to 50% of 1RM appear to provide the most effective stimulus for enhanced power development for this type of training [6].
Similar to ballistic exercises, weightlifting or Olympic lifting (e.g. snatch, power clean) involves high-force, high-velocity training, making it a popular method for power development. The sports-specific nature of weightlifting kinetics is also believed to enhance athletic movements, with an 8-week intervention shown to improve jumping and sprinting performance [9]. Furthermore, weightlifting provides the ideal type of movement for athletes required to generate high-velocities against heavy loads, such as wrestlers or rugby players [6]. Weightlifting loads ranging from 50 to 90% of 1RM have typically been shown to elicit the greatest improvements in maximal power [6].
Plyometric exercises, such as bounding and jumping, involve a rapid SSC with little or no external load. Consequently, this type of training maximises the transfer of training to performance, with movement patterns and velocities similar to those required in most sports. Whilst numerous studies have shown plyometric exercises to improve maximal power output, adaptations are generally restricted to low-load, high velocity movements [6]. These findings emphasise the need to integrate a variety of power training modes for long-term power development. An athlete’s training level, training volume, and periodisation of training load should all be carefully considered when prescribing training for power development. For example, well-trained athletes require a variety of ballistic, weightlifting and plyometric exercises to most effectively improve muscular power, while relatively untrained athletes should first build a substantial base of maximal strength.
Next week we will discuss practial applications and methods often used to devlop explosive power in athletes.
  1.               Pyne, D.B., et al., Fitness testing and career progression in AFL football. J Sci Med Sport, 2005. 8(3): p. 321-32.
2.               Kaneko, M., et al., Training effect of different loads on the force-velocity relationship and mechanical power output in human muscle. Scandinavian Journal of Sports Sciences, 1983. 5: p. 50-55.
3.               Toji, H., K. Suei, and M. Kaneko, Effects of combined training loads on relations among force, velocity, and power development. Can J Appl Physiol, 1997. 22(4): p. 328-36.
4.               Cormie, P., M.R. McGuigan, and R.U. Newton, Developing maximal neuromuscular power: Part 1--biological basis of maximal power production. Sports Med, 2011. 41(1): p. 17-38.
5.               McGuigan, M.R., et al., Eccentric utilization ratio: effect of sport and phase of training. J Strength Cond Res, 2006. 20(4): p. 992-5.
6.               Cormie, P., M.R. McGuigan, and R.U. Newton, Developing maximal neuromuscular power: part 2 - training considerations for improving maximal power production. Sports Med, 2011. 41(2): p. 125-46.
7.               Wilson, G., A.J. Murphy, and A.D. Walshe, Performance benefits from weight and plyometric training: effects of initial strength level. Coaching Sport Sci J, 1997. 2(1): p. 3-8.
8.               Cormie, P., et al., Optimal loading for maximal power output during lower-body resistance exercises. Med Sci Sports Exerc, 2007. 39(2): p. 340-9.
9.               Tricoli, V., et al., Short-term effects on lower-body functional power development: weightlifting vs. vertical jump training programs. J Strength Cond Res, 2005. 19(2): p. 433-7.