Complex training (sometimes referred to as contrast training) applies a post activation potentiate training effect by enhancing the neuromuscular system, with the athlete firstly completing a resisted training movement before completing a plyometric based training activity. The theory being that the first resisted movement increases the level of motor unit recruitment, motor neuron firing rate frequency and intra-muscular calcium concentrations, all of which result in an enhanced performance of the proceeding plyometric based exercise. This enhanced training effect is referred to as post activation potentiation and is an area of current active research within sports science.

Complex Training Considerations

Smilios et al (2005) previously investigated the acute effects of complex training involving loaded half squats and loaded jump squats on counter movement jump performance at varying loads, and found that both loaded half squats and loaded jump squats significantly improved counter movement jump performance in athlete populations. The authors concluded that contrast loading of low to moderate loads can lead to short-term increases in vertical jump performance. Baker has previously highlighted the importance of load selection when implementing complex training methods, suggesting that practitioners should design complex-training training interventions that incorporate exercises from within the speed-strength range of the force-velocity curve profile followed by exercise situated at the high velocity end of the force-velocity curve (Baker and Newton, 2005). This ensures that the motor neuron rate coding is similar for both training modes as both exercises would be performed at high velocities.

Antagonist – Agonist Method

Another complex-training training approach suggested by Baker and Newton (2005) involves performing a movement that recruits the antagonist muscularity vs the proceeding agonist movement. The authors hypothesised that the weak antagonist muscles may limit the speed of movement and that strengthening the weak antagonist may lead to an increase in agonist muscle movement speed and power output. The authors reported that the participant group that performed bench pulls between a bench throw performance test demonstrated significant increases in power output. These findings demonstrate that both agonist-agonist (or same movement pattern) or antagonist-agonist complex training may be advantageous to increasing power production in athletes.

Intra-complex Rest Considerations  

Comyns et al (2006) previously highlighted the importance of adequate recovery periods between each complex exercise (intra-complex) and between each set (intra-set) was of great importance when implementing complex training methods. The authors suggested that the required rest intra-complex rest period may be athlete dependent and should be longer, rather than shorter in duration. Therefore, intra-complex rest periods of 3-4 minutes are recommended when performing complex training with athletes.  

Complex training offers an effective means of increasing power output in athletes, whilst utilising both speed strength and plyometric training within a short time scale, therefore making complex training a time efficient training mode within strength and conditioning practice. However, practitioners should be aware of the neuromuscular demand of complex training, and therefore consider how it is scheduled within the overall periodised training plan and the athletes overall training status.   

The need for replicating the specific ‘repeated sprint demands’ within strength and conditioning practice were previously explored within part one of this article series, including the beneficial adaptations associated with repeated sprint ability training (see ‘Repeated Sprint Ability Part I : Bioenergetic Adaptations). The training considerations when programming repeated sprint ability training is explored within part two of this article series.            

Linear Based RSA

Little and Williams (2007) previously reported that a repeated sprint protocol of 40x15m sprints with a work : rest ratio of 1:4 or 1:6 most closely replicated the repeat physiological demands of competitive soccer. However, the authors also reported that the reduction in sprint performance when implementing a 1:4 work : rest ratio was too great when applied with soccer athletes, suggesting that a greater rest period may be required between sets (e.g. 1:6 work : rest ratio). Furthermore, the authors also recommended that a supra-maximal repeat sprint protocol of 15x40m sprints (with a work : rest ratio of 1:4) may be applicable when periodised appropriately within soccer performance programs. These findings demonstrate the benefits of repeated sprint training protocols that replicate the specific physiological demands of sports performance, and how repeated sprint training can be programmed with the occasional aim of overreaching an athlete as part of an overall periodised plan.   

Change of Direction Based RSA

A potential limitation of such linear ‘straight-line’ repeated sprint protocols is the lack of change of direction during each completed sprint effort. Buchheit et al (2010) previously reported a 30% reduction in sprint performance when comparing 25 m linear sprint performance vs 2 x 12.5 m shuttle sprint performance with the inclusion of a 180 degree turn. Furthermore, the authors also reported variances in measured performance variables between both sprint protocols, with the greater (and therefore more demanding) scores being recorded during the shuttle sprint efforts. The authors suggested that the greater time taken to complete each sprint was a result of the athlete participants having less time to accelerate due to the shorter distance being covered, and the need to decelerate and perform a change of direction before re-accelerating.

The authors also suggested that the differences in sprint performance and the physiological performance variables observed were an indication that the change of direction requirement within the sprint shuttle protocol placed a greater physiological demand on the bodily systems when compared to the linear sprint protocol. Therefore, strength and conditioning coaches should consider implementing repeat sprint training methods that involve a change of direction when working with athlete’s who’s chosen sports involve repeated changes of direction, or as a form of progressive overload within an overall periodised plan.

Aerobic Capacity 

Bogdanis et al (1996) previously demonstrated that aerobic metabolism significantly contributes to repeated sprint performance beyond the first performed sprint. The authors reported that aerobic metabolism demands increased from 31% to 50% beyond the first sprint effort when performing repeated 30 second maximal sprints. The significant increase in aerobic contributions even occurred after a four minute rest period between each repeated sprint. These findings demonstrate that aerobic capacity contributes significantly to repeated high intensity performance, both in the form of recovery and high intensity performance, despite the anaerobic nature such explosive movements Hoff (2005) previously reported a reduction in distance covered and repeated sprint efforts during the second half of competitive games, suggesting a depletion in glycogen scores may contribute to such a reduction in performance. The authors concluded that an improvement in an athlete’s aerobic capacity would lead to an improvement stored fat utilisation, therefore reserving stored glycogen stores for more high intensity efforts.

These findings demonstrate the need for athletes that require repeat high intensity effort capabilities to improve their relative aerobic capacity before embarking on a repeated sprint training protocol, therefore enhancing their aerobic metabolism contribution levels, intermittent recovery performance and glycogen reservation.        

Metabolic conditioning can be considered as an overall umbrella term that encompasses all forms of aerobic and anaerobic based conditioning. Practitioners should ensure any applied metabolic conditioning based training matches the bio-energetic and sports specific demands of an athletes chosen sport whilst periodically training at a sub-maximal and supra-maximal level in-relation to these pre-determined sport demands. Metabolic conditioning based training comes in a variety of forms, one of which is repeated sprint ability.

RSA Demands of Sport

Russell et al (2016) previously investigated the movement demands of premiership soccer players and reported that players covered a mean total distance of 9.5km, a mean high intensity distance of 487 m, and a total of 656 accelerations per 90 minute game. Likewise, rugby league and union players are required to perform repeat high intensity efforts (acceleration, deceleration and changes of direction) including repeat high force collisions (tackles, scrums, etc.) (Till, Scantlebury and Jones, 2017). Spencer et al (2004) reported similar findings, with the mean number of repeat sprints being 4±1 sprints per high intensity bout within competitive elite field hockey. The authors also reported that 95% of the recovery during each repeated sprint was of an active nature.

Similar to the intermittent nature of team sports, racquet sports require athletes to perform repeat high intensity efforts throughout competitive play. However, the distance covered per sprint is reduced when compared to team sport movement demands due to the constrains of the size of the court.  Pluim (2004) previously described tennis as an intermittent exercise, involving alternating short bouts of high intensity exercises lasting 4-10 seconds with short recovery periods of 10-20 seconds with several periods of longer duration activity of 60 to 90 seconds (rallies). These findings demonstrate the repeat sprint demands of team sports and racquet sports, and the need to replicate these demands with training, allowing for the favoured metabolic based adaptations associated with repeated sprint training.

Hydrogen Buffering Efficiency

The accumulation of lactate acid is often referred to as the main factor in diminishing power output and athlete performance. However, it appears that lactic acid itself may be accumulating in an attempt to buffer the accumulation of hydrogen ions that occurs during the process of anaerobic metabolism. The accumulation of hydrogen ions and inorganic phosphates has previously been shown to be key factors contributing to muscle fatigue in repeated sprint performance (Glaister, 2005). Both hydrogen ions and inorganic phosphates inhibit calcium’s effect to activate tropomyosin, therefore reducing the myosin – actin binding mechanism and cross bridge cycle rate. Edge et al (2006) previously reported that athletes with a high level of repeated sprint ability demonstrate a superior ability to buffer the accumulation of hydrogen ions associated with glycolytic metabolism. Therefore, it appears the ability to buffer hydrogen ions is a trainable adaptation in athletes, in addition to an increase in muscle glycolytic enzyme content and activity.

Lactate Shuttling

During the performance of high intensity efforts, skeletal muscle lactate transporters situated within the muscle membrane assist in the reduction of lactate accumulation. In particular, monocarboxylate transporters are critical in the removal of lactate and hydrogen ions. Bishop et al (2011) previously suggested that the responsiveness of monocarboxylate transporters maybe responsive to anaerobic based training methods involving repeated high intensity efforts. Furthermore, previous findings suggest that the transported lactate out of a muscle cell during high intensity training can be oxidised by the adjacent muscle fibres and used as a substrate for energy metabolism (Brooks 2009). Such increase in monocarboxylate transporter adaptations would contribute to an overall reduction in lactate and hydrogen accumulation and increase an athlete’s ability to ‘tolerate’ lactate and muscle buffering capabilities.       

Enhanced Glycolytic Enzyme Activity

An athlete’s anaerobic capacity performance is dependent upon multiple factors, including glycolytic enzyme content and activity. This is due to the glycolytic demands of anaerobic based activity, which taxes the glycolytic energy pathways. Kubukeli et al (2010) previously suggested that the number and activity of glycolytic metabolism based enzymes increase in response to training that stresses an athlete’s anaerobic bio-energetic systems. Such anaerobic based adaptations would obviously favour an athlete’s repeated high intensity effort capabilities.

Such adaptations would favour athletes who require the ability to produce repeated high intensity efforts, therefore providing a sound rationale for the inclusion of anaerobic based training such as repeated sprint based training. The next article within this series will explore the programming of repeated sprint ability within sports performance and what factors should be considered when designing repeated sprint ability-based training (see ‘Repeated Sprint Ability Part II : Training Considerations).

Many sports require athletes to produce ballistic push off movements without any prior landing or eccentric loading including sprint events across a variety of sports (e.g. sprint start, swim start, etc.), explosive takedowns or throws within combat sports (e.g. mixed martial arts, judo, etc.) and acceleration from a standing start (e.g. team sports and racquet sports). The performance of such movements requires athletes to generate large ground reaction forces and peak power production without any prior accentuated eccentric landing. This quality is referred to as ballistic push-off performance, and is vital to competitive sprint performance where races are often won or lost depending on an athlete’s ability to explosively push out of the blocks.

Maximal Strength

Research by McBride et al (2009) previously demonstrated a strong relationship between 1RM back squat performance and the 40 yard and 10-yard dash in American college football athletes. Likewise, Young et al (1995) reported a greater transfer of training effects between maximal strength and acceleration performance when compared to rate of force development qualities. This relationship is explained by biomechanical principles and the need for an athlete to overcome their inertia by generating large horizontal and vertical ground reaction forces over time (300 to 500 milliseconds) and thus, generate large impulses and changes in momentum. Hence, the ability to produce high peak ground reaction forces within ballistic push-off performance is key. These findings demonstrate the importance of improving peak force capabilities (via basic and maximal strength training) in athletes that require ballistic push off performance, in conjunction with other required performance qualities.

Plyometrics  

The explosive nature of ballistic push-off performance demands athletes to perform a large amount of mechanical work (e.g. explode forward and upward by breaking inertia) within a fraction of a second, and therefore requires athletes to produce a high level of power output. Bezodis et al (2010) previously reported that normalised horizontal power was the most appropriate performance measure to quantify sprint start performance (incorporating block velocity and time spent producing the velocity). It would appear that power production is key to ballistic push-off performance, and should therefore be a key focus when aiming to improve ballistic push-off performance in athletes.       

Bishop et al (2009) previously reported that dryland plyometric based training within adolescent swimmers had a positive impact on swim start performance. Potdevin et al (2011) reported similar findings when investigating the effect of plyometric training on swim start and turn performance, with the authors strongly recommending that plyometrics be incorporated within swim performance programs. These findings suggest that plyometric training is an effective training modality for improving ballistic push-off performance in athletes, and should therefore be applied within strength and conditioning programs that look to improve this quality.    

Olympic Weightlifting & Loaded Jumps

The explosive power requirements of Olympic weightlifting and loaded jump training offer an effective method for improving ballistic push-off performance in athletes. In particular, the concentric dominant nature of the Olympic lifts and single effort hex-bar jumps provide an ideal transfer of training effects, matching the kinetic demands of a ballistic push-start within sport. Furthermore, the performance of both the Olympic lifts and loaded jumps require a rapid triple extension at the ankle, knee and hip during the explosive 2nd pull phase (Olympic lifts) and jump phase (loaded jumps), and therefore closely match the kinematics of many sporting actions that involve the same movement demands (e.g. jumping, sprinting, etc.). Hence, the inclusion of Olympic weightlifting and loaded jump training within performance programs may be beneficial towards improving ballistic push-off performance in athletes.

Strength and conditioning coaches need to ensure that maximal strength training and speed strength training methods are correctly implemented within performance programs aimed at improving ballistic push-off performance, therefore improving an athletes ability to explosively push-off from the start blocks, from a deadstart, or any other sporting movement that requires an explosive ballistic push-off action.    

The anterior cruciate ligament connects the femur to the tibia and is often a major injury site amongst athletes across multiple sports. Research by Hawkins and Fuller previously highlighted the incidence of anterior cruciate ligament injuries within elite senior and academy soccer players, with the most severe knee joint injuries (measured by time away from playing activity) involving the anterior cruciate ligament. McManus and Cross (2004) reported similar findings when investigating injury rates within elite senior and academy Australian rugby union players, with knee injuries (e.g. ACL and MCL sprains, etc.) being recorded as the most ‘costliest’ non-contact injuries based on severity of injury and time away from competitive play (e.g. games and training sessions missed). Knee injury occurrence during tackle-based contacts also accounted for a large number of ACL and MCL injuries within competitive games. Berson et al (1981) reported similar findings within squash players, with over one third of injuries being classified as strains or sprains and over half of the reported injuries involving the lower extremities, with the ankle and knee again being the most common injury sites. It is evident that ACL injury occurrence is unfortunately common across multiple sports, and should therefore be a key focus when preparing athletes who compete in such sports.  

Concentric Quadricep vs Eccentric Hamstring Strength

Knee ligament injuries are the result of excessive shear, compressive and torsion forces occurring within the knee ligament connective tissues, leading to ligament strain. Such injuries can be the result of a combination of factors, accumulating overtime and/or occurring within an instant. A lack of eccentric hamstring strength, in-relation to concentric quadricep strength, has previously been identified as a knee ligament injury risk factor (Myer et al, 2008). Upon landing the hamstrings contract in a paradoxical fashion, creating a posterior force upon the knee. This posterior force aids in reducing the shear forces experienced at the anterior knee connective tissues, including the ACL. Myer et al (2008) previously highlighted the importance of specifically developing hamstring strength to ensure the correct dynamic stabilisation of the knee upon impact (e.g. when landing and whilst in locomotion). Therefore, strength and conditioning coaches must ensure programs are structural balanced by including equal amounts of concentric quadricep (e.g. squats, split squats, etc.) and eccentric hamstring (e.g. nordics, glute ham raises, stiff legged deadlifts, etc.) within performance programs.   

Lumbopelvic Control

Zazulak et al (2006) recently highlighted the association between lumbopelvic control and an increased risk of ACL injuries in female athletes, suggesting a lack of torso positional control when performing change of direction or landing based tasks predisposes female athletes to an increased risk of ACL injury. Likewise, Lephart et al (2005) reported similar findings, suggesting the degree of knee and hip flexion upon landing is directly related to the ability of the soft tissue structures to absorb joint forces within the lower limbs. It is therefore apparent that knee ligament injury prevention measures should include lumbopelvic strength and stability-based training that develops the ability to resist torso rotational forces (e.g. anti-rotation based exercises).

Knee Valgus

Excessive knee valgus moments when performing changes of direction and landing has previously been identified as an ACL injury risk factor (Boden et al, 2010). McLean et al (2005) previously investigated peak knee valgus moments in male and female athletes whilst performing a sidestep manoeuvre and reported significantly greater knee valgus moments in the female group when compared to the recorded knee valgus moments within the male group. The authors also reported that the greater knee valgus moments were associated with greater initial hip flexion, internal rotation and knee valgus angles. These findings demonstrate the need for correct hip control when performing sidestep manoeuvres and the ability to resist excessive hip flexion upon impact, especially within female athletes. 

Tendon Stiffness & Proprioception  

Previously documented ACL injury prevention programs have highlighted the importance of tendon stiffness, with the most reduced ACL injury rates seen in those prevention programs that included plyometric based exercises (Hewett et al, 2006). Ingersoll et al (2008) also previously highlighted the importance of proprioceptive based training (e.g. BOSU single leg balance, etc.) within ACL prevention programs, as a means of increasing sensorimotor function within the lower extremities.

Collectively, these findings highlight the importance of reinforcing hip stability, torso control, reduced knee valgus, eccentric hamstring strength, tendon stiffness and proprioceptive feedback within programs aimed at reducing ACL injury rates in athletes.  

A strength and strength conditioning has two primary objectives when preparing athletes for the demands of sport. One : to increase the required performance qualities of an athletes chosen sport, and two : to reduce (where possible) the chance of an athlete gaining an injury within training or competitive play. The second objective involves the identification of any specific injuries associated with the athletes chosen sport, and the implementation of preventive measures to reduce the chance of that injury occurring.

Such ‘injury prevention’ based training is often associated with mobility drills, muscle imbalances, stretching programs, and other corrective exercise-based strategies. However, what is often overlooked is the vital link between improving the required performance qualities within athletes (e.g. force and torque capabilities, tendon stiffness within the musculotendinous unit, etc.) and an athlete’s overall robustness to injury. Eccentric strength capabilities are one such performance quality that is of great importance in regards to injury prevention.  

Co-contraction of Muscle

The biomechanical demands of landing (such as when performing plyometric-based training) places excessive forces on the skeletal structures, including the joints within the lower extremities. Such joint forces can exceed many multiplications of an athlete’s equivalent bodyweight (measured in units of Newtons) due to the effect of local gravitational force. The greater the level of muscular activity and tendon stiffness around the joint upon landing, the greater the reduction in forces experienced on the actual skeletal structures. This is due to the local muscularity being able to efficiently transmit force throughout the muscle, therefore reducing the magnitude of force experienced at the joint itself. This is achieved via the co-contraction of muscles around a joint upon landing. In particular, the ability of a muscle to contract eccentrically.     

Eccentric Muscle Action

When landing from an elevated height, the quadricep muscularity undergo a rapid eccentric contraction, therefore reducing the forces experienced at the knee joint ligaments. Hence, an increase in eccentric force capabilities may reduce the chance of a knee joint ligament injury occurring within athletes and general populations alike. However, previous research by Amiridis et al (1996) clearly highlighted the difference in eccentric force capabilities between eccentrically trained athletes (high jumpers) and sedentary individuals.

The authors investigated the difference in eccentric quadricep torque performance between both groups at varying angular velocities, and found that the high jump group produced significantly higher eccentric torque values when compared to the sedentary group at all recorded angular velocities. The plyometric nature of high jump-based training doesn’t make such results so surprising. However, the results also demonstrate the dangers in novice athletes and sedentary individuals attempting highly demanding plyometrics (such as box jumps from a considerable height) without adequate eccentric strength capabilities.

Interestingly, the authors also demonstrated that the difference seen in eccentric force capabilities between both groups is trainable. The authors had the same participants perform the same eccentric maximal contractions at the same angular velocities, but with an added superimposed contraction. This involved providing an additional electrical stimulus via electrodes placed on the participants quadriceps at the point at which each participant had reached via maximal eccentric contraction torque value. The theory being that if no more motor units were available, then no change in eccentric torque would occur as the additional electrical stimulus was applied, as all the available motor units were already being recruited. The difference between the voluntary and superimposed eccentric contractions in the high jump group showed no significant difference in torque values, meaning the voluntary maximal contractions being performed were of a true ‘maximal nature’. However, a significant increase in eccentric torque performance was evident between the voluntary and superimposed contractions within the sedentary group.

The increase in eccentric torque capabilities seen during the superimposed contractions (within the untrained group) demonstrate that the ability to generate high eccentric torques is a trainable adaptation. The research findings also highlight the inherent dangers of individuals attempting accentuated eccentric landings from a considerable height without any form of plyometric or strength training background, and how a progressive plyometric training program should be considered not only as a form of power-based training, but also as a means of injury prevention.