Rate of force development and the judo athlete – practical considerations for the S&C Coach.
The sport of judo is popularized by competitors unbalancing their opponents in order to drop under their centre of mass (COM) before throwing them. In order to achieve this, a judo player must apply sufficient force to quickly overcome an opponent’s inertia as well as their opponent’s muscular resistance in order to successfully throw them. Judo throws are performed typically either to the front where the thrower rotates 180˚ in to their opponent and throws them forward, examples of this are: Seoi-nage, Tai O Toshi and Harai Goshi.
Image 1: Example of the throw Seoi-nage (Judo-Tao, 2015).
The other attack would be to the back where the competitor will look to attack forwards using their legs and sweep or trip their opponent on to their back.
Image 2: Example of backwards throw Osoto Gari (Moreno, 2011):
This essay aims to evaluate the role of rate of force development (RFD) within the sport of judo and how this impacts the practice of the strength and conditioning coach in order to enhance performance.
Force and rate of force development
Absolute or maximal strength is the amount of muscular force that can be generated during one single all-out effort. However, force is an expression of strength that is time-dependent, and can be defined by the rate in which the mass of a body or object is moved. Therefore, rate of force development (RFD) or explosive muscular strength as it is occasionally referred to, is simply how quickly muscular contractile force can be produced (Aagaard et al, 2002) and is measured as the slope of the torque–time curve which is seen during isometric muscular actions (Andersen & Aagaard, 2006). However, in most sports high outputs of force must occur in a short period of time in order for the athlete to be successful. As can be seen in image 3, comparison of the force capabilities of two athletes show that athlete A would be more successful than athlete B if the activity was time dependant like judo, whereas athlete B would be more successful if there was time limit on force production. Most sporting movements are performed between 0.2 and 0.3 seconds (Zatsiorsky, 1974) but the judo throws Harai Goshi and Osoto Gari, have been found to take between 0.5 and 0.7 seconds respectively (Harter & Bates, 1985; Imamura, Hreljac, Escamilla, Edwards, 2006). Although they appear longer than the 0.2-0.3 seconds for most other sporting movements they are actually broken down in to three components to successfully complete the throw. These are unbalancing the opponent (kuzushi), moving in to their opponents body (tsukuri) before the point of maximum power (kake). Work by Imamura et al (2006) shows the initial unbalancing of the opponent takes place around 0.2 seconds with the other two elements taking roughly the same amount of time. The literature typically divides rate of force development in to three separate elements, starting strength, explosive strength and acceleration strength (Schmidtbleicher, 1992; Miller, 2000; Verkhoshansky, 2006). Starting strength is defined as the force that is generated in the first 30ms and is required to overcome inertia from a dead stop (Bondarchuk, 2014) while the time taken to achieve peak RFD is termed explosive strength. Acceleration strength is defined as the time it takes to reach maximum force once half of the maximum force value has been achieved. Both starting strength and explosive strength have been shown to be important when trying to accelerate lighter loads, with explosive strength increasing in significance as the load increases (Turner, 2009). Both components are relevant to judo performance with starting strength playing a key role in the initial attack and explosive strength being required to complete the throw. However, as highlighted by Turner (2009) the increase in significance of explosive strength as load increases may impact judo players of different weight categories. This could possibly suggest that although S&C coaches would need to develop both qualities they may possibly need to spend more time developing explosive strength with heavyweight athletes in comparison to those in lighter weight classes.
Image 3: Athlete force/time comparison (Turner, 2009)
Work by Cormie, McGuigan and Newton (2011) propose that maximal force production is influenced by both muscle morphology as well as how neural factors of the central nervous system activate the muscle fibres involved. Muscle morphology is comprised of muscle fibre type, cross sectional area, fascicle length and angle of penation. While, neural factors consist of number and type of motor units recruited, motor unit firing frequency, motor unit synchronisation and intermuscular co-ordination. Motor units are typically recruited in a systematic order for graded voluntary contractions, low force contractions will utilise type I fibres which are innervated by small α-motoneurons while type IIa and IIx are innervated by larger α-motoneurons as higher thresholds of force are produced (Burke, 1981). The motor unit firing frequency represents the rate in which neural impulses are transmitted from the α-motoneurons to the muscle fibres. Firing frequency enhances the magnitude of force generated during a contraction along with the high frequency result in a high RFD produced. In addition, the synchronization of motor units has been proposed as a key strategy for inter-muscular co-ordination (Cormie et al., 2011). The final neural factor is inter-muscular co-ordination. This process describes the timing and magnitude of activation for the agonist, antagonist and synergist muscles for a movement. In order for this to be effective, the activation of the agonist muscle group needs to be supported by decreased contraction of the antagonists and an increase in the activity of the synergist (Sale, 2003). A large majority of the throws within judo are initiated with the lower body with judo practitioner’s dropping under their opponents centre of mass (COM) in a low squat stance before initiating the throw (Imamura et al, 2007). This drop under allows for an efficient rate of force development due to the stretch shortening cycle (SSC) preceding it which will benefit concentric muscular action in addition to RFD (Komi, 1984) as well as provide a reduction within the metabolic cost of the movement (Bobbert, Gerritsen, Litjens, & Van Soest, 1996). In order to accomplish this process, a judo player must quickly overcome their opponent’s inertia as well as muscular resistance to block the throw. By producing large amounts of force the attacker will aim to change their opponent’s momentum, which is defined as: p = mass x velocity, and overcome their inertia. In addition by using greater velocity the less time is provided to an opponent to resist the throw and ultimately ensure the attacker greater chances of success (Sacripanti, 2010).
In order to develop all RFD qualities the strength and conditioning coach’s plan must address the individual components of RFD depending on the athletes own specific needs. In order to achieve this, the strength and conditioning should provide a variety of stimuli to address both mass and acceleration components of newton’s 2nd law (Adams et al, 1992).
The use of clustering repetitions within a set allows an athlete to perform more repetitions at a given load than they would be able to achieve in a traditional repetition scheme. By introducing a short rest period of 15-30 seconds between clusters of repetitions athletes are able to partially restore phosphocreatine (PCr) stores and minimise lactate concentration, as it is this resultant increase in lactate production that results in the significant reductions of force generating capacity due to an impairment of ATP production that results in partial changes to contractile characteristics (Haff, Burgess & Stone, 2008). Therefore, the use of repetition clusters allows the athlete to perform repetitions at near maximal output whilst minimising decreases within force production. Work by Haff et al (2003) and Haff et al (2008) found that the average velocity of the barbell in the clean pull was significantly higher in the sets pefromed in a clustered format which utilized 30 seconds of recovery between each repetition with loads of 90-120% 1RM than a traditional set scheme which witnessed a drop in velocity as repetitions increased. The ability to use maximum or near-maximum loads provides increased recruitment of high end motor units along with increased rate coding and synchronization of motor units which will help to provide maximal force output (Dietz & Peterson, 2012). The use of cluster sets provide a large number of potential sequences which allows the strength and conditioning coach to match the set configuration to match the overall goals of that specific phase of training. Typically repetitions are clustered as singles, doubles or triples with roughly 30 seconds of rest between cluster depending on the phase and goals of training (Haff, Burgess, & Stone, 2008).
Maximal strength training:
The role of maximum strength has been shown to play a key role within power output, as power output depends on the ability to exert as high a force as possible (Turner, 2009; Schmidtbleicher, 1992; Stone et al, 2003). Several studies have shown a significant positive correlation between maximal strength and athlete’s peak power output in both lower and upper body performance measures (Baker et al, 2001a; Baker et al, 2001b; Baker & Newton, 2008; Peterson, Alvar & Rhea, 2006). The use of heavy resistance training allows for a more consistent recruitment of the highest threshold motor units along with an increased peak firing rates of motor units allows for a greater force output during muscular contraction which will have a positive impact upon judo throwing performance through heightened neural drive (Sale, 2003; Aagaard et al., 2002). A meta-analysis of the dose response required to develop maximal strength by Peterson, Rhea and Alvar (2005) found that average training intensity to develop maximal strength was 85% 1RM for 6 repetitions or less, twice per week. This work concurs with work by Zatsiorsky and Kraemer (2006) which advocate the use of loads up to 90% 1RM provide sufficient stimuli’s to develop maximal strength development without risking neuromuscular fatigue.
Acceleration and velocity based training:
Considering that judo is a weight category sport where athletes of similar body mass are matched together to provide fairness in competition (Langan-Evans, Close, & Morton, 2011) it can be assumed when considering Newton’s 2nd law of motion that mass is constant. Therefore, when force is equal to mass x acceleration, in order to generate higher levels of force the speed of movement must be emphasised. Originally popularised by powerlifter Dr Fred Hatfield (1989), compensatory acceleration training (CAT) focused on aiming to accelerate the resistance through the full range of motion during concentric contraction in order to enhance force production. Several studies have shown evidence that supports this theory that intent to move a given resistance have a positive effect upon neural drive and force production (Jones et al., 1999; Young & Bilby, 1993; Cormie, McGuigan & Newton, 2011; Behm & Sale, 1993). Typical recommendations are to utilise loads of 50-60% of one repetition maximum (1RM) load for lower body exercises and 50 -70 % 1RM for upper body exercises in a 3 week pendulum wave for 10 sets of 3 repetitions (Simmons, 2007; Baker et al, 2001a; Baker et al, 2001b; Cronin et al., 2001). This percentage loading range concurs with research by Mann (2013) and Gonzalez-Badillo (2010) who found that there is a near perfect relationship between 1RM and the corresponding velocity. Strength and conditioning coaches wishing to use velocity based training should aim to keep mean velocity between 0.75 and 1.0 m/s for the prescribed percentage load as can be seen in table 1.
Table 1: Velocity based training (Mann, 2015).
The use of Olympic weightlifting and its derivatives have been heavily researched for their use in aiding improvements in sports performance due to their high force and high velocity output along with biomechanically similar to several sporting movement patterns (Hori, Newton & Nosaka, 2005; Verkhoshansky, 2006). As previously stated, the explosive strength deficit (ESD) is the difference between maximum force produced and maximum force produced under time constraints. Weightlifting has been shown to increase RFD and reduce ESD allowing an athlete to express more of their maximal force (Comfort, Allen, & Graham-Smith, 2011). Work by Garhammer (1993) suggests that loads of 80% 1RM are best for maximal force production when utilizing Olympic weightlifting movements which is in agreement with work by Haff et al (1997). At this percentage it is best to aim for 2- 4 repetitions per set for a total range of 10-20 repetitions with 15 being optimal. Although, the research into optimal loading for Olympic weightlifting suggests 80% 1RM all testing was completed by competitive weightlifters and may vary with other athletic populations. The strength and conditioning coach can use 80% 1RM as a general reference guide to loading but make adaptations based upon their individual athletes force output. If the strength and conditioning wishes to use Olympic lifts as part of velocity based training it is best to aim for 1.2-1.32 m/s for the clean and 1.52 – 1.67m/s for the snatch (Mann, 2013; Roman, 1986). Generally these lifts will have a higher velocity in comparison to traditional exercises such as squats etc. due to having greater amplitude of motion.
In conclusion, an athlete’s ability to generate a large RFD will provide a positive benefit to judo performance by enabling them quickly overcome their opponent’s inertia and have a greater chance of successfully throwing them. It is up to the strength and conditioning coach to provide an array of training stimuli’s in order to develop both the mass and acceleration components of Newton’s 2nd law and address the individual components of rate of force development. This is in agreement with several studies that have shown that the best result for increased force production are achieved from using a combination of high load and explosive/plyometric type work (Adams et al., 1992; Hakkinen & Hakkinen, 1995; Lyttle, Wilson & Ostrowski, 1996; Harris et al., 2000; Kawamori & Haff, 2004; McBride et al., 1999). There are several training modalities available to the strength and conditioning coach in order to achieve this such as cluster training and maximal intensity strength training which allow for greater loads to be used to address higher threshold motor units as well as acceleration and velocity based training which enhance neural drive and address force output to the time demands of the sport. Through this knowledge the strength and conditioning coach should be able to effectively program modalities to enhance RFD that should aid judo performance.
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