The answer

Now that the shot put techniques have been broken down into different phases, they can be analysed, applying technical aspects to create the optimal technique and achieve greater throw distances.

Long sweeping free leg (rotational technique)

During the initial phase, otherwise referred to as the pre-flight phase, a long sweeping free leg has been identified as optimal for the rotational technique (Young, 2013; Young, 2007).  The reason why this technical aspect is optimal is because it increases the distance of the axis of rotation, which then makes the leg a greater lever for increasing angular velocity and angular momentum. In order for athletes to correctly execute this optimal technique change, they should achieve a greater toe-to-toe distance during the pre-flight phase (figure 1). Athletes should also maintain a free leg sweep that is low enough to the throwing surface to maintain balance, but high enough to allow for an effective toe-to-toe distance (Young, 2013).

Figure 1: The optimal long sweeping free leg, characterised by the toe-to-toe distance (Young, 2007).

Higher centre of mass (glide technique)
During the second phase, otherwise refereed to as the flight phase, a higher centre of mass (COM) has been identified as optimal for the glide technique (Young, 2013; Young, 2007). Previously, the optimal path of a shot putter’s COM was believed to follow a linear path, with the COM starting low and finishing highest at the moment of release (Young, 2013). Through biomechanical analysis however, it is now believed that the S-shaped path of the athlete's COM is optimal, increasing the vertical height of their COM during the flight phase (figure 2). The S-shaped path of the athlete’s COM provides an opportunity for athletes to involuntarily enhance their throws, via the stretch reflex. The stretch reflex is an involuntary reflex contraction of a muscle in response to excessive stretching of an attached tendon or a muscle (Young, 2013). During the shot put glide technique, this involuntary reflex occurs when the rear leg (leg opposite to the side of the shot put) quadriceps tendon is put under excessive load (stretched) created by the increased vertical height of the athlete’s COM during the flight phase. Once the stretch reflex kicks in, it causes the quadriceps to contract, straitening the leg with a much more forceful and powerful muscular contraction than would’ve been achieved under voluntary control only, thus creating a sling shot like action (Young, 2013). It is extremely important to note that this optimal technique requires athletes to have exceptional leg strength. Those athletes who do not have the strength should avoid this technical aspect or complete it to a lesser degree (Young, 2007). 

Figure 2: The optimal COM path (Young, 2007).

Increased rear-knee flexion

During the third phase, otherwise referred to as the rear-foot touchdown (RFTD), a greater rear-knee flexion of approximately 100^o is optimal (figure 3) (Young, 2013; Young, 2007). An optimal rear-knee flexion has been identified by researches as one of the best indicators for success (Young, 2013). It is however important to note that this optimum angle of the rear-knee is highly dependent on the strength of the athlete (Young, 2007). It is also important that slightly greater knee flexion could potentially have greater negative effects on the athletes throw, compared to decreasing the angle of flexion (Young, 2013). The reason why 100^o is optimal for the rear-knee during this phase, is because it provides the greatest opportunity for the muscle groups of the rear leg to function with the most favourable leverage and at full force potential, thus increasing the acceleration of the shot put (Young, 2013). 

 

Figure 3:The optimal 100^o rear-knee flexion during RFTD (Young, 2007).

Increased shoulder-hip separation

During the fourth phase, otherwise referred to as the transition phase, a greater positive shoulder-hip separation is optimal (figure 4) (Young, 2013; Young, 2007). It is optimal for shoulder-hip separation angles to be between 50^o and 70^o, which is achieved through having a long sweeping leg for rotational shot putters in the first phase (Young, 2007). The reason why these ranges of shoulder-hip separation are optimal, is because the increased separation creates greater pretension in the trunk muscle groups. Greater shoulder-hip separation is also optimal because it increases the range of motion of the shot, which increases the amount of time that force can be applied to the shot, thus increasing acceleration (Young, 2007).

Figure 4: shoulder-hip separation (Young, 2007).

Short transition time (glide technique)
Also during the fourth phase, it is optimal for glide technique users to have a shorter transition time or near simultaneous rear-foot and front-foot touchdown (Young, 2013). This is optimal for different glide techniques, as it allows for the force application of both of the athlete’s legs sooner in the throw, and/or it permits athletes to sling themselves over an extended front leg, which both positively affect shot put acceleration (Young, 2013).

Maximised kinetic linking

During the sixth phase, otherwise referred to as the completion phase, it is optimal for athletes to maximise kinetic linking using a throw-like pattern (Young, 2009).  It is optimal for athletes to complete a throw-like movement pattern, because it allows greater momentum to be built and transferred into the shot, through the sequential kinetic chain (figure 5). It is however important to note that only athletes who are strong strong enough (elite), will benefit from using a throw-like movement pattern. Weaker athletes (novice) should apply a push like movement pattern in order to maximise their kinetic chain.

Figure 5: The throw-like movement pattern is performed with a sequential movement pattern where the proximal joints increase their velocity initially, and then the more distal segments increase their velocity later in the movement (Blazevich, 2012).

Front leg extension
During the seventh phase, also referred to as the release phase, it is optimal to extend the front leg fully, just prior to the moment of release (figure 6). The reason why this is optimal is because it will lead to the effective summation of forces. The timing of this is crucial, as early or late front leg extension will create an inefficient summation of forces (figure 7), thus disallowing the transition of momentum from the start to the shot (Young, 2007). 

Figure 6: Front leg extension just prior to the moment of release (Young, 2007).

Figure 7: Summation of forces diagram (Young, 2007).

Greater horizontal release point

Also during the seventh phase, it is optimal for shot putters to increase their horizontal release point, releasing the shot 0.2m to 0.5m in front of the toe board. Although this distance alone only makes up approximately 2-3% of the total measured distance, it does not reflect the benefits of creating a longer acceleration path for the shot put (Young, 2013).  

Neutral shoulder-hip orientation 

Also during the seventh phase, it is optimal for the athlete to have a neutral shoulder-hip orientation (figure 4). The reason why this movement is optimal, is because it allows for a greater transfer of momentum from the non-throwing side to the throwing side, thus increasing the acceleration of the shot (Young, 2013).

Increased height of release

Also during the seventh phase, it is optimal to have a height of release between 2m and 2.2m (Young, 2009). The reason why this height of release range is optimal, is because it creates a longer acceleration path for the shot, thus increasing the acceleration of the shot. It is however important to note that the height of release depends on the height of the thrower. For example, tall throwers will achieve greater heights of release, much like the 2011 Serbian Cup medalists where the tallest thrower was measured at 209cm and had a release height of 2.22m, which was the highest release point (table 1).

Lower release angle
Also during the seventh phase, it is optimal to reduce the release angle, as long as the angle still allows the athlete to achieve elite-level throws. The reason why reducing the angle of release is optimal, is because of the relationship between release angle and release velocity; as the release angle is increased, the release velocity decreases (Young, 2007).  The reason why an increased angle of release decreases the release velocity is due to the inertia of the shot. Athletes are required to apply a greater amount of muscular force, when the release angle is increased, in order to overcome the inertia of the shot (Linthorne, 2001). Due to release velocity being the greatest influencing factor on the horizontal displacement of throws, some athletes will trade off having a decreased angle of release, in order to increase release velocity. A considerable amount of research has analysed the optimal range of the angle of release, but consistency is not evident because the optimal angle of  release is largely determined by the individual athlete. This is because the height of release and the strength of the athlete can greatly influence the optimal release angle (Figure 8) (Lenz & Rappl, 2010). Therefore, an exact value for the optimal release angle that is appropriate for all elite athletes can not be obtained. However, an optimal release angle range from 31^o to 40^o may be considered as a good starting point (Young, 2013;Young, 2009; Young, 2007; Blazevich, 2012; Lenz & Rappl, 2010; Linthorne, 2001).

Figure 8: Mathematical calculation of optimal projection angles, relative to the height of release (Blzevich, 2012)

The question what are the optimal biomechanical principles for shot putters to increase the horizontal displacement of the shot? can be answered by addressing one overarching principle - release velocity. All of the previous optimal technical aspects need to be able to increase the release velocity of the shot, in order to achieve greater horizontal displacement. If the above optimal technical aspects do not quite translate to greater velocities of the shot, they are not worth applying. This is especially relevant to the angle of release, where athletes may not have release angles that fall in the optimal range, but still achieve greater release velocities and greater distances. Therefore, release velocities are a greater determining factor for increased shot put distances. This indicator has been discovered by a variety of researches, who use quantitative results to highlight that release velocity is the key indicator for throw distance (table 1) (Aleksić-Veljković et al., 2011; Čoh, Štuhec, & Supej, 2008; Lenz & Rappl, 2010). Overall, the optimal release velocity that needs to be obtained in order to achieve elite level distances is greater than 13m/s¯¹  (Table 1) (Young, 2013; Young, 2009; Aleksić-Veljković et al., 2011). It is however important to note that increasing the velocity of the athlete or the shot put during one of the earlier phases may have negative effects on the release velocity, thus all of the movements conducted prior to the moment of release need to be conducted in a sequential manner, achieving a successful summation of forces (Young, 2013). 

Table 1: Results from the 2011 Serbian Cup - showing that release velocity is the key indicator for success (Aleksić-Veljković et al., 2011).

References:

Aleksić-Veljković, A., Puletić, M., Raković, A., Stanković, R., Bubanj, S., & Stanković, D. (2011). Comparative kinematic analysis of release of the best Serbian shot putters. Physical Education and Sport, 9(4), pp. 359-364. 

Blazevich, A. J. (2012). Sports biomechanics: the basics: optimising human performance. 2nd Ed. London, A&C Black.

Čoh, M., Štuhec, S., & Supej, M. (2008). Comparative biomechanical analysis of the rotational shot put technique. Collegium antropologicum, 32(1), 249-256.

Lenz, A., & Rappl, F. (2010). The optimal angle of Release in Shot Put. arXiv preprint arXiv:1007.3689.

Linthorne, N. P. (2001). Optimum release angle in the shot put. Journal of Sports Sciences, 19(5), 359-372.

Young, M. (2013). Critical factors in the shot put. Retrieved from http://www.coachr.org/critical_factors_in_the_shot_put.htm

Young, M. (2009). Development and application of an optimization model for elite level shot putting (Doctoral dissertation, Ohio University).

Young, M. (2007). 2007 critical factors research update for the rotational shot put. Retrieved from https://www.usatf.org/groups/Coaches/library/2007/Throws%20Training/2007_Critical_Factors_for_SP.pdf