Acceleration Mechanics in Professional Rugby

  1. Rugby, a sport of speed ?

Speed qualities in rugby are important performance factors for competition in any position. Among the many components of “speed”, the ability to accelerate plays a major role in rugby, which is nowadays described more as a sport of acceleration than of absolute maximum speed, because there are few situations where the player can express himself at Vmax (kick and chase, long crossings without a defender…). Although there are other qualities of important speed (alertness), acceleration is found in a majority of actions during a match and can therefore make significant differences on the performance of the player because it allows the player to acquire the highest possible speed in the shortest possible time.

However, in rugby, accelerations are mostly achieved in thrown situations. That is to say that players are subjected to great speed variations from non-zero speeds (between 2 and 5 m.s for the majority). Moreover, these thrown accelerations can occur with a difference in direction between the initial speed and the acceleration to be produced.

For this paper we will focus on rectilinear acceleration, noting that many mechanical principles are transferable to the quality of vivacity, which will be the subject of a future analysis.

  • What is acceleration ?

First of all, as quickly described above, acceleration is one quality of speed among many others that make up “speeds”. Because there are several types of speeds (cyclic-acyclic) which are expressed through different qualities (starting speed, gesture frequency, velocity, liveliness ….).

From a scientific point of view, acceleration describes changes in speed over time. If an object undergoes an increase in speed as a function of time then we speak of acceleration and vice versa for deceleration. Finally, even if the object is moving at constant speed and undergoes a change of direction, then this will imply an acceleration due to the change of direction of the speed vector (not its norm). Acceleration is expressed in m.s<sup>2</sup> which represents meters second by second (we find our speed expressed as a function of time).

Example : if a player passes from 5 to 15 km.h in 2 seconds then he has made an acceleration of (10 km.h/2s) = 5 km.h.s= 1,39 m.s2 (because 1m.s2= 3,6 km.h.s).

All these data are fundamental to understand that there are a large number of accelerations for the same event. In the case of our example, we have calculated an average acceleration between two distinct times, but what really happened during these 2 seconds ? Did my player consistently accelerate by 1.39 m.s2 ?

Today there are very reliable tools to measure with greater accuracy the speed variations “at any time” like the stalker radars for example. Thus, we obtain the complete spectrum of speeds of our athletes, which allows us to identify precisely the points of improvement.

The purpose of all this demonstration is to understand that each athlete has individual acceleration abilities that are important to consider in acceleration development work.

2.2 How to achieve effective acceleration ?

To give meaning to this part it is relevant to start from a very simple postulate.

What do we want ? That our player reaches his maximum speed in a minimum of time. For this, a certain number of factors are to be observed in our players. Note that each parameter cannot be considered in isolation because they are, for the most part, interconnected. Also, it is important to remember that each athlete has a specific way of moving in space with particular intermuscular and intra-muscular coordinations, so it is not a question of establishing the “perfect technique” but rather of understanding the fundamental principles in order to have all the necessary tools to help our athletes progress individually.

  • Fundamental mechanical principle

This is probably one of the most important principles from which many other principles are derived. To accelerate strongly, it is fundamental to project its center of gravity in the desired direction. In a sprint, the orientation of the forces on the horizontal modality must be high to allow a forward projection of our mass. You can find data that provide information on this capacity when measuring F-V profiles with the Morin and Samozino method under the name RFmax (which represents the maximum capacity of orientation of the developed forces used in a horizontal direction) and DRF (which represents the loss of mechanical efficiency with the increase in speed). It should be noted that the latter is unavoidable, however it can be optimized in some subjects.

  • Secondary mechanical principles

During a sprint, the angle between the trunk and the lower limbs must be reduced, to adopt a posture “inclined” which will allow a better orientation of the forces towards the front and thus a more effective thrust.

This angle will decrease as the speed increases.

Evolution of trunk angulation by the ground. J.Dodoo

Swing Leg : Another factor that influences the quality of acceleration is the return of the swing leg. Directly after the push, the athlete must produce a high retraction speed. An interesting benchmark to see if this leg swing is fast enough is to observe the gap between the two knees at the time of the opposite support (if this gap is small then the leg swing is fast enough). This effort has the effect of limiting the travel time of the back foot and will improve the pushing speed of the supporting foot. Finally, the athlete will have to engage his knee intensely forward and upward (“punch” of the knee), which will improve, (1) the angle of attack of the push, (2) a better horizontal translation of the CG.

Leg thrust : The thrust of the supporting leg must be engaged in a vertical and forward direction. Prior to contact, there is a slight self-organized hip “pre-extension” that allows the body to improve the horizontal transmission of forces. During the contact time, a hip extension (accompanied by a knee extension) will mainly occur, the athlete must have the will to push the leg far back, which is synonymous with a full push. In order to observe this phenomenon one only has to look at the distance between the two hips at the last moment of the push (toe-off), if this distance is large then one has on one side a full push and on the other side a sustained forward engagement with the free leg. Note that in this acceleration phase, the flight time must be less than the thrust time because our body is looking for adherence and reaction from the ground to exert maximum force.

A criterion that can be observed with the naked eye and that results from the majority of these technical principles and to observe the foal of the foot during the race.

It is noted that on the least efficient trajectory, the incomplete thrust added to the low retraction speed does not allow the subject to effectively apply a high percentage of horizontal force. Generally, if your athlete has a technique that is very far from these key points (without trying to reproduce “the ideal technique” because each one has its own motor skills), there will be “cascade” degradation (too high flight time removing grip, incomplete and misdirected thrust…) because they are all intrinsically connected.

Comparison of two foot trajectories (foal) during a sprint. F.Bosch.

Principal physical principles

It is the central physical quality during acceleration, which extension force is able to produce? And at what speed? A weak athlete in the back chain, even optimized, will not be able to reach high acceleration performances (like a car with a low engine capacity). This phenomenon is once again linked to the fundamental mechanical principle of directing a maximum amount of force forward. To ensure this, the gluteal muscles (gluteus maximus) and hamstrings (biceps femoris) are the main powerful hip extenders during the race. As for the quadriceps, they will accompany this movement with a knee extension that remains limited compared to the previous muscles.

Athletes with higher strength ratings on the back chain will potentially be more capable of producing this horizontal force. Obviously it will be necessary to optimize this production of force by an adequate technique and to know what level of force is applied for each speed in this athlete. In general, subjects with powerful hip extensors will have higher F0 and Pmax values. It will also be important to observe for what speed my athlete’s Pmax is expressed and to analyze whether this F-V profile is relevant to the task he is performing.

Often neglected in our collective sports practices, foot work is a major factor in running. It is the foot that will be in contact with the ground, undergoing on one side the forces of a moving body concentrated on a single foot, and on the other side the reaction of the opposite ground. This requires both a great stability of the ankle joint but also a great firmness (called stiffness) of the muscular and tendinous structures in order to be able to receive the great elastic energy and above all to transmit it rapidly to the periphery of the joint, i.e. the forefoot in plantar flexion which will be the last extension of the body before the flight phase.

Note that, as seen above, the axis of attack of the supporting leg is important because it will allow this reaction of the ground to be directed forward.</p> Note that, as seen above, the axis of attack of the supporting leg is important because it will allow this reaction of the ground to be directed forward.

Control of the trunk and pelvis through engagement of the abdominal and pelvic structures is essential during acceleration. Indeed, the athlete capable of creating muscular tension in this area (pelvic crossroads) will have a trunk and pelvis much more stable in space, which has the double advantage of (1) optimizing the transfer of force produced by the muscles from the ground to the center of gravity, (2) improving the horizontal translation of the latter because it erases parasitic movements (trunk flexion-extension for example, which can often be observed in subjects with deficiency in this area). It should be noted that the psoas and the anterior femoral right also play a role in stability, especially during the swing of the free leg.

2.3 How to improve the acceleration capacity of the athletes ?

This game is very open because it is very dependent on the qualities of the players, their tasks in the match and the analysis of the physical trainer.

Let’s take concrete examples for two subjects occupying the same playing position (winger) :

 Subject ASubject B
F0 (N.kg-1)6,829,70
V0 (m.s-1)9,439,21
Pmax (W.kg-1)16,3922,33
FV slope-0,71-1,07
RFmax48,06%58,47%
DRF-6,46%-9,43%
30m Time (s)4,184,17
Comparative table of F-V profile of 2 athletes who have the same 30m sprint time.

For the subject A :

  • We notice a low value of F0 which indicates a lack of starting force
  • A rather low value of Pmax
  • A speed force profile rather in favor of speed
  • A  very low RF max

Subject that lacks extension power as well as low speed thrust technique. The objective will be to increase its extension power and improve its force transfer horizontally. Profil more « velocity ».

Work themes :

  • Power work on hip extensors
  • Moderate speed work which has two objectives, (1) to improve power during sprinting by slightly reducing the power peak to lower speeds, (2) to improve horizontal force transmission.
  • Acceleration technical work by taking up the key points described above

Note that if this topic had been a second line, very heavy under speed work would have been a perfect theme to bring the athlete to express the highest power peak in a station specific speed zone.

For the subject B :

  • Good F0 value
  • Good Pmax value
  • Power peak close to low speeds (F-V slope of -1.07 due to this high F0 value)
  • Good RFmax
  • Drf value too high

Subject that has a great force of extension, which it manages to orient in a horizontal modality. However, this mechanical efficiency degrades strongly as the speed increases. At the same time, this athlete’s peak power is expressed in lower speeds than what he will encounter on the field. Rather “explosive” profile.

Work themes :

  • Maintaining physical qualities of extension
  • Acceleration work launched from specific speeds. Here you can use acceleration with or without resistance with a thrown start, the difficulty of working with resistance will be not to “brake” the athlete so that he remains in the right speed zone defined (if I accelerate my athlete from 4m.s but that by applying the resistance he falls below this speed then my work will be counterproductive in relation to the objectives.
  • Acceleration work from specific speeds.