The rate of force development

The relevance and relative importance of strength and power in sport performance varies considerably between sports. In activities such as weight lifting and the throwing, jumping and sprinting events in track and field strength and power are dominant factors - assuming technique is accounted for. Strength and power also have a major role to play in contact sports or sports that require change of direction – the so called repeated sprint activities (RSA). Today’s seminar contrasts different ways in which strength and power are trained. The aim behind the seminar is to i) look for specificity of adaptation  ii) consider which method is best suited to different sports.  Before reading the papers set there is some background (hopefully) revision and a framework (from Schmidtbleicher) that might help you think about classifying different sports from a strength and power perspective.

Background

Some of this section will be revision, other parts are to ensure you can understand the key aspects of the Hakkinen et al. papers. Before reading the papers it is important to have a clear understanding of various definitions. This will enable you to interpret the papers more precisely, hopefully this will be revision.

Definitions

Strength is usually defined as the peak force or torque (a force that cause rotary movement about an axis of motion) developed during a maximal voluntary contraction (MVC) under a given set of conditions. Force is measured in Newton's (N) and torque in Newton metres  (N∙m). Force is therefore an isometric action while torque can involve movement. Power by contrast is the rate at which mechanical work is performed; P = W/t  (where W = work done; t = time taken to perform the work done) . Additionally, power can be expressed as the product of force and velocity; P = F x v  (where F = force produced; v = velocity of shortening  (NB – at this point you recall the F-V curve and F-P curve from last year’s work on maximal intensity exercise – revise your notes if you’re not sure).  The unit of measurement for power is watts (W); 1W = 1 joule / second (J∙s-¹), whereby 1J of work is completed when the point of application of a force of one Newton moves a distance of 1m  in the direction of the force (NB – joules are the SI unit for work or energy).

Measurements

During a MVC it can take over 500 milliseconds (ms) to generate maximum force (Fmax). In many sporting activities the time available to generate force is less than this. At top speed elite male sprinters have a ground contact time of as little as 80 and 100 ms. Obviously, in this case there is insufficient time to reach Fmax. and the rate of force development (RFD) is becomes  important.  As a general rule, as the time taken to apply force reduces so the rate of force development becomes more important than Fmax  per se.

In the Hakkinen et al. papers they have measured force during a MVC. This method enables a force-time curve to be plotted. From the force-time curve it is possible to derive Fmax, a percentage of Fmax, any absolute level of force production and the RFD. In many sports the performer is required to overcome a fixed resistance – mass of the ball, body mass, opponent and water are just a few examples. By looking at the time take to achieve an absolute force Hakkinen et al. were able to see if by increasing Fmax it is possible to speed up the time taken to achieve a fixed force i.e. whether the RFD has increased. One of the Hakkinen et al. papers examines the effects of traditional strength training while the other examines plyometric training. The force-time curve enables us to contrast the effect of the different training methods on RFD and Fmax. The limitation of using a force-time curve is that it is an isometric test and therefore does not mimic the muscle action(s) used in sport.

Framework

Schmidtbleicher (1992) defines power as ‘the ability of the neuromuscular system to produce the greatest impulse in a given time period’. You will doubtless recall from last year’s notes (lecture on maximal intensity exercise) that impulse is F x t; where F = force and t = time. Schmidtbleicher suggests that the time available to generate the impulse is dependent upon the resistance or load the performer has to work against or accelerate. He could have included decelerate in regards to RSA, where performers have to slow down and change direction. In his paper Schmidtbleicher sub-divides the notion of RFD into initial rate of force development (IRFD) (sometimes referred to as starting strength) and maximal rate of force production (MRFD) (often referred to as explosive strength). He suggests that in events where time to apply force is less than 250 ms IRFD and MRFD are the predominating factors. Towards the shorter end of this range IRFD is more important and vice versa for MRFD. Where there is more than 250 ms to generate force, then Fmax is the predominant factor. IRFD would include sports such as boxing, martial arts and fencing where the initial speed is important. One point to consider is that there may be multiple requirements within one sport. In the 100m (track) sprint consider the amount of time force is applied to the starting block, the ground contact time as the runner accelerates and then ground contact times at top speed.

When considering the role of RFD the type of muscle action(s) needs to be considered. At this point, it is important to remember that stretch-shortening cycles (SSC) should be regarded in its own right, not just as a shortening followed by lengthening muscle action. Schmidtbleicher identifies two type of SSC –  long and short. Short SSCs are characterized by ground contact times between 100 – 250 ms, typically these actions have small angular displacements at the ankle, knee and hip, for example long and high jump (contact time ~ 120 – 140 ms). The long SSCs have ground contact times in excess of 250 ms with large angular displacement at the hip, knee and  ankle for example a vertical jump to head  (or catch)  a ball, or to block a shot in volleyball. You may recall from last year, that this fits with Young’s work on reactive strength; in fact the two types of SSC are synonymous with Young’s two types of reactive strength measures.

Schmidtbleicher, D (1992). Training for power events. In Strength and Power in Sport (Komi, P.V. – Ed). Blackwell Scientific Publications. Oxford.

PH July 08