UNDERSTNADING RATE OF FORCE DEVELOPMENT

Today we continue our series on Strength and Conditioning (S&C). This series started out as a standalone piece written by senior tutor Nick Parke, however we quickly realized that the topic has far too many nuances to fit in a single article - and so, the series was born.

Rate of force development (RFD) is a key factor in determining performance. Simply put, it is the ability to produce force in a short period of time. RFD has important consequences as it determines the explosive power of an athlete, and the ability to produce a large amount of force in a short period of time correlates strongly with numerous sport performance measures including sprinting, jumping, and lifting weights.

The stretch shortening cycle (SSC) is the countermovement that is seen in most human movements - such as a jump - and is commonly viewed as the first stage of force development. The pre-movement allows the human body to utilize potential energy stored in the muscles, tendons, and ligaments (2).

A common example of the role of the SSC is a comparison of jump height performance in the counter movement jump (CMJ) and squat jump (SJ). Individuals typically display an average 3.4cm greater height in the CMJ (3), highlighting the contribution of the SSC. The stretch shortening cycle is often compared to the compression of a spring; during the eccentric muscle contraction the spring is placed under pressure, and reduces in size (i.e. bunches up). During the concentric muscle contraction, the pressure in the spring is released. In addition to the force created through the muscle contraction, the potential energy stored in the spring is also released. The amount of potential energy stored in the spring will be increased with the speed and force applied; this theoretical concept can be applied to the jump example by taking a run-up before the jump.

The SSC does not only occur in running/jumping movements; it is utilized in any movement in which a limb/body part changes direction, including abduction of the arm and twisting of the torso. As the SSC is utilized in such a wide variety of movements it is useful to classify them into two categories, the slow SSC and fast SSC. As I am sure you can tell, these types of SSC are described based on the duration of the cycle (2).

The example of the CMJ mentioned earlier would be classified as a slow SSC, however if we incorporated a run into the jumping movement it would be classified as a fast SSC. Other common examples of the slow and fast SSC are walking and sprinting, respectively.

After reading through the classifications of slow and fast stretch shortening cycles, you might be wondering how you can tell if a movement has a SSC greater or less than 250 milliseconds. There is a general set of defined guidelines which look at the level of joint displacement. Larger levels of joint displacement are typically slow SSC, and movements with small amounts of joint displacement are fast SSC.

There are two shifts which can occur to increase RFD, the first being the total amount of force produced, and decreasing the time in which the force is produced. This is where we introduce the force-time curve.

It is commonly thought that for individuals to increase RFD it is essential to train all aspects of the force-time curve. If only a specific part of force development is trained then there will be an increase in that specific capacity, but the overall level of RFD will remain unchanged. The aspects of the force-time curve are:

Maximum strength
Strength-speed
Power
Speed-strength
Speed

This theory is often missed by amateur-level S&C coaches, who see an athlete that wants to improve their jump or their sprint time, and think getting stronger in the squat will do this. As a result a huge emphasis is placed on improving maximum strength in the squatting movement. As expected, the individual will observe an increase in maximum strength - however the ability to jump higher, run faster, etc. remains unchanged. Therefore it is essential for a S&C coach to design training which targets strength and power production, rather than simply focusing on speed or strength training alone.

Written by George Pollitt.