(This post has specific strength and conditioning nomenclature which might mean doing some further research of yourself).
What is Leg stiffness?
A large part of strength and conditioning is getting athletes quicker. In getting athletes quick, coaches talk about increasing leg “stiffness”. In the scientific literature I haven't come across a simplistic explination for leg stiffness in layman terms. I’ve spent some time exploring this area and breaking down some of the scientific jargon to get a clearer understanding of leg stiffness.
Research Into Stiffness
Stiffness can refer to the lower and upper body, but here I’ll discuss the lower body as it’s easier to visualise in practical terms.
First, watch this video as it takes Tyreek Hill through some fun exercises. Tyreek would be classed as having good leg stiffness.
Researchers use the word leg “stiffness” in the context of plyometric, speed and agility training. It is also used interchangeably with “ankle stiffness”. Leg stiffness appears to be narrowed down to the ankle joint, but is stiffness in relation to a tight ankle?
Leg “stiffness” is also used in medical literature when talking about “tight” muscles or a lack of flexibility. So in one world, stiffness is indicative of better performance, and in another world, it suggests possibility of injury.
Muscle Stiffness In Strength and Conditioning
Many articles manage to offer a scientific explanation of muscle stiffness but fail to put it into practical terms. For example:
“The mechanics of muscle function have often been described by the property of the system to resist the applied stretch. This so-called stiffness (k) is calculated by dividing the change in force by the change in length (ΔF/Δ1). Different methods have been used to study the stiffness of the lower leg, including different spring-mass models. The stiffness calculations have also been extended to single joints or even to single muscles.”
Unless you understand the nomenclature, this is hard to interpret.
Anthony Turner, who is the Associate Professor in Strength and Conditioning at Middlesex University and Associate Editor at the Strength and Conditioning Journal, has a chapter dedicated to this topic in his Advanced Strength and Conditioning: An Evidence-based Approach book.
Turner suggests that leg stiffness is a broad term used to describe athletes lower body power or speed. Simply, it’s how much bend in ankle, knee and hip is seen during a change of direction/speed of movement. If an athlete has a large bend in the ankle, knee and hip, it means the athlete is spending more time on the ground. Spending more time on the ground results in being slower.
It becomes harder to see greater bends in the lower body when working with elite athletes. To understand this further, hi-tech equipment is needed such as a Fusion Sports jump mats.
Leg Stiffness = muscles resisting a stretch through a isometric contraction/co-contraction and making the tendon stretch and utilise its elastic properties to do the work.
Athletes that are less stiff have what is called a compliant tendon. A compliant tendon is used when more force is needed. For example, when jumping up to a small box the movement can happen quickly (stiff) as you don’t need a deep squat to generate more height. When jumping on a very high box the movement is slower with a deeper squat to generate more force for the height of the box.
Can Muscle Stiffness Be Trained?
Through training, stiffness can be improved and voluntarily utilised depending on the task ahead. Research studies have shown that resistance training (weight of 80%+ of 1RM) and plyometric training together increase the stiffness of athletes more than just resistance training alone. Therefore doing strength training while performing jumps and sprint work will increase the stiffness of athletes.
Muscle Stiffness and Medical World?
The medical world says that you should increase the flexibility of your muscles to absorb more force and therefore be able to reduce the chance of injury. This means they want to see a slower movement with more bend in the ankle, knee and hip.
The argument here is that it will keep you injury free. However, in 2004 there was a great article published in the Journal of Physical Therapy. They discuss this subject and state:
The suggestion here is that a strong muscle with a flexible tendon is ideal. A strong muscle means that more of the stretch goes to the tendon utilising the elastic energy making you more efficient.
If you have a weak muscle and an inflexible tendon, then your muscle will have to do more by stretching. In stretching the muscle and not the tendon, you increase the risk of a muscle strain or tear.
In my practical experience, some coaches believe tight muscles are a good thing because it makes you more stiff, meaning quicker. I have also spoken with physical therapists that say a big bend in the ankle, knee and hip is a good thing for injury prevention.
It appears that the medical and strength and conditioning professions in research are aligned, however, in the day to day world it's slightly different.
To summarise, in some circumstances (changing direction, jumping etc.) we want to be stiff meaning we have a quick movement resulting from an isometric contraction of the muscles and letting the tendons do the work. We also want to have the flexibility of both the tendon and muscle but utilise a strong isometric contraction to ensure the stretch goes to the tendon. When we are inflexible or tight the muscle has to stretch which could result in injury.