This piece first appeared in Athletics Weekly, Sept. 22, 2016.
By John Shepherd
In the sprints, hurdles, throws and jumps events, power and speed are obviously paramount. An athlete in these events may have the best technique, but unless he/she can generate and apply force rapidly then the athlete is not going to maximize potential.
“Athletic horsepower” is all about overcoming resistance. An obvious example is the high jumper who needs to overcome gravity to soar skyward. It appears that athletes who are able to generate the most power have the most effective human springs—an ability which the sports scientists say reflects lower limb stiffness.
Simply put, an athlete wants to have limbs—or more specifically muscles, ligaments and tendons around the relevant joints and even cartilage and bone—that are able to produce maximum power usually in the shortest amount of time. This is certainly the case for sprinters, sprint hurdlers and horizontal jumpers.
High jumpers and throwers, for example, may need to moderate their speed to an optimum performance- producing level. Sprinting flat-out to throw the javelin, or perform a high jump takeoff, for example, is unlikely to result in the most effective throw or jump. Nevertheless, their ability to “fire” their leg muscles quickly is very important. This relies very much on lower limb stiffness—from now on in this article referred to as stiffness.
Essentially, our legs are like springs. If a spring compresses under a force it will store energy that’s rapidly released when it uncoils and, the stiffer the spring, the more powerful the return of energy. Well, the legs work in a similar way around their joints. The coiling equates to an eccentric muscular action whereby the relevant muscles lengthen under load. Meanwhile, the uncoiling is a concentric action, whereby the relevant muscles shorten under load. The quicker the transition from lengthening to shortening, the more power generated. This reaction is also known as the “stretch reflex” or “stretch-shortening cycle”.
Measuring Leg Stiffness
Stiffness can be measured in several ways depending on the type being measured and the resources available.
It is beyond the scope of this article to go into full detail but in relevant overview. Vertical stiffness refers to the movements involved in hops and jumps that occur in a vertical direction and can be determined by calculating the force used to jump and the height gained. Leg stiffness uses a much more complex evaluative methodology, for example, accounting for horizontal velocity, contact time and changes in vertical leg length in regard to maximum vertical force. Leg stiffness is used most relevantly to assess running. But it’s a little complicated as vertical stiffness can also be an influencer on horizontal speed, as will be identified.
Research shows that each joint in the legs can have a different degree of natural and conditioned stiffness. As we are dealing with human beings and not an inanimate object such as a mechanical spring, there’s a neurological dimension that also
needs to be factored in—the ability of the central nervous system to react to the forces involved.
Researchers have looked at stiffness across different leg joints—the ankle and knee in sprinters. In one study, 10 male sprinters had their ground-reaction forces, kinematic, and EMG parameters (electrical activity in muscles) measured when sprinting over a force plate at speeds of 70%, 80%, 90% and 100%.
Results showed that with increased running speed average ankle-joint stiffness was constant but increased in the knee joint. This, the researchers argued, could reflect the role of the Achilles tendon in firing the calf muscles, and that the equivalent knee tendons were less activated by slower speeds. This led them to identify the need to develop greater knee-joint stiffness and vertical stiffness as a means to improve speed (vertical jumps and drop jumps are useful training tools in this respect).
Another trial considered the development of leg stiffness over six months of specific training. Nine male athletes performed maximal-effort 60m sprints before and after the completion of six months of winter training. The researchers looked at the sprinters’ techniques at peak velocity and their ground-reaction force (GRF). Sprinting speed was significantly developed through longer step length. In terms of stiffness it was determined that vertical stiffness significantly increased. However, to add some slight confusion in terms of the previous research it was noted that, although knee joint stiffness remained constant, ankle-joint stiffness was significantly developed but leg stiffness was not. They put this down to a respective increase and decrease in ankle plantar flexion (toes down) movement and ankle dorsiflexion (toes up) angle respectively. Interestingly, it’s been identified that increased stride length—though potentially increasing sprint speed—does not necessarily reflect a direct increase in leg stiffness or vertical stiffness (see Analysis of Bolt panel above).
How To Improve Stiffness
This is the multi-million dollar question. One more or less constant truth is that the faster you run, the greater the amount of stiffness across all joints. This is also the case when it comes to hops or drop jumps—i.e., the quickest ground reactions generate superior stiffness. Therefore, sprinting will promote enhanced stiffness in itself. This is a useful consideration for an endurance athlete (see Leg Stiffness and Endurance Performance panel above) in regard to developing increased running economy (RE).
When it comes to using plyometrics to enhance stiffness, as well as emphasising the speed of reaction, jumps should be performed for distance as well as height, and single and multiple ground contacts should be used.
ANALYSIS OF BOLT
Researchers reporting in the International Journal of Sports Medicine looked at the 2009 Berlin World Championships performances of Bolt, Tyson Gay and Asafa Powell. Despite Bolt’s win in a world record of 9.58, they calculated that he had the least amount of leg stiffness and vertical stiffness compared to his two rivals, even though his peak speed of 12.3 m/sec was superior. We’ve seen how this can be explained in terms of stride length, a longer (but still very quick) contact time (0.091 sec) and lower step frequency (4.59 per still second).
LEG STIFFNESS AND ENDURANCE PERFORMANCE
Running economy (RE) is seen to be in part reflexive of stiffness. RE relates to the ability of an athlete to move along at a comfortable but fast sustained speed using less energy. If, like Mo Farah, you have a high top speed, then slower paces will be much easier to maintain. Farah is therefore more than likely to have greater stiffness and to have trained specifically to develop this. Forefoot striking may also be relevant here as this leads to greater stiffness, particularly at the knee when compared to heel-striking.
POWER OF ACCELERATION
Researchers looked into how leg strength and leg stiffness affected the acceleration, peak speed and the deceleration phases in the 100m sprint—30m-60m-100m. Regional to national-level sprinters were the subjects (100m PBs ranged from 10.72 to 12.87). The half-squat and counter-movement jump were used to determine leg strength and hopping leg stiffness. It was discovered that the half-squat and counter-movement jump were the best predictors of speed over the 0-30m phase and hopping of the last two phases. When it comes to conditioning, specific methods are needed to improve the overall sprint.
John Shepherd is coach to top UK athletes. See johnshepherdfitness.com