Stiff legs make for faster sprinting, as John Shepherd explains

If you look at a top-class sprinter in full-flight in slow-motion you’ll see that any degree of bending at the knee joint after ground contact on foot-strike in terms of an absorbent reaction is virtually non-existent.

An elite male sprinter’s foot will only be in contact with the ground for around 0.08 seconds at maximal velocity, yet in that time they will overcome more than three times bodyweight as they reach near to 30mph. This highlights just how important leg stiffness is as, without it, sprint velocity would be very much dampened.

In a review of research on leg stiffness the following definition and way of assessing stiffness was provided: “Human running can be modelled as either a spring-mass model or multiple springs in series. A force is required to stretch or compress the spring and thus the stiffness can be calculated from the ratio of this force to the change in spring length.”

Leg stiffness involves a very strong eccentric reaction and then a lightening quick concentric action – however, there should be virtually no delay between the “stretch” and “reflex”. These muscular actions constitute the stretch-shortening cycle and are key to plyometrics. Plyometric muscular actions are akin to the firing of a bow or spring. Pull the spring to maximum length (akin to the eccentric muscular action) and, if you let it go, it will compress with great power and energy release (akin to the concentric muscular action). This response in relation to sprinting produces the dynamic extension of the ankle, knee and hip joints (triple extension) needed to propel the sprinter forward at great velocity.

Research: Leg stiffness and training methods

Leg stiffness can be developed via numerous training methods. These include weight training, plyometrics, sprinting and resisted running, such as sled-towing. However, looking at the research, it appears that developing leg stiffness via various training methods is complex and does not occur as one might think. In fact, some of the methods contribute to a different facet of speed development. For example, some seem better suited to developing sprint acceleration rather than top-end speed.

Researchers looked at the relationship between leg power and leg stiffness as they related to sprint performance. Eleven subjects aged 15-17 participated in the study and acceleration and maximum speed were analysed during a 40m sprint using radar measurement. Leg power was analysed using a treadmill equipped with speed and force transducers from which linear power was calculated. A hopping test was also used on a force plate and leg stiffness calculated using flight and contact times.

The researchers discovered that forward leg power was correlated with both initial acceleration and maximum velocity running during track sprinting. However, crucially in the light of the subject matter of this article it was noted that “leg stiffness as calculated from hopping was significantly correlated with maximal velocity but not with acceleration”.

New Zealand researchers studied the ground-reaction forces (GRF) involved when accelerating in a sprint. Thirty-six male athletes performed maximal-effort sprints from which video data and GRF data was collected at the 16m mark. The team discovered that the faster-accelerating athletes displayed less vertical impulse (more force was directed horizontally to push them forwards). The quicker accelerators also had the faster ground-contact times.

Similar findings on acceleration were made by a further highly relevant piece of research that directly considered leg stiffness in relation to sled-towing training. It involved 22 sprint-trained subjects. Two groups were created, one that used a weighted sled (WS) and one that did not (UR). Sled-towing is a commonly used sprint training method and is one that many believe will enhance leg stiffness. The WS group used a resistance that reduced their maximal speed by 7.5%. After a three-week programme of training designed to level the participants’ condition they were randomly split into two groups (WS & UR) and both groups sprint-trained twice a week for four weeks. The only difference between the programmes was that the WS group trained with the weighted sled and the other without.

Before and after the test the participants were tested in regard to sprint technique, muscular strength (including, isokinetic and plyometric) and sprinting leg stiffness. The team discovered that the sled-towing sprinters improved their velocity in the transition phase (this is the phase after block clearance when the runner builds to maximum speed – the phase that the previous researchers had seen not to benefit from hopping) and the non-resisted sprinters improved their maximum speed.

It seems that the additional loading of the sled, rather than producing stronger and stiffer legs that would improve top-end sprint speed, actually hindered the development of this quality, although boosting acceleration. Sled-towing had more of a concentric strength developing role as opposed to an increasing leg-stiffness role as it relates to the stretch-shortening cycle and top-end speed.

These findings were correlated by researchers who specifically analysed the performances of 19 regional to national-level sprinters over 100m. They also wanted to consider the importance of leg strength and stiffness over three phases of a 100m sprint: 0-30m, 30-60m and 60-100m, using video analysis. They matched performances based on leg strength using a concentric half-squat – a counter-movement jump and hopping.

As before, the hops were used as the predictor of leg stiffness. The times achieved by the subject sprinters ranged from 10.72-12.87. Specifically, it was discovered that the concentric half-squat was related most strongly to the mean speed of each phase. The counter-movement jump was related to the first 0-30m, while the hopping test was the best predictor of the two last phases. So again, we see that different means of training have different outcomes and that the plyometric exercise, when used as a means to enhance top-end speed, seems highly correlated to leg stiffness – that’s to say an athlete with greater leg stiffness should have the faster top-end speed (although they might not be the best accelerators).

From the research we can see that varying training methods (weights, sleds, plyometrics) affect the development of speed and leg stiffness differently. Although it would be true to say that all methods can improve sprint speed and leg stiffness, it is crucial to note that some, notably hopping, develop and reflect a heightened level of leg stiffness in terms of activating a superior stretch- shortening cycle, benefitting top-end speed. Simply put, they reduce the “give” in the sprinter’s legs.

Coaches and athletes must therefore be mindful to train in ways that will optimise all phases of the sprint, while enhancing leg stiffness. For example, heavy-load concentric squats could be used to develop the first few steps of acceleration and then sled-towing the remainder of the acceleration (transition) phase and plyometrics to enhance top-end speed.

The role of drop jumps in developing leg stiffness

Drop (or depth) jumps are designed to enhance plyometric ability. A drop jump requires the performer to step, run or jump off a platform at a suitable height and land and perform a jump or series of jumps from one or two legs.

drop jumps

Crucial to the optimum performance of a drop jump is the transition between the eccentric (landing) and concentric (jump) parts. To maximise the power return the speed between these two muscular actions should be minimised and this is where leg stiffness becomes vital.

Researchers looked specifically at this method of training and its effects on 20 professional rugby union players. They were tested for 0-10m acceleration and for top-end speed using a flying 10m-sprint (a run where they had reached top-speed) and strength (using a three repetition maximum squat), reactive strength, counter-movement jump, drop jump and leg spring stiffness.

In keeping with the other findings of the research provided so far it was discovered that leg spring stiffness and drop jump performance was related to the flying 10m time. So again, it’s the plyometric exercise, albeit this time the drop jump, that most benefits top-end speed and developed leg stiffness.

It’s complicated

Leg stiffness is dependent, as noted, on the stretch-shortening capacities of muscles and therefore of the individual. The author has observed when training his group of sprinters and jumpers that some have natural greater leg stiffness than others and that this seems to be affected by the way they can “fire” their muscles. Each athlete, though powerful, generates their power slightly differently to others – some require a greater degree of flexion, for example, when performing drop jumps to generate power. The stretch-shortening capacity is an automatic one (although one that can be enhanced) and it relies on the functioning of the athlete’s neuromuscular system.

The stretch-shortening capacity is also actually variable across different joints (when jumping there are stretch-shortening cycles occurring at the ankle, knee and hip complexes and the degrees of reactivity and stiffness can vary across these joints). The stretch shortening cycle can also vary in regard to the angles and speeds of the jump being performed and the degree of central nervous system fatigue and general fatigue. Research corroborates some of these practical thoughts as Kuitunen et al write: “Our results suggest that leg and joint stiffness … is mainly adjusted by centrally programmed motor commands and the contribution to stretch reflexes to muscle force output is muscle-dependent.”

Leg stiffness, the “human spring response”, can be trained by varying methods in the pursuit of greater sprint speed. It’s crucial that coach and athlete realise that different conditioning methods as they affect leg stiffness can create different “sprint responses” (sled-towing will improve acceleration more than top-end speed, for example). Plyometric exercises appear to be key to leg stiffness. For example, an athlete who has good hopping and drop jump ability will have good high-end sprint capacity and leg stiffness.

Eccentric and concentric drills, practices, maturation and the inherent stretch shortening cycle ability of athletes must all be taken into account when conditioning for greater speed against the variable of leg stiffness.

» This article was first published in the October 2 edition of Athletics Weekly magazine and is one of a number of features included in the performance section of the magazine each week. Subscribe now to make sure you never miss out – subscribeme.to/athletics-weekly