Acceleration Performance & Soleus Injury

Summary

Sled training has become an increasingly popular addition to strength and conditioning programs, particularly for team sport athletes. It offers a practical means of developing acceleration, and when loaded appropriately, can also serve as a useful injury prevention tool. However, the relationship between acceleration development and lower limb injury — particularly soleus strain — is worth exploring in more depth.

Performance & prevention paradox

In theory, we typically all subscribe to the principle that performance enhancement and injury prevention training are synonymous. In practice, these two objectives often get separated. Acceleration performance is a much desired and distinguishing quality in all field sports.

Injury trends

Whilst only one muscle, the soleus plays a significant role in the transfer of force into the ground – a force that generated by the entire kinetic chain. The Australian Football League (AFL) is perhaps the most professional league in the world when it comes to documenting injuries. Every year they share that information publicly, and hamstring injuries are consistently the leading soft tissue injury. But close behind are injuries to the calf complex – chiefly soleus and the numbers are increasing. Green and Pizzari (2017) conducted a systematic review of calf muscle injuries in sport and found that soccer had the highest reported incidence rates among field sports. Interestingly the incidence of calf injuries in the athletics world is comparatively low – particularly in the sprint and jump events. Why the disparity between sports?

When it comes to the increasing challenge of calf injuries, a reductionist approach, is still quite widespread: surprisingly the first line of enquiry is strength. The solution: make the calf stronger and it will no longer break. Rather than fixing the problem this has just created a paradox: we focus more and more on calf strength in our training, but injury rates appear to be nevertheless increasing in many sports. Commonly, many practitioners working in field sports do not even examine athletes’ running mechanics as a potential source of injury.

The role of the calf with acceleration

Let’s start with a short review of anatomy. We often think of the calf as one complex, when in fact it is a connected muscular system consisting of the gastrocnemius, soleus, and plantaris muscles, with attached tendons and planter aponeurosis unit. This is mentioned because it highlights the connectedness and complexity of the anatomy involved. Focusing on one muscle overlooks the rest of the system and how it interacts.

Mobility matters

To accelerate effectively, an athlete requires adequate mobility in 3 primary joints:

• The hip - a lack of hip extension is a driver of many injuries. An athlete who under-extends at the hip during the propulsion phase must generate more plantarflexion force at the ankle to achieve the same forward momentum. The soleus becomes a compensatory power source for a hip that is not doing its share of the work. This pattern is well-established in sprint biomechanics literature. Athletes with restricted hip extension during the stance phase tend to show greater ankle joint contribution to propulsion - and greater injury exposure to the distal lower limb.
• The ankle – horizontal force production is critical to acceleration performance. However, force application is compromised without good dorsi flexion of the ankle. A knee-to-wall range of motion of 10cm would be a minimum standard
• The first metatarsophalangeal joint (MTPJ) – a lack of big toe dorsi flexion seems like a small matter, but it can lead to big problems. Some of which can be achilles issues. Mobility to through MTPJ utilises what is known as a ‘windlass mechanism’, since in humans, dorsiflexion at the MTPJs causes functional tension of the plantar aponeurosis, which originates from the calcaneal tuberosity and inserts distally on the proximal phalangeal bases. The windlass mechanism at the MTPJs requires sufficient dorsiflexion to operate.

Mechanics matter

Timing and coordination of the swing leg and arms during initial acceleration sees the close alignment of foot contact and centre of mass over the vertical axis. An athlete who doesn’t time and coordinate the swing leg forward will tend to have a flatter foot position at initial contact - making the first milliseconds of ground contact a period of rapid, uncontrolled dorsiflexion. The soleus is not designed for this. Under these conditions it is loaded eccentrically before it has been able to pre-activate.

Good runners have an active ankle plantar flexion anticipating immediately prior to foot contact. However, this plantar flexion is not observable but manifested as stiffness during the contact phase. Conversely, forces associated with mistiming and/or misplacement of the foot, that are either excessive or poorly absorbed, can lead to calf and achilles injuries. In other words, poor mechanics and poor ankle stiffness often sees an athlete waiting to push off the ground - rather than reactive off it. As such, the quality of force application is determined by the timing of the planter flexors in generating force, rather than the force itself.

It is the last part of the stance phase with initial acceleration where the majority of propulsive forces are delivered. Horizontal forces needed to accelerate the body demand a rigid lever. The windlass mechanism tightens the plantar aponeurosis to lock the foot bones together, transforming the foot into that rigid lever. A well-functioning ankle and MTPJ has been shown to allow the plantar aponeurosis to return energy that amounts to 5-10% of the combined lower limb joint forces during late stance via the windlass mechanism.

Without doubt the calf complex must tolerate significant forces when sprinting. Research by both Komi et al (1992) and Dorn (2012) drew similar conclusions that calf and achilles tendon in series, experiences tensile forces up to 12 times body weight at speed. So, without a doubt, strength is important.

A systems approach would look at the calf complex in the context of running, related timing and coordination as well as strength. If we accept that muscles set up forces, but it is the serial elastic components that store energy and convey forces into the ground with high-intensity sprinting, then this will dictate criteria for our performance enhancement and injury prevention programming.

Training & specificity

We now understand how the calf complex is built and functions. The next step is how to train it. Specificity is a foundational principle of training. It is assumed that in high level sports, more specific training will transfer better from training to the sport. Concepts like Dynamic Correspondence try to define specificity in more detail, but the key concept is whether or not the function of the training exercise and the function of the training goal are aligned. This means that we aren’t just defining specificity as focusing on the same muscles, but also on training similar coordinative demands as well. Specific training requires muscles to perform in a similar context as they will be required to in the sport.

Let’s take an example from calf training. As we described above the primary function of the calf complex is bracing. While exercises like loaded calf raises train similar muscles, they are doing so in a completely different context and therefore can be considered less specific. The forces and loads seen in sprinting also can’t be matched with traditional strength training; calf raises feature moderate loads for long duration rather than high peak loads for short durations. Furthermore, to optimise the function of the calf complex, all training and testing should involve knee extension (Bosch, 2015). This is why more specificity can be found in locomotive exercises such as advanced running drills, plyometrics, and resisted sled training. Locomotive exercises not only allow the muscles to be trained in the right context, but they allow athletes to train and improve the coordinative elements at the same time.

Testing=Training & Training=Testing

The DynaSled is an innovative piece of technology that can be utilized for acceleration performance, injury prevention and rehabilitation. The DynaSled can offer a much more functional means of real time testing and training calf strength. Mainly as a modality it is by default coordination training with resistance.

When an athlete sprints with a friction-resistance sled, the external load at the hips implicitly demands for hip extension. The sled does not move unless the athlete pushes backward through the ground. Athletes who compensate with insufficient hip drive will feel this immediately: the sled does not move efficiently, and the effort required is disproportionate to the speed achieved. Pulling with a waist harness, as opposed to pushing the sled requires the arms and the swing leg to coordinate synergistically and has significantly greater demand on the core in comparison to pushing.

It is important to make the distinction between resisted acceleration and horizontal mechanical power when using a loaded sled. Generally speaking, it is best to polarize training:

• Resisted acceleration – should involve loads of around 10-20% body weight. With these loads acceleration mechanics are not compromised
• Horizontal mechanical power development – typically intended for advanced and well-conditioned athletes, should involve loads of up 80-100% bodyweight over 5-10m that elicits a 50% decrement in speed

Sled training also offers power development without axial loading. Often the time investment and transfer to on-field performance sometimes sees Olympic lifting and their derivatives as not being worthwhile with all athletes.

Return to play and testing

The same principles of specificity are also relevant when it comes to testing. Understandably professionals involved in preparing running-based athletes are inclined to want to test strength qualities – specifically calf strength. However, we shouldn’t forget that we need to measure what is important, rather than making something important simply because we can measure it. Our assessment of strength must also adhere to Dynamic Correspondence criteria. Seated calf raises on a force platform may well assess intramuscular coordination, but are completely devoid of intermuscular coordination and many other essential criteria for acceleration performance. One of the easiest and simplest means of identifying ankle stiffness is with the use of the ‘slo-mo’ video function on an iPhone when sprinting. Of course, it could be argued this is purely subjective. However, ankle stiffness is now easily quantifiable.

We no longer need to be constrained to a force plate and a single plane of motion. The DynaSled captures the full horizontal force profile — greater dynamic range, sharper impulse peaks, and far more pronounced left–right asymmetry.

Often in the return to run progression we introduce the continuum of resisted walking, then resisted marching and then eventually resisted running with a sled for the specificity it offers. We can now do the same with live diagnostics.

This is where DynaSled delivers its most actionable data: asymmetry that would be invisible on a force plate, and a force–velocity profile that reflects exactly what the athlete produces stride by stride.

The training environment now becomes a lab:

• Feedback to athletes can be instant
• Faster decisions on progressing an athlete back from calf injuries along with other lower limb injuries
• More accurate and functional decisions on return to sprinting and competition
• Training and testing become synonymous

Soleus strains in soccer and AFL are not random events. They are the product of specific mechanical patterns that can be identified and modified. The tools to do this -- force-velocity profiling and friction-resistance sled training with direct force measurement -- are available to field-based practitioners. The injury does not have to be inevitable.

References

Bramah, C., Preece, S. J., Gill, N., & Herrington, L. (2021). Kinematic characteristics of male runners with a history of recurrent calf muscle strain injury. International Journal of Sports Physical Therapy, 16(3), 732.

Bosch, F & IJzerman, J. (2015). Running mechanics in injury prevention and performance. In D. Joyce & D. Lewindon (Eds), Sports Injury Prevention and Rehabilitation (pp. 106-120). Routledge.

Dorn, T. W., Schache, A. G., & Pandy, M. G. (2012). Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance. Journal of Experimental Biology, 215(11), 1944-1956.

Green, B., & Pizzari, T. (2017). Calf muscle strain injuries in sport: a systematic review of risk factors for injury. British Journal of Sports Medicine, 51(16), 1189-1194.

Hunter, J. P., Marshall, R. N., & McNair, P. J. (2004). Interaction of step length and step rate during sprint running. Medicine and Science in Sports and Exercise, 36(2), 261-271.

Komi, P. V., Fukashiro, S., & Järvinen, M. (1992). Biomechanical loading of Achilles tendon during normal locomotion. Clinics in sports medicine, 11(3), 521-531.

Morin, J. B., Petrakos, G., Jimenez-Reyes, P., Brown, S. R., Samozino, P., & Cross, M. R. (2017). Very-heavy sled training for improving horizontal force output in soccer players. International Journal of Sports Physiology and Performance, 12(6), 840-844.

Thompson, P. J. L. (2016). Current perspectives of biokinetics in middle and long distance running-an examination of the ‘elastic response.’. New Studies in Athletics, 31, 25-40.