Physiological Responses to Linear/Non-Linear Soccer Simulations.

Phys Responses JSCR

Our recent study investigated the Loughborough Intermittent Shuttle Test (LIST) (Nicholas et al., 2000) and it’s effects on neuromuscular fatigue when adjusted.

The problem with this ‘gold standard’ simulation is that, though it mimics the demands of a typical soccer game, it’s ecological validity was to be questioned.. how could it mimic the demands of a soccer game if the testing was completely linear (running in one straight direction)? Nonlinear running is frequently performed during soccer matches, with professional players completing 727 (+/-) 203 swerves and turns within a single match (Bloomfield et al., 2007). Such swerves require a great deal of rapid changes of direction and decelerations.

But to successfully complete these actions in soccer, repetitive demanding muscle contractions are necessary, particularly those of an eccentric nature and the cumulative effect of performing these type of contractions ultimately leads to fatigue, less efficient movement patterns (Small et al., 2009) and an increased risk of injury (Greig & Seigler, 2009). Research has also suggested that nonlinear running induces greater heart rate (HR), perceptual, and metabolic responses than straight-line (linear) running (Dellal et al., 2010), but this has also been questioned in recent studies, where they found similar drops in jump performance regardless of the type of running (Hader et al., 2016).

JSCR Infographic

Summary of study. Credit to Adam Johnson @PreventionPhys

The aim of our study was see if a modified version of the LIST could be incorporated mimic the demands of soccer while also achieving a similar amount of swerves and turns as reported in Bloomfield’s paper. The modified LIST allowed for the inclusion of 6 x 90degree and 1 x 180degree changes of direction (COD) per run. This meant that there was a total of 720-1008 COD’s within the whole protocol without altering total simulation time or activity time during the simulation.


8 university football/rugby athletes too part measures including blood lactate, heart rate (maximum and average), jump height, peak landing and isometric force, and perceived fatigue were measured across the testing period. The running was monitored through Global Positioning System (GPS) trackers attached to each participant.


Though participants found the modified LIST harder through perceived fatigue and heart rate measures, it portrayed no changes to jump height and isometric/landing forces. The modifed LIST did not change the total distance covered in the LIST but did change the amount of decelerations and the high speed running (HSR) distance. However,  greater HSR distances were directly related to reductions in isometric hamstring force.

What does this mean?

COD’s do seem to induce higher ratings of perceived fatigue but HSR distance plays a role in hamstring fatigue, indicating how important this GPS variable is for sport scientists and coaches alike when it comes to monitoring training load and managing fatigue. It also allows for changes to training (particularly related to small sided games or metabolic conditioning) by incorporating COD’s which could reduce HSR distance but still lead to higher or similar metabolic demands.


The modification to the LIST was based on multi-directional running at 1.5x the timing of the maximal effort run timing of each participant. The COD’s were positioned in a manner that did not alter total distance covered by the original LIST. However, such timings and COD positionings may have given participants enough time to run and change directions at a pace which did not require a great deal of side cutting mechanics but full body turns following lesser speeds of running.

The study would ideally have aimed to recruit a larger sample size of professional athletes to see if these results reflected the larger population of professional soccer players.

Full study available here:

An Introduction to Concussion within Combat Sports (Part 1)

Disclaimer: This article may change from time to time to reflect recent/relevant advances in the field of concussion within combat sport…


All of these animals have shock absorbers built into their bodies. The woodpecker’s tongue extends through the back of the mouth out of the nostril, encircling the entire cranium. It is the anatomical equivalent of a safety belt for its brain. Human beings? Not a single piece of our anatomy protects us from those types of collisions. A human being will get concussed at sixty G’s (G- force). A common head-to-head contact on a football field? One hundred G’s.

From the film ‘Concussion’ in 2015 based on some real life events and people.

Now while the whole film (let alone the quote), may not be scientifically accurate (or even about concussions!), it is a profound statement allowing us to recognize a crucial matter: every organism has an amazing, unique and intricate design, and we are extremely fragile and dependent upon these designs.

The brain sits in a pool of cerebrospinal fluid without any connection to the skull. As shown in the movie clip; the example of an apple in a slightly filled jar of water gives us a parable of how the brain moves with each impact to the cranium (in this case, the jar). With each impact to the jar, it leads to the loss of integrity of the apple, eventually causing it to crack. In the case of the brain? It’s a lot more complex than that.. a neatly packed ball of jelly with specific functions, hitting onto the cranium and causing the jelly to be stretched and sheared at a microscopic level. The makings of this jelly are over a 100 billion neurons and glial cells, all involved in governing your body systems.

SO what is a ‘Concussion’?

Concussion, also known as Mild Traumatic Brain Injury (mTBI), is an injury through impact to the head (brain) which causes temporary losses in brain function.

It has been previously described as “transient, temporary, and neurological dysfunction resulting from head injury with temporary loss of brain function” (Anderson & Macleod, 2006)

It makes up 80-90% of TBI cases but is notably distinguishable from other forms of TBI because

a) There are no gross pathologies such as a hemorrhage

b) No identifiable damage of the neural structure through conventional neuroimaging techniques such as MRI (Noble & Hesdorffer, 2013).

This means it is quite difficult to diagnose and has many generic symptoms which can easily make it a condition which is misdiagnosed or more importantly, ignored or unnoticed.

How does it happen/What causes it?


Concussion is caused by acceleration, deceleration and rotational forces applied directly or indirectly to the head, leading to the damage of brain tissue. These forces can occur from direct or indirect impacts, hence concussions are not only the result of impact to the head, but from impact to the body resulting in head movement (e.g. car accidents involving a whiplash {coup-contrecoup})

(Note: The amount of movement in the picture above is not an accurate representation of brain movement within the cranium, but gives us an idea of the acceleration/deceleration forces on the brain during impact that leads to stretching of brain tissue)

Image result for concussion

The principal causes of head trauma in combat sports include straight impact blows to the face (e.g. jabs and front kicks) leading to linear acceleration of the head or impact from an angle (e.g. roundhouse kicks, hooks and uppercuts) leading to rotational acceleration (Unterharn-scheidt, 1995). High incidences are also seen within sports such as American Football, Hockey, Rugby, Football (soccer) and Basketball (Harmon et al., 2013). These are the results of head on head collisions and collisions with the ball or ground.

The movement of the brain within the cranium can lead to shearing of cells along with stretching and disruptions at the structure of the neurons within the brain. The stronger the impact, the larger the stretch and deeper it may affect the brain. It is said that a 15% stretch of the neuron length will result in biochemical changes (such as membrane depolarization and deregulated release of neurotransmitters) resulting in an acute failure of neurons leading to a concussion (Gaetz, 2004).

Further research with the use of finite element modelling (Patton, Mckintosh & Kleiven, 2015) and various alternative methods (Jang et al., 2019) have pointed towards a deep structure within the brain, particularly being effected by head impacts. This structure/region is known as the Corpus Callosum, which is ultimately a bundle of nerve fibers that bridges/connects information between the right and left hemispheres of the brain. Due to this potential disruption in connectivity, it is suspected to be a cause of cognitive deficits at the onset or post-concussion, but has yet to be fully understood. Watch below and observe the red portion of the brain during this simulated head movement based on an American Football tackle. This is the corpus callosum being stretched by ~50% of it’s original length.


While acute changes in neural function are a scare factor within bouts, long term consequences are not always considered by athletes and coaches. 204 participants from the Professional Fighters Brain Health Study were observed for long term brain changes; repetitive head impacts correlated with larger losses in brain volume (atrophy), particularly at the  left thalamus and regions of the corpus callosum. MMA fighters portrayed a similar pattern to a slightly lesser extent. On the other hand, retired boxers showed large declines in left & right amygdala and right hippocampus. In addition, neurofilament light chain (Nfl) was examined within this study. This is a protein found within the cerebrospinal fluid and blood when damage to the neuron (specifically axons) occur (Gaetani et al., 2019). Higher levels of Nfl were found in relation to larger loss in volume of the brain, indicating higher levels of neuronal damage. Read more about this 2019 study here or below.

Embedded Image

To help familarize yourself with some of the affected regions of the brain;

a) the amygdala plays a key role in emotion, stress, fear etc

b) the hippocampus is responsible for long and term memory

c) the thalamus is the gatekeeper of sensory information before reaching the brain and allowing you to respond.

Perhaps this may now aid you to understand where the fear of dementia (particularly Alzheimers), memory loss in general, poor reaction times and increased negative mood states has originated from. However, much is yet to be proven and it is dangerous to just affiliate all the above to being hit on the head!

Other have observed changes in neural function even after standard header practices within soccer, which may suggest that with each exposure to head impact, there is an increased risk of concussion or acute changes in brain excitability.

Through studies observing biomechanical forces to the brain, severity of impact to the brain has been observed to increase with weight class. In addition, it has been reported that the punch of a professional boxer may generate forces on impact that are equivalent to a 6kg bowling ball rolling at 20mph (Athaet et al., 1985). So though you may be entertained by head shots or knockouts, it is vital to understand the neurlogical consequences to constantly exposing themselves to this level of impact. (Safety first!)

How prevalent is it?

The Centre for Disease Control and Prevention (CDC) reports an estimate of 135,000 patients per year to arrive at emergency departments with concussions from sports or recreational activity (Laker, 2011).

Within combat sports, particularly boxing and kickboxing, concussions account for approximately a third (33%) of injuries experienced by fighters . Similarly, 28.3% of fight stoppages within MMA have been reported due to head trauma with a rate of 15-16 concussions per 100 bouts (Curran-Sills & Abedin, 2018; Hutchison et al., 2014; Buse, 2006). A study that examined injuries sustained in professional kickboxing over a 16-year period showed an injury rate of 19.2 concussions per 1000 fight participations (Zazryn et al., 2003). Elite amateur boxers over a 5-year period in the UK showed an incidence of 0.53 concussions for every 1000 hours of competing/training (Loosemore et al, 2015). Note; these studies have reported diagnosed concussions, but this does not reflect quantity and magnitude of impacts taken by each athlete.

It is most frequently observed in American football, especially linemen and linebackers, who are reported to be exposed to more than 1000 impacts per season (Crisco et al., 2010). Even the sport of soccer/football has been observed to have significant rates of concussion with reports of 1/5th of injuries within high school games to be due to concussion (Kerr et al., 2018).

What are the symptoms?

Typically, clinical symptoms straight after a concussion include:

  • Somatic symptoms (eg, headache, nausea/vomiting, light/noise sensitivity and vision problems).

  • Cognitive symptoms (eg, foggy feeling, memory problems and difficulty concentrating).

  • Emotional symptoms (eg, labile mood).

  • Physical signs (eg, loss of consciousness, confusion, difficulty making eye contact, slurred/slow speech and sluggish/change in fighting style).

  • Balance impairment (eg, dizziness, difficulty walking and stumbling).

  • Behavioural changes (eg, irritability).

  • Cognitive impairment (eg, slow to react, slow to answer questions and not oriented to person, location or date/round).

  • Sleep/wake disturbance (eg, somnolence and drowsiness).

(McCrory et al., 2016)

Most recover within days to weeks. However, 10-15% may experience persistent issues even after a year (Hall et al., 2005; Sterr et al., 2006).

Though loss of consciousness (e.g. Knockouts) is generally attributed to concussion, it is important to note that this is only known to occur in 10-20% of cases (Giza & Hovda, 2001), indicating it still may occur 80-90% of the time, regardless of an obvious sign.

However, even if these symptoms are eventually not apparent.. or concussion is not diagnosed..

with newer electro-physiological techniques, researchers now question has the brain actually recovered? Yes, the symptoms can cease to exist, but are there any unknown/residual effects? Can the brain structures look fine but something still be wrong?

Short answer: We’re not quite sure yet!

Less shorter answer: The risk of further concussions and musculoskeletal injuries have been reported to increase in athletes with previous concussions (Guskiewicz et al., 2003; Herman et al., 2016; Nordstrom et al., 2014), which suggests that some of these concussion-related changes to the brain have not yet truly healed. Some research also suggests that in some cases, the brain to muscle pathway still portrays signs of disrupted neural pathways even after 9 months post concussion (Lewine et al., 2007; Huang et al., 2009; Lewis et al., 2017).

Therefore the question is: How much do we still not actually know about the effects and potential causes of concussion in sports, let alone the sports which involve which involve repetitive head impact?


  • Part 2: Concussion diagnosis tools and management

An Introduction to Acute Fatigue within Sport


Muscle fatigue most often occurs during strenuous dynamic physical activity (James, Scheurmann & Smith, 2010) and within sport, can be explained as a reduction of maximal force or power that is associated with sustained exercise, or the inability to produce or maintain a required force or power output (Celine et al., 2011).


Neuromuscular fatigue is a vast topic of research and though many of the causes are still obscure, it is agreed to be the result of a combination of central and peripheral factors (Allen, Lamb, & Westerblad, 2008; Enoka & Duchateau, 2008; Fitts, 1994; Waldron & Highton, 2014).

Vaguely put, central fatigue is related to the brain and central nervous system while peripheral fatigue is to do with the muscles themselves and the substances related to contracting them.

In a few more words, central fatigue is related to reduced central motor drive of the central nervous system (CNS) due to changes in the synaptic concentration of neurotransmitters and therefore its ability to recruit available motor units (Davis and Bailey, 1997).

Peripheral fatigue on the other hand, relates to limitations of muscle capacity through biochemical changes (Waldron & Highton, 2014), such as an increase in blood lactate concentration and hydrogen ion accumulation (Mohr, Krustup & Bangsbo, 2003). It occurs at, or distal to, the neuromuscular junction and is thought to be linked to several mechanisms such as failure of excitation-contraction coupling, a disruption in neuromuscular transmission and the inhibition of muscle contraction through either the build-up or depletion of metabolites (Kent-Braun, 1999).


Lactate and hydrogen ions (H+) production is the result of glucose breakdown. If a sufficient amount of oxygen is not available to the working muscles then hydrogen ion concentrations begin to rise and the muscles, including blood flow to the muscles, become acidic. This acidic environment blocks neuromuscular transmission (signal to and from the brain and muscle), slowing down the individual in order to allow more oxygen to get to the working muscles.

Decreased work rate within sports has been affiliated with reduction in muscle glycogen content (Saltin, 1991) while other factors such as physiological changes within the muscle cell, dehydration and a reduction of recruitable muscle fibres for force production have also been suggested (Saltin, 1973; Jacobs et al., 1982; Bangsbo, 1994). Sjersted and Sjoogard (2000) hypothesized that the sarcolemma was one of the sites of fatigue due to its inability of maintaining sodium and potassium concentrations during repeated stimulation. As a result of inefficiency of the sodium/potassium pump, potassium concentrations increase outside the membrane, in turn decreasing the amount within the membrane. This leads to depolarization of the cell, thus reducing action potential amplitude.

Recovery from peripheral fatigue where it relates to metabolic factors and impairments in neuromuscular transmission is known as high frequency fatigue and has been shown to occur within shorts periods of time following exhaustive exercise (Bruton, Lannergren, & Westerblad, 1998; Saltin, Radegran, Koskolou, & Roach, 1998). Low frequency fatigue on the other hand, is related to peripheral fatigue due to impairments within the excitation-contraction coupling process, and occurs as a result of repeated short intervals of repeated muscle stimulation. This type of fatigue is more prominent following eccentric exercise and is likely the cause of a longer lasting neuromuscular fatigue (Keeton & Binder-Macleod, 2006).


Fatigue within sports is primarily identified through a decline in performance (Reilly, 1994). A fatigue effect has been reported to be noticeable in soccer, particularly within the second half due to an observation of decreased work rate (Reilly and Thomas, 1976). Bangsbo et al (1991) reported a 5% greater distance covered within the first half by professional soccer players,

Fatigue been shown to impair sprinting mechanics and joint stability. Such a combination of factors is likely to predispose players to injury during tasks that have high mechanical demands. (Greig, 2009). Fatigue leads to altered movement patterns of athletes into more stressful positions (Borotikar et al., 2008), and has been reported to be a direct contributor towards injuries such anterior cruciate ligament (ACL) injury via promotion of high risk biomechanics (McLean & Samorezov, 2009)

With advancements in team sports performance analysis such as player tracking technology, fatigue is a popular topic of interest and has usually been associated with variables such as running performance, distance covered from baseline, along with reported changes in neuromuscular force or sprinting ability (Rampinini et al., 2011). Basic physiological measures such as heart rate and blood lactate provide a useful index of overall physiological strain and anaerobic energy yield in response to high intensity situations respectively (Reilly & Thomas, 1997; Reilly, 1997). Thus, combining these variables with a measurement in decrease of muscular force overtime within match play may help indicate acute neuromuscular fatigue.


Other ways include monitoring training load through session RPE’s (rate of perceived exertion) or/and RSI (Reactive strength index). These are easily obtained through a set of numbers based on session time and perceived exertion or using a jump mat and monitoring jump performance which is custom for most professional athletes particularly within team sports. An example and some uses for these two methods are summarised in the below picture.


Sport scientists and S&C coaches may prefer to use other methods of further analysis through a combination of the above mentioned factors or equations such as the Fatigue Index equation (Fitzsimons, Dawson, Ward & Wilkinson, 1993) based on peak running speed/peak force produced and sum of general running speeds or general force produced, depending on the athlete being monitored.