Abstract
The Covid-19 pandemic has forced worldwide shutdowns including major sporting leagues for at least 8 weeks, where athletes have been forced to train at home with remote coaching. The amount of detraining that has occurred during this break is an important factor in guiding retraining post Covid-19 shutdown. The pattern of detraining in fitness components is predicted using previous studies findings on the detraining process of specific physiological elements. The patterns are used to foresee what we can expect to happen physically during this break. The more highly trained the athlete the more vulnerable they are to detrain, as they require high level of training to maintain these characteristics. Aerobic fitness declines rapidly with training cessation but can be retrained rapidly and maintained with lower volumes. Similarly, muscular size and strength can be maintained with reduced loads in younger populations. It is harder to maintain power, agility and ‘match fitness’ during the break due to lack of facilities and competitive training. Strength and conditioning coaches need to identify the detraining that has occurred over the break and strategically prepare athletes for match play in as short as 3 weeks. A balance between preparing the athletes as fast as possible and reducing potential injury risk needs to be carefully decided, using predisposing factors to injury risk such as previous injury and information from wellness monitoring to guide the volume and intensity of training prescription for each individual. An increase in injuries could be expected as workload volumes will soar post Covid-19 break, those especially vulnerable are inexperienced athletes and those with previous soft-tissue and stress related injuries.
Keywords Covid-19, Coronavirus, Detraining, Return to play and Injury
The world has been largely affected by a novel viral pathogen, COVID-19, in 2020 and 2021. Most industries around the world have suffered ramifications from the virus, some on a larger scale than others, and sport has not been spared. Sporting events and leagues have been cancelled and postponed all over the world including the Toyko Olympics, National Basketball Association (United States of America), English Premier League, Formula 1, Australian Football League (AFL) and Nation Rugby League (Australia) (9). These sports began to shut down mid-March 2020 and remained shut down in mid-May. Competitions have been closed for about 8 weeks with sports making players train remotely. In Australia, COVID-19 restrictions did not allow players to train with more than one person and training had to be conducted outdoors, meaning athletes could not use club gyms or indoor facilities to train. Players in the AFL were also not allowed to be tracked via GPS trackers. This gave High Performance teams across the world a great challenge of maintaining players fitness remotely with minimal equipment in some instances. It can be assumed that programs had been simplified from the regular programming as coaches were not able to guide players through the program face to face and had an absence of objective data. It can also be assumed that players had lost match conditioning, as they are only able to train with one other in a non-contact fashion. Sports like rugby and AFL would find it very hard to emulate physiological requirements of matches with two-person training without contact. The goal of the Covid-19 break programs should have been to minimise the detraining of physiological elements, focusing on training what you can train easily remotely, such as aerobic fitness, repeat sprint ability and linear speed, while maintaining the components that would be much harder to train such as maximal strength, power and agility.
This article will focus on the considerations of returning to sport after the Covid-19 break for mainly team sports including AFL, Rugby, Soccer and Basketball from Australia, but the principles can be applied to many other team and individual sports. The considerations are not just for top-level sport but for sub-elite, second tier and community sports. The considerations may pertain even more to lower levels as the ability to obtain necessary equipment, coaching and motivation to maintain high-level of fitness may have even more barriers at the lower levels.
The goal of programs given to athletes during the break would be to minimise reversibility of the trained physiological adaptations obtained over pre-season. Rugby Union, Rugby League and AFL had just begun their season when they were interrupted by the pandemic, with soccer and Basketball close to the completion of their seasons. The adherence to break programs could vary between players as they do not have to physically complete it in front of coaches. This would be even more prevalent at lower levels of sport as it is not their main job, hence their motivation to maintain fitness may be diminished. It would be useful to fitness test the athletes when they get back from the shutdown to see what condition they are in comparison to their most recent test results. This will help identify the specific physiological parameters that have detrained and guide coaches in planning return to sport training. The higher level of fitness of the athlete the quicker these attributes will detrain, which needs to be strongly considered as most athletes were at their peak after the pre-season, so they will be most susceptible to detraining (12). Significant conditioning is lost in 2-6weeks of insufficient training with a decline in VO2 max of 4-20% seen (10). There is also a reduction in blood volume which is the main determinant of rapid decline in cardiac function (10,15). The lactate threshold is also found at a lower percentage of VO2 when detraining has occurred (15). A study on detraining post soccer season found that Yo-Yo IR2 and repeated sprint performance dropped after a two-week detraining (cessation) period but improved from baseline after two-weeks of High-Intensity aerobic training. While a period of three weeks was required to return to the initial level of sprint performance. The study also observed that a reduced amount of high-intensity training could maintain Yo-Yo IR2 scores (12). Several weeks of detraining saw a 25-45% decline in oxidative enzyme activities, which resulted in reduced ATP production in skeletal muscle (15). After 8 weeks of training cessation, decrements in all physiological parameters were seen in an elite rower with a decline in VO2 peak (-8%), pVO2 peak (-20%), power at lactate 2mM (-27%) and 4mM (-22%) (10). This shows that aerobic fitness declines with full training cessation but can be maintained with smaller high-intensity aerobic training. It would be expected that most athletes would have done some training and been able to maintain a lot of their aerobic fitness if they followed a well-designed program. Aerobic fitness also showed it was quick to retrain after introducing some high-intensity aerobic training.
Rugby, AFL, Soccer, Basketball and all team sports have a combination of aerobic and anaerobic requirements, so detraining of both these factors need to be observed to gain a picture of what is the best way to retrain athletes after Covid-19 induced break. It is widely known that a person with a relatively higher proportion of fast-twitch fibres will be able to achieve higher muscle force and power output during fast movements then the athlete with lower proportion of fast-twitch fibres (3). A cessation of resistance training will induce, or re-induce a switch from Type IIA fibres to Type IIX Fibres, which on the surface may look like a benefit as Type IIX has the greatest velocity but an increase of contractile strength, power, and rate of force development (RFD) of a trained muscle outweighs these benefits. Type IIX fibres produce very high amounts of energy in short spaces of time but can only withstand such effort over a short time (seconds). Type IIX fibres need to rest to avoid exhaustion, which they would not be able to obtain in any of the major ball sports (3). Therefore, it is beneficial for ball sport athletes to have a greater amount of Type IIA fibres, obtained through resistance training, due to the simultaneous increase in contractile strength, power, and RFD. In a strength training study of young healthy males, it was observed that Type IIX fibres of the vastus lateralis muscle decreased from 9 to 2% in three months of training. After a 3 month period of detraining the Type IIX fibres increased to 17% of vastus lateralis muscle (2).
The positive physiological adaptations to resistance training are reversed when training is ceased in any population. Muscle cross-sectional area and myofiber size decreases with detraining as well as muscular strength (4,7). In a study of young males, 12 weeks of detraining had shown significant losses in strength and myofibre size. Despite the evidence of reversibility in physiological adaptations obtained through resistance training, the amount of resistance training needed to maintain training adaptations is much less then training volume needed to stimulate adaptations. It is also widely researched that strength decline occurs slower than the gains obtained through resistance training (4). A study that looked at the exercise dosing to maintain resistance gains in younger and older adults, found that once per week dose of resistance training was enough to maintain positive neuromuscular adaptations. In the young populations one-third of volume dosing lead to continued muscular hypertrophy and strength gains, whereas one-ninth of volume, maintained improvements in contrast to strict detraining. Among older populations neither dosing scheme was enough to maintain gains in size, but strength was largely maintained by both prescriptions. A dose of one-third of the volume of resistance training prevented the reversal of Type IIA fibres to Type IIX fibres (4). A detraining period of 4 weeks after a 16-week resistance training led to decreases in 1RM of Parallel Squat and Bench Press and decreases in power output of upper and lower extremities. A tapering period (volume was progressively lowered, and intensity increased) saw further increases in maximal strength but no changes in power output of upper and lower body. Power gains in strength trained athletes seem to be lost at greater rate than strength is diminished. Not unlike aerobic characteristics the more highly trained an athlete the more susceptible they become to performance decrements in muscular power (11).
The positive adaptations to resistance training are not only experienced in the muscle and neuromuscular pathways, there is a positive effect in connective tissue. Tendons which connect muscles to bone experience an increase in stiffness after 12-16 weeks training. Positive adaptations to resistance training for tendons and muscle CSA are slower than those of muscular strength and neural activation and inversely feel the effects of detraining quicker than the latter. Tendon stiffness did not change after the first two months of training protocol but increased by 54% at the end of three months. During the training period there was no elongation of tendon found despite large increases in maximal strength. During the detraining period it was found that tendon elongation occurred at every test period of 1 month especially in the first month (18). Tendon stiffness in the Achilles is positively correlated with running economy and shorter ground contact time. There is evidence that tendon stiffness improves standing vertical jump and counter movement jump. A study demonstrated that subjects with stiffer Achilles tendons had shorter ground contact time after 40cm drop jump (1). This is advantageous to sports that involve jumping and sprinting such as the ball sports we described earlier. It is also pointed out that a stiff tendon is able to transfer forces faster to bones then a complaint tendon, hence assuming that a stiff tendon absorbs less energy then a complaint tendon allowing the muscle to provide less energy for contraction (1).
With the Covid-19 pandemic being such an unprecedented event having something to look back on for guidance is very hard to find, although we can take some information from altering the normal season in sports. NFL experienced a compromised off-season during 2011 due to NFL lockout. During the lockout they did not have access to their teams’ healthcare providers, strength and conditioning professionals, and high-level coaches. There was a rapid transition from the start of camp to start of pre-season competition of seventeen days. Ten Achilles injuries occurred over the first 12 days of training camp, with 2 additional injuries occurring in the next 17 days which included the pre-season competition. Injury data states that you would expect between 1 and 3 Achilles tendon injuries in 6 weeks including camp and pre-season competition (16). The compromised preparation to the season showed a significant increase in injuries especially among inexperienced athletes. The average experience of athletes who suffered an Achilles tendon rupture was only 1.4 years. The average age of Achilles tendon rupture was 23.9 years in this instance compared to league average of 29 years. This indicates a shortened preparation may lead to rookies suffering more injuries from a lack of preparation then more experienced athletes as they are more familiar with rigors of training and how to prepare their body to cope with demands (16).
The volume and intensity of training become a vital factor in returning to play after the Covid-19 break as most of these athletes were at the start of their season or right in amongst the season, so they would have undergone a thorough preparation (been close to peak fitness) but then experienced a break. Some athletes would have followed a break program stringently and minimised detraining, but it could be expected that some athletes have done minimal training especially at community level. The common pattern of loading we could expect is a gradual build-up of volume and intensity to a peak at the end of preseason and then a drop for the Covid-19 break, which will vary dependent on the amount of detraining that has occurred. The job is now to get athletes back up to that peak in shortest space of time. It is now the coaches, conditioning staff and medical teams’ task to plan and design how to get athletes back to peak fitness, skill performance, tactical awareness and competitiveness all in a 2-3-week period before games. There must be a balance between training too hard and promoting injuries and training hard enough to retrain these factors and gain competitive advantage. Susceptible demographics such as rookies, those with previous soft tissue and stress -related injuries need to be watched extra closely for response to spikes in load. The acute:chronic workload ratio appears to be a valid tool to assess an athlete’s level of readiness to train and compete, and their injury risk (5,13). Acute:chronic workload ratios using moderate speed running and a 3-day and 6-day acute window and a 21-day and 28-day chronic time window were best able to explain non-contact injuries in matches, matches and the next 2 days and matches and next 5 days in AFL (5). The ratio of minimum injury risk has been found to be 0.8-1.0 and the injury risk increases with workloads either side of this range. The model predicted that injury risk doubled from 1.8% to 3.6% if the workload deviated from 1 to 1.4 or 0.5 (5). It has also been proven that moderate weekly loads (≥ 1400 and ≤1900 arbitrary units) offered a protective effect for athletes in both pre-season and in-season. Larger absolute changes weekly changes in load (>1000 arbitrary units) were shown to increase the chance of injury in comparison to lower deviations (13).
In the German professional soccer league, the winter break was shortened in the 2009-2010 season from six and half weeks to three and half weeks (8). The overall findings were the shortened break did not effect the overall and match injury incidences. Despite this there was an increase in training and knee injuries and the risk of a more severe injury was also increased after the shortened break. This indicates accumulation of fatigue and lack of some general preparation time during the winter break increased severity of injury risk. As an athlete fatigues knee- and hip-flexor angles decrease, and proximal tibial anterior shear force and knee-varus movement increase when performing stop-jump tasks or side-step cutting under fatigued conditions. Also, in a fatigued state muscles can absorb less force prior to being stretched to injury. So, a rapid return to training intensity and volume could see high levels of fatigue which could make athletes vulnerable to injury. Also, a lack of preparation for gameplay will also induce high levels of fatigue and subsequent injury risk (8). The AFL has tried to control the fatigue factor by shortening quarters from 20 to 16.5 minutes. To add to the pressures of planning a return to play it is highly likely that coaches could have increased intensity and competitiveness of training during these shortened preparation periods due to time constraints. The break and short retraining period does not allow coaches the time to gradually increase intensity and volume (8,16). A preseason period of just three weeks training and only two weeks of contact is being undertaken in the AFL and NRL, with a reduced number of staff due to Covid-19 layoffs. It would be assumed that we are likely to see more injuries due to lack of a comprehensive and gradual preparation usually afforded to coaches and strength and conditioning staff.
Tracking overall training loads is important in mitigating injury risk, further to this identifying specific measures of matches and training loads ultimately prepares athletes for match play. Maximal velocity running abilities are required of players during competition to beat opposition players to possession and gain advantage in attacking and defensive situations. Hence players require regular exposure to high-speed running in training environment (13,14). Lower-limb injuries are associated with excessive high-speed running during training environments and the predominant mechanism of hamstring injury is high-speed running. Despite this the risk seems to be reduced when players have a well-developed aerobic fitness and chronic workloads (13). Players with a higher chronic training load were able to tolerate greater distances at maximum velocity with a reduced injury risk then those players with a lower chronic training load (14). Players who performed significantly more than their two-yearly average on the amount of high-speed running above (24km/h) in 4 weeks prior to injury had a greater risk of hamstring injury then players who did not. To reduce the build-up of fatigue over 4 weeks of increased high-speed running coaches can taper the athletes in the fourth week to offer a balance between injury prevention and performance (6). This may be hard for coaches to program in the current situation as they only have 3 weeks to retrain athletes before match play and will struggle to taper high-speed running loads when matches start. You would expect high-speed running loads to spike when matches start and as it would be hard to emulate the volume of high-speed match loads at training and during running programs completed in Coviid-19 break.
The process preceding an injury is a multifactorial and one factor alone cannot predict injury, it is important for practitioners to look at things such as workload spike, physical qualities, playing experience, and previous injury that might increase (or decrease) the likelihood of further injury (13). It has been found that high acute:chronic workloads ratio in rugby league has mediated injury risk for non-contact injuries, it has also been shown that high aerobic fitness has moderated the risk of injury in soccer and Gaelic football (13). This shows that each sport might have different factors that mitigate or increase injury risk such as in AFL it was found that acute:chronic ratio of distance at moderate speed running (18-24 km/h) was best predictor of injury risk (5). Lowerbody strength, faster speed and repeat sprint ability were also found as moderators of injury risk (13).
It would be expected that athletes had a large deviation in workloads during the Covid-19 break followed by an increase in workload at the return of training, so it would be expected that there would be an increased risk of injury upon return to train and play. Workloads need to be practically managed, so they mitigate as much injury risk, by staying as close to the acute:chronic ratio of 1 as practical while trying to obtain a training stimulus. The more aggressive the workloads the more attention that needs to be paid to previous injury, age, perceived muscle soreness, fatigue, mood, sleep ratings and psychological stressors (13). Athletes that have negative scores on any of these factors need to be monitored closely especially if they have below average aerobic fitness, lower-body strength, speed and repeat-sprint ability which have all been shown to mediate injury risk.
The lack of team training and competition can lead to variety of physiological parameters detraining, but as well as the physical parameters the ‘detraining’ of mindset and psychological factors need to be considered. This extended break can be compared with an injury layoff as players cannot compete in both situations, but it is unique that players are fully healthy, although they have been completing rehab like programs at home for approximately 8 weeks. As well as a change of routine they also have the stress that everyone is experiencing, to different degree, regarding the impacts of COVID-19 on normal life. A longitudinal study investigating competitive athletes return to sport after serious injury can help shed some light on athletes thought processes during their return. There has to be an awareness that physical and psychological readiness to return to sport are not synonymous and can be highly individual (17). There is a general excitement and anticipation with the return to training/competition, with the motives including achieving personal goals, loving the game, socializing with teammates and identity being restored (17). Fears and concerns about returning to training/competition could include ability to perform after a break, ability to withstand the rigors of the sport, lack of adequate preparation leading to injury and possibility of catching Covid-19 impacting their families, team and sport. Upon a return to play athletes described that the body was simply not used the bumping and grinding on football field and regaining “match fitness” was also a difficult hurdle to overcome for athletes. The Covid-19 break could lead to a better appreciation for sport and a renewed perspective of the importance of sport in our lives, as we have now experienced a world without it (17).
Practical Implications
- Athletes have been forced to alter their training in lockdown, as they were not allowed to use their usual facilities, have access to coaches physically and were only able to train with one other for a significant period.
- Fitness testing should be conducted upon the return from break to identify the reversibility of physiological parameters and guide retraining programs.
- Aerobic fitness, repeat sprint ability and linear speed is easier to train in lockdown then maximal strength, power and reactive agility due to equipment and training partner restrictions, so the latter components should be prioritised upon return.
- Athletes who had just completed preseason would have been at peak fitness making them highly susceptible to detraining during the break.
- Aerobic fitness is lost in 2-6 weeks of inadequate training but can be retrained quite rapidly in 2-3weeks and maintained with a reduced amount of high-intensity training.
- Detraining leads to a switch back to Type IIX fibres which could indicate the muscles could fatigue more quickly until retrained.
- Shortened preparation times can lead to more fatigue experienced during training and match play which increases the injury risk, due to altering biomechanics and the lack of ability of muscles to absorb force.
- Muscle strength and myofiber size decline following detraining but it is slower than the gains obtained through resistance training.
- The maintenance dose for strength in younger populations could be as little as 1/9th of volume. It is achievable to maintain strength and limit detraining during Covid-19 break with well-programmed and resourced at home training.
- Power gains are lost more rapidly than strength gains and the more well trained the athlete in muscular power the more susceptible to decreases in power output.
- Resistance training promotes tendon stiffness whereas detraining negatively affect it.
- Volume loads of return to play training need to be managed carefully, especially for susceptible groups such as rookies and those with previous injury.
- Acute:chronic workload ratio close to 1, higher overall chronic workload, higher aerobic fitness, more lower-body strength, faster speed, and greater repeat sprint ability are meditators of injury risk and should be used to identify athletes who are at greater risk of injury then those who aren’t and have their loads altered accordingly.
- Injuries are multifactorial and the relationship between workload spikes, physical qualities, playing experience and previous injury need to be taking into consideration when assessing future injury risk.
- The immediate task for strength and conditioning staff is to educate the coaches about the fine equilibrium between the intensity and volume of training and the injury risk. Every team will have a different strategic approach, but the cost benefit ratio of their training periodization approach needs to be taken into consideration. If the athletes are being overloaded due to short preparation times, strength and conditioning staff need look at wellness monitoring data closely to try to identify patterns that indicate possible injury.
- Physical readiness and psychological readiness are not equal, so psychological readiness needs to be paid adequate attention, to make sure players are well supported in their return to play.
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