Tuesday, December 27, 2011

Are children more intolerant to exercise in the heat compared with adults?

The view that children are more intolerant to exercise in the heat than adults is based on early studies of Drinkwater et al (1977) and Wagner et al (1972). In the first study, young and adult females were asked to walk until exhaustion at various ambient conditions: 28oC and 45% relative humidity (rh), 35oC and 65% rh and 48oC and 10% rh. Their findings showed that more women completed the exercise task (2 times of 50min) compared with girls. Their conclusion was that young girls were more intolerant to exercise in the heat than women. However, in this study as well as in that of Wagner et al (1972) exercise intensity was not strictly controlled so that girls exercised at higher relative intensity than women and this could explain their relative inability to complete the exercise bouts in the heat.

What do recent studies show?
Recent studies suggest that no difference exists between children and adults in exercise tolerance in the heat when exercise is performed at similar relative intensity. For instance, Rowland et al (2008) asked 12 year old boys and 32 years old men to exercise at 65% of maximal oxygen uptake in the heat (31oC). Increases in core temperature, heart rate and cardiac index were similar in both groups. Exercise duration was also similar in the two age groups. Recently, Bergeron and colleagues (2009) showed that physiological stain (core temperature and heart rate) did not differ between 12-13 years old and 16-17 years old boys doing prolonged exercise in the heat (33oC).

Are children at increased risk for heat illness?
The traditional view is that children are at an increased risk for heat illness (heat stroke and heat exhaustion) compared with adults. This is based mainly on two observations: 1) thermoregulatory mechanisms of heat dissipation are lower in children, and 2) incidence of heat stroke is higher in the pediatric population during periods of heat waves. With regard to the first point, recent evidence shows that thermoregulatory responses do not differ between young boys and girls and adults when exercising at the same relative intensity. Regarding the latter point, there are indeed reports of elevated risk for heat stroke in infants and small children (<4 years old) primarily due to parental negligence.

In fact, there is very limited if any report in the literature of heat illness while children exercise in the heat.

  • Recent scientific evidence shows that there are NO differences in exercise tolerance and physiological strain between children and adults when exercising at the same relative intensity in the heat.
  • This challenges the traditional view of higher physiological stress and more intolerance to exercise in prepubertal boys and girls.
  • It seems that child athletes are not more susceptible to heat injuries than adults.

For more reading
-Bergeron et al. (2009). Repeated-bout exercise in the heat in young athletes: physiological strain and perceptual responses. Journal of Applied Physiology 106:476-485.
-Drinkwater et al. (1977). Response of prepubertal girls and college women to work in the heat. Journal of Applied Physiology 43:1046-1053.
-Rowland et al. (2008). Exercise tolerance and thermoregulatory responses during cycling in boys and men. Medicine & Science in Sports & Exercise 40:282-287.
-Wagner et al. (1972). Heat tolerance and acclimatization to work in the heat in relation to age. Journal of Applied Physiology 33:616-622.

Wednesday, December 14, 2011

Do talented boys become elite adults?

In searching for future elite players, coaches are looking for young players who have the potential or talent and present the highest probability to become elite players in the future.

Don't stay on current performance
According to many studies in talents selection, coaches and other experts tend to focus on current performance. Indeed, it has been shown that players born early in the selection year are likely to be recognized as talented, transferred to higher level teams and consequently to receive higher quality coaching. The advantage of these young players, however, might be due to their advanced maturation level and not to their true potential. This kind of selection does not guarantee that these players will develop to elite performers since the current advantage will disappear at the age of 17-18 years. It seems that methods of selecting adolescent soccer players may not fulfill the intention to develop “talented future players”.

Biological vs chronological age
Biological age is one of the key factors in talents selection. Children with similar chronological age might differ in their biological age. Thus, children with early maturation and higher biological than chronological age will be stronger, faster and with higher endurance than their teammates of similar chronological age who are late maturers. Players that mature later may not lag behind their peers in maturation in the future. Thus coaches should be encouraged to consider not only present but also future performance.

To improve talent identification, sports science may contribute a lot. At first, current performance must be evaluated taking into consideration the biological age. One valid method of maturity assessment is the examination of the wrist-hand x-ray. By evaluating the bone age, sports scientists can calculate the skeletal and biological age. In addition, using appropriate models the height at 18 years of age can be estimated. Adult height prediction is an important part of talents selection since height is crucial for certain playing positions in modern football.  A more simplistic approach for maturity level estimation is based on anthropometric measurements. Besides selection, talent development should be evaluated bearing in mind the maturity stage of the player. For instance, at the period when the abrupt change in body height occurs performance will be negatively affected and this should be kept in mind in assessing players’ potential.

From this discussion, it appears that more attention should be given to individual biological maturation in the selection and development of young football players in order to avoid excluding players with delayed biological maturation who might have the potential to become elite players in the future.

How we treat early maturers?

It depends on his maturity stage. If let say he is 15 years old but his development has almost been completed we can apply more advanced training like strength training with external loads and anaerobic lactic acid training. One key aspect is his mental and “psychological” maturity. Early biological maturers may not present an advanced "psychological" maturity and thus may not be able to cope with more stress. To read a comment on this please visit British Medical Journal (Nassis G. Do talented boys become elite adults?  http://www.bmj.com/rapid-response/2011/11/02/do-talented-young-football-players-become-elite-adults).

George P Nassis, BSc, Dipl, MSc, PhD is a Sports Scientist-Exercise Physiologist, Head of Panathinaikos F.C Performance Lab and Lecturer of Applied Sports Physiology at the University of Athens, Greece. Part of his post-graduate studies has been conducted at Loughborough University, UK. He has published 20 papers in peer-reviewed international journals on basic and applied research. He is also working with elite athletes on physical conditioning.

Thursday, December 8, 2011

Low fat chocolate milk: is it an effective recovery drink for football players?

Current recommendations to improve rate of muscle glycogen resynthesis after exercise suggest consuming either 1.2 gr carbohydrate per kilogram of body weight (kg BW) per hour (h) for 2-4 hours after exhaustive exercise or 0.8 gr carbohydrate/ Kg BW/h in combination with amino acids or protein. Although solid and liquid supplements are suggested, liquids are more easily consumed by athletes due to exercise-induced appetite suppression in the hour(s) following training.

Recent research shows that low fat chocolate milk may be as effective as a carbohydrate-electrolyte beverage at speeding recovery after hard training. However, most research so far has been conducted on the cycle ergometer and this might limit the practical applications. To my knowledge, very limited research exists on football palyers. One interesting study was published few days ago in the Journal of Strength and Conditioning Research by Spaccarotella and Andzel from Kean University, USA.

What was their protocol?
13 young 19-20 years old football players completed their regular pre-season training in the morning and afternoon. Performance was assessed with the 20-m shuttle run test in the afternoon session. After the morning session, the players received either low fat chocolate milk or an equivalent volume of carbohydrate-electrolyte beverage.

Table. Composition of the beverages used (in 100ml).

Low fat chocolate milk
Energy (kcal)
Protein (g)
Fat (g)
Carbohydrate (g)
 Modified from Spaccarotella and Andzel (2011)

Main finding
Time to fatigue in the 20-m shuttle run test did not differ between conditions.

Practical applications
  • Low fat chocolate milk might be as effective as a carbohydrate-electrolyte solution at speeding recovery after a training session.
  • The benefits of low fat chocolate milk as a recovery drink are:
    • It contains large amounts of sodium
    • It is cheap
    • It can be easily found

Points to consider
Although this is an interesting area of research with obvious practical applications, readers should keep in mind that training load was not controlled in the morning session in the above mentioned study. Besides this, I think that the findings of Spaccarotella and Andzel’s study add to current knowledge. Future research should examine the effect of this recovery drink under more controlled conditions.

Related posts in this blog

OMEGA-3 FATTY ACIDS SUPPLEMENTATION TO FACILITATE RECOVERY AND ANABOLISM? http://georgenassis.blogspot.com/2011/07/omega-3-fatty-acids-supplementation-to.html


Tuesday, December 6, 2011

Effect of whole-body vibration on subsequent sprint performance

Whole-body vibration (WBV) intervention is a common practice in training and before game in some clubs as a means to improve anaerobic performance. What is the evidence behind WBV use? Does this intervention improve performance?

Whole-body vibration before sprinting
In a recent study, Ronnestad and Ellefsen (2011) had their soccer players perform either 30sec half-squat with WBV at 30 Hz or half squat without WBV 1min before a 40-m run. Their results showed that performance was significantly improved by almost 1% after WBV. Interestingly, no change in performance was observed when WBV frequency was 50Hz.

What this study shows?
1.    Sprint performance might improve when combining half squat with WBV for 30sec.
2.    Optimal frequency for this intervention seems to be 30Hz.

Unanswered question
  • How long does this beneficial effect of WBV last? Ronnestad and Ellefsen (2011) had their soccer players sprint 1 min after the intervention so we do not know what would happen if the sprint was performed after 5, 10 or 20 min which is a more realistic scenario in football.

What is the ideal combination of WBV frequency and duration?
This was the research question set by Da Silva-Grigoletto and colleagues (2011) in a group of health but non well-trained volunteers. In their study, individuals performed half squat with WBV frequency at 30 Hz. Duration varied from 30 to 90 sec. Also number of sets was 3, 6 or 9. WBV with half squat were performed before countermovement and squat jump and power output test at half squat.

Main findings
  1. WBV for 60 sec produced the best results.
  2. Number of sets affected subsequent performance. Performing six 60-sec sets of WBV resulted in greater improvement in power output in subsequent testing.
It is important to keep in mind that these results were in non well-trained athletes. Thus, we do not know if they directly apply to well-trained football players. 

Wednesday, November 30, 2011

Does injury rate differ when playing football on artificial turf?

Recent evidence on this issue has been provided by Ekstrand, Hagglund and Fuller from Linkoping University, Sweden and the University of Nottingham, UK.  Twenty five elite teams (15 male and 5 female) playing on 3rd generation artificial turf were followed and their injury risk was compared with the risk when playing on grass. 

The results showed that injuries rate did not differ between artificial turf and grass for both men and women. Study has been published in the December 2011 issue of the Scandinavian Journal of Medicine & Science in Sports.

Monday, November 28, 2011

Quercetin supplementation: does it enhance performance?

Quercetin is classified as a flavonoid, a substance found in considerable amounts in fruits and vegetables. Basic research has showed that quercetin may act as antioxidant and anti-inflammatory. Recently, two more roles of quercetin ingestion on human body have been investigated: 1) its biological role in alleviating the negative effects of intense training on immune system, 2) its effect in enhancing sports performance.

Does quercetin supplementation enhance performance?
Claims on the beneficial effect of quercetin supplementation in performance are based on the fact that quercetin has been shown to enhance the number of mitochondria in animal studies. Let me remind you that mitochondria are the main sites for aerobic energy production. What is the truth? Although studies showed elevated mitochondrial biogenesis with quercetin supplementation in animals, no such strong evidence exists in humans.

Besides mitochondrial biogenesis, quercetin may also act with a mechanism similar to caffeine. Again, no such strong evidence exists in humans. Some studies have showed performance improvements after 1-3 weeks of quercetin supplementation in untrained subjects. However, most studies with elite and trained subjects failed to show improved performance with quercetin supplementation. One explanation might be that trained subjects have already maximized their mitochondrial aerobic capacity with training.

 In the December 2011 issue of the Medicine & Science in Sports & Exercise, Kressler and colleagues published a meta-analysis on the effect of quercetin ingestion on endurance capacity. Their conclusion was that “On average, quercetin provides a statistically significant benefit in human endurance capacity (VO2max and endurance exercise performance), but the effect is between trivial and small.

To summarize, there is no strong evidence that quercetin supplementation enhances endurance performance in well-trained athletes.

Can it be used as a means to prevent illness after intense training?
Although evidence is limited in humans, it seems that quercetin supplementation can strengthen some aspects of the immune system and thus protect athletes from some illnesses during periods of intense training.

Current use
Quercetin is available as a sports supplement claimed to enhance performance. Most studies involve supplementation with 1,000 mg daily, distributed in 2 doses. Typical supplementation period is 1-3 weeks but it may extend up to 6 weeks.

For more reading
  • ·     Kressler et al. Quercetin and endurance exercise capacity: a systematic review and meta-analysis. Med Sci Sports Exerc. 43:2396-2404, 2011.
  • ·     Nieman et al. Quercetin reduces illness but not immune perturbations after intensive exercise. Med Sci Sports Exerc. 39:1561-9, 2007.
  • ·     Nieman et al. Quercetin's influence on exercise performance and muscle mitochondrial biogenesis. Med Sci Sports Exerc. 42:338-45, 2010.
  • ·     Rodriguez et al. American Dietetic Association; Dietitians of Canada; American College of Sports Medicine. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. J Am Diet Assoc. 109:509-27, 2009.

Sunday, November 20, 2011

How to improve first steps speed in players-Part 2

Assisted or resisted sprint training? Which is the most effective in improving acceleration?  In a recent study, Dr Upton from Texas Christian University evaluated the effect of a 4-week training program with assisted, resisted or traditional sprint training on performance indices. Athletes, female soccer players separated in 3 groups, performed 10 maximal effort sprints of 18.3 m (20yd) with their training program. Training was performed 3 times per week.

Training program characteristics are shown in table 1.

Table 1. Training program characteristics.

Assisted Sprint Training
Resisted Sprint Training
Traditional Sprint Training
Assistive/Resistive force
14.7% of body mass
12.6% of body mass
Sprint distance (m)
Deceleration jog distance (m)
Rest between reps (min)
Rest between training sessions (hrs)

Main findings
  1. Acceleration (0-4.6m) was improved by 19% in the ASSISTED Sprint Training group.
  2. No changes were observed in the other 2 groups.

Take home message
Although this study was in female soccer players, findings suggest that assisted sprint training is the most effective means, compared with resisted and traditional sprint training, to improve first steps speed. In my opinion, this type of training could be supplemented with plyometrics to maximize the benefits (http://georgenassis.blogspot.com/2011/10/effect-of-plyometric-training-on-first.html).

Wednesday, November 16, 2011

Young high-level players present fast recovery after a game

This is the conclusion of a study published in this month issue of the Medicine & Science in Sports & Exercise by Rampinini and his colleagues.

What they did?
Twenty two 19 yr old professional players from an Italian Serie A team were tested for sprint, strength and short-passing ability before, immediately after, 24 h and 48 h post a 90min friendly match. Total distance (TD) and high intensity distance (HID) covered during the game were 11.764m and 2664m, respectively. TD and HID were lower in the 2nd than in the 1st half. The mean heart rate was 88% and 82,5% of maximal in the 1st and 2nd half, respectively.

What did they find?
  • Legs maximum strength and 40m run time recovered to pre-game values in 48 h of recovery.
  • Short-passing ability was not affected by fatigue and was similar immediately post-match compared with the pre-game values.

Practical applications
  1. Match fatigue may not affect short-passing ability, at least in high-level players.
  2. Certain fitness parameters may return to pre-game values within 48 h after the match. This finding might indicate a faster recovery rate in young compared with adult players. I am not aware of such data in the literature and this is an interesting question for further investigation.
  3. Young high-level players may be ready for intense training 48 h after a game.
  4. High-level player of this age might be able to cope with 2 games per week.

One more thing. We do not know whether there is a cumulative effect on fatigue with several games played every 3-4 days, as the authors also state in their paper.

Sunday, November 13, 2011

Are there regional differences in injury rate and severity in European top-level football teams?

Walden and colleagues from Linkoping University, Sweden, investigated regional differences in injury incidence in men’s football teams in Europe. Their study was recently published in the Scandinavian Journal of Medicine and Science in Sports. A total of 26 top-level European football clubs were followed at various periods between 2001 and 2010. The different regions were categorized according to the climate type using an international climate classification system (Koppen-Geiger climate classification system).

The clubs in the northern and southern Europe group included in this study appear below.

Northern Europe group
  • AC Milan
  • AFC Ajax
  • Arsenal FC
  • BVB Dortmund
  • Chelsea FC
  • Club Brugge KV
  • FC Bayern Munchen
  • FC Inter
  • Hamburger SV
  • Juventus FC
  • Liverpool FC
  • Manchester United FC
  • Newcastle United FC
  • Olympique Lyonnais
  • Paris St Germain FC
  • PSV Eindhoven
  • Rangers FC
  • RC Lens
  • RSC Anderlecht
  • Stade Rennais FC

Southern Europe group
  • AFC Fiorentina
  • FC Barcelona
  • FC Porto
  • Real Madrid CF
  • SL Benfica

Injury was defined as “any physical complaint sustained by a player that results from a football training or match and leads to the player being unable to participate in future football training or match play” (Walden et al., 2011).

Main findings
  1. Overall, injury incidence was significantly higher for the northern group teams.
  2. Injury severity was also significantly higher for the northern group teams.
  3. Upper body, trunk and Achilles tendinopathy were more common in the northern group teams.
  4. No difference in injury rate was detected for hip/groin, thigh, knee and ankle.
  5. Anterior cruciate ligament (ACL) injuries rate was significantly higher in the southern group.

The authors concluded that there were regional differences in injuries rate in top-level teams in Europe. The higher overall injury rate in northern group was attributed to several factors, such as tactics, playing style and playing intensity. One possible explanation for the higher ACL injury incidence in southern part clubs was higher shoe-surface traction because of the warm weather, according to the researchers. 

Sunday, November 6, 2011

Effect of high intensity aerobic interval training with the ball on aerobic fitness

Previous studies have shown that maximum oxygen uptake (aerobic fitness) can be improved with high intensity interval running in football players. Running intensity should be 90-95% of maximum heart rate during this type of training and the bouts should last 4 minutes.

Can we do the same while dribbling a soccer ball?
In a study by McMillan et al (2005), 11 17-year old football players supplemented their regular football training with high intensity aerobic training for 10 weeks. Aerobic training consisted of 4 4-min high intensity periods by dribbling the ball in a football specific track. Heart rate was 90-95% of maximal during these periods. High intensity periods were separated by 3-min jogging at 70% of maximum heart rate.  This training was performed 2 times a week at the end of the session.

Figure. Soccer specific dribbling track used for high intensity interval training sessions (Chamari et al., BJSM 2005).

Main finding
  • Aerobic fitness improved by about 10% in these players.
  • This is a big improvement considering the high initial value for maximum oxygen uptake in these players (63.4 ml/kg/min).

Take home messages
  1. It seems that maximum oxygen uptake can be improved with high intensity interval training while dribbling the ball in high level players.
  2. Heart rate should be 90-95% of maximum in this training and this requires high technical skills.
  3. Accordingly, I think that this is an effective training for high level players. However, the effectiveness of this training in improving aerobic fitness in lower level players should be examined.

McMillan et al (2005). Physiological adaptations to soccer specific endurance training in professional youth soccer players. Br J Sports Med 39(5): 273–277.

Wednesday, November 2, 2011


As you may know, hamstring muscle injury is the most frequent one in football with its prevalence ranging from 12% to 16% of all injuries. The incidence of this type of injury is 0.5-1.5 per 1000 hours of football match and training. It seems also that the risk of hamstring muscle injury re-occurrence is very high. Indeed, re-injury rate is around 22% within the next 2 months of the first injury.

The majority of the prevention programs concentrate on the eccentric strengthening of the hamstring muscles. The Nordic hamstring exercise is a simple eccentric hamstring strength exercise that is being used in a number of prevention routines. However, there is a limited number of randomized controlled studies examining the effect of eccentric muscle strengthening on hamstring injury rate in football players.

One such study was published in the November issue of the American Journal of Sports Medicine by Dr Petersen and colleagues (Petersen et al., 2011, pages 2296-2303). 942 professional and amateur football players were randomly allocated into 2 groups: the intervention and the control group. The intervention group trained with the Nordic hamstring exercise for 10 weeks whereas the control group followed only the regular team’s training. Eccentric strength training was performed once per week in the first week, 2 times in the 2nd week and 3 times per week for the weeks 3-10. The team coach decided on the time of these exercises in the training session. The mean injury registration period for these players was 318 days.

The Nordic hamstring exercise
  • Start in a kneeling position
  • The partner pushes the lower leg or heels of the exercising player to keep them in contact with the ground
  • The exercising player resists the forward movement of his torso using his hamstring muscles for as long as possible
  • The player uses arms and hands to slow down before contact with the ground
  • Immediately after touching the ground with chest, the athlete pushes with his hands to get back to the starting position

Main findings
  1. A total of 67 acute hamstring injuries were registered with 15 injuries in the intervention and 52 injuries in the control group
  2. Injury rate of new injuries was 60% lower in the eccentric training group compared with the control
  3. Rate of recurrent injuries was reduced by about 85% in the intervention group
  4. No injuries occurred in performing the Nordic hamstring exercise

Take home messages
  • It is possible to reduce the incidence of hamstring muscle injury in professional and amateur football players by supplementing their regular training with a simple exercise that focuses on increasing eccentric hamstring muscle strength.
  • This training must be performed 1-3 times per week for a minimum of 10 weeks. Coaches decide on the time of this exercise in the training session.
  • The Nordic exercise seems to be a safe method for improving muscle-specific strength.

Saturday, October 29, 2011

The science of happiness

by Harvard psychologist Professor Dan Gilbert on TED

Chronic co-ingestion of L-carnitine and carbohydrate improves performance

Carnitine is a molecule involved in fatty acid translocation in the mitochondria (Figure 2 below). Theoretically, increased carnitine levels within the muscle would increase fatty acid metabolism thus sparing muscle glycogen. Glycogen sparing will improve endurance performance. However, research over the past 30 year was unsuccessful in elevating muscle carnitine levels with oral carnitine ingestion.

What’s new?
Few months ago a research group from the University of Nottingham Medical School, UK, showed elevated muscle carnitine levels after chronic carnitine ingestion in healthy subjects (Wall et al., 2011). Subjects were divided into two groups: a) the carnitine group, ingested 80g of carbohydrate (CHO) containing 2 gr of L-carnitine tartrate for 5,5 months, and b) the control group that received only CHO for the same period. Supplements were ingested at breakfast and 4 hours later. Physiological measurements and performance test took place before and at the end of the supplementation period.

The main findings were
  • 21% increase in muscle carnitine level with oral ingestion
  • lower muscle glycogen utilization during submaximal exercise in the carnitine group
  • 35% improvement in the performance test in the carnitine group (Performance was defined as total work output in 30-min cycling in the laboratory).

Although this is a laboratory study, the findings suggest that chronic co-ingestion of L-carnitine and carbohydrate may improve endurance performance. This is the first study to show increased muscle carnitine levels with oral supplementation. To my opinion, these findings have important implications for athletic performance.


Wall et al (2011). Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. Journal of Physiology 589: 963-973.

Monday, October 24, 2011

Post-training muscle cooling may attenuate training effects

Today, I would like to comment more on a point I made few weeks ago in a post on cryotherapy after training (http://georgenassis.blogspot.com/2011/07/some-thoughts-on-use-of-water-immersion.html). Please let me remind you that cryotherapy is used by many athletes as a means to minimize muscle damage and muscle soreness and thus speed up recovery after hard training and game. In my latest post I set the hypothesis that cryotherapy-induced reduction in muscle damage and inflammation might suppress training adaptations.

One of the blog reader emailed  me the paper by Yamane et al (1996) published in European Journal of Applied Physiology. In their study, researchers had their subjects cool one leg and arm with water immersion after training whereas the other limb was the control (no cooling). One group performed arm strength training and the other endurance cycling training. Training was performed 3-4 times per week for 4-6 weeks.

Did cryotherapy improve performance more that the control?
No. In this study, control and cooled legs improved endurance performance time at similar degree. However, maximal oxygen uptake was improved only in the control leg. In addition, maximal arm strength was improved at a significantly greater magnitude in the control than in the cooled arms group.

Take-home message
I am not saying that cryotherapy is not good for recovery. There are a lot of data in the literature suggesting that cryotherapy might speed up, at least, the perception of effort by athletes. If you feel good you perform well in the next session. If you perform well you gain more adaptations. However, from this study it appears that the effect of cryotherapy on LONG-TERM training-induced adaptations requires further investigation.

Yamane et al. (2006). Post-exercise leg and forearm flexor muscle cooling in humans attenuates endurance and resistance training effects on muscle performance and on circulatory adaptations. Eur J Appl Physiol 96: 572-580.