Saturday, August 30, 2014

Halftime, Carbohydrates and a Re-Warm Up

Halftime of a soccer match is a short break between two periods of intense activity. It’s also a time for players to recover from the first period, re-hydrate, take in a few carbohydrates and a receive feedback and instructions from their coach. All squeezed into a 10-15 minute break.  As such, different coaches may approach halftime differently.  What is the best strategy for halftime?  What should players do to recovery quickly and prepare themselves for the next period of play?  Two recent studies shed some light on these questions.  They give some insight on diet, hydration and physical activity practices that may help players maximize their second-half performance.

In the first study, Dr. Mark Russell and his colleagues looked at how carbohydrate ingestion during a match (sports drink) affects blood glucose (Russell et al., 2014).  They found that drinking 150-200 ml (5-7 oz) before and every 15 minutes during the match and at halftime had a positive effect on blood glucose.  But, this effect was found only at the end of the first period.  During the second period, when blood glucose levels begin to wane, the carbohydrate drink had no effect.

One of the more interesting aspects of the study is the finding that blood glucose declined during the halftime break, despite the fact that the players drank a carbohydrate beverage five minutes into the 15-minute break.  Thus, the players started the second period with fairly low blood glucose levels – not considered hypoglycemic, but close.  Low blood glucose at the start of the second period could have a several negative impacts on their performance in the initial minutes.  Slow, lethargic and fewer high intensity efforts are characteristic of low blood glucose.

Blood glucose during exercise and recovery is influenced by a number of factors.  Insulin is a potent hormone that is released in response to high blood glucose.  This promotes glucose uptake by fat and liver cells. Epinephrine (adrenalin) has the opposite effect, it stimulates the liver to break down its glycogen and release it into the blood as glucose.  Cortisol has a similar effect.  During exercise, especially intense exercise, epinephrine and cortisol levels in the blood are increased.  In addition, insulin release is suppressed.  This combination stimulates glucose release from the liver and prevents its uptake by fat cells.  Thus, blood glucose levels are elevated.  This provides the muscle with an ample supply of carbohydrate for its energy needs.  In addition, hypoglycemia is prevented.

For the most part, the body views halftime as recovery.  Things change during recovery.  Within minutes of stopping exercise, epinephrine and cortisol levels subside and insulin levels rise.  This stimulates glucose to move into the live (and muscle) to be stored as glycogen.  When a carbohydrate sports drink is taken in and glucose enters the blood, the insulin response may be greater.  This in turn, could lead to greater glucose uptake by the liver and reduced blood glucose or a “transient hypoglycemia”.  This may be what happened during the halftime period of Dr. Russell’s study.  In this case, the carbohydrate drink per se did not cause hypoglycemia, but it did not prevent the drop in glucose during recovery.

So, what should a player to do during halftime – drink carbohydrates or not?  How can an athlete re-hydrate and replenish energy stores without sacrificing performance.  Some insight might be gained from the second study.   

This study focused on the benefits of a re-warm up during halftime (Edholm et al., 2014).  During the 15-minute halftime of a competitive match, the researchers asked one-half of the players to take on a traditional passive recovery (control group).  The other players were asked to rest for 7 minutes then do some low to moderate intensity jogging and calisthenics for the remaining 7 minutes (re-warm up group).  In a second match, the players reversed their halftime strategy.

The researchers found that sprint and vertical jump performance declined during the first half.  This was expected.  However, during the 15-minute halftime interval, performance further declined in the control group but not in the re-warm up group.  Thus, the re-warm up group was better prepared for the second period. Also, the re-warm up group maintained greater ball possession and had less defensive running than the control group.  Thus a 7-minute, active re-warm up during halftime benefited player performance.  Other studies have shown similar benefits of a re-warm up.  In terms of match performance, it’s easy to envision how being physically ready to compete at the start of the second half could lead to a fast start and goal scoring opportunities.  

One concern of an active re-warm up is that the amount of energy expended could impact performance later in the match.  The researchers (and others) found that this is not the case.  Seven minutes of low to moderate intensity activity had no negative impact on markers of fatigue such as running distance and intensity or exercising heart rate in the later stages of a match.

So, what do these two studies offer for a halftime strategy?  Instead of having players enter “recovery mode” during halftime, they simply transition to and from a period of light activity.  A key benefit of this approach to halftime might be suppression of insulin and maintenance of epinephrine.  Combined, this could help the player maintain blood glucose levels, especially if he/she is taking in a carbohydrate beverage.  Thus, the low blood glucose state that Dr. Russell found might be prevented by an active re-warm up.  It should be pointed out that there is no research on combining a re-warm up AND sports drinks during halftime.  So, this idea remains a bit untested.

As for a halftime strategy…  First, blood glucose levels decline during the halftime recovery and this could affect performance at the start of the next period.  Second, a halftime re-warm up seems to benefit performance, particularly at the start of the second period without negatively affecting the later stages of the match.  It is possible that this effect is due, in part to maintaining blood glucose and avoiding the negative effects of hypoglycemia.  Based on this, coaches should consider the following strategy.  Provide the players with a carbohydrate beverage at the start of halftime.  Have them rest and recover for the first few minutes while they begin drinking.  Then, begin a low intensity re-warm up while they finish their drink.  This should better prepare them for the second half compared to the traditional passive halftime.  They should be ready for a fast start once the whistle blows.  However, as with any new strategy, it’s always best to test this out on players during training and scrimmage matches.  Sometimes, the theory doesn’t always translate into reality!    

References:

Russell M, Benton D, Kingsley M (2014) Carbohydrate ingestion before and during soccer match play and blood glucose and lactate concentrations, Journal of Athletic Training, 49(4):447-453.

Edholm P, Krustrup P, Randers MB (2014) Half-time re-warm up increases performance capacity in male elite soccer players, Scandinavian Journal of Medicine and Science in Sports, in press, doi: 10.1111/sms.1223