Humans have an innate ability to survive in extremes of heat and cold. Furthermore, humans also have the ability to exercise in such extremes. However, exercise in the heat is arguably one of the most common and difficult challenges that face competitive athletes. Humans are homeotherms (i.e., body temperature functions independently of the surrounding environmental temperature and must be maintained within a fairly narrow range). 

However, under conditions of high heat and humidity, the challenge to maintain the core temperature between 36.5–37.5^oC is difficult and potentially life threatening. It is of fundamental importance for strength and conditioning coaches and athletes to understand the challenges and the limitations of exercise in the heat and, above all, know how to recognize early signs of heat stress to allow timely and safe responses. 

The continued deaths of athletes due to heat-related stresses reinforce the fact that current knowledge and practices are still inadequate. A continued focus on heat stress deaths, specifically sickle cell-related incidences, increases the urgency and need for continued education in this area.

Recent research reported that in the last decade 16 deaths have occurred from exercise conditioning for American football (1,3). According to this research, these 16 deaths occurred specifically from conditioning and not from actual playing in a game situation (1). 

Ten of these 16 deaths were attributed to sickle cell trait (SCT). With appropriate education and management, those 10 deaths may have been preventable. The Center for Disease Control estimates that over 7.5 million students participate regularly in high school sports and data from the National High School Sports Related Injury Surveillance Study for the period 2005–2009, shows that 118 heat illnesses were reported among the 100 schools sampled (2). 

This equates to 116 per 100,000 athletes exposed. Moreover, when the data is viewed specifically for American football, the incidence of illness increases to 4.5 per 100,000 athletes exposed (2). This represents an almost 10 times higher incidence than other reported sports. Sickle cell trait is the most prevalent condition in American football-related heat stress injuries (1,2).

What is Sickle Cell Trait?  

Sickle cell trait is a condition in which an individual inherits a normal (Ha) and an abnormal gene (Hs) for hemoglobin (Has) as opposed to two normal hemoglobin genes (Haa). Hemoglobin “Ha” is a normal functioning hemoglobin molecule, whereas hemoglobin “Hs” is an abnormal molecule. The Hs molecules change normally round, disc shaped, red blood cells into curved, or “sickle” shaped cells. 

These cells have an impaired ability to the flow and transportation of oxygen, and hence the term “logjam” is often used to describe their clotting or bottle-necking action within the circulatory system. This can lead to rhabdomyolysis, a condition where damaged blood cells release a toxic substance, myoglobin, which can cause kidney failure, and ultimately, death. It is important to note that the presence of SCT doesn’t necessarily mean these events will occur. However, there is an increased likelihood that they can occur. Individuals of African American descent are most at risk for SCT, as SCT occurs in 8–10% of the African American population versus less than .0002% in the Caucasian population (2).

Causes of Sickling 

The obvious cause of sickling is the presence of hemoglobin “Hs.” However, other conditions often trigger catastrophic sickling and education in this area is crucial for all types of sports personnel (e.g., coaches, athletes, and trainers). The most common cause of severe sickling in athletes is exertional related, high-intensity exercise (3). When coupled with high ambient temperature, this exercise causes significant increases in body temperature which can cause sickling (1,3).

Preventing Athlete Complications 

The most effective approaches to preventing collapse and sickling are proactive and merely require the coach, athlete, trainer, etc. to be educated and knowledgeable in advance. Here are some guidelines to help coaches (1,3):

1. Know the SCT status of the athlete—this will allow for closer monitoring of the individual athlete.
2. Keep a log of the athlete’s normal physiological responses to the exercise routine—this will help determine an atypical response.
3. Allow identified “at-risk” athletes slightly longer recovery times between maximal or near maximal efforts and modify exercises that place such demands on the athletes.
4. Monitor hydration and be sure “at-risk” athletes are appropriately hydrated—this will help regulate core temperature.
5. Avoid punishment exercises or “toughness” in presenting workouts. Instead, preach sensibility and being realistic about effort.
6. Be intuitive so when an otherwise healthy and fit athlete complains of fatigue, breathlessness, or is generally struggling to complete a routine set of exercises, you can act immediately and appropriately.
7. Allow identified “at-risk” athletes longer warm-up or build-up times to maximal bouts to allow appropriate circulatory adjustment.
8. Advocate for a slow and progressive off-season and pre-season conditioning program especially if it conducted in a hot environment.
9. Refrain from having an athlete exercise if they are sick, especially with any condition involving a fever.
10. Communicate constantly with medical personnel and be adequately prepared to deal with emergencies.
11. Have a specific emergency protocol in place to deal with heat stress.

 Treatment Response to Suspected Heat Stress Sickling 

• Stop all exercise immediately and call emergency help
• Remove athlete from heat/sunlight immediately
• Cool down athlete (Having an ice bath present would be a practical precaution)
• Remove excessive clothing, padding, helmet, shirt, etc.
• Check vital signs and refer to qualified emergency professionals if necessary

Sickling collapse and death can be avoided. Coaches and athletes must recognize the severity of the condition and then educate themselves to recognize the early signs and symptoms of this heat stress. An emergency medical plan specific to SCT is advised in warmer climates and where there is an increased “at-risk” population. Note that the presence of SCT is not reason to have a “white gloves” or “soft” approach to conditioning athletes. It is simply a more realistic and safer approach to maximizing an athlete’s potential without putting them at undue risk. 

References 

1. Anzalone, ML, Green, VS, Buja, M, Sanchez, LA, Harrykissoon, RI, and Eichner, R. Sickle cell trait and fatal rhabdomyolysis in football training: A case study. Medicine and Science in Sports and Exercise 42(1): 3–7, 2010.
2. Centers for Disease Control and Prevention (CDC). Heat illness among high school athletes. MMWR Morb Mortal Wkly 59(32): 1009–1013, 2009.
3. Eichner, ER. Sickle cell trait in sports. Current Sports Med 9(6): 347–351, 2010.

This article is from the NSCA's Performance Training Journal 11.2. Gain access to more journal articles with an NSCA membership. 

About the Author

Declan Connolly is a Professor at the University of Vermont, and Director of the Human Performance Laboratory. Dr. Connolly has published over 350 articles in sports performance, muscle adaptation, and injury with training. He has published one book ‘The Science of Heart Rate Training’, is currently writing another on ’Basic and Applied Kinesiology”. He is a past President of the New England American College of Sports Medicine. He is past Physiology Section Editor of Research Quarterly for Sports & Exercise, a Fellow of the American College of Sports Medicine, and a Certified Strength and Conditioning Specialist. He is also an Assistant Editor for the Journal of Sports Medicine and Physical Fitness, an Associate Editor for Journal of Strength and Conditioning Research and Director of Certification for the Collegiate Strength and Conditioning Coaches Association.