EFFECTS OF GENDER, AGE, CIRCADIAN RHYTHMS, AND SLEEP LOSS ON THERMAL RESPONSES DURING EXERCISE

1. EFFECTS OF GENDER, AGE, CIRCADIAN RHYTHMS, AND SLEEP LOSS ON THERMAL RESPONSES DURING EXERCISE

2. REVIEW • HUMAN ORGANISM IS A REGULATING ORGANISM GOVERNED BY A PROPORTIONAL CONTROL SYSTEM: GRADED RESPONSE TO A SIGNAL WHICH INCREASES OR DECREASES IN PROPORTION TO THE INTENSITY OF THE STIMULUS.

3. REVIEW • THRESHOLD - TEMPERATURE ABOVE WHICH OR BELOW WHICH EFFECTOR RESPONSE IS DIFFERENT FROM THE BASELINE RESPONSE AT REST. • SLOPE (GAIN) OR THERMOSENSITIVITY - DIFFERENCE IN EFFECTOR RESPONSE PER UNIT OF CHANGE IN CORE TEMPERATURE.

5. GENDER DIFFERENCES IN THERMOREGULATION

8. GENDER DIFFERENCES RELATED TO THE MENSTRUAL CYCLE

9. FOLLICULR PHASE • FOLLICLE STIMULATING HORMONE (FSH) OF ANTERIOR PITUITARY STIMULATES DEVELOPMENT OF FOLLICLES (OVA), WHICH SECRETE INCREASING AMOUNTS OF ESTROGEN AND EVENTUALLY SMALL AMOUNTS OF PROGESTERONE.

10. INCREASED ESTROGEN STIMULATES: • PROLIFERATION OF UTERUS ENDOMETRIUM (MUCOUS MEMBRANCE OF UTERUS).

11. INCREASED ESTROGEN STIMULATES: • SECRETION OF LHRF FROM HYPOTHALAMUS, WHICH STIMULATES INCREASED RELEASE OF LH AND FSH (13TH-15TH DAY) FROM ANTERIOR PITUITARY; INCREASED LH LEVELS STIMULATE FOLLICLES TO MATURE AND BREAK THROUGH THE OVARIAN WALL 14-24 HOURS AFTER THE LH SURGE (OVULATION).

12. LUTEAL PHASE • SUPPORTING STRUCTURE OF FOLLICLES FORM CORPUS LUTEUM (YELLOW GLANDULAR MASS IN OVARY FORMED BY THE OVARIAN FOLLICLES), WHICH STIMULATES PROGESTERONE SECRETION THAT INDUCES SECRETORY CHANGES IN THE UTERUS. • LH AND FSH SECRETION DECREASES.

13. IF FERTILIZATION DOES NOT OCCUR, THE CORPUS LUTEUM DEGENERATES RESULTING IN DECREASED PROGESTERONE SECRETION, WHICH LEADS TO ENDOMETRIUM DEGENERATION AND MENES BEGINS.

14. NOTE: THERE ARE HIGHER CORE TEMPERATURES (+0.4oC) DURING THE LUTEAL PHASE OF THE MENESTRUAL CYCLE WHEN PROGESTERONE LEVELS ARE ELEVATED. ALSO, DURING THE ENTIRE MENSTRUAL CYCLE, CORE TEMPERATURES ARE HIGHER THE PM THAN DURING THE EARLY AM.

15. ELEVATION OF CORE TEMPERATURE OF ABOUT 0.4o C DURING LUTEAL PHASE EFFECTS: THERMOREGULATORY SET POINT IN TWO WAYS: • EARLIER ONSET OF SHIVERING AND HEAT PRODUCTION (I.E., HEAT CONSERVING MECHANISMS). • INCREASED CORE TEMPERATURE THRESHOLD FOR ONSET OF HEAT LOSS MECHANISMS SUCH AS SWEATING AND CUTANEOUS VASODILATION. HEAT LOSS MECHANISMS DO NOT BEGIN UNTIL HIGHER CORE TEMPERATURE IS REACHED.

17. NOTE: PARALLEL CHANGES IN SUDOMOTOR (SWEATING) AND VASOMOTOR (VASODILATION) RESPONSES INDICATES THAT THERE IS A CENTRAL ALTERATION IN THERMOREGULATORY CONTROL WITHIN THE HYPOTHALAMUS.

18. •ELEVATION OF CORE TEMPERATURE OF ABOUT 0.4o C DURING LUTEAL PHASE INCREASES THE CORE TEMPERATURE AT WHICH THERMAL COMFORT IS PERCEIVED, WHICH MAY IN PART BE RELATED TO THE SLIGHTLY HIGHER SKIN TEMPERATURES OBSERVED DURING THE LUTEAL PHASE.

19. WORK-HEAT TOLERANCE

20. BEFORE HEAT ADAPTATION, HEAT STORAGE IS INCREASED DURING THE LUTEAL PHASE DUE TO DELAYED ONSET OF SWEATING AS THE THRESHOLD FOR ONSET OF SWEATING IS INCREASED; ALSO, SWEATING SENSITIVITY MAY ALSO BE SUPPRESSED DUE TO: • INCREASED EFFECTS OF HIDROMEIOSIS (??). • DECREASED SHIFT OF FLUID OUT OF THE VASCULAR COMPARTMENT AS HEMOCONCENTRATION OCCURS LESS RAPIDLY IN THE LUTEAL PHASE.

21. AFTER HEAT ADAPTATION, MENSTRUAL CYCLE HAS MINIMAL EFFECTS ON THE WORK-HEAT TOLERANCE OF WOMEN DURING EITHER THE FOLLICULAR OR LUTEAL PHASE; HEAT ADAPTATION BRINGS ON FASTER SWEATING RESPONSE BY DECREASING THE CORE TEMPERATURE THRESHOLD FOR THE ONSET OF SWEATING; SWEATING SENSITIVITY MAY ALSO BE IMPROVED BUT IT DOES NOT FURTHER INCREASE SENSITIVITY INDUCED BY TRAINING. HEAT ADAPTATION ALSO DECREASES THE THRESHOLD FOR THE ONSET OF SKIN (CUTANEOUS) BLOOD FLOW AND INCREASES THE SENSITIVITY OF THE SKIN BLOOD FLOW RESPONSE.

22. • ALSO, TRAINING DECREASES THE THRESHOLD FOR THE ONSET OF SWEATING AND INCREASES THE SENSITIVITY OF THE SWEAT RATE RESPONSE. TRAINING ALSO DECREASES THE THRESHOLD FOR THE ONSET OF SKIN (CUTANEOUS) BLOOD FLOW.

24. GENDER DIFFERENCES

25. PERFORMANCE OF LOW INTENSITY EXERCISE IN BOTH DRY AND WET ENVIRONMENTAL CONDITIONS ELICITS QUITE SIMILAR RESPONSES IN BOTH FEMALES AND MALES, PARTICULARLY IF FACTORS SUCH AS BODY SURFACE AREA, FITNESS LEVEL, BODY COMPOSITION, BODY SIZE, AND MENSTRUAL CYCLE PHASES ARE CONTROLLED.

26. HOWEVER, DURING THE LUTEAL PHASE OF THE MENSTRUAL CYCLE WHEN CORE TEMPERATURE IS ELEVATED AND HEAT STORAGE IS INCREASED, PERFORMANCE MAY BE LIMITED DURING HIGH INTENSITY PERFORMANCE UNDER CERTAIN HYPERTHERMIC CONDITIONS.

27. CARDIORESPIRATORY FITNESS LEVEL (I.E., MAXIMAL OXYGEN UPTAKE RATE) AND HEAT TOLERANCE

28. BOTH FEMALE AND MALE RESPONSES TO HEAT STRESS APPEAR TO BE HIGHLY DEPENDENT ON FITNESS LEVEL; FOR EXAMPLE, VO2MAX IS SINGLE BEST DETERMINANT OF DIFFERENCES IN SWEAT SECRETION PRODUCED BY AN INCREASE IN CORE TEMPERATURE. ALSO, INDIVIDUALS WITH A HIGH MAXIMAL OXYGEN UPTAKE RATE HAVE A LOWER STEADY-STATE CORE TEMPERATURE DURING HEAT STRESS AND CAN ADAPT TO HEAT STRESS FASTER (I.E., FEW DAYS) AS EVIDENCED BY A EARLIER PLATEAU IN CORE TEMPERATURE DURING LONG-TERM HEAT EXPOSURE.

29. AGE AND HEAT TOLERANCE

30. AGE AND HEAT TOLERANCE

31. OLDER ADULTS - LOWER HEAT TOLERANCE

32. DECREASED BLOOD VOLUME. • INCREASED BODY FAT. • LOWER FITNESS LEVEL (VO2MAX), WHICH RESULTS IN A LOWER LEVEL OF SWEAT SECRETION FOR A GIVEN INCREASE IN CORE TEMPERATURE, HIGHER STEADY-STATE TC, AND DECREASED ABILITY TO ADAPT TO HEAT STRESS.

33. LOWER CARDIAC OUTPUT DUE TO LOWER STROKE VOLUME AND LOWER MAXIMAL HEART RATE. • INCREASED POTENTIAL FOR CARDIOVASCULAR STRAIN. • THRESHOLD FOR ONSET OF SWEATING AND SWEATING SENSITIVITY (??).

34. DECREASED SWEATING CAPACITY DUE TO A DECREASE IN TOTAL BODY WATER. • DECREASED CONVECTIVE, EVAPORATIVE, AND RADIANT HEAT LOSS. • REDUCED ABILITY TO ADAPT TO HEAT.

35. YOUTH - LOWER HEAT TOLERANCE

36. LOWER B0DY SURFACE AREA (BSA). • GREATER BSA/BW RATIO WHICH INCREASES THE POTENTIAL FOR HEAT LOSS IF AIR TEMPERATURE IS LESS THAN SKIN TEMPERATURE OR INCREASES THE POTENTIAL FOR HEAT GAIN IF AIR TEMPERATURE IS GREATER THAN SKIN TEMPERATURE. ALSO, INCREASES THE GRADIENT FOR RADIANT HEAT GAIN. • LOWER BLOOD VOLUME.

37. HIGHER PERCENT BODY FAT. • HIGHER THRESHOLD FOR ONSET OF SWEATING (??). • SWEATING SENSITIVITY (??). • POSSIBLY A DECREASED CAPACITY FOR SWEATING DUE LOWER TOTAL BODY WATER.

38. LOWER SWEATING RATE AT REST AND DURING EXERCISE, WHICH POTENTIALLY LOWERS THE CAPACITY FOR EVAPORATIVE HEAT COOLING. • GREATER ENERGY EXPENDITURE DURING WALKING AND RUNNING FOR A GIVEN ABSOLUTE WORKLOAD, WHICH RESULTS IN GREATER METABOLIC HEAT PRODUCTION PER KILOGRAM OF BODY WEIGHT. • REDUCED ANATOMICAL VOLUME OF HEART AND REDUCED STROKE VOLUME.

39. INCREASED POTENTIAL FOR CARDIOVASCULAR STRAIN. • LOWER CARDIAC OUTPUT AT A GIVEN METABOLIC LEVEL, WHICH LOWERS THE CAPACITY FOR CONVECTIVE HEAT TRANSFER FROM THE BODY CORE TO THE PERIPHERAL SKIN.

40. DECREASED POTENTIAL FOR CONVECTIVE, EVAPORATIVE, AND RADIANT HEAT LOSS. • REDUCED ABILITY TO ADAPT TO THE HEAT. • HIGHER BASAL METABOLIC RATE WHICH RESULTS IN GREATER RELATIVE HEAT PRODUCTION.

42. CIRCADIAN RHYTHMS

43. ZEITGEBERS • EXTERNAL RHYTHMIC INFLUENCES THAT PROVIDE TIME CUES, WHICH SYNCHRONIZE CIRCADIAN RHYTHMS WITHIN AN INDIVIDUAL. • NORMAL SYSTEMS OSCILLATE IN A 24 HOUR CYCLE.

44. ZEITGEBERS • WITHOUT ZEITGEBERS, CIRCADIAN RHYTHMS BECOME FREE RUNNING: - CORE TEMPERATURE FOLLOWS 25 HOUR CYCLE. - SLEEP/WAKE 25 HOUR CYCLE.

46. EXAMPLES OF EXTERNAL RHYTHMIC INFLUENCES • LIGHT/DARK CYCLE. • SLEEP/WAKEFULNESS CYCLE. • SOCIAL ACTIVITY. • FEEDING/FASTING CYCLE.

47. TWO PACEMAKERS • X - LIGHT DARK/CYCLE (4 TIMES AS STRONG); WILL OVER-RIDE OR CONTROL Y PACEMAKER BECAUSE IT IS FOUR TIMES AS STRONG. • Y - FEEDING/FASTING AND ACTIVITY CYCLE.

49. EFFECTS ON THERMOREGULATION

50. CIRCADIAN RHYTHMS CHANGE CORE TEMPERATURE THRESHOLD FOR THE ONSET OF SUDOMOTOR (I.E., SWEATING) AND VASOMOTOR (I.E., BLOOD FLOW) RESPONSES. • AS LONG AS HEAT LOSS EFFECTOR MECHANISMS (I.E., SWEATING AND VASODILATION OF THE SKIN VASCULATURE) ARE TIGHTLY COUPLED TO CORE TEMPERATURE RHYTHM, THERE IS NO EVIDENCE THAT THE CIRCADIAN CYCLE IMPAIRS THE HOMEOSTATIC MECHANISMS OF THERMOREGULATION.