Sunday, May 4, 2008

The Physiology and Anatomy of Some Relevant Sex Differences, With Reference to Rowers

Female Athletes – The Physiology and Anatomy of Some Relevant Sex Differences, With Reference to Rowers
By Prof. Craig Sharp

1.1 Introduction

This section will concentrate mainly on those anatomical and physiological aspects which influence physical performance in dance, physical activity or sport, with a brief introduction to sexual dimorphism, or how we become two sexes. The important point to grasp in any description of such physical differences is that, although on average one sex may have one attribute more highly developed than the other, there is very broad overlap. The average height of British men and women is 5’9” (1.7m) and 5’4” (1.6m) respectively. Yet very many women (e.g. rowers, basketball players) are taller than the male average, and vise versa. And to give a performance example, the best male Marathon runners in the world run the distance in about 2 hours and 6 minutes, and their female counterparts run under 2 hours and 20 minutes. Nevertheless, those elite women runners will beat all but a very small percentage of all male marathoners. In its day, the East German women’s athletics team could have beaten the men’s teams of many of the smaller nations.

For much of this paper, I am talking about average differences between groups of men and women. It is important to remember that much of the differences between people in general relate to the genetic hand that they are dealt at conception. To take one example, maximal oxygen uptake (VO2 max). If one were to measure this value in the laboratory on a random selection of 100 20-year old untrained but healthy young women who were all the same height and weight, it could be found to vary between 1.5 and 3.5 liters of oxygen used per minute. The reasons for the wide range would be largely genetic. And if one took the lower quarter of the sample, and subjected them to two years of Olympic-style rowing training, they might improve as much as ~25% - say to about 2 liters of oxygen per minute. In other words, even with training, they would not come near to the better genetically endowed upper group. This aspect of genetics is not a fact to get depressed about, especially in an activity needing several attributes such as rowing. It simply means that one has to identify one’s strengths, and use them to the full. In the example just quoted, it may very well be those with the low VO2 Max might be at the upper end of the scale in terms of muscle strength and local muscle endurance.

One can alter one’s physical aspects very considerably, but always in the context of one’s genetic programme – which in physical terms is mediated through anatomy and physiology.

1.2 Gender Formation

Gender aspects as a whole in sport and exercise, embrace genetic, hormonal, anatomical, physiological, psychological and sociological factors. The term ‘sexual dimorphism’ refers to the two sexes which we grow into, triggered initially by genetic and then hormonal factors operating on a fetus.

Gender is coded for in a specific pair of the 23 human chromosomes. Termed the ‘sex chromosomes’ they are designated as X and Y. XX codes for women and XY for men. It is important to note that all mammals are basically female unless specifically masculinised (vice versa with birds, in whom the males have the pair of identical sex chromosomes). In humans the sexing process is initiated in the sixth week of fetal life. If the chromosomes are XX, then the female development automatically follows the pre-destined programme; i.e. the outer layer of the two small clumps of cells which form the undifferentiated sex gland or gonad, begin their development into an ovary. A double set of tubules known as the Mullerian ducts then form the fallopian tubes, uterus and upper vagina.

However, following coded instructions from the male Y-chromosome, the medulla or middle of the undifferentiated gonads is stimulated into becoming the male sex glands or testes. Within days, these start to secrete two hormones. The ‘male’ testosterone promotes growth of another tubular system known as the Wolffian ducts to form the male tubing, in the form of the epididymis, vas deferens and ejaculatory duct, all concerned with the passage of semen. The second hormone secreted at this time by the embryonic and still abdominal testes is ‘Mullerian inhibitor’ which actively inhibits growth of the Mullerian ducts. So, not only is the male tubing actively promoted by one hormone, testosterone, but the embryonic female tubing is actively suppressed by another hormone, the Mullerian inhibitor.

So much for the internal genitalia, what of the external? These in the early embryo consist of two cellular aggregates in the pubic region. If there is no Y-chromosome, then these will form into a vagina and a clitoris respectively. But, if Y-induced testosterone is present, then the would be vagina will heal up, as it were and form the scrotum. And instead of a clitoris, a penis will form. Note that the skin of both the scrotum and the external vagina is similarly pigmented, wrinkled and hairy; also the scrotum has a central line (the ‘median raphe’), which is where the original vagina healed over (Money and Erhardt 1971 Sharp 1997).

1.3 Anatomical Aspects

1.3.1 Body Dimensions

The average British woman is 1.6m (5’4”) tall, compared to the 1.7m (5’9”) for men, but whatever the height of a race or tribe, men are about 7% taller. This applies also virtually throughout the animal kingdom, with the main exception of the spotted hyena, in which the bitches are larger – probably and evolutionary adaptation to the fact that the males have a tendency to cannibalize the pups. Girls may be briefly larger – and stronger – between 10 and 12 years, due to their earlier growth spurt, which occurs just over two years earlier in girls. By around 13, the boys forge ahead on average as their growth spurt carries them up to about 10% beyond the girls in many of the physical dimensions.

1.3.2 Upper Body

Men end up with broader shoulders, longer arms, and narrower hips, both in terms of absolute measures, and relative to body height. The shoulder and arm difference usually leads to men being relatively stronger than women in the upper body compared to the lower, and the longer arms give better biomechanical leverage, which is shown to particular effect in throwing events and racket games, where the terminal velocity of the hand, or the head of the racquet, is the critical factor in determining the speed of missiles leaving either. Longer arms – and broader shoulders – also give men a leverage advantage in rowing and canoeing events.

Many women tend to have more of a “valgus angle” to their arms than men, whereby their arm is not as straight as men’s. That is, if the arm is held by the side with palms facing forwards, their lower arm angles away from the body. It is one reason why many women tend to throw objects, such as balls, stones and snowballs ‘round arm’. The feature is due to a greater male development of the lateral humeral epicondylar cartilage at the elbow. This cartilage has receptor sites specifically programmed to respond to testosterone at puberty, and acts to straighten out the male arm. A woman with a pronounced valgus or carrying angle, who wishes to become a good javelin thrower, might find herself predisposed to elbow injuries, and the same may apply to a lesser extent in rowing. Of the three main athletic throwing events, shot, javelin and discus, and taking into account the different weights of implements which the two sexes throw, women are furthest behind in the javelin and closest to men in the discus. The latter is what one might expect, given that women’s greater spinal flexibility allows more rotation, which is especially important in the discus event.

1.3.3 Lower Body

The broader hips of women, both in absolute measurements in many cases and indeed as a proportion of body height, result from a broader pelvis (due t its cells in turn bearing receptor sites responsive to oestrogen). This leads in general to a woman’s femur having to make a greater angle medially (the Q-angle) as it inclines towards her knee, which is the main reason why many untrained women throw their heals out when they run. In athletic clubs, such a running style is modified, if necessary.

However, a more important implication of this greater medial angulation of the femur relates to the angle of force of the powerful quadriceps muscles as they insert onto the patella or kneecap. The bulk of this muscle group is located laterally, i.e. on the outer side of the thigh. Thus when it contracts it exerts a strong ‘bowstring’ effect on the patella tending to pull it sideways (known as ‘sub-laxation’), out of the intercondylar groove on the femur in which it normally tracks. This misalignment may lead to excessive wear in the cartilage underside of the patella (the retro-patellar cartilage), resulting in the aching condition of ‘chondromalacia patellae’. Although often known as ‘runner’s knee’, it may occur in rowers, and it is more common in broader hipped women.

This may also tend to happen in women with a tendency to ‘knock knees’ who have too large a ‘Q-angle’, which is measured as follows: if you draw a line from the anterior supra-iliac spine (the front of your hip bone) to the centre of your patella, and another one from the centre of your patella to your ‘tibial tuberocity’ (the bony bump just below your knee), the angle between these two lines make with each other id the Q-angle. In men it should be less than 10degrees and in women less than 15 degrees. If it is greater than these, this creates increases the mechanical advantage of the outside quads, as mentioned above, and it lessens the ability of the one inner quadriceps to counter the bowstring force – the vastus medialis. Thus, overlarge Q-angles lead to bad patellar alignment and tracking in the groove, and increase the possibility of patellar sub-luxation and of chondromalacia patellae. So, ideally a women rower should have, proportionally reasonably narrow hips, and fairly straight legs.

The vastus medialis just mentioned is one member of the quadriceps muscle group, which inserts into the patella from the opposite direction – medially rather than laterally. And it acts to stabilize the patella in its groove, and to counter the bowstring effect to some extent. If you put your hand on the inner side of your knee, and slowly straighten your leg into full extension, then you will fee the vastus medialis tensing up as it comes into action just before full extension. Thus, exercises to strengthen the medialis must always involve straightening the leg fully, and slowly, so that there is no swing effect initiated by the other quadriceps muscles. Straightening the leg while seated, and holding it hard isometrically (i.e. tensed but not moving) will also strengthen medialis. This should be done 5 times on each side, and held for 10-15 seconds, two or three times daily. Otherwise a visit to the ‘quads station’ of a multigym will provide effective training.

1.3.4 Body Fat

Young women have a considerably greater percentage of their body fat, compared to men. Expressing body fat as a percentage is not especially reliable, but can be a useful rough guide. In my laboratories at the BOMC and elsewhere, the leanest subjects we have measured, mainly elite distance runners and gymnasts, have been in the range of from 5-8% for men, with very few equivalent women below 16%, and ranging up to 21% (compared to a normal student population of between 12-18% for men, and 22-30% for women). Until the age of around 10, there is little difference in body fat between boys and girls, but when both sexes go through puberty, the boys tend to lower their fat percentage, and the girls tend to gain fat. The gain in the women is a natural hormonally induced part of the growth process. The difference in body fat between the ‘average’ young man and woman is in the order of 70 000 kilocalories (or 300 000 kilojoules), which is just about the energy cost of producing a full term human infant. Rowers tend to vary from 18-25% for heavyweight women, with some lightweights being lower. The men tend to range from 10-15% again with some lightweights being lower.

Body fat is primarily an energy store. And men and women tend to store their fat differently. In men, the main fat store is in the abdomen, so a fat man will have a varying degree of ‘beer belly’, even though he may have quite slim legs. For women the fat deposits are thighs, hips, bust and back of the arm. It makes sense for women not to store fat on their abdomen, which becomes full during pregnancy.

This gender difference in average body fat certainly aids survival in extremes of cold and starvation, indeed Scott of the Antarctic may well have reached the South Pole had he and the team been women. It often leads to better performance by women in long distance sea, lake and loch swims, in many of which women hold the all-comers records. In part this is because the greater amount of fat acts as an insulation under the skin, and in part it floats women higher in the water, as fat is lighter than water. (Drop a piece of lean meat and a piece of fat – such as butter – in water; and see which sinks and which floats!). Floating higher makes for easier swimming. However the greater fat is a handicap in weight bearing activities involving running or jumping. This is one of several pressures towards a degree of female leanness, as in lightweight rowing, which may be very harmful if taken to extremes. In rowing and canoeing, extra weight lowers the racing shell, and will increase its surface area, and hence friction. Nevertheless, the ‘weight-supported’ rower can afford to carry an extra kg or two of fat, compared to the runner of gymnast! In women, this may help to minimize or delay the onset of sports osteoporosis – which is seen more in lightweight rowers, as is the associated amenorrhea, or absence of periods.

The gender difference in body shape, much of it accounted for by body fat distribution, leads to many women having a lower centre of gravity, and may be part of the reason for their better balance – as seen on the balance beam in gymnastics, a discipline for women but not for men, and possible shown in sailing, where women crew members are often noted as being better balanced in their movements around the boat.

A certain degree of fat is essential to the body; partly this is in terms of acting as a packing material around vital organs, the ovaries for example, and for helping keep the eye firmly in its socket. Also, fat is important in hormone-processing. It is thought that one reason why women who are very thin stop menstruating is that they do not have enough fat t activate their osterogen precursors.

1.3.5 Flexibility

Women have greater flexibility than men, as may be seen in asking a group of untrained men and women to planter flex their foot, i.e. ‘point their toe’. This enhanced flexibility, is of course, much featured in gymnastics and many forms of dance (and in modern circuses e.g. Chinese National Circus, and the ‘Cirque du Soleil’). In part the flexibility of women is due to slight differences in their joints, and in part it may be due to the presence of the hormone ‘relaxin’, which appears to act on the ground substance of collagen, the vital structural element in ligaments and tendon, imparting a greater degree of elasticity. Relaxin comes into its own during childbirth, when it has a major function in acting on the symphasis pubis joint. This is where the two pubic bones meet each other at the bony floor of the pelvis – between the legs. Normally there is very little space between the two pubic bones, nor do they normally move, but under the influence of relaxin, the connecting collagen may allow a considerable widening between the pubic bones, thus enlarging the birth canal. The main application in rowing is hamstring flexibility, which may be associated with low back pain injury if the hamstrings are too inflexible.

1.4 Physiological Aspects

1.4.1 Heart, Lungs and Blood – the Cardio-Respiratory System

Women have proportionally less blood (65ml/kg bodyweight compared to 75ml) than men, and lower hemoglobin concentration (13.9g/100ml compared to 15.8g). Working maximally aerobically, women need 7 liters of blood to carry a liter of oxygen, compared to men’s 6 liters, yet proportionally their hearts are about 8% smaller, although maximum heart rates are the same. The net effect is on the maximal oxygen uptake, which at elite rowing levels is up to ~75ml/kg bodyweight for men, and ~65ml for women. It is this VO2 Max which is responsible for ‘aerobic fitness’ or overall stamina – the ability to train on land or water for one to two hours – or more. The other important aerobic aspect is the anaerobic threshold, which is the rate of work which a rower can sustain without incurring a sudden marked rise in blood lactic acid. This threshold indicates the rate of work which can be sustained for relatively long periods. The anaerobic threshold is usually accompanied by a heart rate within a set range, so can be used as a training guide. The overall aerobic sex difference matters little, in that women rowers do not compete with men.

1.4.2 Muscle

Between 10 and 11, many girls are stronger than their boy peers, but boys end up on average stronger. This is partly because the cross sectional area of their muscles is greater, through androgen hormone effects, among other factors – and partly due to the longer levers of their limb bones. There is little difference in muscle quality between the sexes; both tend to generate about 30 Newton’s of force per square cm of untrained muscle tissue. 30 Newton’s is a force of about 3kg per square cm.

In terms of muscle endurance, women appear to have better low-grade local muscle endurance, for example on repetitions of 50% of their maximum muscle force. This may be of benefit in sports such as swimming, cycling and indeed rowing, which consist of very large numbers of relatively low-grade contractions. And each is a sport where women approach closer to men’s performances than weight bearing sports such as running –where the force at each stride are much greater. Although not all related to rowing, it is of interest to briefly note that women have much better fine manipulative skills, e.g. as in keyboards, or electronic assembly.

1.4.3 Heat Regulation

Exercise such as rowing generates considerable heat. About two-thirds of food energy appears unavoidably as heat in muscular work, and this must be lost almost as fast as it is generated, or collapse from heat stroke would occur. We live closer to heat death than to cold death. Normal core body temperature is around 37 degrees, but much above 43 degrees may be quickly fatal. To lose heat in exercise, men tend to sweat more per square meter of skin surface than women (e.g. 800ml/hour/m2 compared to 600ml/hour/m2). However, women tend to loose more heat by radiation. This benefits women in very humid conditions, where sweat cannot evaporate very readily into air already saturated with water vapor. Under such conditions, and not being so reliant on sweating, women tend to radiate more of their heat away, through a warm skin, red with dilated blood vessels radiating the heat away like tiny electric fires.. Men benefit in dry heat, as their sweat can be evaporated. Sweat which drops of is simply wasted, as far as cooling is concerned, because it is when the sweat changes from liquid into water vapor (i.e. steam), that the heat energy is taken form the skin, or, more accurately, from the warm blood circulating through the skin.

So, women thermo regulate better in wet heat, and men better in dry heat. Men will tend to be more severely affected on a hot humid course, and women on a hot dry one. In life threatening environments, however, their greater sweat production implies that mend tend to die quicker than women from dehydration, for example in desert conditions, or if shipwrecked in mid ocean (and having to ‘take to the boats’ for too long). In both sexes, sweat patterns change through exercise and training, covering more extensive areas of skin, and occurring sooner. It seems surprising, that fit people sweat sooner, as dancers or athletes notice at parties or receptions. In both sexes the level of salts (or electrolytes) in sweat drop markedly, the higher the fitness levels. In other words the fitter you are, the better you conserve your body salts.

1.5 Bibliography and References

Most texts of exercise physiology have chapters on gender issues, but a very good comprehensive account is: Christine Wells. Women, Sport and Performance: A Physiological Perspective. Human Kinetics. 1995

Money, J and Erhardt, A. Man, Woman, Boy, Girl. John Hopkins. 1971

Mittwoch, U. Genetics of Sex Determination. Academic Press. 1976.

Muir, R. Textbook of Pathology. RNM McSween and K Whaley (eds). Edward Arnold. 1992. Chapter 22.

Sharp, NCC. The New Sexual Dimorphism. B. J. Sports Med. 1997. 31, 82-83.

Sharp, NCC. Body fat and weight management. Chapter 6 in Bean A and Wellington P (eds) Sports Nutrition for Women. A and C Black. 1995.

Wilmore, JH and Costill, DL. Physiology of Sport and Exercise. Human Kinetics. 1994. Chapter 19, Gender Issues and the female athlete.

No comments: