Should ‘moist wound healing’ principles be applied to cracked nipples?

pot-of-vaselineMaintaining a slightly damp, rather than dry, environment under a dressing has been shown in many studies to help wounds heal faster. Moisture prevents a scab from forming, which allows new epithelium cells to move across the surface of the wound more quickly, and reduces the time it takes it to close1.

As the technique appears to improve the healing rates of a range of skin lesions2, it has been suggested that its benefits may extend to cracked nipples3, and there are now several off-the-shelf creams and dressings that claim to maintain a moist environment, and thus help the healing process. The inference is not, at first glance, unreasonable – if moisture helps skin to regenerate more quickly, then it may well help injured nipples to recover too. It is important to remember, however, that nipples are subject to a unique set of circumstances when a mother is breastfeeding, so it is also quite possible that the application of the technique might not be appropriate. Is there any scientific evidence that applying moist wound healing principles can aid the recovery of cracked or injured nipples?

In a study comparing dressings made from hydrogel with lanolin (both classified by the Breastfeeding Network as having ‘moist wound healing’ properties)4, 106 mothers were allocated at random to one of two groups: the first was given hydrogel dressings to use between feeds; the second was given lanolin cream. Mothers started using the treatments within 24 hours of giving birth, and their effectiveness was assessed via telephone interviews 3, 4 or 5, 7, 10 and 12 days later. The researchers found mothers reported significantly lower pain scores (a difference of just under 1 point, on a scale of 1-5) in the hydrogel group on days 10 and 12.

There were a couple of problems with this study (actually, there were several, but I’ll limit it to the major ones). Firstly, there was no baseline group of mothers not applying anything to their nipples, so it is not possible to say whether either treatment was better than simply leaving nipples alone. A second issue is that the people conducting the interview were aware, when they spoke to the participants, which treatment they were receiving. The researchers claim that to have conducted the study blind would have been ‘impossible’, an unsubstantiated and somewhat odd statement, as it would appear to be completely possible to interview a mother over the phone without knowing what she had on her nipples. Knowledge of the treatment group in this type of study is a problem if there is any chance that the researcher may have a bias towards a particular treatment, as they may subconsciously influence the patients’ responses. It may be worth mentioning at this point that the research was funded by Tyco, the manufacturers of the dressings.

These criticisms are, however, a digression. The main thing to note about this research is that it did not test whether the dressings actually helped wounds to heal. Although moist wound healing is touted in the introduction as the ‘science bit’ justifying the use of the dressings, it is not mentioned anywhere in the procedure or the results.

A hospital-funded study comparing hydrogel and lanolin – this time documenting the impact that the treatments had on bleeding and cracked nipples – did not find the dressings to be quite so effective5. Researchers who were blind to the treatment group rated nipples as healing significantly better when women used lanolin with breast shells, rather than hydrogel dressings. Self-reported measures of pain were also significantly lower in the lanolin group. A final point worth mentioning is that the study was halted early, due to a third of the 21 women in the hydrogel dressing group developing an infection.

Although this study compared two treatments, it is again compromised by the lack of a proper control group. We can see that lanolin appears to result in improved healing and lower pain scores when it is compared with hydrogel, but we still do not have any evidence that moist wound healing techniques are useful for treating injured nipples when breastfeeding – to ascertain this requires a control group where mothers keep their nipples dry.

So far, there appear to be only two studies that have looked at this issue. An experiment published in 1995 examined whether using a polyethylene adhesive dressing had any effect on the development of nipple redness, fissures and pain6. 50 mothers took part in the study, using a dressing on one nipple, and leaving the other untreated. Although the mothers reported less pain when feeding with the treated nipples, the researchers caution that this may simply have been because of the ‘Hawthorne Effect’ (the mere fact there is an intervention is enough to cause an improvement.) The dressings made no difference to the development or healing of fissures or redness, as reported by observers blind to the treatment group. 16% of the participants dropped out due to finding the dressings uncomfortable, and 66% said they found it uncomfortable to remove them – something they had to do before every feed.

A more recent study looked specifically at the effect of lanolin on the healing of nipple fissures7. 225 women, all with fissures, were randomly allocated to one of three groups: in the first group mothers applied lanolin 3 times a day; in the second they applied breast milk after each feed; in the third they applied nothing. The appearance of their nipples was assessed 3, 5, 7 and 10 days after starting the treatment by researchers who did not know which group the mothers were assigned to. There was no significant difference in healing time between the breast milk and no-treatment groups. The nipples of the women using lanolin, however, took significantly longer to heal (45% of this group took longer than 7 days, as opposed to 32% of the milk group, and 25% of the no-treatment group).

There is another area of research, which does not examine the use of moist wound healing directly, but is still relevant to the debate. Broken skin makes nipples vulnerable to infection8, which may mean there is an additional problem with keeping nipples damp, rather than dry: organisms like thrush are known to thrive in warm, moist environments9.

Given the possible risk of infection, and the evidence that maintaining a moist environment around cracked nipples may potentially delay the healing process, it seems to unwise to recommend the application of the products described above to mothers with cracked or fissured nipples. In spite of this, they continue to be promoted by both commercial companies and health professionals on scientific grounds. Until evidence that genuinely supports its use is found, presenting moist wound healing to mothers as a clinically tested treatment is at best misguided, and at worst dishonest.

  1. Nature. 1962 Jan 20;193:293-4.
  2. Br J Nurs. 2008 Aug 14-Sep 10;17(15):S4, S6, S8 passim.
  3. J Hum Lact. 1997 Dec;13(4):313-8.
  4. J Obstet Gynecol Neonatal Nurs. 2003 Jul-Aug;32(4):486-94.
  5. Arch Pediatr Adolesc Med. 1998 Nov;152(11):1077-82.
  6. Nurs Res. 1995 Nov-Dec;44(6):347-51
  7. Saudi Med J. 2005 Aug;26(8):1231-4
  8. J Hum Lact. 1991 Dec;7(4):177-81.
  9. Hum Lact. 1999 Dec;15(4):281-8.

Breastfeeding and being sick

thermometerAt 3.00 am, C started coughing. The cough turned into a whine, and as my heart sank, a full-blown wail. As I got out of bed to comfort her, I felt a bit odd, but put it down to the early hour and my chronic tiredness.

I sat down to feed C, who was still grumbling, hoping that I’d feel better shortly. I didn’t. After what seemed like an age, C finally had enough milk to send her back into a doze and I put her back into her cot. I then bolted to the bathroom. I only just got there in time.

After I had vomited every last bit of the lovely meal my husband had cooked the night before, I collapsed on to the floor filled with worry. It was one thing for me to be sick, but what if C was too? What if we all were? My thoughts then turned to breastfeeding. To avoid C catching a virus (if that’s what it was) surely the best thing to do would be to steer clear of her? But this would also mean not passing on valuable antibodies, not to mention ruining my milk supply and risking engorgement or mastitis. It was also highly likely that C had already been exposed to any infectious illness before I started exhibiting symptoms, so continuing to nurse as I usually did was really the only sensible option.

I carried on breastfeeding C as normal, washing my hands first and trying to minimize the germs she encountered. Or at least I tried to feed her as normal. After half a day of this, I realised that germs were only a small part of the problem. My energy levels were at rock bottom, I couldn’t keep anything down and I was massively dehydrated. Engorgement was the least of my worries! My let-down reflex took an age to kick in and C ended up bawling in frustration. I’m still trying to work out why exactly my ability to breastfeed was so depleted, but I can only assume it was related to my poor ability to keep food or drink down, and the resulting plunge in fluid levels and blood sugar. Assuming I had norovirus – the winter vomiting bug – this acute phase of sickness shouldn’t last more than 24 hours. I sincerely hoped this would be the case.

Thankfully, 6 hours later, I was sick for the last time. Although it took me several days to recover completely, my milk supply soon picked up and C remained impressively illness-free.  Whether she had developed sufficient immunity through exposure to antibodies in my milk, or simply didn’t come into contact with the virus I don’t know, but she didn’t show a hint of being sick. Unfortunately, the same couldn’t be said for my husband, who started racing to the bathroom the minute I stopped…

Can breastfeeding affect your sex life?

unmade-bedIn the initial period after giving birth, sex tends not to be a top priority. Slowly, as you start to feel less ‘delicate’, and days become distinguishable from nights again, it creeps back up the agenda, and by 6-12 weeks, most women have recommenced intimate relations. The speed with which you get back to sex may be influenced by the way in which you feed your baby, however: there is evidence that if women formula feed, they may be up for it significantly sooner than if they breastfeed1.

Determining how exactly breastfeeding affects a mother’s sex life is a complicated business. The body of scientific literature looking at this issue spans several decades, and whilst it contains some interesting observations, it is also littered with apparently contradictory results. In their 1966 work, ‘Human Sexual Response,’ William Masters and Virginia Johnson reported that all 24 of the breastfeeding women they studied experienced ‘enhanced sexuality’ compared to non-nursing mothers. This result does appear to be something of an anomaly, however; whilst it is certainly not the case that all breastfeeding women experience a pronounced drop in libido, there is a tendency for them to be less interested in sex, and less likely to engage in it than formula feeders.

Of five studies comparing the sexual experiences of breast feeding and formula feeding women, one reported that feeding method did not appear to make any difference2. The remaining four found that breastfeeding mothers were less interested in having sex3,4, less likely to be having it1,3, and more likely to find it painful when they did5.

A study that monitored women during the weaning process also suggests nursing may suppress sexual activity6. Mothers who had been breastfeeding for at least 6 months completed a daily questionnaire about their health and how they were feeling until a month after they had weaned their babies. The responses they gave in the 4 weeks before weaning were then compared with their responses in the 4 weeks after.

Within 3-4 weeks of stopping breastfeeding, the mothers in the study reported a significant increase in sexual activity. They also reported an improvement in mood and a decline in fatigue, and the authors suggest that this may in part explain the women’s increased appetite for sex. Other research has reported that the lower sex drive of breastfeeding women exists irrespective of tiredness or depression, however, so it is likely that there are other factors underlying the issue4.

One potential culprit is the differing hormone levels that occur in women who are nursing. High prolactin and low oestrogen levels are thought to reduce vaginal lubrication, and as these hormone levels are frequently found in breastfeeding mothers, the theory goes that this may in turn reduce their sexual activity1. Although this is a reasonable hypothesis, a direct link between prolactin/oestrogen levels and lower sexual desire in breastfeeding women has yet to be demonstrated: in a study monitoring levels of these hormones directly, no correlation was found between the two7. The study did find a relationship between very low levels of the sex hormones testosterone and androstenedione and ‘a severe reduction in sexual interest’, which affected five women in the breastfeeding group in the study, and none in the formula feeding group. It is important to point out, however, that the formula group was much smaller (14 breastfeeders were compared with only 6 formula feeders), so while the results indicate that breastfeeding may result in lower levels of these hormones, they don’t provide conclusive proof.

An alternative explanation is that breastfeeding women may be less interested in sex as they have an ‘intimate touching need’ satisfied by nursing, and are therefore less likely to seek its fulfilment through sex1. Again, however, this is speculation: it’s possible it may have an impact in some relationships, but it has not been evaluated scientifically.

So far, the research looking at the relationship between breastfeeding and sex hasn’t provided any clear answers. There is evidence that breastfeeding women are more likely to report a lack of interest in sex, or find it less comfortable, but this is not something that affects nursing mothers across the board – many report enjoying sex, and some have even said that their sexual relationship during this period is better than before they gave birth8! Whether this because of, or in spite of breastfeeding is impossible to know, but it indicates that nursing your baby will not necessarily condemn you to a poor sex life. Having said that, the libido of some mothers does appear to be seriously diminished during breastfeeding. The reason for this isn’t clear, but it could be because these women are experiencing particularly low levels of certain sex hormones.

If you do find sex particularly unappealing while you’re breastfeeding, the good news is that it doesn’t appear to last forever. A large study found that although breastfeeding women reported less interest in sex than formula feeders to start with, this difference had disappeared by 12 months4, so however long you intend to breastfeed, it seems your shouldn’t have to wait more than a year for your libido to get back to normal.

  1. J Fam Pract. 1998 Oct;47(4):305-8.
  2. Br J Obstet Gynaecol. 1981 Sep;88(9):882-9.
  3. J Sex Res. 2002 May;39(2):94-103.
  4. Br J Obstet Gynaecol. 1997 Mar;104(3):330-5.
  5. BJOG. 2000 Feb;107(2):186-95
  6. Obstet Gynecol. 1994 Nov;84(5):872-6.
  7. Br J Psychiatry. 1986 Jan;148:74-9.
  8. J Midwifery Womens Health. 2000 May-Jun;45(3):227-37.

Can ‘full term’ babies born before 40 weeks find it harder to breastfeed?

stork-carrying-babyThe fact that premature, or preterm, babies can experience difficulties breastfeeding is well documented. Neurological immaturity, poor muscle tone and underdeveloped reflexes can all affect the ability to perform suckling movements, so babies born early generally don’t find it as easy to feed directly from the breast as babies born at full term1. The length of time that constitutes ‘full term’ is slightly hazy, however. Although the average pregnancy lasts 40 weeks, it is usually from 37 weeks gestation that babies are regarded as being sufficiently mature to qualify as term. There is some evidence, however, that babies born after 37 weeks, but before 40, may not always be as well developed as those born later, and a large study in Australia suggests that this could potentially make it more difficult for them to breastfeed2.

The study looked at the impact that length of gestation had on breastfeeding rates in a sample of 3600 children born over the period of a year. Babies were divided into three groups: those born before 37 weeks (preterm); those born at 37-39 weeks (early full term); and those born at 40 weeks and over (late full term). Preterm babies were the least likely to be breastfed – only 88% of mothers in this group initiated breastfeeding (compared to 92% in the early full term group and 94% in the late full term group), but this could have been due to social factors such as the age and level of education. Although there appeared to be a small difference in the initial breastfeeding rates of the early and late full term babies, it was not statistically significant. 6 months later, however, when breastfeeding status was next documented, the gap had widened to a significant level.

Pre-term babies were still the least likely to be breastfed – only 41% were still being nursed at this point, compared to 61% of the late full term babies. 55% of the early full term babies were still breastfed at 6 months – more than in the preterm group, but significantly fewer than in the older full term group. The fact that there was a difference between the full term groups at 6 months, but not initially, is important as it indicates that early full term babies may be at greater risk of breastfeeding failure: mothers of the younger babies were as likely to start breastfeeding, but they were more likely to stop before their babies reached 6 months. This difference can’t be explained by the other factors recorded in the study, and the underlying reasons for it aren’t clear, but the authors suggest that it may have arisen due to subtle immaturity, such as underdeveloped mouth muscles, that can affect babies born before 40 weeks, even when they have passed the 37 week milestone.

If your baby is born slightly early, the chances are, of course, that she won’t have any major problems breastfeeding. In the study reported above, the majority of babies born after 36 weeks were still breastfeeding 6 months later, so the prognosis is good. It’s still worth noting that a gestation period under 40 weeks might decrease the chance of a mother continuing to breastfeed, however, so any difficulties that do arise can be picked up and addressed before it’s too late. With a little patience and practice even very preterm babies can adjust to breastfeeding as they get older1, so dealing with problems caused by mild immaturity is comparatively straightforward. The key is being aware they may occur: recognizing that for some babies born between 37 and 39 weeks breastfeeding may be difficult for a short time – and providing the right support to mothers through this period – would help ensure that many more babies born slightly early are able to successfully achieve exclusive breastfeeding.

  1. Infant, 2005 July; 1(4):111-115.
  2. Arch Dis Child Fetal Neonatal Ed. 2008 Nov;93(6):F448-50.

Breastfeeding in public: is nine months really the end of the road?

bibAlthough the NHS recommend breastfeeding for at least a year, and the World Health Organization for two years and beyond, I’m well aware that Western culture doesn’t really allow for this. Thanks possibly to hard-line health awareness campaigns, it now seems generally acceptable to be seen feeding very little babies in public, but it’s also still acceptable for people to vociferously object to anyone breastfeeding an older child. Gauging the point at which breastfeeding goes from ‘good’ to ‘bad’ is a tricky business, however. When does your gorgeous little infant suddenly lose their innocent penchant for breast milk, and turn into an ‘older child’, apparently in danger of being psychologically damaged by continued nursing?

Many people I know have given me their opinion on breastfeeding beyond this (as yet undefined) ‘baby’ stage.

After a television programme on extended breastfeeding, a fairly inebriated friend of my husband held forth about how how unpleasant it was to see people breastfeeding children (which doesn’t entirely explain why he spent an hour watching it on television). It wasn’t clear why he held this view, but the fact he is an avid reader of such publications as The Sun, Nuts, FHM and Maxim perhaps gives some indication of his attitude towards, and personal interest in, breasts. ‘Surely it’s got to do some long term psychological damage – f*** up your attitude towards breasts,’ is an argument that is wheeled out quite frequently (as demonstrated in this discussion of Nell McAndrew’s decision to breastfeed her toddler), although interestingly, I’ve yet to hear it from a woman. It is of course possible to turn this argument on its head – in many other countries, breasts are viewed as primarily practical, rather than sexual, so if you must view these things in black and white, you could argue that this is the ‘right’ way round – but such men seem strangely unreceptive to this possibility.

The idea that a child may be negatively affected by a memory of breastfeeding is another charge that comes up quite frequently. I’ve never got to the bottom of quite why this would be the case, but it seems, again, to be to based on the premise that breasts are for grown-ups, and getting them out for children is slightly suspect.

I had a recent discussion about feeding older children with a couple of very good friends, and although a concrete age was never mentioned, the topic came up when I started to breastfeed C, who is now nine months old. Although obviously still a baby (she can’t yet walk), she is able to sit up, wave, clap and generally communicate. It’s presently quite unusual for babies in Britain to still be breastfed at this point – according to the latest Infant Feeding Survey, only 20% of mothers make it to nine months. The timing of the conversation may have been entirely coincidental, of course, and nothing to do with me giving C an afternoon snack, but I found it hard to dismiss the thought that there was a coded message in there.

I’ve since talked to my friend about this, and while she was adamant this wasn’t the case, she also admitted she has a bit of a problem with breastfeeding toddlers. When she asked me how long I was planning to feed C, I said I wasn’t sure, but I couldn’t dismiss the possibility of continuing for another few months. She understood my reasons for this, and agreed that this was, in theory, a positive thing, but it clearly wasn’t something she felt entirely comfortable with. I’m ashamed to say that it isn’t something I feel entirely comfortable with either. The idea of feeding C (behind closed doors) is lovely, but the thought of admitting to anyone that I’m ‘still’ doing it is less appealing. I am, however, convinced that this is something I have to address: it isn’t much good complaining about our society’s attitude to breastfeeding, unless I’m prepared to challenge it myself.

The antibacterial properties of breast milk

lab_technicianMany years ago, breast milk was thought to be sterile. While this is far from being the case (it actually contains all manner of germs1), the role it plays in helping keep babies free from harmful disease means it does display some pretty impressive bug-busting capacities.

The immunological components of breast milk help to protect both a mother’s breast and her baby from infection during feeding, as well as aiding the development of the baby’s immune system2. They also have another useful consequence, however: protecting breast milk from disease for some time after it has been expressed, enabling it to be stored. Several studies have examined whether it is safe to keep expressed milk for short periods, and there is general agreement that it can be stored for 8 hours at room temperature (25 degrees C), for three days in the fridge (4 degrees C) and for up to a year in the freezer (-20 degrees C) without any increase in the levels of pathogens (harmful bacteria) it contains3.

Not only does breast milk inhibit the growth of pathogens, however – it actively reduces them. This was convincingly demonstrated in a piece of research examining what happened to milk during short term storage4. Milk was collected from 9 mothers and divided into three samples: the first was analyzed the same day; the second was refrigerated (at 4 to 6 degrees C) for 48 hours; and the third was refrigerated for 72 hours. Each sample was then contaminated with an E.coli solution (the kind of nasty bacteria that dwells in toilets) and left for two hours. When the samples were tested, levels of E.coli had reduced by 80% in both the milk that was fresh and the milk that was 48 hours old. Levels had also diminished in the 3 day-old milk, but only by around 10%, indicating that the antibacterial properties, whilst still present, had started to degrade by this point.

If your baby needs to feed from a bottle or cup, a considerable body of research indicates that it’s safe to give him breast milk that has been stored in the fridge for up to three days, or in the freezer for several months. There is also evidence that if the milk you express does come into contact with germs (keeping pumping equipment sterile in your bag at work isn’t always easy), then the bactericidal components of breast milk should be able to take care of them, providing the milk is under two days old. There may still be potential issues associated with feeding stored, rather than fresh breast milk to your baby: various chemical changes occur in milk once it has left the body, and it’s possible some of these may affect its nutritional value3,5,6. Nevertheless, expressed breast milk remains a healthy alternative to formula, and as a result of its antibacterial qualities, you can rest assured that if your baby can’t feed from you directly, he still has a safe source of food and drink.

  1. Mastitis: causes and management. World Health Organization; 2000.
  2. Adv Food Nutr Res. 2008;54:45-80.
  3. Acta Paediatr Suppl. 1999 Aug;88(430):14-8.
  4. J Pediatr Gastroenterol Nutr. 2007 Aug;45(2):275-7.
  5. Acta Paediatr. 2001 Jul;90(7):813-5.
  6. Biofactors. 2004;20(3):129-37.

Could persistent nipple pain be caused by the strength of your baby’s suck?

barracudaUp to 90% of women experience nipple pain or soreness in the initial stage of breastfeeding, with the pain peaking in the first week, then gradually subsiding1. But what if, after 6 weeks, breastfeeding still hurts? You have been observed by what seems like hundreds of lactation professionals, and everything looks fine: your baby is latching on properly and you don’t have an infection. It simply feels to you like she is just a very ‘enthusiastic’ feeder, demonstrated by her toe-curlingly strong suck. Surely that couldn’t be the problem… could it?

Very little research has investigated the causes of chronic pain during breastfeeding, but one interesting study in this area has found a link between nipple pain and a baby’s ‘intra-oral vacuum’, or suck2. The study looked at two groups of women: 30 mothers who were experiencing persistent, unexplained nipple pain (without injury), despite help from lactation specialists; and a control group of 30 mothers with no problems breastfeeding. The vacuum produced by each baby whilst on the breast was measured using a small tube taped to the nipple and attached to a pressure sensor. The amount of milk the babies consumed was also measured, by weighing the babies before and after the feed.

The results were startling. The babies of the mothers who experienced pain when feeding exerted a vacuum when they were ‘actively’ sucking (taking in milk) that was more than 50% higher than the babies in the control group. In between these periods, when they were resting, the vacuum produced by the babies in the pain group was more than twice as high. Unfortunately, a stronger suck did not translate into more milk: babies in the pain group consumed on average 42% less milk, despite feeding for a similar length of time.

The cause of the lower milk intake wasn’t clear. There is a possibility that it was due to chance, or the experimental set-up, although the amount consumed in the control group babies matched that recorded in previous research, making this less likely. As pain can interfere with the let-down reflex, it’s possible that the simple fact that it hurt was enough to stop the milk from flowing properly3. This may in turn have affected milk production, as the amount of milk a baby consumes determines the rate at which it is produced4. It is important to point out, however, that all the babies in the study were gaining weight sufficiently, so the lower milk consumption documented in this single feed did not appear to translate into a more general nutrition problem.

The reason for the higher vacuum is also elusive. It may in some way be the effect rather than the cause of the restriction in milk flow, although this is purely speculative, and how and why this would happen isn’t clear. It’s also possible that the babies in the study may have been experiencing some other feeding difficulty that they compensated for with a stronger suck, although this had not been identified by any of the health professionals who had come into contact with them.

The study data indicate quite clearly that the women suffering from persistent, unexplained nipple pain had babies who exerted a significantly higher intra-oral vacuum on the breast when feeding. Although the data can’t prove the stronger suck caused the pain, it’s likely the two are related. Could this be the reason why for some women, breastfeeding never really seems to become comfortable? If you’re on the receiving end of high suction, then it’s easy to see how you could feel ambivalent about these results. On the one hand, it may be a relief to know that breastfeeding can be painful as a result of the way that your baby suckles, and not because of something that you are doing wrong. On the other hand, the prognosis may be a little disheartening, as it isn’t immediately clear how you solve a problem like this.

At present, such a diagnosis is unlikely, as intra-oral vacuum is rarely tested. The results of this study, however, suggest that in situations where chronic nipple pain has no obvious cause, that it probably should be (the authors certainly think so). Discomfort when breastfeeding is a difficult and stressful situation to deal with, and only with more research in this area can a cause (and hopefully a treatment) be identified. In the meantime, it seems that affected mothers need to carry on gritting their teeth, and perhaps reach for the pain killers…

  1. Acta Paediatr. 2008 Sep;97(9):1205-9.
  2. J Obstet Gynecol Neonatal Nurs. 2005 Jul-Aug;34(4):428-37.
  3. J Pediatr. 1948 Dec;33(6):698-704.
  4. J Exp Physiol. 1996 Sep;81(5):861-75.