Growth and Motor Development Components in Bush and Mountain Leadership Training
By Lyn Day, S. A. C. A. E., Underdale [Implications in italics by Bob Geary]
Wherever children or adolescents have been taken into the outdoors to participate in walking trips, one can cite examples of accidents, or sometimes even deaths that in all probability would not have occurred had the leader of the group understood the abilities of the young people that they were responsible for. Possibly as worrying are the many other negative outcomes from such experiences, which do not come to our attention because they do not show up at the time. These may result in later medical problems, or rejection of the activity and environment because of the poor quality of the experience.
The first step in preventing any of these occurrences is for leaders to fully understand the physical and physiological capacities and social and emotional needs of the youngsters they work with. The second is to be able to apply this understanding to the real conditions and situations.
The major group considered in this paper will be the adolescent, but since increasing numbers of children of upper primary age are being taken into extended outdoor situations, I will first highlight some special factors relating to this younger age group.
1. Muscle Mass:
Muscle mass is related to strength, endurance, effective support of major joint structures and effective covering or padding for underlying structures as protection from the pressures of load bearing. Just prior to the onset of puberty, the young child, on average, has just less than 50 percent of the muscle mass of an adult.
If we were considering walking without load carrying then the musculature would not be a cause for concern since the child also has less total body weight to shift. In certain tasks such as pull ups for example, this can even give the child an advantage because their strength relative to their body weight is quite high. However, as soon as we introduce load carrying, the child is disadvantaged because of its comparatively smaller muscle bulk. Hence, we must ensure that the load carried is in proportion to its muscular development.
In a similar way, the smaller muscle mass places the child at a disadvantage in regard to the other factors mentioned as soon as load carrying is introduced. Thus, the greater load on the small muscle mass will cause fatigue more quickly and hence endurance will be affected negatively. If major additional stress is placed on joint structures such as might occur when climbing a steep gradient or jumping down from a rock, the chance of injury would be higher because of the smaller muscle mass controlling and protecting joint movement. Likewise, bruising of deep tissue and even injury to nerve pathways can result if loads apply point pressures rather than being spread by sufficient surface tissue.
Any load carrying by children must be in proportion to their total muscle mass. The usual maximum load recommended for adults is one third of total body weight. For children, therefore, pack weight should not exceed one quarter of total body weight, as an absolute maximum and ideally will be less than that.
The load needs to be distributed as effectively as possible. Therefore, children’s packs should have wide, well padded waist belts.
2. Male/Female differences in performance:
Prior to puberty one can expect both sexes to perform on an equal basis. Individual considerations such as obesity, personal fitness levels and physical size are much more important performance considerations than are sex differences.
3. Aerobic Capacity
Children’s aerobic capacity is equivalent to adults when differences in body weight are allowed for.
Given normal fitness levels a child’s aerobic capacity will not limit performance and no special allowance needs to be made in this regard. However, individual differences in fitness levels will be apparent and should be planned for just as with adults.
4. Mechanical efficiency of children:
A shorter stride length and poorer skill levels at load carrying because of less practice and opportunity to modify their gait means that overall children have to expand more energy to do the same amount of work as an adult.
Children will need smaller work to rest ratios ? perhaps something like 30 minutes walking followed by 10 minutes rest.
Children will need shorter total work hours in a day when load carrying. Three is suggested as satisfactory, with a maximum of four.
If the above factors are observed, children can partake of quite long trips. However, if they are pressed too hard, fatigue will be cumulative, energy will be quickly depleted and injury and morale problems will result.
Once the child enters puberty it is entering a phase of relatively rapid growth; in fact the second fastest period of their lives. Their year of most rapid growth generally occurs two years after the onset of puberty and may result in growth of 10?12cm. in the year. Such a rapid change in size, correlated as it is with changes in body proportions often leads to some minor difficulties for the adolescent in terms of motor performance. Furthermore, the leader of bush and mountain walking activities is faced with further complications because of the wide range of individual differences in time of onset of puberty. In the first instance, there is a sex difference, with girls on average being about 18 months ahead of boys at age of onset. However, age of onset can vary over a 5 ? 6 year range so that in any given group of 14 year olds, there are likely to be large differences in stage of physical development. Because these differences will affect performance on any walk which involves load carrying, allowances must be made to cater for the individual who would be disadvantaged at their particular stage of development. The following factors related to physical growth should be considered.
Increase in body size:
The rapid changes in body size through the growth spurt mean that corresponding perceptual adjustments have to be made when performing motor skills. The situation is further complicated because different body segments grow at different times. In addition, sex differences in body proportions develop at this time. For example, male shoulder width increases relative to the female, but female hip width increases relative to the male.
The ‘clumsy adolescent’ is a reality. As they adjust to new body size they can be expected to slightly misjudge clearance on branches and rocks as they walk, to stumble on slight path irregularities that would ordinarily cause no problems, or to land awkwardly as they jump from one level to another.
On the other hand, bushwalking is an excellent activity to help them adjust to their new body dimensions. They are constantly being forced to make easy perceptual judgements without major time constraints and this kind of practice allows continuous accommodation to new proportions.
Differences in typical pressure points and Pack adjustment can be expected between the sexes. For example, because of greater hip width, chaffing and hot spots can develop more readily from hip belt irritation with females. A further example is provided by trunk length. In males and females of equal stature, the female will normally have the longer trunk length and this obviously needs to be reflected in pack size and adjustment
Both males and females show a strength increase through adolescence, but the male steroids give them a strength advantage by the end of the growth spurt. However, in the process of gaining this advantage, there are two factors that a leader needs to be aware of.
a) Because girls enter the growth spurt to up 18 months earlier than boys, then early in adolescence a number of girls will be as strong as, and sometimes even slightly stronger than many of the boys.
b) Increase in size, either of body or muscle, doesn’t immediately mean increase in strength. In fact, the strength gains lag at least 6 ? 12 months behind the size gains, as the adolescent learns the co?ordination to produce large strength outputs from the bigger muscles.
The rapidly growing adolescent can even be at a disadvantage in some strength related tasks for this reason. They increase their body size and are heavier, but initially can’t produce the corresponding strength output.
The big adolescent is not always the strong adolescent, especially if the increase in size has just occurred. Therefore, by virtue of size alone they shouldn’t be expected to perform the ‘heavy’ tasks or extra load carrying.
The age group 12 ? 16 is a very difficult one to lead because of the large difference in performance potential.
12 ? 18 months after the year of maximum growth males should be stronger than females, and would be expected to perform better on load carrying kinds of tasks. However, other factors, and in particular, fitness and training can moderate this generalisation. Furthermore, the difference between males and females should not be a factor if pack loads are kept to one?quarter of total body weight.
3. Changes in body composition:
Mention has already been made of the gains in muscle that occur at adolescence, especially for the male. The female hormones on the other hand predispose the female toward greater development of fat tissue. Hence, by the end of the growth spurt the female typically has a slightly thicker layer of fat just under the skin than the male. In addition, fat pads are associated with breast development, an obvious sex characteristic which needs to be considered in view of some of the chest harnesses incorporated into recent pack design.
Participant’s needs to be considered on an individual basis once again since there are obviously large variations within each sex and obvious overlap in values once one approaches the end range in each sex.
The presence of fat has implications for temperature regulation, energy expenditure and reserves and protective padding of underlying structures in terms of pressure from pack straps and waist belts. The slim person is obviously at greater risk in regard to this latter factor and prolonged numbness (for weeks after the event) can result from point pressure on nerves caused by inadequately padded and narrow shoulder straps. Where chest straps are used on pack harnesses they need to be raised above breast level for females. Care also needs to be taken that the chest strap is not pulled on too tightly when work output is high, or respiratory problems can arise.
4. Growth related injury potential:
There is no hard evidence which indicates that backpacking for children or adolescents leads to growth related injuries. However, this is not to say that they don’t, or won’t happen. In fact, evidence collected about children involved in other sports activities indicate that they face a greater risk from stress related injuries than adults because of the growth processes they are experiencing. Further, some of the damage may not be evident until later in their adult life when further wear or stress causes it to reach a pain threshold.
A stress fracture is an overuse syndrome. It is a partial or complete fracture of bone due to its inability to withstand non-violent stress that is applied in a rhythmic and repeated sub?threshold manner.
There are a number of factors which are considered to predispose children and adolescents to structural damage.
The existence of the epiphyseal or growth plate particularly in the long bones. This relatively sensitive area of cartilage?like material is weaker than fully formed bone around it and is consequently more susceptible to impact or stress fracture.
The relative weakness of supporting muscle tissue. This can have implications in two ways. First, loads and stress that are ordinarily absorbed by the musculature are gradually transferred to the bone structure as the muscle fatigues. Secondly, when severe stresses are imposed the muscle is not strong enough to accommodate it and hence it is once again transferred directly to the bone structures. Such stresses have their most severe effects at the point of attachment of tendons and ligaments to bone, at weaker points in the bone, such as the epiphyseal plate, and at joints where the longitudinal transmission of force compresses joint tissues to cause wear and tear on them. It has been suggested that this kind of degeneration may be a contributing factor to arthritis in later life.
Postural defects. These are often caused by muscle imbalance and if present, can be severely by stresses imposed by load carrying.
Growth related injuries can be seen to be a resultant of a combination of factors which impose an overload on the structural system. These factors are ?
LOAD carried. One quarter total body weight as a maximum as already discussed.
PERIOD of load carrying. This relates to the length of time in any day that the adolescent is load bearing. This is obviously related to the fatigue factor discussed above, and again capacity will be related to individual development. However, four hours per day of actual walking/load carrying is suggested as a maximum for people less than 16 years of age.
FREQUENCY of load carrying. This refers to how often one walks with a pack and it probably is not a factor for concern with most school age children.
SPECIAL STRESSES, such as those imposed when walking downhill, or jumping down from rocks etc. Most students this age will not be involved in situations of prolonged descent, but if they were, pace should be controlled, and rests forced to give stabilising musculature a chance to recover. Similarly routes involving load carrying over long stretches of boulders, as for example, along a rocky foreshore are probably better left for later age groups.
Some postural defects could lead to severe back pain as a result of the increased stresses of load carrying. Collection of pre?trip details should include questions relating to back pain or injury, and it may be necessary to exclude some students if the condition is severe.
Any complaints of pain or discomfort while on the trip should be thoroughly investigated and not set aside as the whingings of a spoilt child. Many stress-related injuries are very difficult for even a physician to recognise, and in most cases, the person can still perform the tasks required, despite some pain. Therefore, whenever suspicion exists that an injury may be present, the problem should be approached conservatively, i.e. load lightened and redistributed and physical support provided if needed, followed by referral to a physician upon return.
The next section considers the physiological changes occurring during adolescence. These changes represent a transitional phase from the functional responses of childhood to those of adulthood. Each day following the year of maximum growth in adolescence brings the individual nearer to the mature adult response and therefore more able to handle the work demands of advanced bush walking.
1. Capacity to perform work:
If we ignore the contribution of muscle mass to strength which has been discussed already, then the main factor defining performance in a bushwalking type task will be the adolescent’s ability to use oxygen. In this regard, individual differences in fitness levels are a much more important factor to consider than are age differences. Certainly, the adolescent uses more oxygen and can do more total work than can the pre?adolescent. Their larger size and corresponding increase in physiological functions ensures this. Thus, cardiac output is greater, they have more haemoglobin to combine with and transport oxygen to working muscles and more myoglobin to store oxygen at the muscle site.
However, the larger person also has to expend more energy just to move their own body weight around. Therefore, to determine one’s relative ability to perform aerobic work it is really necessary to divide the oxygen used by the person’s body weight. When this is done, the 10 ? 12 year old child comes out just as well as the adult in terms of their ability to perform work relative to their body weight. Male/female differences in ability to use oxygen after the year of maximum growth in adolescent do favour the male if one uses average population figures as the norm. However, this difference is largely influenced by culturally induced activity patterns. Hence when groups are compared who have followed similar training programmes the difference tends to disappear
The leader of bushwalking activities needs to be aware of the individual fitness levels of participants. Given loads relative to their size, it is this rather than age which determines their potential work capacity in regard to ability to use oxygen as the limiting factor. The fitness level modifications to Naismith’s rule for route planning provide an indication of the kinds of allowances that should be made.
Individual fitness levels are a much more important consideration than male female differences in route planning decisions which involve aerobic capacity.
2. Ability to regulate body temperature:
Until adult body proportions are attained, the adolescent, and to a greater extent, the child, are at a disadvantage in very hot or very cold environments. There are a number of reasons for this.
Body heat is dissipated by radiation, convection, conduction evaporation, all of which depend on the surface area of the skin. Conversely, the amount of heat transferred from
a very hot environment to the body will likewise be proportional to the surface area of the skin.
Therefore, the larger the skin surface area to body mass ratio the greater will be the heat transfer to or from the body in extreme environments. Thus when one compares the ratio of 360?380cm.2/kg for an 8 year old child to the 240?260cm. 2/kg ratio of an adult it is clear that the greater heat transfer will be a clear liability until adult proportions are attained.
At younger ages, metabolic rate is higher and therefore heat produced through metabolism is greater. Prior to puberty for example, the metabolic rate is 20 ? 30 percent greater than for an adult. Obviously, this will be a disadvantage to the younger person when the climate is hot, and exercise levels are high.
Energy reserves in both the child and adolescent are less than in the adult. Therefore this combined with the higher metabolic rate, mean that energy reserves for heat production over a prolonged period are less.
At a given level of exercise, cardiac output is lower for children than for adults. Therefore, it seems reasonable to assume that available blood flow to the skin will be correspondingly less, thus limiting convective transfer of heat from core to periphery.
Throughout adolescence, adaptations to adult physiological responses are occurring, and sweating patterns are no exception. Children perspire approximately 50 percent less than adults while performing similar tasks in hot environments. This doesn’t necessarily mean that cooling by this means is 50 percent less effective since children do have more sweat glands per surface area of skin. However, it no doubt contributes to the poorer heat tolerance of children.
Loss of body fluids through dehydration causes core temperature to rise significantly. For example, a 1 percent fluid loss in a child causes a rise of 0.28°C in core temperature and this is two to three times the increase one would expect in an adult.
Since even adults exercising in heat cannot recognise dehydration and compensate appropriately even when fluids are readily available we must expect that children and adolescents will voluntarily dehydrate. Therefore, whenever they exercise in hot environments they must be encouraged to drink fluids beyond their subjective need.
Acclimatisation to heat stress does occur in both adults and children, but the response for children is slightly different. First, their level of acclimatisation in lower than for adults. Secondly, their rate of acclimatisation is physiologically slower, but subjectively faster than for adults. This may give them a false confidence and lead to heat stress.
On the other hand, in cold environments once a drop in core temperature begins in a child or young adolescent its rate of decline is much greater than for the older adolescent or adult, and it rarely plateaus out as they do.
Avoid planning trips to environments where temperature extremes are likely to be encountered for participants under the age of 16 -17 years.
Ensure that for any group in hot environments (adults included) fluid intake is maintained to prevent any onset of dehydration. Recommended level at 35˚C = 5 l/day
3. Effect Of menstruation on females:
For most healthy girls, the menstrual period will not affect their ability to participate in bushwalking and associated activities. However, heavy exertion at this time can cause pain in the lower abdomen in some girls. Others, particularly those who are not very fit or not used to the demands of pack carrying may show signs of undue fatigue such as shortness of breath and tiredness.
The leader should bear in mind the possible effects of menstruation, particularly through adolescence, and consider the above factors in any diagnosis made should pain or fatigue symptoms occur for any girl on a trip.
Girls whose history indicates that they experience severe pain at the time of menstruation may have to miss the trip if time of onset of the period overlaps with trip dates.
1. Rarick, G.L. Physical activity: Human Growth and Development. Academic Press, New York. 1973.
2. Espenschade, A. & Eckert, H. Motor Development. Merrill, Ohio. 1978.
3. Bar Or, 0. Climate and the exercising child ? A Review Intl. J.Sports Medicine. 1980, No. 1, pp.53?65
4. Davies, C.T.M. Thermal responses to exercise in children. Ergonomics, 1981, Vol. 24, No. 1. pp.55?61.
5. Gullestad, R. Temperature regulations in children during exercise. Acta Paediat. Scand. 1975. 64: pp.257?263
6. Risks in Long Distance Running for Children. The Physician and Sports
Medicine. Vol. 10, No. 8. August, 1982.
7. Wilkins, K. The Uniqueness of the Young Athlete: Musculoskeletal
Injuries. Am. J. Sports Medicine.. 1980. Vol. 8, No. 5
8. De Haven, K. Athletic Injuries in Adolescents. Pediatrics Annals.
1978.Vol. 7 (16).
This article by Bob Geary and Lyn Day is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.