Read the following the other day: Women work harder than men – our anthropological study reveals why
An interesting article, however it would have more value if it used a more relevant measure of work effort than counting footsteps. A count of 12,000 steps versus 9,000 steps is not an indicator of more work (1.33 times), nor harder work. As with time measurement foot step counting is a poor indicator of work effort.
Also electronic foot step counters do not actually count foot steps, they have sensors which sense vibration, acceleration and movement, which is equated to a foot step. It is possible to be sat down but not still, and achieve a foot step count, it is also possible to be walking and get zero foot steps, they maybe fun but they are not reliable.
Similarly fitness trackers estimates of heart rate and calories burned are based on lots of statistical wrangling and assumptions, two different brands of trackers are likely to give different results. The point of counting steps is to estimate distance based on averaged length of stride, even though the tracker is calibrated to the individuals stride, a persons stride is not constant and is likely to vary during a trip. A measuring wheel will provide a better measure of distance, or a map will provide a reasonable estimate. Basically a fitness tracker is just a wrist watch, measuring chunks of time and making assumptions about that time and calculating various things on the basis of assumptions about feedback from various sensors. Probably do as well with an ordinary wrist watch and do own calculations: point of tracker is it saves some effort and provides a more instantaneous result rather than at some time afterwards. It thus provides feedback allowing adjustment of behavior.
Also a few months back I read an article by an health and fitness writer, she was discussing the issues of couples exercising together and the 10,000 step recommendation. Basically if they go for a walk together, and they have different strides, then if one achieves the 10,000 steps the other will either be behind on their step count or in front. Assume that an average stride is less than 1m, then 10,000 steps is a distance less than 10km. Average walking speed is typically taken as 5km/h or the slightly less 80m/min, additionally typically expect to change mode of movement from walking to jogging/running at around 7km/h.
So assume a couple both complete a 5km trip in 1 hour, then on the face of it they are both walking at 5km/h. Except their strides are not the same, so similar situation to different diameter wheels, the smaller wheel has to do more revolutions per minute than the larger wheel. Say one has a stride of 1m and the other 0.8m, a ratio of 1.25. Then one has made 5000 steps, whilst the other has made 6250 steps, or 83 steps/min compared to 104 steps/min.
So whilst completing the same external work, travelling 5km, the internal work of their bodies is different. Their elevated heart rates will be different, and the fuel each burns will be different. Do we also have a situation of one walking at 5km/h and the other running at 5km/h?
So what do we measure as work, and is it the useful external work of importance or is it the internal work of our bodies we need to measure or both?
Now in physics (mechanics), work done (energy) is the product of force(F) and distance(s) to give (W=F.s) and is measured in Joules(Nm=J). Whilst power is work done divided by time (P=W/t=F.s/t=F.v where v=s/t) and measured in Watts (J/s=W). Also friction force is the product of the force normal to the surface and a friction coefficient (F=mu.N), the normal force is typically the weight of the object in Newtons (Weight=m.g=m 9.81). The friction coefficient is typically less than one, therefore less force is required to slide something than to lift it off the ground. Also rolling resistance is typically less than sliding resistance, so even less force is required to push/pull something on wheels.
Now the calories count on most trackers and gym equipment is an estimate of fuel used, not useful external work. The human body has an efficiency of around 25%, that is only 25% of the fuel used produces useful external work. So from the fitness tracker we can get calories, distance and time. We can estimate external work done (W) as 25% of the calories, we get velocity from distance and time, and can get an estimate of the force exerted from F=W/s, and power from P=W/t. The person with the higher force and the higher power is working harder. {NB; Working longer isn't considered working harder}
Now would expect the heavier person to exert the higher force, since distance is constant for both people, therefore expect work done W=F.s to be higher, and the time is the same for each therefore P=W/t will be higher.
Except the person with the shorter stride is doing a lot more internal work, a lot more leg movements, possibly higher heart rate, and therefore expect to be burning more calories, but not expecting a higher force, and as over all speed is the same, not expecting higher power P=F.v.
But would expect similar power outputs only if smaller force is combined with higher speed. So would require looking at the mechanics of the activity in more detail, for example walking involves both vertical and horizontal movement, lifting an heavy body off the ground requires more force than lifting a lighter body, also sliding or rolling a heavy body requires more force than a lighter body. So who does burn the most calories for the task? Also who is operating closer to their maximum heart rate (MHR), as they would also be working their body harder.
Also need to take into consideration the environment. I recollect reading a book which indicated that traditional Canadian lumberjacks, with an axe, burned some 10,000 Calories (kcal) per day. But this isn't from the work alone, this is mostly just being in the cold. So two people operating in different environments will be burning different amounts of calories.
It is to be noted that sport, health and fitness have theories and equations to do some numbers on maximum heart rate (MHR) and calories. For example based on the various equations for MHR mine varies between 162.5 to 174.2 beats per minute. Exercise at about 90% of MHR is anaerobic metabolism, exercise is typically set around 65% to 85% of MHR. Would therefore expect work to be less than this as typically lasts for longer than exercise. MHR is dependent on age, with younger having higher MHR.
Energy needs are typically based on base metabolic rate (BMR) or resting metabolic rate (RMR), the equations for BMR are dependent on gender, weight, height and age. Total daily energy expenditure (TDEE) is then determined by using a multiplier to account for level of activity. With females typically having a lower BMR than males, so if all other parameters are equal women need less energy. Further given required energy reduces with weight and height, and women typically lighter and shorter, then even less energy required.
This suggests that calorie counting possibly no better than step counting, as expect females to count more steps and burn less fuel for the same task compared to males. On the other hand if males are typically heavier then the force required to shift their weight is greater, so W=25%C, where C is the calorie count, may give a valid estimate of work done, and then a valid estimate of force from F=W/s. This poses a different issue: heavier people have to do more work for a given task. Therefore need to measure the task by some value which is constant, such as travelling 5km, or moving 90 L of water. Also consider weight ratio's and various other ratio's which assist in an equitable comparison. For example an average fit person can generate about 3 Watts/kg for an hour, so the heavier they are the more power they can generate, but the more power they need to shift their weight. If weight has a negative impact on a task then the extra power may not be a benefit.
As well as calculating MHR it is also possible to estimate VO2 max, which is a measure of peak oxygen consumption during activity, oxygen being required for combustion of food/fuel. This being measured in ml/kg/min, which for a given body weight can be converted to L/min, and kcal/min and Watts, from VO2 max, exercise levels, or in our case work, can be based on oxygen uptake reserve (VO2R). If concerned with exercise type work, then there are formulae available for walking, running, cycling, arm cycling, and stepping. For other activities can use metabolic equivalents MET's from the compendium of physical activity. Ironing clothes for example is 1.8 MET's, cooking and food preparation is 2 MET's. carpentry general workshop work is 3 MET's, walking at 3 mph (4.8km/h) with light load 4.5 MET's.
So from activity sampling, and time measurement can get an assessment of daily energy requirements from calculated MET's. Again this is based on fuel, not useful external work, again can estimate useful work at 25%. Note that in calculating energy in kcal or kJ from MET's that an allowance is made for weight. So the heavier person is using more fuel.
The sociologists work was with cultures in Asia, and one noticeable trait is that the males and females are typically not significantly different in height, breadth and weight: that is their basic skeletal frames are similar proportions. Any difference likely due to bone density and muscle mass, that is the males likely slightly heavier due to more muscle mass with stronger muscles wrapped around thicker heaver bones. So are the equations for BMR which vary by gender, valid for such culture? Males and females do have different body chemistry, but is it significant enough to alter BMR?
It does appear that there is no simple single measure of work. Work needs to be defined by multiple characteristics, such as distance, time, weight, energy and power. Also need to reference external work, and the internal work of the human body. If internal work being considered, then MHR and VO2 max, and fuel energy used are also useful measures.
However, hard work should be considered high force or high power, with duration also considered with respect to fatigue from work. Also working at a high percentage of MHR for longer durations may also be considered hard work.
However, time alone does not equate to hard work, 20 hours work versus 8 hours does not make the longer duration hard work. Similarly more foot steps per day does not equate to more work or hard work.
It does however seem that industrial engineering could benefit from the work of sports and fitness medicine, as can the design of human powered machines.
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Revisions:
[31/01/2023: 15:55] : Original
