There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

So far in this course we have dealt entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. There are notes on the course website about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but we didn’t discuss them. In every example we’ve considered we’ve imagined that we could understand something about evolution by examining the evolution of a single gene. That’s the domain of classical population genetics. For the next few weeks we’re going to be exploring a field that’s actually older than classical population genetics, although the approach we’ll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel’s work in 1900 there was a heated debate between the “biometricians” (e.g., Galton and Pearson) and the “Mendelians” (e.g., de Vries, Correns, Bateson, and Morgan). Biometricians asserted that the really important variation in evolution didn’t follow Mendelian rules. Height, weight, skin color, and similar traits seemed to

Human biochemical genetics

The purpose of this Position Stand is to provide guidance to professionals who counsel and prescribe individualized exercise to apparently healthy adults of all ages. These recommendations also may apply to adults with certain chronic diseases or disabilities, when appropriately evaluated and advised by a health professional. This document supersedes the 1998 American College of Sports Medicine (ACSM) Position Stand, "The Recommended Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory and Muscular Fitness, and Flexibility in Healthy Adults." The scientific evidence demonstrating the beneficial effects of exercise is indisputable, and the benefits of exercise far outweigh the risks in most adults. A program of regular exercise that includes cardiorespiratory, resistance, flexibility, and neuromotor exercise training beyond activities of daily living to improve and maintain physical fitness and health is essential for most adults. The ACSM recommends that most adults engage in moderate-intensity cardiorespiratory exercise training for ≥30 min·d on ≥5 d·wk for a total of ≥150 min·wk, vigorous-intensity cardiorespiratory exercise training for ≥20 min·d on ≥3 d·wk (≥75 min·wk), or a combination of moderate- and vigorous-intensity exercise to achieve a total energy expenditure of ≥500-1000 MET·min·wk. On 2-3 d·wk, adults should also perform resistance exercises for each of the major muscle groups, and neuromotor exercise involving balance, agility, and coordination. Crucial to maintaining joint range of movement, completing a series of flexibility exercises for each the major muscle-tendon groups (a total of 60 s per exercise) on ≥2 d·wk is recommended. The exercise program should be modified according to an individual's habitual physical activity, physical function, health status, exercise responses, and stated goals. Adults who are unable or unwilling to meet the exercise targets outlined here still can benefit from engaging in amounts of exercise less than recommended. In addition to exercising regularly, there are health benefits in concurrently reducing total time engaged in sedentary pursuits and also by interspersing frequent, short bouts of standing and physical activity between periods of sedentary activity, even in physically active adults. Behaviorally based exercise interventions, the use of behavior change strategies, supervision by an experienced fitness instructor, and exercise that is pleasant and enjoyable can improve adoption and adherence to prescribed exercise programs. Educating adults about and screening for signs and symptoms of CHD and gradual progression of exercise intensity and volume may reduce the risks of exercise. Consultations with a medical professional and diagnostic exercise testing for CHD are useful when clinically indicated but are not recommended for universal screening to enhance the safety of exercise.

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Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory, Musculoskeletal, and Neuromotor Fitness in Apparently Healthy Adults: Guidance for Prescribing Exercise

The primary purpose of this narrative review was to evaluate the current literature and to provide further insight into the role physical inactivity plays in the development of chronic disease and premature death. We confirm that there is irrefutable evidence of the effectiveness of regular physical activity in the primary and secondary prevention of several chronic diseases (e.g., cardiovascular disease, diabetes, cancer, hypertension, obesity, depression and osteoporosis) and premature death. We also reveal that the current Health Canada physical activity guidelines are sufficient to elicit health benefits, especially in previously sedentary people. There appears to be a linear relation between physical activity and health status, such that a further increase in physical activity and fitness will lead to additional improvements in health status.

/pdf/health-benefits-of-physical-activity-the-evidence-17tsp10zcy.pdf

Health benefits of physical activity: the evidence

The purpose was to: 1) perform a systematic review of studies examining the relation between physical activity, fitness, and health in school-aged children and youth, and 2) make recommendations based on the findings. The systematic review was limited to 7 health indicators: high blood cholesterol, high blood pressure, the metabolic syndrome, obesity, low bone density, depression, and injuries. Literature searches were conducted using predefined keywords in 6 key databases. A total of 11,088 potential papers were identified. The abstracts and full-text articles of potentially relevant papers were screened to determine eligibility. Data was abstracted for 113 outcomes from the 86 eligible papers. The evidence was graded for each health outcome using established criteria based on the quantity and quality of studies and strength of effect. The volume, intensity, and type of physical activity were considered. Physical activity was associated with numerous health benefits. The dose-response relations observed in observational studies indicate that the more physical activity, the greater the health benefit. Results from experimental studies indicate that even modest amounts of physical activity can have health benefits in high-risk youngsters (e.g., obese). To achieve substantive health benefits, the physical activity should be of at least a moderate intensity. Vigorous intensity activities may provide even greater benefit. Aerobic-based activities had the greatest health benefit, other than for bone health, in which case high-impact weight bearing activities were required. The following recommendations were made: 1) Children and youth 5-17 years of age should accumulate an average of at least 60 minutes per day and up to several hours of at least moderate intensity physical activity. Some of the health benefits can be achieved through an average of 30 minutes per day. [Level 2, Grade A]. 2) More vigorous intensity activities should be incorporated or added when possible, including activities that strengthen muscle and bone [Level 3, Grade B]. 3) Aerobic activities should make up the majority of the physical activity. Muscle and bone strengthening activities should be incorporated on at least 3 days of the week [Level 2, Grade A].

/pdf/systematic-review-of-the-health-benefits-of-physical-4z2vkcynlg.pdf

Systematic review of the health benefits of physical activity and fitness in school-aged children and youth

Background Physical activity has been related to reduced risk of osteoporotic hip fractures, but the evidence among men is weak. Objective To determine the association between baseline leisure physical activity and future risk of osteoporotic hip fracture in men. Methods At baseline in 1975 our prospective study cohort included 3262 men who were 44 years or older and did not have chronic disease restricting their ability to exercise. At baseline, physical activity was assessed by a questionnaire. Hip fractures were followed for 21 years, or from the age of 50 years for subjects who were initially younger than 50 years. Results The hazard ratio of osteoporotic hip fracture, adjusted for other possible predictors (height, body mass index, baseline diseases, smoking, use of alcohol, work-related physical activity, and occupational group), in men participating in vigorous physical activity compared with men not participating was 0.38 (95% confidence interval, 0.16-0.91) (P=.03). Conclusion These results provide further evidence that there is an inverse association between baseline physical activity and future hip fracture risk among men.

https://jamanetwork.com/journals/jamainternalmedicine/articlepdf/416048/ioi90183.pdf

Physical activity and osteoporotic hip fracture risk in men.

Does childhood and adolescence provide a unique opportunity for exercise to strengthen the skeleton

The effect of tenotomy and of immobilisation in lengthened and shortened positions on the intramuscular connective tissue of the calf muscles of the rat was studied morphometrically and with a scanning electron microscope. Both tenotomy and immobilisation produced a marked increase in both the endomysial and the perimysial collagen networks, with a simultaneous decrease in intramuscular capillary density. The increase in connective tissue volume was more pronounced and occurred more rapidly in the soleus, which consists mainly of type I, slow-twitch fibres than in the gastrocnemius, which is mainly of type II, fast-twitch fibres. The relative volume of connective tissue increased in parallel with the duration of immobilisation or after tenotomy. There was slightly more increase after immobilisation in a shortened rather than in a lengthened position.

The effect of tenotomy and immobilisation on intramuscular connective tissue. A morphometric and microscopic study in rat calf muscles

Mineral mass, size, and estimated mechanical strength of triple jumpers’ lower limb

Physical training may be able to improve bone strength through site-specific changes in the composition, size and structure of the bone without notable increases in volumetric density. To address this possibility specifically, we compared 14 competitive female weightlifters with 14 female physical therapy students. Peripheral quantitative computed tomographic scans (pQCT) were taken from the distal radius, radial shaft, distal femur, and tibial midshaft of the dominant limb. Analysis of covariance (ANCOVA) was used to estimate the intergroup differences, using body weight and age as covariates. Cortical density did not differ between the weightlifters and controls at any site, whereas trabecular density was greater in the weightlifters, the benefit being 10% (P = 0.186) at the distal radius and 11% (P = 0.040) at the distal femur compared with the controls. Weightlifters' cortical cross-sectional area was 38% (P = 0.029) larger at the distal radius, 26% larger (P = 0.001) at the radial shaft, and 9% larger at the tibial midshaft (P = 0.034). Consequently, the weightlifters' forearm bone strength indices were also significantly higher, the intergroup difference being 41% (P = 0.001) at the distal radius and 43% (P = 0.004) at the radial shaft. Thus, the observed intergroup difference at the distal radius was mainly due to enlarged bone, particularly its cortex, rather than higher volumetric bone density. Findings at the radial shaft were similar. In contrast, weightlifters' trabecular tissue at the distal femur was denser but the bone per se was not clearly bigger than that of the controls' (intergroup difference 5%, P = 0.117). We suggest that bones subjected to exceptionally high bending-loading (distal radius and radial shaft) are larger than their normal counterparts while at sites experiencing axial, compressive-loading (e.g., distal femur), a denser trabecular structure (more load-carrying area) may be sufficient and any substantial enlargement in bone size may not be necessary.

Site-specific skeletal response to long-term weight training seems to be attributable to principal loading modality: a pQCT study of female weightlifters.

Pekka Kannus

Papers

Physical activity and osteoporotic hip fracture risk in men.

Does childhood and adolescence provide a unique opportunity for exercise to strengthen the skeleton

The effect of tenotomy and immobilisation on intramuscular connective tissue. A morphometric and microscopic study in rat calf muscles

Mineral mass, size, and estimated mechanical strength of triple jumpers’ lower limb

Site-specific skeletal response to long-term weight training seems to be attributable to principal loading modality: a pQCT study of female weightlifters.