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Human factors of the biomechanics of function and injury

This is an excerpt from Sports Injuries Guidebook by Robert Gotlin.

By Adam Gotlin, BS, MS

It would be irresponsible to present an overview of the biomechanics of function and injury without considering the centerpiece of the entire discussion: the athlete. Specifically, an athlete's age, sex, nutrition, and training all have significant effects on the biomechanics of movement. Even further, genetics, disease, and history of prior injury all influence how people move, the forces they create, and how materials in the body respond to loading. As previously discussed, injury occurs when loads exceed the tolerance of a given tissue. We now discuss the factors contributing to an individual's fitness and risk of injury.

Age

As humans grow, the musculoskeletal system undergoes many dramatic structural changes. During childhood and into the early 20s, human tissue generally develops and grows larger and stronger. In later adulthood, tissue begins to degenerate as wear and tear, toxins, and debris accumulate and outpace the healing powers of the body. The load-bearing capacity of tissue changes dramatically with age. Woo and colleagues tested the tensile strength of cadaver anterior cruciate ligaments for young adults (subject ages 22-35) and older adults (60-97). The young adult tendons could withstand roughly 2,160 N of tensile force before tearing, whereas the average peak force for older adults was 658 N (Woo et al. 1991). This is one of the primary reasons why surgeons commonly repair torn anterior cruciate ligaments using allografts, or donor ligament tissue, in older adults, and autografts, or graft tissue from another location in the patient's body, in young adults. In addition to changes in strength, different structures exist for youth athletes that must be considered when studying the biomechanics of injury. Growth plates are cartilaginous areas of long bones that act as sites for bone growth. Injury to growth plates can yield additional complications. Fractures to the area around growth plates can lead to growth that is misaligned with the normal skeleton or premature closure of the growth plate. Abnormal skeletal geometry can lead to suboptimal force distribution, which can lead to concentrated stresses and future injury.

Sex

There are many factors that contribute to differences in typical biomechanics of males and females. Sex has a large influence on structural anatomy, muscle mass, and skeletal geometry, all of which greatly influence how external forces are transmitted throughout the body. Further, hormones, sociological factors, and activity patterns may dictate why one sex is at greater risk of injury than the other. This should be taken into consideration when prescribing training and nutritional programs, as well as evaluating whether recommendations from research studies apply to one or both sexes. As an example, women are roughly five times more likely to tear an anterior cruciate ligament than men in similar sports. This seems to be due to anatomical differences combined with quadriceps dominance in muscle cocontraction over the knee during landing tasks in females (Ford et al. 2011). Sex affects not only the dimensions of the skeleton but also the coordination strategies adopted by athletes.

Nutrition

We are what we eat. Diet has a direct influence on the elements and minerals present in the body that are pivotal to proper function of the musculoskeletal system. Bone relies on calcium, an inorganic compound that the body does not produce on its own, to improve its strength and structural rigidity. Since calcium is secreted throughout the day, an adequate supply is needed in order to maintain mechanical strength. Women stop gaining new bone mass around the age of 30, so it is particularly important for them to maximize calcium intake prior to this age to reduce the chance of osteopenia or osteoporosis (diseases characterized by weak and fragile bones). Further, a core part of the adaptation and healing process is replenishing chemical and mineral deficiencies in the injured area. After an intense workout, athletes are encouraged to consume meals that contain high levels of protein and carbohydrate to facilitate the rebuilding of muscle. Proper levels of key proteins, fats, and other nutrients must exist in the body to facilitate the strengthening and healing of tissue.

Exercise

As discussed earlier, physical activity and normal loading lead to natural modeling and remodeling of body tissue. Bone responds to everyday loading by building new bone mass and remodeling existing bone mass to withstand future similar loading profiles. Normal loading through exercise also causes tendons and ligaments to grow larger or denser, increasing their stiffness and mechanical strength. Muscles respond directly to training and exercise by adapting their fiber structure and material composition. Resistance strength training will cause muscle to add fibers in series, increasing overall contractile strength; stretching will cause muscle fibers to be added in parallel, reducing stiffness and passive forces.

Other Factors

Genetics, disease, and drugs also have substantial effects on the mechanical and material properties of the musculoskeletal system. We already saw that the fast- to slow-twitch muscle fiber ratio is largely determined by genetics. Further, specific genes have been directly linked to muscle mass regulation in the body (Lee 2004). Various diseases can influence the biomechanics of movement by impeding the normal function of musculoskeletal elements. For example, rheumatoid arthritis is a disease in which the body mistakenly attacks its own joint cartilage, which can lead to pain and subsequent suboptimal gait modifications and loading patterns. Pharmaceutical drugs can have drastically positive and negative effects on an individual's biomechanics. Sometimes, the intention behind the drug is to influence biomechanics (i.e., ibuprofen to reduce pain from the inflammatory response), whereas other times the impact is a side effect (e.g., fluoroquinolones antibiotics increase the risk of tendon tear).