When you go to sea, whether it’s calm or blowing hard, you are giving your knees a workout. The knee is a bony joint that is a complex collection of bones, muscles, ligaments (soft tissue connecting bone-to-bone), tendons (soft tissue connecting muscle to bone), and cartilage (lubricating bone-to-bone surfaces). The knee enables the common leg motions of walking, jogging, stair ascent and descent, squatting, kneeling, and maintaining a static stance, especially on a moving surface. It is a “synovial joint,” meaning it is encapsulated in fluid that lubricates the bone-to-bone movement.
The biomechanics of knee motion consist of swinging and shock absorption. Swinging consists of a push off from the toes, a swinging motion, and then a heel strike. The knee’s absorbing power while walking is much greater than its force generating power. The peak load through the knee joint is two to three times body weight (BW) during walking, two to five times BW getting up from a sitting to standing position, four to six times BW during stair climbing, and seven to twelve times BW during running. Maintaining stability while working on a lobster boat could fall into a range that includes all of these, from two to twelve times BW.
The knee is often thought of as a hinge. However, because of the complicated shape of the surfaces of the knee bones that slide against each other, the knee’s motion cannot be modeled as a simple hinge. At its center, the knee joint motion is a combination of rolling and sliding bones. Also, it lacks the kind of stability seen in other types of joints like the hip (ball and socket), and the foot (mortise and tenon).
Ligaments provide structural strength
The structural strength of the knee is provided by the ligaments, which are soft tissue connecting bone-to-bone. “Collateral” ligaments, located on either side of the knee joint, control the inward rotation or outward bowing of knees, and bear the weight under those conditions. “Cruciate” ligaments join the shin bone and thigh bone to one another within the internal structure of the knee and prevent any forward/ backward motion of these bones in relation to one another. Athletes and sports fans know that the Anterior Cruciate Ligament (ACL) plays a role in controlling deceleration from jumping and quick changes of direction.
Shock absorption is assisted by a soft tissue called a “meniscus” that lies between the ends of the thigh and shin bones. This tissue produces a fluid that lubricates the articular cartilage that covers the ends of each bone. Like a gasket, the meniscus is the interface between two differently shaped bone surfaces. In the knee this gasket cushions the interface and increases stability by distributing weight across the surfaces of the two bones.
Knee injury
High impact pressure, like a bump or hit to the knee, can cause acute trauma -- tears, abrasions to internal tissue. Longer exposure to pressure in a specific location can cause the same damage over a period of time, especially when the tissue does not have adequate time to regenerate between injuries. Cartilage and ligaments in and around the kneecap have limited blood supply, and thus typically take longer to recover from injury. Contact stress on the knee occurs when kneeling or when pressing the kneecap on the boat hull while reaching overboard to haul in a trap.
Opposing leg muscles, the quadriceps, and the hamstrings, exert force on the knee joint. When the quadriceps contract, the knee straightens; when the hamstrings contract, the knee flexes or bends.
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