The body as a system
The human body is a system made up of lots of different subsystems. They rely on each other to function, so it’s important that they work together effectively. We’ll focus on the nervous, skeletal, and muscular systems.
Nervous system
The nervous system communicates messages. These messages travel between the brain and different parts of the body. Our brain is made up of nervous tissue, which is formed of groups of specialized cells. These nerve cells (also known as neurons) communicate with each other. They do this by sending signals to and receiving signals from other systems.
The nervous system works with the muscular system. When an athlete wants to kick a ball, the brain decides which parts of the body need to move—in this case, parts of the leg. The brain then sends a signal along the athlete’s nerves via the nervous system. This signal will contract the muscles used in the desired movement.
Skeletal system
The skeletal system is made up of bones and cartilage. It provides structure to the human body and protects internal organs. It also enables movement alongside the muscular system.
Bones and cartilage are both made up of tissue. Bone (known as osseous tissue) is a hard, dense, connective tissue that forms the structural elements of the skeleton. Cartilage is a semi-rigid connective tissue that aids movement. It normally covers the ends of bones as a smooth layer that enables movement, stabilizing joints and absorbing shock. Joints form where two bones meet, and are bound together by ligaments.
Muscular system
The muscular system works with the skeletal system to perform movement, such as kicking a ball. Each muscle is made from groups of long, thin cells called muscle fibers. These muscle fibers receive a signal from the nerve and the muscle contracts in response. It relaxes when there is no signal. Muscles are grouped together in opposing pairs. When one muscle in a pair contracts, the other relaxes. This is what creates movement. One example of a muscle pairing is that of the hamstrings and quadriceps. Both of these muscles are found in the thigh and would contract and relax in order for a leg to kick a ball.
There are many subsystems in the muscular system, including muscles, tendons, ligaments, and other connective tissues. Muscles are attached to bones by rope-like structures called tendons. Both tendons and ligaments add stability to a joint by preventing twisting, wear and tear, and excess movement.
Prostheses and systems
So, how do these body systems relate to prostheses? Well, a prosthesis is an artificial substitute for an absent or malfunctioning body part. Its function can be practical, helping the wearer perform daily tasks. It can also be cosmetic, meaning that it changes the wearer's appearance.
To design prostheses, engineers draw inspiration from the way natural limbs work to create a device that mimics body movement. Prostheses also have to connect to and work well with existing body systems. The point where a prosthesis attaches to the body is called the anchor point.
Prostheses and the nervous system
Prostheses aren’t directly connected to the nervous system. But nerves are very important, because they carry signals to the body system controlling the prosthesis.
Prostheses are fitted during multiple physical therapy sessions. In these sessions, the wearer retrains their nervous system. People with limb differences use repetitive actions to change message pathways in their nervous systems, rewiring the signal types sent to and from the brain. This creates new movement patterns.
In high-tech prostheses, the electrical wiring tries to replicate the nervous system.. The wires send electrical signals to an externally-powered, motorized prosthesis.
Prostheses and the muscular system
Some types of prostheses rely on muscles to enable movement. Body-powered prostheses can be controlled by a harness system. They connect to body parts such as the shoulder, wrist, elbow, or leg. Flexing these body parts creates tension that moves the prosthesis. These types of prostheses rely on opposing pairs of muscles contracting and relaxing.
Myoelectric prostheses need an external power source. They use electrical signals generated by muscles to create movement. The wearer puts pressure on their prosthetic through the flexing and contracting of muscles. This action creates movement. In a prosthesis with multiple joints, one muscle might control a sequence of actions. This can help the wearer feel more in sync with their movements.
Prostheses and the skeletal system
Technical challenges increase depending on how much of the limb is being substituted. A below-the-knee (transtibial) prosthetic leg must replace the calf, ankle, and foot. An above-the-knee (transfemoral) prosthesis must also replace the knee, and possibly the thigh. The design of both types of prostheses are inspired by the structure, strength, and coordination of the skeletal system.
An endoskeletal prosthesis copies the purpose and properties of the skeletal system. This type of prosthesis is often made up of metal or carbon fiber structures. They provide structural integrity and support, similar to the skeletal system.
Conclusion
It’s essential that engineers and designers consider body systems when designing prostheses. These systems provide design inspiration, and are often used as anchor points. Here are a few examples:
- In the back-and-forth motion enabled by knee-like hinge joints in transtibial legs
- In the use of muscles to facilitate movement
- In the training of nerve pathways to adjust to new limbs
Our understanding of the human body and its subsystems is vital for the development of prostheses. By understanding this, engineers can create designs that work in harmony with the body.
Glossary
anchor point the place where a prosthesis attaches to the user’s body.
muscle fiber a cell that makes up muscles.
myoelectric an adjective used to describe the electric impulses generated by muscles.
neuron a cell within the nervous system, also known as a nerve cell—its job is to carry information from one part of the body to another.
osseous tissue the hard, dense connective tissue that forms bones.
prosthesis an artificial substitute for an absent or malfunctioning body part.
transfemoral an adjective used to describe an above-the-knee prosthesis or amputation.
transtibial an adjective used to describe a below-the-knee prosthesis or amputation.