Human movement is produced by the skeletal acting as simple lever machines. The physics of a lever system can be directly compared to that of a limb.
In general terms the muscles and bones of the spine (red) are force magnifiers. This force is used to stabilize the skeleton and provide a stable platform(red) for the movement of the limbs. Such lever produce very little range of movement but a great deal of force.
The muscles and bones of the limbs are generally arranged into 3rd class levers and in such a way to become distance magnifiers. The reason for this is to provide range of movement for the limb rather than strength.
The image illustrates the concept of 'range of movement' discussed above.
These simple ideas of machines can be applied to the skeletal system and human movement.
A. Humerus (upper arm) bone.
B. Synovial membrane that encloses the joint capsule and produces synovial fluid.
C. Synovial fluid (reduces friction and absorbs pressure).
D. Ulna (radius) the levers in the flexion and extension of the arm.
E. Cartilage (red) living tissue that reduces the friction at joints.
F. Ligaments that connect bone to bone and produce stability at the joint.
To produce movement at a joint muscles work in pairs.
Muscles can only actively contract and shorten. They cannot actively lengthen.
One muscles bends the limb at the joint (flexor) which in the elbow is the biceps.
One muscles straightens the limb at the joint (extensor) which in the elbow is the triceps.
1. Humerus forms the shoulder joint also the origin for each of the two biceps tendons
2. Biceps (flexor) muscle provides force for an arm flexion (bending). As the main muscle it is known as the agonist.
3. Biceps insertion on the radius of the forearm
4. Elbow joint which is the fulcrum or pivot for arm movement
5. Ulna one of two levers of the forearm
Technically in a flexion like this the Biceps performs a concentric contraction.
6. Triceps muscle is the extensor whose contraction straightens the arm.
7. Elbow joint which is also the pivot (fulcrum)for this movement.
It should be noted that the description of movement is fairly complex. A true Triceps extension takes place against gravity.
Exercise: Bend your arm in a flexion. Point your elbow upwards vertically. Raise your hand vertically above your head. This is a true concentric contraction of the Triceps
Pick up a heavy object in concentric Biceps flexion. Now lower and straighten your arm. You should feel your Biceps contracted but Triceps relaxed. That an eccentric contraction of the Biceps This just shows how complex movement can be!
Comparison of movement at the hip and knee joint:
The knee joint is an example of a hinge joint.
The pivot is the knee joint.
The lever is the tibia and fibula of the lower leg.
A knee extension is powered by the quadriceps muscles.
A knee flexion is powered by the hamstring muscles.
Movement is one plane only.
The Hip Joint:
Rotation is in all planes and axis of movement.
The lever is the femur and the fulcrum is the hip joint.
The effort is provided by the muscles of quadriceps, hamstring and gluteus.
The shoulder is a ball and socket joint.
The humerus is the lever.
The shoulder (scapula and clavicle) form the pivot joint.
Force is provided by the deltoids, trapezius and pectorals.
Movement is in all planes.
1. Tendon connecting muscle to bone. These are non-elastic structures which transmit the contractile force to the bond.
2. The muscle is surrounded by a membrane which forms the tendons at its ends.
3. Muscle bundle which contains a number of muscle cells(4) (Fibres) bound together. These are the strands we see in cooked meat. The plasma membrane of a muscle cell is called the sarcolemma and the membrane reticulum is called the sarcoplasmic reticulum.
4. The muscle fibre Cell)shown here and above is multinucleated
There are many parallel protein structures inside called myofibrils.
Myofibrils are combinations of two filaments of protein called actin and myosin.
The filaments of actin and myosin overlap to give a distinct banding pattern when seen with an electron microscope.
This model show the arrangement of the actin and myosin filaments in a myofibril
Note how the thick myosin filaments overlap with the thinner actin filaments.
Myofibril cross section:
a) Actin only
b) Myosin only
c) Myosin attachment region adds stability
d) Actin and myosin overlap in cross sections
A sarcomere is a repeating unit of the muscle myofibrils.
defined by the distance between two Z lines
large number of mitochondria
1. An action potential arrives at the end of a motor neuron, at the neuromuscular junction.
2. This causes the release of the neurotransmitter acetylcholine.
3 This initiates an action potential in the muscle cell membrane.
4. This action potential is carried quickly throughout the large muscle cell by invaginations in the cell membrane called T-tubules.
5. The action potential causes the sarcoplasmic reticulum (large membrane vesicles) to release its store of calcium into the myofibrils.
6. Myosin filaments have cross bridge lateral extensions.
7. Cross bridges include an ATPase which can oxidise ATP and release energy.
8.The cross bridges can link across to the parallel actin filaments.
9. Actin polymer is associated with tropomyosin that occupies the binding sites to which myosin binds in a contraction.
10. When relaxed the tropomyosin sits on the outside of the actin blocking the binding sites.
11. Myosin cannot cross bridges with actin until the tropomyosin moves into the groove.
12. The calcium binds to troponin on the thin filament, which changes shape, moving tropomyosin into the groove in the process.
13. Myosin cross bridges can now attach and the cross bridge cycle can take place.
Cross Bridge Cycle:
The energy for the cycle is produced by the ATPase section of the crossbridge structure. This energy temporarily changes the shape of the crossbridge which is now attached to the actin polymer. The two slide relative to each other giving an overall shortening
1. The cross bridge swings out from the thick filament and attaches to the thin filament.
2. The cross bridge changes shape and rotates through 45°, causing the filaments to slide. The energy from ATP splitting is used for this “power stroke” step, and the products (ADP + Pi) are released.
3. A new ATP molecule binds to myosin and the cross bridge detaches from the thin filament.
4. The cross bridge changes back to its original shape, while detached (so as not to push the filaments back again). It is now ready to start a new cycle, but further along the thin filament.
This model is for one myosin molecule cross bridging to one actin. Looking at some of the diagrams above we can see that there must be many cross bridges formed but not quite together.
If electron micrographs of a relaxed and contracted myofibril are compared it can be seen that:
These show that each sarcomere gets shorter (Z-Z) when the muscle contracts, so the whole muscle gets shorter.
But the dark band, which represents the thick filament, does not change in length.
This shows that the filaments don’t contract themselves, but instead they slide past each other.