Key concepts in musculoskeletal anatomy

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This post is first in a series of posts that will cover the upper and lower limbs; however, this particular post will be looking at some key concepts which are universal to any part of the musculoskeletal system, and will hopefully make learning musculoskeletal anatomy easier. Try and understand these concepts, before moving on to the anatomy of specific regions in the limbs.

Predicting muscle action

The first principle is that knowing a muscle’s origin and insertion is key to appreciating the movement of the muscle, take tibialis anterior, it originates on the ventral surface of the tibia as its name suggests, it then inserts on to the plantar aspect of the 1st metatarsal and the medial cuneiform. Thus, the tendon goes down the leg over the dorsum of the foot and then travels over the medial edge of the foot to insert on the plantar aspect. Now imagine as the muscle contracts, pulling on the tendon upwards, the dorsum of the foot will rise towards the shin, i.e. it will dorsiflex, but as it is attached to the bottom of the foot, it will also cause the foot to invert at the subtalar joint. Try and visualise the movement in your mind’s eye.

Form and function

Don’t ignore the shape of the muscle you are trying to memorise the function of; ask yourself in which directions do the fibres run? Are all the muscle fibres running parallel in one direction, if so then the movement will be in that line of direction, for example, in biceps, triceps and sartorius etc. However, fascicles (bundles of muscle fibres) can also be arranged in a circular orientation, convergent (pectoralis major) or pennate. The circular orientation is found in muscles typically surrounding an opening. Hence they are often referred to as sphincters, e.g. orbicularis oris, oculi, external urethral sphincter etc. In pennate muscles, the fascicles are attached obliquely to a central tendon, there are different types of pennate muscles, uni, bi and multipennate, look them up and see if the arrangement of their fibres help you understand their function?

More than one possible action

The third important concept, is that origin and insertion points can be reversed, an excellent example to consider here is the psoas major muscle, it originates from the transverse processes of the lumbar spine and inserts on the lesser trochanter of the femur, so imagine as the muscle contracts, it flexes the hip. However, imagine the brain stabilises the hip and prevents it from moving, by contracting the hip extensors, if the brain sends signals for the psoas to contract, the lesser trochanter is fixed and now acts as an origin, when the muscle shortens, the lumbar spine is caused to flex, bending us forwards (flexion) at the hip. Another example is the sternocleidomastoid muscles, contracting individually they rotate the neck to the contralateral side, however, if the neck is stabilised and prevented from moving, contraction of both sternocleidomastoids elevates the sternum, allowing the sternocleidomastoids to act as an accessory muscle of inspiration.

Joint & muscle relationships

When you are trying to remember what action a muscle performs, as well as considering it’s origin and insertion, remind yourself which joint or joints the muscle crosses. Let’s us take the quadriceps, it consists of rectus femoris and the three vasti. The vasti all originate on the femur and converge on the quadriceps tendon, effectively crossing the knee joint. Hence they extend the knee only. Rectus femoris originates from the anterior inferior iliac spine, and just above the acetabulum, it then inserts in the quadriceps tendon; thereby crossing both the hip and knee joints; thus it acts at two joints, it can, therefore, flex the hip or it can extend the knee.

Hilton’s law

Hilton’s law, states that the innervation of the joint and skin is the same as the innervation of the muscles that cross the joint. Hence when trying to recall the innervation of the knee, one needs to consider the muscles acting on the knee joint. The quadriceps are innervated by the femoral nerve, the hamstring group by the tibial and common fibular nerves and the gracilis muscle from the adductor group acts on the knee and is innervated by the obturator nerve. Hence following Hilton’s law, the innervation of the knee is provided by all of the aforementioned nerves.

Polo mints

Don’t forget polo mints, you may have already learnt that the bony pelvis behaves like a polo mint, any attempt to break it in one area, results in a second break elsewhere in the pelvis. However, the pelvis is not the only area to behave this way, the mandible is similar, a fracture in one area should result in searching for a second fracture. Bones joined together by interosseous membrane also behave similarly, so the radius and ulna. A fracture in one will result in dislocation and or fracture in the other. The tibia and fibula can be considered in the same light. Look up Monteggia and Galeazzi fractures.

Functional approach

Before learning an entire list of muscles, their origins, insertions and nerves, try and appreciate what movements occur at each joint. Learning them merely as a list with no functional understanding will be firstly dull and secondly will only stay in your cerebrum for the exam if you’re lucky.

Let’s take the hand, for simplicity we will ignore the thumb (you can do this yourself later). The main joints are the wrist (radiocarpal), metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints. Examine your own hand now; from a functional perspective we need the following:

  • Flexors of the fingers (MCP, PIP and DIP)
  • Flexors of the wrist (Radiocarpal)
  • Extensors of the fingers (MCP, PIP and DIP)
  • Extensors of the wrist (Radiocarpal)
  • Abductors and adductor of the wrist and fingers

Let’s take flexors of the fingers, we have both flexor digitorium profundus (FDP) and flexor digitorum superficialis (FDS). Applying the principles we covered earlier, these muscles travel across the radiocarpal joint to get to the DIP and PIP joints respectively, therefore as well as flexing the fingers they will also weakly flex the MCP and wrist joints.

However, we also have specific wrist flexors, these muscles have the word carpi (carpal bones) in them. Flexor carpi radialis and flexor carpi ulnaris. No prizes for guessing which sides of forearm they run down. Interestingly both of these muscles also produce radial and ulnar abduction at the wrist.

Let’s take a brief look at extensors, note that unlike the flexors of the fingers there are no superficialis and profundus, just extensor digitorum communis. This is most likely because we need far more dexterity and finesse of movement in flexing the fingers than extension. The extensors can be thought of as muscles which reset the fingers before the next needed flexion.

The extensors as a whole do have another important function. Essentially if we were unable to extend our wrist, our wrist would be in dropped wrist position. With your wrist in this position, try and grip something. You will find that it’s tough to use your flexors to grasp anything properly while your wrist is also flexed. This is because there needs to be some tension in the flexor tendons at the wrist for adequate flexion at the fingers. To grip something well, you need to have your wrist in the neutral position (which requires your wrist extensors) to use your finger flexors adequately.

Try applying this functional approach (study the movements of your limbs and ask yourself, what type of muscle do I need to do this?) to the muscles of the thumb, the hip, ankle and feet. You will find it much easier to recall the muscles if you have a functional understanding.

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