Question

Find and describe two examples of 2-joint muscles in either the lower or upper extremity (such as biceps femoris long head, rectus femoris, biceps brachii, gastrocnemius, etc). Explain to me which joints these muscles cross and how they function in moving those specific joints.

Answer 1

• A two joint muscle is which crosses two joint and it produce actions on both joints.
• But the simultaneous action of both joints results in a inability to produce enough force in the muscle.
• This is called as insufficiency.​​​​​​​

   1. Active insufficiency

2. Passive insufficiency

Before seeing the two joint muscle & its function, should understand the concept of insufficiency.

This insufficiency is because of the length - tension relationship in the muscle.

If a muscle presents in its optimal length, it will produce maximum tension. If it is lengthened more, the tension will decrease. But this lengthening increase the passive tension in the parallel group of muscles.

This is exactly happens when a two joint muscle is simultaneously try to produce both actions.

1.Rectus femoris:

Rectus femoris one of the two joint muscle which presents in the group of quadriceps femoris. All other muscles are single joint muscle.

Rectus femoris is arises from the ASIS of iliac crest & crosses hip joint.

In the front of thigh it forms quadriceps femoris group with other ( vatus medialis, vastus intermedialis & vastus lateralis) muscles.

Then it crosses the knee joint & inserted into patellar tendon.

Action of rectus femoris:

It flexes the hip & extends the knee joint.

Active insufficiency of rectus femoris:

Maximal flexion of hip joint is restricted with the knee extension and knee extension is restricted with the maximal hip flexion.

This is because of the shortening of the rectus femoris in one joint causes the reduce the movement in the other. It called as Active insufficiency.

Also this causes the increase of passive tension in the parallel group muscle hamstrings. The knee extension with hip flexion lengthens the hamstrings ( increase in passive tension) which inturn restricts the movement. This is called as Passive insufficiency.

2. Biceps brachii:

It has two heads. The long head arises from the supraglenoid tubercle of scapula & the short head arises from the tip of corocoid process of scapula.

Both heads joined & crosses the shoulder joint anteriorly. It forms a muscle bulk in the anterior compartment of arm. Then it crosses the elbow joint & inserted into radial tuberosity.

Action of biceps femoris:

It flexes the shoulder & elbow joint.

Active insufficiency :

With a flexed elbow, shoulder flexion is restricted. Because a simultaneous action at both joint shortens the muscle & restricts the movement in other joint. This is called active insufficiency.

At the same time a passive tension created in the parallel group (triceps). When attempting both flexion of shoulder & elbow, the triceps extremely lengthens causes the muscle to become passively insufficient to produce force. This is called as Paasive insufficiency.

Conclusion :

Two joint muscle / multi joint muscle

a)simultaneous action at both joints - active tension - muscle shortening - active insufficiency

b) active tension - causes passive tension in parallel group - muscle lengthening - passive insufficiency

* compare it with forearm felxors & extensors. Common example for the length - tension relationship of muscles.