1.1.7–1.1.11 The muscular system GapFill

Skeletal muscles do not work individually to cause movement during sport and physical activity; they work in  antagonisticminorgrossagonist pairs. To put this simply, as one muscle in a pair contracts, the other  growsshortensrelaxeshurts.

The  antagonistagonistgross musclecontraction is the term given to the primary 'instigator' of a joint movement. The contraction of this 'instigator' causes movement at the articulating joint. The  contractionagonistgross muscleantagonist is the term given to the muscle which opposes the motion of its paired 'instigator'. 

Let's explore these muscle terms further with some sporting examples. Firstly, the hamstring and quadriceps are an example of an antagonistic muscle pair. During  abductionextensionadductionflexion at the knee joint (e.g. when pulling the leg back in preparation to kick a ball), the hamstring acts as the  antagonistmajoragonistgross muscle, while the quadriceps acts as the  agonistcontractionantagonistgross muscle. However, they reverse roles when the leg extends (e.g. to kick a ball). 

The biceps and triceps are another example of an antagonistic muscle pair. During   flexionextensionabductionadduction at the elbow (e.g. upwards phase of a biceps curl), the biceps acts as the  antagonistcontractoragonistgross muscle, contracting to reduce the angle between the upper arm and lower arm and lift the weight, while the  tricepslatissimus dorsigastrocnemiuspectorals acts as the antagonist. However, in this example, when lowering the weight in a controlled way the biceps still acts as the agonist, otherwise the weight would come down too quickly. 

Other examples of antagonistic pairs include:

• the gastrocnemius and   hamstringssoleusgluetus maximustibialis anterior during plantar flexion / dorsiflexion at the ankle
• the hip flexors and   gluetus maximussoleustibialis anteriorexternal obliques during flexion/extension at the hip