Deltoids

#Function

Three heads, one shared insertion, three different jobs. Anterior (front) delt flexes the shoulder — raises the arm forward. Lateral (middle/medial) delt abducts the shoulder — raises the arm out to the side. Posterior (rear) delt extends and horizontally abducts the shoulder — pulls a raised arm back (covered in the previous lesson, since it functions with the upper back).

#Origin and insertion

Origin spans the clavicle (anterior fibres), the acromion (lateral fibres) and the spine of the scapula (posterior fibres) — three distinct starting points converging on one insertion, the deltoid tuberosity of the humerus.

#The lateral delt's dumbbell problem

With a dumbbell held at your side and raised laterally, the resistance is gravity acting straight down. At the bottom of the movement, the arm hangs close to vertical — the moment arm between the weight and the shoulder joint is close to zero, and the exercise is nearly unloaded there. Resistance only becomes meaningful once the arm is well away from the body.

This means a standard dumbbell lateral raise trains roughly the top two-thirds of the range and barely trains the bottom third at all — precisely the region where the deltoid is longest and, by the same length-tension logic used throughout this course, where mechanical tension would otherwise be highest.

The fix: a cable set up so the line of pull crosses the body, or a machine lateral raise, keeps meaningful resistance on the muscle even in the bottom, lengthened position. This is why cable laterals appear ahead of dumbbell laterals in the programs on this site — not as a preference, but because the resistance profile genuinely covers more of the muscle's useful range.

#Anterior delt — usually already covered

Nearly every pressing movement (incline press, overhead press, flat press) already loads the anterior delt heavily as a secondary mover. Direct anterior raise work is rarely necessary and is one of the more commonly over-programmed isolation exercises in commercial plans.

#Exercise selection

Source tier: the moment-arm analysis of a dumbbell lateral raise is directly measurable biomechanics — this is one of the more rigorously demonstrated resistance-profile arguments in the whole course, not an inference.