Appearance
6.6 — Muscles of the Lower Limb
Standing still is not passive. Your calf muscles are firing continuously to stop you toppling forward, because your body's centre of mass sits slightly in front of your ankles. Small corrections run several times a second, and you are unaware of every one of them.
The lower limb muscles are built for that kind of work — sustained, load-bearing, mostly resisting gravity rather than generating movement. They are the largest muscles in the body, they contain the highest proportion of slow fibres, and one of them doubles as a second heart.
The gluteal muscles
Gluteus maximus is the largest muscle in the body by volume. It extends the hip and laterally rotates it.
It is barely used in ordinary walking on the flat. It comes into play in climbing stairs, standing up from sitting, running, and sprinting. This is why it is often weak in people who sit for a living, and why hip extension weakness shows up when someone struggles to rise from a low chair without using their arms.
Gluteus medius and minimus are the ones that matter more than their reputation suggests. They run from the side of the pelvis to the greater trochanter of the femur, and they abduct the hip — which in practice means they stop the pelvis dropping when you stand on one leg.
Chapter 5.7 gave the mechanics: standing on one leg, body weight acts at the midline while the supporting hip is off to the side, and these muscles must generate the counter-force. They are working every single step you take, on alternating sides, for the whole of the stance phase.
When they are weak or their nerve is damaged, the pelvis drops on the opposite side during single-leg stance — the Trendelenburg sign. The person compensates by leaning their trunk over the standing leg, producing a rolling gait.
And this is why gluteal injections must be given in the upper outer quadrant of the buttock. The sciatic nerve runs through the lower medial part, and injecting there causes permanent damage. The safer modern practice is to use the ventrogluteal site — on the side of the hip — or the deltoid where possible.
Gluteal tendinopathy, where these tendons attach at the greater trochanter, is a very common cause of lateral hip pain, particularly in women over 40. It is frequently misdiagnosed as "bursitis" or as arthritis, and the distinguishing feature is that it hurts to lie on that side at night and to cross the legs. Treatment is load management and progressive strengthening, not injection alone.
The hip flexors
Iliopsoas — actually two muscles, psoas major from the lumbar vertebrae and iliacus from the inside of the pelvis, joining to insert on the femur. The most powerful hip flexor, and it is unusual in crossing from the spine directly to the leg.
Two consequences of that route. Tight hip flexors from prolonged sitting pull on the lumbar spine and increase lumbar curvature, which is one contributor to the low back pain of sedentary work.
And the psoas sign is a genuine clinical test. Because psoas lies against the back of the abdominal cavity, inflammation touching it — a retrocaecal appendix, a psoas abscess — causes pain when the muscle is stretched. Extending the hip with the patient lying on their side produces pain, and it localises the problem in a way that palpating the abdomen does not.
Rectus femoris — one of the quadriceps, and the only one crossing the hip as well as the knee, so it flexes the hip too.
Sartorius — the longest muscle in the body, running diagonally from the front of the pelvis across the thigh to the inner knee. Named from the Latin for tailor, after the cross-legged sitting position it produces.
The quadriceps
Four muscles on the front of the thigh — rectus femoris and three vastus muscles — converging into one tendon that encloses the patella and continues to the tibia.
They extend the knee, and they control its flexion eccentrically, which is the larger part of what they do. Going downstairs, sitting down slowly, and landing from a jump are all quadriceps working eccentrically while lengthening.
Quadriceps wasting happens remarkably fast. Measurable loss occurs within days of immobilisation or knee injury, largely because pain and swelling in the knee reflexively inhibit the muscle. This is why rehabilitation after any knee problem starts with quadriceps activation immediately, and why the muscle should be worked before, not only after, knee surgery.
Vastus medialis, the innermost, is important for patellar tracking — it pulls the patella medially against the tendency of the other three to pull it laterally.
The hamstrings
Three muscles on the back of the thigh — biceps femoris, semitendinosus, semimembranosus — running from the ischial tuberosity to below the knee.
They cross two joints, extending the hip and flexing the knee, and that is exactly what makes them vulnerable.
Hamstring strain is the commonest injury in sprinting sports. The mechanism is specific: during the late swing phase of a sprint, the hip is flexing forward while the knee is extending, so the hamstring is being stretched at both ends at once while contracting eccentrically to decelerate the leg. Peak force occurs at peak length.
It is a two-joint muscle problem, and the same vulnerability applies to rectus femoris and gastrocnemius.
Recurrence rates are high — around 15 to 30 percent within a year — largely because rehabilitation is often stopped when pain resolves rather than when strength and length are restored. The Nordic hamstring exercise — an eccentric exercise — reduces hamstring injury rates by roughly half in trials, and it is one of the best-evidenced injury prevention measures in sport, and one of the least used because it is unpleasant.
Hamstring flexibility and back pain are linked. Tight hamstrings limit forward bending at the hip, so the lumbar spine bends more to compensate, increasing disc loading (Chapter 5.3).
The adductors
Five muscles on the inner thigh, pulling the leg toward the midline and stabilising the pelvis during gait.
Adductor strain — "groin strain" — is common in sports with rapid direction change: football, hockey, ice skating. Chronic groin pain in athletes is genuinely difficult, because the adductor origin, the hip joint, the abdominal wall and the pubic symphysis all sit within a few centimetres and all produce similar pain.
Gracilis, one of the adductors, is long, thin and relatively expendable, so it is commonly harvested as a tendon graft for ACL reconstruction along with semitendinosus.
The calf
Gastrocnemius — the visible bulge, with two heads crossing both the knee and the ankle. Powerful, fast, and predominantly fast-twitch. Because it crosses the knee, it is at its strongest when the knee is straight, which is why the calf raise feels different straight-legged and bent-legged.
Soleus — deeper and broader, crossing only the ankle. Predominantly slow-twitch and built for endurance, and it is the muscle doing the continuous postural work described at the start of this page.
Both join the Achilles tendon.
Testing them separately is straightforward and diagnostic: a calf raise with the knee straight tests mainly gastrocnemius, with the knee bent tests mainly soleus.
The calf muscle pump
This is the most important thing in the chapter and the one least often taught.
Veins in the legs must return blood upward against gravity, and they have very little pressure to do it with — the arterial pressure has almost all been dissipated by the time blood reaches the venules.
The calf muscles are the pump. When they contract, they compress the deep veins running between them, squeezing blood upward. One-way valves in the veins prevent it falling back, so each contraction advances the column. Relaxation lets the veins refill from the superficial system.
The numbers are substantial: walking reduces the pressure in the leg veins from around 90 mmHg standing still to about 20 to 30 mmHg. A single calf contraction can move 40 to 60 percent of the blood in the calf veins.
The consequences of that pump failing run through several chapters.
Standing still for long periods allows venous pressure to stay high, which is why guardsmen faint on parade and why people feel their legs swell after a day standing.
Immobility is the central risk factor for deep vein thrombosis. No pump, stagnant blood, clot forms (Chapter 18.10). This is why long flights, long car journeys, hospital bed rest and post-operative immobility all raise the risk, and why the advice is to walk and to move the ankles. Calf pump exercises done in a seat genuinely work.
Chronic venous insufficiency results when the valves fail, so blood falls back with each relaxation. The high pressure damages skin over the ankle, producing pigmentation, thickening and eventually venous ulcers. Compression stockings work by supporting the veins so the pump becomes effective again, and they are the mainstay of treatment.
And this is why compression stockings and calf compression devices are used after surgery and in immobile hospital patients — they substitute mechanically for a pump that is not running.
The foot muscles
Extrinsic — in the leg, with tendons crossing the ankle.
Tibialis anterior lifts the front of the foot (dorsiflexion). Its failure produces foot drop, from common peroneal nerve injury (Chapter 5.7) or an L5 root problem.
Tibialis posterior supports the medial arch. Its failure causes progressive flat foot in adults, a genuinely disabling condition that is frequently missed early when it is treatable.
Peroneal muscles on the outside evert the foot and stabilise the ankle laterally.
Intrinsic — around 20 small muscles within the foot, supporting the arches and controlling the toes finely.
"Shin splints" — medial tibial stress syndrome — is pain along the inner border of the shin from repetitive loading, common in runners increasing distance too quickly. The important thing is to distinguish it from a tibial stress fracture, which is more localised, worse, and needs rest rather than modified training. Localised bone tenderness over a small area, and pain at rest or at night, points to a stress fracture.
Where the nerves run
Two nerves supply almost everything below the knee, and both are testable in seconds.
The sciatic nerve — the largest nerve in the body, about as thick as a thumb where it leaves the pelvis. It supplies the hamstrings and then divides behind the knee into the tibial and common peroneal nerves.
Tibial nerve — the back of the leg and the sole. Test: plantarflexion — push the foot down against resistance.
Common peroneal nerve — the front and outer leg and the top of the foot. Test: dorsiflexion — pull the foot up. The most commonly injured nerve in the leg, at the fibular neck.
Femoral nerve — the quadriceps and the skin over the front of the thigh. Test: knee extension.
And the L4, L5 and S1 nerve roots have quick tests that localise a disc herniation (Chapter 5.3): L4 gives knee extension and the knee reflex, L5 gives dorsiflexion and big toe extension, S1 gives plantarflexion and the ankle reflex.
What the next page fixes
The Part has covered how muscle works and what each group does. Chapter 6.7 covers what goes wrong: the everyday problems — cramp, strain, delayed soreness — and the serious ones, including the muscular dystrophies, rhabdomyolysis, and the reason a statin side effect occasionally becomes a medical emergency.