9.2 Movement, Skeletons and Joints

Movement is a change in position of an organism or part of it. Locomotion is movement of the whole organism from one place to another. A plant leaf changing orientation, a flower opening, a sea anemone retracting its tentacles and a frog projecting its tongue are movements without locomotion. This distinction matters because sessile organisms can perform rapid and biologically valuable movements even though they remain attached to one location.

Many plant movements arise through differential growth. In phototropism, unequal auxin distribution causes unequal cell elongation, producing curvature toward light in a shoot. Heliotropic leaf positioning follows the sun and may improve light interception while limiting overlap. Other plant movements are faster and depend on changes in turgor or mechanically unstable tissues: touch can trigger Mimosa leaflets to fold, while repeated stimulation of trigger hairs closes a Venus flytrap.

Sessile animals also move locally. A sea anemone extends tentacles to intercept prey and retracts them from threats. Remaining in one place reduces the energetic and structural demands of locomotion, but it makes food delivery, dispersal and predator avoidance dependent on water currents, mobile life stages or local body movements. Sedentary is therefore not equivalent to inactive.

An arthropod exoskeleton is a hard external cuticle. Muscles attach to its inner surface, and flexible membrane at joints permits adjacent rigid segments to move. The covering supports and protects, reduces water loss in terrestrial species and supplies broad attachment sites. Its external position also creates a growth problem: because the cuticle cannot expand continuously, it must be shed and replaced during moulting, temporarily reducing protection.

A vertebrate endoskeleton lies inside the body. Bone supports soft tissues, protects organs, stores calcium and phosphate, contains marrow where blood cells form, and supplies rigid levers for movement. Internal bones grow with the animal and do not require whole-body shedding. At a joint, muscle tension applied through a tendon rotates a bone about a pivot. The skeleton does not create force; it redirects and transmits force generated by muscle.

Neither skeleton is universally superior. An exoskeleton can be a strong protective shell at modest body size, while an endoskeleton supports continuous growth and can carry large masses without enclosing every tissue in cuticle. Both solve the same mechanical need: muscles shorten by pulling, so they require firm anchorage and articulated elements against which tension can act.

A joint is where two or more bones meet. In a synovial joint, articular cartilage covers bone ends and distributes load while reducing friction. A capsule encloses the joint; its inner synovial membrane secretes lubricating fluid. Ligaments connect bone to bone and resist excessive displacement. Tendons connect muscle to bone and transmit tension. Nerves include motor fibres to muscle and sensory fibres from proprioceptors that report position and strain.

The hip is a ball-and-socket joint: the rounded head of the femur fits a pelvic socket. Its geometry permits flexion, extension, abduction, adduction and rotation across multiple planes. The elbow and knee are principally hinge joints and therefore allow a much narrower range, mainly flexion and extension. A large range of motion can reduce intrinsic stability, so socket depth, capsule, ligaments and surrounding muscles are important stabilizers.

Muscle produces active force by contracting; it cannot actively push its attachment away. Movement in opposite directions therefore commonly requires an antagonistic pair. When one muscle acts as agonist and shortens, its antagonist relaxes and lengthens. To reverse the movement, their roles exchange. The labels agonist and antagonist depend on the movement being described rather than being permanent properties of a muscle.

Breathing demonstrates antagonism without a limb. During inspiration, external intercostal muscles contract and elevate the ribs while the diaphragm contracts and flattens, increasing thoracic volume. Quiet expiration is largely passive as inspiratory muscles relax and elastic tissues recoil. During forced expiration, internal intercostal and abdominal muscles contract, decreasing thoracic volume. Opposing fibre directions help the intercostal layers produce opposing rib movements.

Partial activation of some fibres can generate muscle tone without visible joint movement. Tone stabilizes posture and keeps the musculoskeletal system ready to respond. Co-contraction of antagonists can also stiffen a joint: both sides generate tension, and although their turning effects may balance, resistance to disturbance increases. Antagonism therefore supports precision and stability as well as reversal.