Locomotion and Movement — Practice Questions with Answers

Skeletal muscle: Striated, voluntary, typically attached to bones, responsible for body movement.

Smooth muscle: Non-striated, involuntary, found in the walls of internal organs (like the gut and blood vessels).

Cardiac muscle: Striated but involuntary, found exclusively in the heart, responsible for its continuous, rhythmic contraction.

The origin of a muscle is its point of attachment to a relatively fixed or stationary bone, which does not move (or moves comparatively less) during muscle contraction.

The insertion of a muscle is its point of attachment to the bone that is actually moved when the muscle contracts — the insertion point moves toward the origin during contraction.

Upon nerve stimulation, calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm (muscle cell cytoplasm).

These calcium ions bind to troponin (a regulatory protein associated with the actin filament), causing a conformational change that shifts tropomyosin away from the myosin-binding sites on actin — this exposes the binding sites, allowing myosin heads to attach to actin and initiate the cross-bridge cycling responsible for muscle contraction.

The axial skeleton forms the main, central body axis, comprising the skull, vertebral column, ribs, and sternum.

The appendicular skeleton comprises the bones of the limbs (arms and legs) and their girdles (the pectoral girdle, connecting the arms to the axial skeleton, and the pelvic girdle, connecting the legs).

Synovial joints are freely movable joints, lubricated by synovial fluid, allowing varying degrees and types of movement.

Hinge joint (movement in one plane only): Example — the elbow joint or knee joint.

Ball-and-socket joint (movement in multiple planes, allowing rotation): Example — the shoulder joint or hip joint.

Muscular dystrophy is a group of genetic disorders characterised by progressive degeneration and weakening of skeletal muscle tissue over time, typically caused by mutations in genes responsible for producing proteins essential for maintaining normal muscle fibre structure (such as dystrophin).

As muscle tissue progressively degenerates, affected individuals experience gradually worsening muscle weakness, mobility difficulties, and in severe forms, can eventually affect breathing and heart function, since these organs also rely on functioning muscle tissue.

Tendons are tough, fibrous connective tissue structures that connect skeletal muscle to bone, transmitting the force of muscle contraction to produce movement of the bone.

Ligaments are similarly tough, fibrous connective tissue structures, but connect bone to bone (typically at a joint), providing stability and helping to restrict/guide the range of movement possible at that joint.

Rigor mortis is the stiffening of skeletal muscles that occurs a few hours after death, caused by the loss of ATP production in the now-non-living muscle tissue.

Without a continuous supply of ATP (required to allow the myosin heads to detach from actin and end the cross-bridge cycle), the actin-myosin cross-bridges formed during the last contraction become permanently locked in place, since the muscle can no longer actively relax — this results in the characteristic temporary stiffening of the body observed after death.

The pectoral girdle (comprising the scapula and clavicle on each side) connects the bones of the upper limb (arm) to the axial skeleton, and provides attachment points for numerous muscles that move the arm and shoulder.

The pelvic girdle (comprising the hip bones) connects the bones of the lower limb (leg) to the axial skeleton (specifically the sacrum of the vertebral column), providing a strong, stable base for weight-bearing and attachment points for muscles that move the leg.

Since a single muscle can only actively pull (contract), never push, movement in opposite directions at a joint requires pairs of muscles working in opposition to each other — this arrangement is called an antagonistic pair.

At the elbow joint, the biceps muscle contracts to flex (bend) the arm, while the triceps muscle contracts to extend (straighten) the arm — when one muscle of the pair contracts, the other must relax, allowing coordinated, controlled movement in both directions at the joint.

The sliding filament theory explains that muscle contraction occurs due to the thin actin filaments sliding over the thick myosin filaments within each sarcomere, without either filament type itself changing in length — this sliding shortens the overall sarcomere, and the cumulative shortening of many sarcomeres in series produces the observed muscle contraction.

Troponin and tropomyosin are regulatory proteins associated with the actin filament — in a relaxed muscle, tropomyosin blocks the myosin-binding sites on actin, preventing cross-bridge formation. Upon nerve stimulation and calcium release, calcium binds to troponin, causing tropomyosin to shift away and expose the binding sites.

Cross-bridge cycle: Myosin heads (energised by ATP hydrolysis) then bind to the newly exposed sites on actin, forming cross-bridges. The myosin head then undergoes a "power stroke," pulling the actin filament inward — ATP subsequently binds to myosin, causing it to detach from actin, ready to repeat the cycle as long as calcium and ATP remain available, progressively sliding the actin filament further along the myosin filament.

A skeletal muscle fibre is a single, elongated, multinucleated muscle cell, enclosed by a plasma membrane called the sarcolemma.

Within each muscle fibre are numerous, parallel, thread-like structures called myofibrils, which run the length of the fibre and are responsible for the actual contractile activity of the muscle.

Each myofibril is composed of repeating units called sarcomeres, arranged end-to-end — each sarcomere (bounded by Z-lines) contains the precisely organised, overlapping arrangement of thick myosin filaments and thin actin filaments responsible for the striated appearance of skeletal muscle under a microscope.

The sarcoplasmic reticulum is a specialised, extensive network of membranous tubules surrounding each myofibril, serving as the primary storage site for calcium ions — upon nerve stimulation, this calcium is rapidly released into the sarcoplasm, triggering the contraction process described by the sliding filament theory.

A neuromuscular junction is the specialised synapse between a motor neuron's axon terminal and a skeletal muscle fibre.

Step 1: When a nerve impulse (action potential) reaches the axon terminal, it triggers the release of the neurotransmitter acetylcholine into the small gap (synaptic cleft) between the neuron and the muscle fibre.

Step 2: Acetylcholine diffuses across this gap and binds to specific receptors on the muscle fibre's sarcolemma (motor end plate), triggering a localised electrical change (an action potential) in the muscle fibre membrane.

Step 3: This action potential rapidly spreads across the entire sarcolemma and deep into the muscle fibre via specialised tubular invaginations (T-tubules), ultimately triggering the release of calcium ions from the sarcoplasmic reticulum.

Step 4: This released calcium then initiates the sliding filament mechanism of contraction, as previously described — this entire sequence, from nerve impulse to calcium release, ensures that muscle contraction occurs rapidly and precisely in direct response to nervous system signalling.

Fibrous joints are immovable joints, where adjacent bones are tightly bound together by dense fibrous connective tissue, allowing essentially no movement — example: the sutures of the skull, joining the individual cranial bones.

Cartilaginous joints allow only limited, slight movement, with adjacent bones connected by a pad or plate of cartilage — example: the intervertebral discs between adjacent vertebrae in the spine, which allow slight bending/flexibility of the vertebral column.

Synovial joints are freely movable joints, characterised by a fluid-filled joint cavity (containing lubricating synovial fluid) enclosed within a joint capsule — example: the knee joint (a hinge-type synovial joint) or the shoulder joint (a ball-and-socket synovial joint), both allowing extensive, free movement.

This range of joint types, from completely immovable to freely movable, reflects the varying functional requirements of different regions of the skeleton — some areas require rigid stability and protection (like the skull), while others require extensive mobility for movement and locomotion (like the limbs).

Amoeboid movement: Movement achieved through the formation and extension of temporary cytoplasmic projections called pseudopodia — example: Amoeba, and certain human white blood cells (macrophages) during immune response.

Ciliary movement: Movement achieved through the coordinated, rhythmic beating of numerous short, hair-like cilia — example: movement of mucus along the respiratory tract lining (trachea), aided by ciliated epithelial cells.

Flagellar movement: Movement achieved through the whip-like, undulating motion of one or a few longer flagella — example: movement of human sperm cells, propelled by a single flagellum (tail).

Muscular movement: Movement achieved through the coordinated contraction of specialised muscle tissue, typically acting on a skeletal system — example: locomotion (walking, running) in humans and other vertebrates, achieved through skeletal muscle contraction acting on the bones and joints of the limbs.

Osteoporosis is a condition characterised by a progressive decrease in bone mass and density, resulting in bones becoming increasingly porous, fragile, and susceptible to fracture, even from relatively minor stresses or falls.

Cause: Bone tissue is continuously being broken down and rebuilt throughout life (bone remodelling) — osteoporosis occurs when the rate of bone breakdown (resorption) exceeds the rate of new bone formation, leading to a net, progressive loss of bone density and mineral content over time.

Effects: Weakened, porous bones are significantly more prone to fractures, particularly in weight-bearing bones like the hip, spine, and wrist — even minor falls or everyday activities can result in serious fractures in advanced cases.

Risk factors: Advancing age (particularly post-menopausal women, due to declining oestrogen levels, which normally help maintain bone density), inadequate dietary calcium/vitamin D intake, insufficient physical (weight-bearing) exercise, and certain genetic/hormonal factors are all recognised risk factors contributing to the development of osteoporosis.

Once nerve stimulation ceases, calcium ions are actively pumped back into the sarcoplasmic reticulum (an active transport process requiring ATP), lowering the calcium concentration within the sarcoplasm.

With reduced calcium available, calcium detaches from troponin, allowing tropomyosin to move back into its original position, once again blocking the myosin-binding sites on the actin filament.

Without access to these binding sites, myosin heads can no longer form new cross-bridges with actin — since the sliding filament mechanism can no longer be actively driven forward, and the muscle fibre's inherent elasticity (along with the pull of any antagonistic muscle) helps return the actin and myosin filaments to their original, relatively less-overlapped resting position, the muscle fibre relaxes and returns to its resting length.

Bone is a specialised, rigid connective tissue, composed of living bone cells (osteocytes) embedded within a hard, mineralised extracellular matrix, primarily composed of collagen fibres (providing flexibility/tensile strength) reinforced with calcium phosphate mineral deposits (providing rigidity/compressive strength).

Compact bone is the dense, solid outer layer of bone, composed of tightly packed structural units called osteons (Haversian systems), each consisting of concentric rings of bone matrix surrounding a central canal containing blood vessels and nerves — compact bone provides strength and protection, forming the outer shaft of long bones.

Spongy (cancellous) bone is found in the interior of bones, particularly at the ends of long bones and within flat bones — it has a porous, lattice-like structure (composed of thin, interconnected struts called trabeculae), which is lighter in weight than compact bone while still providing structural support, and often contains red bone marrow (responsible for blood cell production) within its interconnected spaces.

The human vertebral column (spine) consists of 33 individual vertebrae, divided into five distinct regions: 7 cervical vertebrae (neck region), 12 thoracic vertebrae (upper back, each articulating with a pair of ribs), 5 lumbar vertebrae (lower back), 5 fused sacral vertebrae (forming the sacrum), and 4 fused coccygeal vertebrae (forming the coccyx/tailbone).

Functional significance: The vertebral column serves multiple critical functions — it provides the main structural, axial support for the body (allowing an upright posture in humans); it protects the delicate spinal cord, which runs through a central canal formed by the vertebrae; it provides attachment points for numerous back and postural muscles; and the presence of cartilaginous intervertebral discs between adjacent vertebrae provides flexibility (allowing bending and twisting movements) while also acting as effective shock absorbers, cushioning the vertebral column during physical activity.

Slow-twitch (Type I) fibres contract relatively slowly but are highly resistant to fatigue, relying primarily on aerobic (oxygen-dependent) metabolism for sustained ATP production — these fibres contain abundant mitochondria and myoglobin (giving them a characteristic reddish colour), making them well-suited for sustained, endurance-type activities (such as long-distance running or maintaining posture).

Fast-twitch (Type II) fibres contract rapidly and generate greater force, but fatigue relatively quickly, relying more heavily on anaerobic metabolism (glycolysis) for rapid ATP production — these fibres contain fewer mitochondria and less myoglobin (appearing paler in colour), making them well-suited for short-duration, high-intensity, powerful activities (such as sprinting or weightlifting).

Most human skeletal muscles contain a mixture of both fibre types in varying proportions, with the specific ratio often influenced by genetics as well as the type of physical training/activity an individual regularly undertakes — athletes in endurance sports often show a higher proportion of slow-twitch fibres, while athletes in explosive power/sprint sports often show a higher proportion of fast-twitch fibres.