I-band – The light section of a sarcomere that contains only actin filaments.

Iliacus – A hip flexor muscle that helps lift the thigh toward the body.

Iliopsoas – A powerful hip flexor made up of the psoas major and iliacus muscles.

Imagery – Mentally rehearsing a movement or situation to improve confidence and performance.

Immobilisation – Restricting movement to allow healing after injury, often using a cast or brace.

Impact force – The force generated when the body or an object strikes a surface.

Impairment – A loss or limitation in physical or mental function.

Impedance – The resistance to electrical flow, often measured in body composition testing.

Implementation intention – A mental plan linking a situation with a specific behavioural response (“if–then” planning).

Imposed demand – The specific physical stress placed on the body by an activity or training method.

Improvement rate – The speed at which an athlete shows progress in skill or fitness.

Impulse – The product of force and time; determines the change in momentum of a body.

Impulse-momentum relationship – The principle that impulse applied equals the change in momentum.

In vitro – A test or process performed outside the body, often in a lab setting.

In vivo – A test or process performed within a living organism.

Incentive motivation – Drive created by external rewards such as trophies or recognition.

Incidental activity – Unstructured physical movement that occurs naturally during daily tasks.

Inclusion model – A teaching or coaching approach that ensures all participants are meaningfully involved.

Inclusive fitness – A term in evolutionary theory, but in sport, often refers to training suitable for diverse participants.

Incremental testing – Fitness testing where intensity increases progressively until exhaustion.

Independent variable – The variable a researcher changes to observe its effect on performance.

Index of coordination – A measurement of the timing and efficiency of movements, often in swimming or cycling.

Indirect calorimetry – A method of estimating energy expenditure based on oxygen consumption and CO₂ production.

Individual zones of optimal functioning (IZOF) – A theory suggesting each athlete has their own ideal arousal range for peak performance.

Individualisation – Adapting training to suit an athlete’s unique needs and responses.

Induced drag – Resistance caused by lift forces, especially in sports involving flight (e.g. javelin, ski jumping).

Inertia – The tendency of an object to resist a change in motion or direction.

Inferior – A directional term meaning below or lower in the body.

Information processing – The model describing how athletes receive, interpret, and respond to stimuli.

Informed consent – The process of explaining a study or activity so participants can agree voluntarily and ethically.

Infraspinatus – A rotator cuff muscle that helps rotate the arm externally.

Infrared thermography – A method of measuring surface temperature using heat-sensitive cameras.

Inhalation – The process of drawing air into the lungs.
Innate ability – A natural talent or genetic advantage in a specific area of performance.

Innervation ratio – The number of muscle fibres controlled by one motor neuron.

Input – The information received by the senses before decision-making in a performance context.

Insertion – The point where a muscle attaches to a movable bone.

Inspiratory reserve volume (IRV) – The additional air that can be inhaled after a normal breath.

Instability training – Exercises performed on unstable surfaces to improve balance and core strength.

Integrated periodisation – Combining different training components (e.g. strength, endurance) into one structured plan.

Integrative neuromuscular training – A programme combining strength, balance, and movement technique, especially in youth development.

Integrity – Acting with honesty, fairness, and ethical responsibility in sport and coaching.

Intensity – The level of physical effort or exertion in training or performance.

Intercostal muscles – Muscles between the ribs that help with breathing.

Interdisciplinary team – A group of specialists (e.g. coach, physio, psychologist) working together to support an athlete.

Interference effect – When one type of training (e.g. endurance) negatively affects the outcome of another (e.g. strength).

Internal motivation – Drive to perform based on internal satisfaction or personal goals.

Internal rotation – Rotating a limb toward the body’s midline.

Interval training – A method alternating between periods of work and rest to build fitness.

Intrinsic feedback – Sensory information perceived internally during or after a movement.

Ischaemia – A restricted blood flow to tissues, often resulting in pain or reduced performance.