J-curve hypothesis – The theory that moderate exercise strengthens the immune system, but excessive training may suppress it.

Jackknife stretch – A dynamic or static hamstring stretch involving hip and spine flexion.

Jargon – Specialist language used in sport science or coaching that may be unfamiliar to beginners.

Jaw reflex – A basic neural reflex used in concussion and neurological assessments.

Jerk (Olympic lift) – The second explosive movement in the clean and jerk, lifting the bar overhead.

Jet lag – A disruption of the body’s circadian rhythm caused by crossing time zones.

Joint – A point where two or more bones meet and allow movement.

Joint action – The type of movement that occurs at a joint (e.g., flexion, rotation).

Joint angle – The angle between two bones at a joint, influencing force and range of motion.

Joint capsule – A fibrous sac that encloses a synovial joint, stabilising and protecting it.

Joint congruency – How well the surfaces of bones fit together at a joint.

Joint integrity – The condition of joint structures like ligaments and cartilage that support function.

Joint laxity – Looseness or instability in a joint, often increasing injury risk.

Joint mobility – The range and ease of movement available at a joint.

Joint proprioception – The sense of position and movement in a joint, important for balance and coordination.

Joint reaction force – The force produced between surfaces within a joint during movement.

Joint stability – The ability of a joint to resist displacement and maintain alignment.

Jog-to-sprint drills – Conditioning exercises alternating jogging and sprinting to develop speed and aerobic fitness.

Jogging – A form of low-intensity running often used in warm-ups and aerobic conditioning.

Joint-specific warm-up – Movements designed to prepare a particular joint for activity.

Jump height – A measurable performance metric assessing explosive lower body power.

Jump landing mechanics – The technique and control used to safely absorb forces when landing from a jump.

Jump rope – A coordination and conditioning tool used in agility and cardiovascular training.

Jump squat – A plyometric strength exercise combining squatting and explosive upward movement.

Jumping jack – A full-body aerobic exercise involving leg and arm abduction and adduction.

Juxtaposition training – Alternating exercises that use contrasting movement patterns or muscle groups.

Jerk reaction – A reflexive or uncontrolled response to a sudden external stimulus.

Jerk technique analysis – A breakdown of body position and timing in Olympic weightlifting performance.

J-loop band – A resistance band shaped like a loop, often used in strength or rehab work.

Jump test – A fitness test assessing explosive leg power (e.g. vertical jump or broad jump).

Junctional fatigue – A reduction in neuromuscular transmission efficiency at the neuromuscular junction.

Joint loading – The amount of stress or force applied to a joint during movement.

Joule – A unit of energy used in biomechanics to quantify work or force output.

Joint distraction – A mobilisation technique used in physiotherapy to increase joint space and mobility.

Judgment – The decision-making ability of athletes under pressure during gameplay.

Just-noticeable difference (JND) – The smallest change in stimulus intensity that can be detected.

Joint kinesiology – The study of how joints move and what limits or enables that movement.

Jab step – A quick, small step used in sport to fake or create space from an opponent.

Joint lock – A position used in martial arts to control or immobilise a joint.

Judgmental feedback – Performance information that includes evaluative statements, which may affect motivation.

Joint distraction force – A force pulling joint surfaces away from one another, increasing mobility.

Joint orientation – The position or alignment of bones forming a joint during movement.

Jitter – Minor variations in performance or signal transmission, sometimes relevant in sport technology.

Juvenile idiopathic arthritis – A chronic condition in children affecting joints and potentially limiting physical activity.

Jumping asymmetry – Uneven power or technique between left and right sides during jump tests.

Jumping economy – The energy efficiency of an athlete's jumping technique.

Joint effusion – Swelling within a joint due to injury or inflammation.

Jigsaw method – A cooperative learning strategy where students become “experts” on different topics and share knowledge.

Joint misalignment – When a joint moves out of its normal position, affecting movement and load distribution.