This part of the bodybuilding glossary covers four letters at once — I, J, K, and L — and brings in some terms that were long overdue, including the kettlebell, lat pulldown, lateral raise, and jerk. You’ll also find core training concepts such as isolation exercises, training intensity, and linear periodization, along with nutrition terms such as IIFYM and leucine. Browse the full list below, or use the navigation links to jump straight to a specific term.
- IIFYM (If It Fits Your Macros)
- A flexible dieting approach in which food choices are based primarily on hitting daily macronutrient targets — protein, carbohydrate, and fat — rather than restricting specific foods or food groups, as long as the totals fit within the plan. → The ultimate guide to IIFYM dieting strategy
- Imbalance
- A difference in strength, size, or development between opposing muscle groups, such as quads versus hamstrings, or between the left and right sides of the body. Left unaddressed, imbalances can increase injury risk and limit progress on certain lifts.
- Incline bench
- A bench set at an upward angle relative to vertical, used primarily to emphasize the upper portion of the chest during pressing and fly exercises. → Dumbbell curl over incline bench
- Incline curl
- A biceps curl variation performed seated on an incline bench, which places the arm behind the body and puts the biceps under stretch at the start of the movement. This position emphasizes the long head of the biceps more than a standing curl. → Incline dumbbell curl: the ultimate stretch exercise for biceps growth
- Injuries
- In a bodybuilding context, any damage to bone, muscle, or connective tissue sustained during training. The most common bodybuilding injuries are muscle strains, though tendon and joint injuries also occur, particularly with poor form or excessive load. → Recognizing and managing weight lifting injuries
- Insertion
- The point where a muscle attaches to the bone it moves — typically the attachment point furthest from the body’s midline or center, as opposed to the origin, which is the more fixed attachment point closer to the midline. → Muscle anatomy basics: origin, insertion & innervation
- Insulin
- A hormone secreted by the pancreas that regulates carbohydrate metabolism. Insulin determines whether the carbohydrates you eat are used immediately for energy, stored in muscle cells as glycogen, or — when blood sugar is chronically elevated — converted and stored as body fat. → Insulin: double-edged sword hormone
- Intensity (training intensity)
- In a bodybuilding context, the relative difficulty of a given set or exercise, often expressed as a percentage of a lifter’s one-rep max, or described qualitatively by how close a set comes to muscle failure. Intensity is one of the core training variables, alongside volume and frequency.
- Intermediate strength trainer
- A person with roughly 6 to 12 months of consistent bodybuilding or strength-training experience, generally past the rapid “newbie gains” stage but not yet at an advanced level of strength or technical mastery.
- Isokinetic
- A type of muscle contraction performed at a constant speed throughout the range of motion, typically using specialized machines that automatically adjust resistance to match the force applied. Isokinetic training is more common in rehabilitation and research settings than in typical gym training.
- Isolation exercise
- An exercise that involves only one muscle group and movement at a single joint — also called a single-joint exercise. Common examples include the dumbbell fly, lateral raise, and leg extension, each targeting one muscle with minimal involvement from others. → Compound and isolation exercises in strength training
- Isometric
- A type of muscle contraction in which the muscle generates force without changing length — for example, holding a plank or a wall sit. Isometric training was popularized in the mid-20th century by figures such as Charles Atlas. → Types of muscle actions: isotonic vs. isometric
- Isotension
- A posing and conditioning technique in which continuous muscular tension is held in a given muscle for an extended period — for example, contracting and holding the quadriceps for 10 to 20 seconds or more. Bodybuilders use isotension during pre-competition preparation to improve muscle separation and control.
- Isotonic
- A type of muscle contraction in which the muscle changes length while producing force — either shortening (concentric) or lengthening (eccentric), as in a “negative” rep. Most standard bodybuilding exercises, such as the biceps curl or squat, involve isotonic contraction. → Types of muscle actions: isotonic vs. isometric
- Jerk
- The second phase of the Olympic clean and jerk lift, in which a barbell already racked on the shoulders is explosively driven overhead while the lifter splits or squats slightly to receive it in a locked-out position above the head.
- Joint
- The point at which two bones meet. Most joints used in weight training, such as the elbow, knee, and shoulder, function as hinge or ball-and-socket structures that allow the connected bones to move relative to one another. → Joint health supplement review
- Junk volume
- Training volume performed with poor form, insufficient intensity, or so much accumulated fatigue that it contributes little to muscle growth while still adding to overall recovery cost. The term is often used to caution against simply adding more sets without regard for their quality or purpose.
- Kettlebell
- A cast-iron or cast-steel weight shaped like a ball with a handle, used for swinging, ballistic, and dynamic exercises that differ from typical barbell or dumbbell movements. Kettlebells are especially popular for conditioning and full-body explosive training. → Kettlebells: most popular alternative training tool
- Kettlebell swing
- A ballistic, hip-hinge-driven exercise in which a kettlebell is swung from between the legs up to roughly chest or shoulder height, primarily using power generated by the hips and glutes rather than the arms. It is a foundational kettlebell movement for building explosive hip power and conditioning. → Two-arm kettlebell swing: illustrated exercise guide
- Kinesiology
- The scientific study of human movement, encompassing biomechanics, anatomy, and physiology. Kinesiology principles underpin much of exercise science, including how specific exercises load and recruit particular muscles and joints. → Kinesiology: what it is and is it a popular degree today?
- Kipping
- A technique, common in CrossFit-style training, in which a swinging, momentum-driven motion of the hips and legs is used to assist a pull-up, allowing more repetitions to be completed than with a strict, dead-hang pull-up. Kipping prioritizes total reps over strict muscular control.
- Knee wrap
- A long, elastic band of fabric wound tightly around the knee joint, used to provide compressive support and a small amount of elastic rebound during heavy squats and other demanding leg exercises. → Knee wraps: the benefits and drawbacks
- Knurling
- A grooved or roughened pattern machined into the gripping section of a barbell or dumbbell handle, designed to reduce the tendency of the hand to slip during heavy lifts.
- Lactic acid
- A byproduct of anaerobic energy production during intense exercise. Lactic acid was once thought to be purely a fatigue-causing waste product, but research now suggests its related compound, lactate, is also reused by the liver to help replenish glycogen stores.
- Lat pulldown
- A cable exercise performed by pulling a bar down toward the upper chest while seated, primarily targeting the latissimus dorsi along with the biceps and rear shoulders. It is one of the most common machine-based alternatives to the pull-up. → Wide-grip lat pulldown: complete guide for building back width
- Lateral raise
- An isolation exercise performed by raising a dumbbell or cable handle out to the side until the arm is roughly parallel with the floor, primarily targeting the lateral (side) head of the deltoid muscle. → Standing dumbbell lateral raise: proper form & tips
- Latissimus dorsi
- Commonly called the lats, these large, fan-shaped muscles span the sides of the back and, when well developed, give a bodybuilder the characteristic V-taper shape. The lats function primarily to pull the arms down and back, as in a pulldown or row. → The anatomy of the latissimus dorsi: the largest back muscle
- Layoff
- Any extended period of time away from training, sometimes referred to informally as a “training vacation.” A planned layoff differs from a deload in that it typically involves no training at all, rather than just reduced intensity or volume.
- Lean body mass
- Total body mass minus fat mass — including muscle, bone, organs, and water content. Lean body mass is often used as a more meaningful reference point than total body weight when tracking muscle gain or assessing calorie and protein needs. → Lean body mass: definition, calculation & importance
- Leucine
- One of the three branched-chain amino acids (alongside isoleucine and valine), widely regarded as the most directly important for triggering muscle protein synthesis. Leucine intake is often used as a practical marker for whether a meal contains enough protein to stimulate muscle growth. → BCAA supplements guide for bodybuilding
- Lifter
- A general slang term referring to a person who regularly engages in strength training, regardless of their specific discipline (bodybuilding, powerlifting, Olympic weightlifting, or general fitness).
- Ligament
- A band of fibrous connective tissue that joins one bone to another, providing stability at a joint. Ligaments are distinct from tendons, which connect muscle to bone, and ligament injuries such as sprains typically take longer to heal due to limited blood supply. → Ligament sprains: tearing or stretching of ligaments
- Linear periodization
- A training structure in which intensity (weight lifted) gradually increases over weeks or months while training volume (sets and reps) gradually decreases, typically building toward a planned peak in strength or performance. It is one of the most common periodization models used in strength sports.
- Lower back
- The lumbar region of the spine and its surrounding musculature, including the erector spinae. The lower back is heavily involved, directly or for stabilization, in nearly every major compound lift, making its strength and mobility important for both performance and injury prevention. → The anatomy of lower back muscles: erector spinae
- Lower body
- A general term referring to the muscles below the waist — primarily the quadriceps, hamstrings, glutes, and calves — as distinct from the upper body. Many training splits dedicate specific days to lower body work.
- Lunge
- A single-leg exercise performed by stepping forward, backward, or to the side and lowering the body until both knees are bent, then returning to standing. Lunges build strength and stability in the quads, glutes, and hamstrings while also challenging balance and coordination. → Lateral lunge exercise guide
