Biomechanics provides a science for understanding human movement. We can draw upon principles of mechanics to understand why certain exercises rank ahead of others. When combined with our knowledge of anatomy, it seems clear that we can safely handle heavy loads only when with compound movements. Isolation has no place in lifting weights.
Our own physical body possesses a wisdom which we who inhabit the body lack. We give it orders which make no sense.
- Henry Miller
The force-length relationship shows that muscles have an optimal joint angle. They produce the greatest force at the point. Tension develops best in a whole muscle when it works in a position slightly longer than resting length (110-120 %). The thick (myosin) and thin (actin) filaments of muscle responsible for contraction must reside close enough to attach. Fewer connections form when the filaments operate too closely or too far apart. At excessive lengths, less overlap of these filaments occurs as the potential actin sites extend beyond the reach of most of the myosin cross-bridge heads. With excessive shortening, too much overlap occurs at the binding sites. This prevents contraction. The muscle acts like a ball of yarn.
Muscles have elasticity. They store energy when stretched. This stretch creates passive tension. Active tension represents the force generated purely by the contractile elements of a muscle. This operates most strongly at resting length. Using both of these forms of tension properly to enhance training performance affects our exercise form.
The optimal joint angle depends on the muscle’s insertion points. Muscles evolved to produce force in the positions they would most likely encounter the greatest loads. This generally meant when the resistance directly opposed gravity or when we needed it most. This occurs at intermediate muscle lengths. This affects our exercise selection. We must allow passage through these optimal angles. You will generate a stronger sensation when contracting your muscles roughly halfway between the endpoints of any movement you attempt.
Multi-articulate muscles that attach to multiple joints change length depending on the positions of all the associated joints. These muscles have passive and active insufficiencies. The body normally seeks to avoid these positions. We sometimes force it to do otherwise. The body will attempt to lengthen in one joint before reaching a shortened position in another joint. These keeps ideal length-tension. Elbow flexion and extension of the shoulder maintains the best length-tension for the biceps brachii. Shoulder flexion and elbow extension maintains the best length-tension for the long head of the triceps. These both naturally occur when we pull or push any object.
Multi-joint muscles also act best on the joints where they have the largest moment arms. This takes place when the muscle resides farther from the active joint. For example, the hamstrings primarily creates hip extension. Knee flexion occurs less strongly. The greater moment arm at the hip explains why. The gastrocnemius produces plantarflexion at the ankle better than flexion at the knee joint. It resides durther away from the ankle. The moment arm concept receives further attention later in this article. Both hip extension and plantarflexion occur naturally during a squat. Multi-joint muscles can also better store and release the elastic energy during multi-joint movements. They generate passive tension by allowing positive work at one joint and negative work at the adjacent joint.
Passive insufficiency occurs when the muscle fails to extend enough. This happens since a full stretch at multiple joints cannot occur. This position represents the longest possible length of a muscle. The muscle will not generate any great active tension while remaining in this position. It does generate great passive tension. Anytime you feel a deep stretch in the muscles, such when reaching to touch your toes, you feel passive insufficiency.
Active insufficiency occurs when the muscle fails to contract due to shortening. You may feel this as a cramping sensation. This results in little active or passive tension. This addresses the incorrect notion that maximal shortening of a muscle would lead to the greatest force production. A poor exercise makes the resistance feel heavier in this position. Many machine options incorrectly modify mechanics to challenge this position more than a free weight would.
You can perform some simple examples that demonstrate this concept of active insufficiency. Flex your wrist completely and attempt to tighten your fist. You will notice the impossibility of closing the fist completely or with considerable force compared to closing your hand in a slightly extended position, as when normally grasping an object. The multi-joint long finger flexors enter a position of active insufficiency, while the long finger extensor also enters passive insufficiency. While sitting on a chair, extend your legs fully while also bending forward at the waist. When touching your quadriceps, you will notice the inability to intensely contract the long band of muscle located between the outer part of the quadriceps muscle group (vastus lateralis) and the inside of your leg (various adductor muscles). This muscle of the quadriceps, known as the rectus femoris, enters active insufficiency in this position, since it shortens at both the hip and the knee. Lying on the floor and bridging, or lifting your hips up and off the floor, places the hamstrings in active insufficiency, which may result in a tight sensation that sometimes causes spasms.
Active and passive insufficiencies highlight that muscles become progressively weaker as they move further away from an ideal position. Any position away from the ideal medium length though causes a decrease in the possible amount of tension, which reduces the possible strength and size benefits, as tension represents the primary stimulus for muscular size and strength improvement.
Depending on the angle of insertion for the muscle, some muscular force created during a contraction contributes toward stabilizing or destabilizing the segment by pulling the bone into or away from the joint. This indicates that not all muscular force works to create movement. Muscular force primarily directs along the length of the bone and into the joint when at a very acute angle or when the muscle has excessively stretched. The moving force again begins to decrease and the force directs along the length of the bone, acting to pull the bone away from the joint as the angle becomes very obtuse or when the muscle has excessively shortened. This dislocating force takes place in the elbow and shoulder joints when a high degree of flexion occurs.
A good example involves the elbow. When the forearm extends, the tendon of the biceps brachii inserts into the radius at a low angle. Most of the force generated by the biceps brachii directs into the elbow rather than into moving the segments around the joint, which may enhance stability but fails to produce movement. This makes sense when we extend our arms to push on an object through our lower body, but this position wastes muscular tension if attempting to move at the elbow. If the elbow extends at an acute angle greater than 90° of flexion, tension produced by the biceps tends to pull the radius away from its articulation with the humerus, lessening the stability of the elbow due to the dislocating force.
As segments move through the midrange of the joint motion, the angle of insertion allows more of the muscular force to move the segment. This, in addition to the force-length relationship already described, provides further evidence that movement becomes much less efficient away from the ideal medium position.
Therefore, muscle force most efficiently converts to torque when the angle of the force applies perpendicularly or it equals roughly 90° to the long axis of the bone, which again represents a medium length. Multi-joint movements inherently prevent excessive wasting of force as it limits the range of motion possible in the involved joints.
Multi-Joint or Compound Movements
Although commonly referred to as compound or structural movements, multi-joint better describes this exercise category as isolating a specific muscle seems impossible in a functional sense. All movements are compound movements, just differing in the degree. Multiple rotary motions of limb segments produce translatory or linear motion which allows the body segments to move resistances in mostly straight and uniform lines or curves. A good example includes pushing or pulling with a weight. Furthermore, multi-articulate muscles produce force at both attachments when they contract, so undesired movement at one joint must be counterbalanced by other muscles. To understand this, quickly twist your wrist so that your palm faces upward. You’ll notice the triceps brachii muscles contracting during this forceful action called supination of the wrist. This occurs as the triceps brachii has to prevent the biceps brachii from moving the arm at the elbow, since this represents an undesired action. Stabilizer muscles throughout the body also work to allow muscles to contract harder, a phenomenon termed concurrent potentiation activation. These forceful movements contract more muscle even seemingly unrelated to the activity to counteract other movements or stabilize joints. When squeezing your fist very tightly, notice that the surrounding muscles of the arm also begin to contract intensely.
Many fitness sources simply state that compound movements function best because you can lift heavier weights. It would seem only natural that involving additional muscles would allow heavier weights, so this argument tends to simplify more important reasons why these movements function better. These functional exercises ultimately have greater value as they better stress multiple systems of the body safely and represent movements our bodies perform naturally.
Multi-joint exercises stimulate more efficiently since they involve more musculature. Additional muscle involved forces the cardiovascular system to work harder, conferring greater conditioning benefits and also may stimulate hormonal improvements due to the greater absolute stress imposed on the body, which in turn increases overall strength and size beyond the muscles worked through a movement. They facilitate the development of greater balance by developing stabilizing muscles and coordination. Compound movements allow stimulation of muscle groups often not emphasized when focusing only on superficial muscles.
Compound movements represents gross motor skills that are performed naturally for activities requiring strength and speed, and require less skill compared with fine motor skills. This gross body movement allows you to concentrate on fairly straightforward actions after establishing proper form as opposed to learning more complex behavior associated with more complicated choices. If the body loses degrees of freedom or possible movements due to injured muscles, it must rely on compensatory motions that increase energy expenditure and can result in microtrauma and dysfunction. When performing multi-joint movements though, the greater degrees of freedom present allow other muscles to temporarily compensate while the injured muscle recovers.
Multi-joint movements allow synergy. For example, the pectoralis major muscle of the chest can assist in elbow extension as it adducts the shoulder when pushing an object away from you. The gluteus maximus muscle in the rear high portion of the thigh allows the leg to travel backward at the hip to perform extension, but also can aid in extending the knee since part of the muscle attaches to the femur of thigh bone, and another part inserts into the iliotibial tract, a thick tendon band on the outer side of the thigh when squatting. The latissimus dorsi muscle works to extend the shoulder, which in turn leads to elbow flexion when pulling an object toward you. All of these examples can be completed to illustrate these concepts. This proves that these muscles function as units, and attempting to isolate their functions limit their ability to improve performance at multiple joints.
A muscle action can also accelerate and create movement at all joints, whether the muscle spans the joint or not. For example, one of the plantarflexion muscles of the ankle named the soleus can force the knee into extension despite not connecting to it. When standing, the soleus contracts and necessitates extension of the knee joint. Test this notion for yourself. If you relax the hamstrings, the large muscles on the back of your thigh, you notice that when standing on your toes with slightly bent knees that this motion forces your knees to extend. Therefore, compound movements often receive additional help from muscles seemingly uninvolved when viewing purely the attachment sites of our muscles.
Closed chain exercises have the distal segment of the chain fixed, usually involving the hand or foot remaining in contact with an immobile structure or surface. Open chain exercises have the distal segment moving freely throughout space. Although not always, closed chain exercises tend to associate with multi-joint movements and open chain with single-joint movements, especially when analyzing the lower body. Closed chain exercises have the reputation to challenge the core muscles to a greater extent since they require our bodies to move through space.
Exercises with greater loading appear to stimulate greater growth, especially heavy weight-bearing, closed chain activities that involve the feet planted to the ground. Greater loading always occurs through multi-joint movements.
The growth of the muscles in the neck represents a great example of the indirect effect. The indirect effect represents a secondary growth effect that enlarges other muscles despite them seemingly appearing less involved in a particular exercise. For example, trainees generally develop larger necks than average despite avoiding isolation work for these muscles. This seemingly mysterious phenomenon appears best explained by hormonal changes, although it also arises from other aforementioned reasons that explain greater involvement of more muscles than we may expect. Enough tension produced an a stabilizing role for a muscle, which represents the primary stimulus for greater strength and size, may occur in multi-joint movements to stabilize and control other segments seemingly unrelated. Therefore the indirect effect likely exists for multiple reasons.
Isolation or Single-Joint Movements
The shape of a muscle is entirely determined by heredity and by its existing degree of development, in relation to the overall fatty-tissue to muscle-tissue ratio.
- Arthur Jones
The appearance and popularity of isolation or single-joint exercises appears fixed in bodybuilding methodology. A good example involves curling or flexing your elbow only with a weight in hand. The concept arises of weak links throughout multi-joint movements, which certain muscles fail in receiving sufficient exercise and require more direct work. Isolation exercises appear meant to target weaker muscles or parts of muscles. This involves the difference between facing a greater absolute intensity versus a greater relative intensity, with some advocating that both sorts of movements have their place in strength training. They assume that since this strikes a middle ground between two firm stances either completely for or against either type of movement, they automatically seems correct, an argument that relies on faulty logic.
Many authorities advocated, including Arthur Jones, that the smaller muscles tired first during compound movements. For example, the muscle of the arms would tire first before the larger muscles of the back when performing a pulling exercise. In actuality, this seems unfounded since muscles contribute to the total work involved if the exercise is performed correctly.
In general, the largest muscle groups activate to perform the movement foremost when analyzing a multi-joint movement. Although larger muscles groups may contribute a larger range of motion and therefore perform additional work that leads to greater sensations experienced such as burning or tenderness, keep in mind that tension represents the primary stimulus for greater size and strength. Therefore, multi-joint movements better stimulate improvement by keeping muscles within their ideal length-tension range by limiting extremes in range of motion and allowing multi-articulate muscles to avoid approaching active and passive insufficiencies. Tension appears more important than range of motion for greater size and strength. So, although when pushing on an object you may feel the large chest muscles working more than the triceps brachii or the back of the arm extending the elbow, the triceps still require enough tension to stimulate improvement. Also when using proper form, these muscles will contribute similar work as the chest, which changes when the hands space very widely apart. I address these issues more deeply when discussing exercise form for specific choices.
Isolation does not exist in practice. Muscles would never contract alone since this would produce no functional movement. Even attempting to isolate a specific joint action unavoidably involves multiple muscle groups to stabilize and counteract force applied to other joints. Placing the body in unusual positions and movements may seem to increase the relative stress of specific muscles, but these remain non-functional and actually reduce the tension possible during contraction. The fascia represents a sheath that intertwines, separates, and binds muscular structures throughout the body, interconnecting the muscle. This tissue also prevents isolation.
From a functional point of view, single-joint movements exist to alter positioning when unloaded or dealing with very light loads. A good example includes kicking a ball. The extension of the knee may occur mostly by itself in order to reposition the foot prior to delivering a powerful kick. The elbow may extend by itself in order to reposition the hand for writing. Neither of these movements requires substantial force.
Many isolation exercises approach their optimal muscle-length relationship as the moment arm becomes greater. Poor exercises feel heaviest where the muscle becomes biomechanically weak. They must contract intensely with an unfavorable length-tension. People often confuse these with more effective exercises but instead they lead to less muscular tension. Understanding why involves a brief discussion of levers. Levers provide important clues about the functions of specific muscles.
A lever serves as a semi-rigid or rigid body that exerts force on any object impeding its tendency to rotate. This occurs when subjected to a force that does not pass through its pivot point. It functions as a simple machine that adjusts the force, speed, or a combination of each during movement. A joint represents two bones coming together to serve as an axis, fulcrum, or pivot. The class of lever depends upon the arrangement of the axis of rotation, resistance, and the force.
Force-Axis-Resistance (FAR) represents a first-class lever. This represents a balanced setup compared to the other levers. Elbow extension serves as an example.
Axis-Resistance-Force (ARF) represents a second-class lever. The muscle and resistive forces act on the same side as the fulcrum. Plantarflexion of the ankle represents this movement form. The long moment arm allows for less muscular forces compared with resistive forces. Wheelbarrows and crowbars provide other examples of these types of levers beyond the human body.
Axis-Force-Resistance (AFR) represents a third-class lever. These levers are built for range of motion and speed and represent most of the levers in the body. Elbow flexion represents one good example. These muscles operate at a mechanical disadvantage, leading to a large difference in internal versus external forces.
Mechanical advantage represents the tradeoff between distance and force. A mechanical advantage (better termed disadvantage) of less than 1 requires greater muscle forces then resistive forces for the same torque. Therefore, this type of lever may require a greater distance moved in return for increased force applied. Humans appear designed for greater velocities as opposed to generating great forces, since we consist mainly of third-class levers. A mechanical advantage greater than 1 allows muscular force to be less than the resistive force to produce an equal amount of torque.
Think of playing on a seesaw. When one kid, despite his weight, positioned himself or herself further away from the center, the other kid was more likely to rise higher. The weights of each kid did not change, but their forces changed as they moved further or closer to the center of the seesaw. This provides a practical demonstration of levers.
Torque equals magnitude of force times the length of its moment arm. The moment arm refers to the perpendicular distance from the line of action of a force to the fulcrum.
Increase in the length of the force arm or decrease of the weight or resistance arm results in a greater advantage or disadvantage. This represents why resistance training machines with listed weights may mislead. It also shows why the resistances other people use, given their own body mechanics, should not compare to your own baseline. For our purposes here, it shows that isolation exercises uses inefficient levers and as described later, maximizes harmful shearing forces on our segments. Isolation exercises reduce mechanical advantages and often also make the weights feel heaviest at weaker muscles lengths.
As with many issues though, analyzing multiple perspectives with fair-mindedness benefits us by realizing that few debates are one-sided. Some EMG studies show differences in muscle activation among different heads or portions of the same muscle, providing some credence to bodybuilder viewpoints. With very light loads, selective parts of a muscle may be emphasized, but this occurs by putting other heads or parts of the muscle in a non-functional state usually at a weaker length. Generally muscle fibers do run the entire length of the overall fiber, but some fibers do not and this may give support to different exercises working certain portions better. Heavy loads force involvement of the entire muscle safer than trying to use awkward exercises at unusual angles.
Variation in motor unit recruitment order or the way our muscles activate in different patterns provides an argument made for stimulating muscle groups from different angles. As mentioned more deeply in later articles though, variety for its own sake may promote harmful practices. Remember that the anatomical origin and insertion points do not change by altering the placement of various segments, calling into question how much significant change could result muscles by exercising muscles at different angles. Genetics determines muscle shape, while exercise selection has a negligible effect on shape but a significant possible change in size, with only small changes possible when the muscle mechanics allows for a widespread insertion. A muscle shape becomes more noticeable as the size increases, which may mislead a trainee into thinking a specific exercise has changed the shape.
Even considering the higher orders of work for muscle groups that may become possible with single-joint exercises, this threshold may have already been reached safely when using a multi-joint exercise, and you also have to accept all the negative consequences for any possible minor benefits.
Isolation may cause strength gains out of proportion with normal capabilities, which can allow strength imbalances to develop quickly. Isolation exercises often ignore parts of a functional kinematic chain throughout the body, possibly overdeveloping one region while ignoring another, predisposing us to injury.
A program of purely isolation exercises would impractically require countless exercises to train the whole body.
Single-joint exercises still may have rehabilitative uses under rare circumstances, but this extends beyond the scope of this series for developing the best program for greater muscular strength and size.
Finally, muscles exhibit plasticity and attempt to adapt to imposed demands, and may even change force-length relationships. Although probably overblown given that ideal length-tension in muscles seems nearly permanent, this provides a possible argument given against using machines and non-functional movements, since they would have a negative carryover to everyday life.
The final argument made by this article in favor of multi-joint movements involves force application. Force represents any influence causing an object to undergo change. The evidence analyzed shows that our bodies tolerate certain forces better than others when encountering heavy resistances especially.
Bending represents a force applied to bone that places maximum tensile forces on the convex surface of the bent object and maximum compression forces on the concave surface of the bent object. Bones already function strongest in the areas meant to handle bending forces.
Torsion represents a twisting force that directs force with one end of the bone fixed. This force can especially harm the low back, although resisting torsion appears safer. Tension pulls to make an object longer and thinner along the line of force. This force pulls or stretches bone to cause lengthening and narrowing, with maximal stress on the plane perpendicular to the applied load. This occurs usually with the pull of a contracting muscle on its tendons. Tension forces usually cause strains and sprains. The fibula represents a strong bone at handling tensile forces.
Compression deforms objects through pushing to make an object shorter or thicker along the line of force. This force presses the ends of our bones together to cause widening and shortening, with maximal stress on the plane perpendicular to the applied load. This happens when our muscles oppose the pull caused through gravity or weight-bearing activity forces. Specific bones handle large compressive forces, with examples including the tibia and femur of the lower body functioning strong against compressive forces. Bones can withstand greater compressive versus tensile forces.
Long bones extend longer than wide and provide the primary levers that allow muscles to create movement. They serve as columns to support loads along their long axis.
They appear beam-shaped to create a stronger structure for minimizing loads imposed on them through bending. A long bone acts strongest when stressed by forces acting along the long axis of the bone. Multi-joint movements that have several joints rotating to move in a linear path allow bones to bare the load along the long axis. Bone suffers when handling forces applied transversely across its surface. This concept has major implications in choosing safe exercises. Major bones within this category include the humerus, radius, ulna, femur, tibia, and fibula. Ligaments handle tensile stress well, and similar to bones do not handle shearing stress well.
Shear consists of two equal and opposite forces acting parallel that tend to displace an object between the lines of force in parallel. Shear strain occurs as molecules slip past each other, as opposed to compressive stress caused by loads pushing molecules together.
Bones function strongest in compression and weakest in shear. Single-joint exercises and various exercise machines maximize shearing over compressive forces which harms our joints. Multi-joint movements compress the bones bearing the heavy loads.
Short bones otherwise resemble long bones though with smaller sizes and greater flexibility. These bones assist in shock absorption and the transmission of forces. A sesamoid bone, a specific type of short bone, embeds in a tendon or joint capsule to alter the angle of insertion for the muscle and to diminish friction created by the muscle. The patella at the knee joint serves as a good example, and also fails to act properly if loaded transversely. Other sesamoid bones include the phalanges, metacarpals, and metatarsals.
Flat bones protect internal structures and offer thin, broad surfaces for muscular attachment. They serve mainly as protection for vital areas. These bones include the cranium, rib cage, sternum, and the innominate (ilium, ishium, pubis) of the pelvic girdle.
Irregular bones consist of cancellous or spongy bone with a thin exterior of additional bone referred to as cortical. The irregular bones perform a variety of specialties, including supporting weight, dissipating loads, protecting the spinal cord, contributing to movement, and providing sites for muscular attachment. They usually appear compact and distributed throughout the body. These include the skull, pelvis, vertebrae, carpals, and tarsals.
Articular or hyaline cartilage serves to transmit compressive forces over a joint, allowing smooth motion with minimal friction and wear and the redistribution of contact stress over a larger area. Bursae serve as liquid-filled membranes that protect soft tissues from bony projections. Fibrocartilage serves as an intermediary between other tissues and articular cartilage, often occurring where articular cartilage meets ligaments or tendons. This tissue exists in locations requiring tensile and high pressure strength such as intervertebral discs and the knee joint, while also improving the fit between articulating bones.
Short, flat, and irregular bones along with cartilage exist throughout the body precisely whereas needed, and fail to perform their functions adequately when performing single-joint movements that place forces on areas our bodies never adapted to handle.
The force-length relationship establishes that our muscles have an ideal intermediate length where they function most strongly.
Multi-articulate muscles function with greater tension during multi-joint movements.
Tension represents the primary stimulus for muscular strength and size improvements.
Multi-joint exercises have many advantages over single-joint exercises beyond the simple reasoning that more weight can be used. More than several pieces of evidence support compound movements in the compound versus isolation exercises debate.