Excerpted from:  (Page, Frank, Lardner.  Assessment and Treatment of Muscle Imbalance, The Janda Approach.  2010, Human Kinetics, Champagne IL.)

Muscles must be able to respond to a variety of simultaneous factors such as gravity, repetitive movement, and upright posture.  Muscles are influenced by both neurological reflexes and biomechanical demands, and therefore can be considered to be a window into the function of the sensorimotor system.  Janda found that patients with chronic musculoskeletal pain (most notably, chronic low back pain) exhibit the same patterns of muscle tightness and weakness as patients with CNS disorders exhibit.  This finding indicates a link between muscle imbalance and the CNS.

Tonic and Phasic Systems

The tonic system is the first used by the human body, as it is responsible for maintaining the fetal posture in newborn infants.  The phasic system soon is activated as the infant learns to lift her head for visual orientation.  The development of normal movement patterns utilizes reflexive coactivation of the tonic and phasic systems.  Muscles that are phylogenetically tonic demonstrate increased tone, while muscles that are phylogenetically phasic demonstrate decreased tone.  In patients with chronic musculoskeletal pain, this pattern of muscle imbalance manifests as tightness and weakness in the tonic and phasic muscles, respectively.  This finding supports Janda’s observation that chronic musculoskeletal pain is mediated by the CNS and is reflected in the sensorimotor system throughout the body.  It also allows us to predict typical muscle responses because of these neurodevelopmental chains.

Janda conceptualized muscle imbalance as an impaired relationship between muscles prone to tightness/shortness and muscles prone to inhibition.  More specifically, he believed that muscles predominantly static, tonic, or postural in function have a tendency to get tight and are readily activated in various movements—more so than muscles that are predominantly dynamic and phasic, which have a tendency to grow weak.

Tonic and Phasic Muscle systems:

Janda believed that muscles should not be classified based on the two-leg stance.  He preferred to consider the function of the muscle in relation to a one-leg stance, noting that muscles involved in maintaining upright posture during a single-leg stance (balancing) show a tendency toward tightness.

No muscle is exclusively tonic or phasic; some muscles may exhibit both characteristics.  Muscles do however have a tendency to be tight or weak in dysfunction.

Janda’s Classification of Muscles Prone to Tightness or Weakness

Czech physiotherapist Pavel Kolar (Kolar, 2001) expanded on Janda’s original list of tonic and phasic muscles from a more neurodevelopmental perspective adding to the tonic muscle list the coracobrachialis, brachioradialis, subscapularis, and teres major; and phasic muscle list the   rectus capitus anterior, suprspinatus, infraspinatus, teres minor, and deltoid.

Kolar also noted that the latissimus dorsi may be either tonic or phasic.  In contrast to Janda, Kolar categorized the piriformis and gastrocnemius as phasic muscles and suggested that the biceps, triceps and hip adductors exhibit both tonic and phasic portions.  Specifically, the long head of the triceps and short head of the biceps are tonic, while the medial and lateral triceps and long head of the biceps are phasic.  The short adductors are tonic, while the long adductors are phasic.

Faulty Movement Patterns

Janda theorized the facilitation of antagonists (flexors) and inhibition of agonists (extensors) in response to pain.  The subsequent muscle imbalances lead to changes in movement patterns.  Altered recruitment patterns typically begin with the delayed activation of a primary mover or stabilizer, along with early facilitation of a synergist.  Muscle tightness leads to overactivation of certain muscles, while muscles that should be activated are not, possibly due to inhibition or motor reprogramming.  Janda noted that altered peripheral input due to pain leads to these changes in muscle activation, causing faulty movement patterns that eventually become centralized in the motor program.

Imbalances in children begin in the upper extremity as opposed to the lower extremity, as is seen in adults.  These patterns of muscle imbalance are systematic and predictable because of the innate function of the sensorimotor system.  Subsequently, adaptive changes within the sensorimotor system (either vertical or horizontal) affect the entire system, most often progressing proximally to distally.  This muscular reaction is specific for each joint, suggesting a strong relationship between joint dysfunction and muscle imbalance.

Although Janda is considered the father of the neurological paradigm of imbalance, he recognized that muscle imbalances also occur as a result of biomechanical mechanisms.  Lifestyle often contributes to muscle imbalance as well.  Janda felt that the muscle imbalance in today’s society is compounded by stress, fatigue, and insufficient movement through regular physical activity as well as a lack of variety of movement.  This lack of variety contributes to repetitive movement disorders.  Janda noted that most repetitive movements reinforce the postural system, neglecting the phasic system, and lead to imbalance.