Muscle Imbalance Syndromes in Older Adults (excerpt)

Muscle imbalances are characterized by a relative difference in the strength or flexibility of opposing (agonist and antagonist) muscles. Muscle imbalance is associated with a variety of clinical conditions such as low back pain or shoulder impingement. Either weakness or shortness can cause faulty postural alignment (Kendall et al., 2005). Faulty movement patterns may also result, which can eventually lead to musculoskeletal pain syndromes. The actual cause of muscle imbalance is not clear, although two schools of thought exist: biomechanical and functional muscle imbalances.

Biomechanical imbalances. Muscle imbalance may be caused by prolonged postures (Kendall et al., 2005) or repetitive movements (Sahrmann, 2002). By placing muscular structures in sustained positions or repetitive movements for extended periods, muscle will generally adapt to those biomechanical conditions. “Adaptive shortening” occurs when muscles are overused in a certain direction and subsequently become tight and short (Kendall et al., 2005). In contrast, “stretch weakness” occurs when muscles opposing the direction of repetitive movements, or those placed on prolonged stretch in sustained postures, become longer and weak (Kendall et al., 2005).

Functional imbalances. The late Czech professor Vladimir Janda, MD, identified two groups of muscles based on their functional roles (Janda, 1987). Functionally, muscles can be classified as “tonic” or “phasic.” Tonic muscles are responsible for the flexor synergy (combined flexion, adduction, and internal rotation), and are developmentally older and dominant. Tonic muscles are involved in repetitive or rhythmic activity (Umphred, 2001), and mainly function for stabilization. Phasic muscles are responsible for the extensor synergy (combined extension, abduction, and external rotation), and are active shortly after the flexor system in newborns. Phasic muscles work eccentrically against the force of gravity (Umphred, 2001) and function mainly for movement; however, they may also have secondary dynamic stabilizing roles.

Normal movement patterns require balance between opposing tonic and phasic muscles. For example, the hamstrings (tonic) must have adequate flexibility while the quadriceps (phasic) need sufficient strength to allow the knee to extend during gait. More important, however, are the speed of contraction and relaxation of tonic and phasic muscles, and the coordination of such. Muscle strength is not as important as the ability to turn a muscle on or off at the correct time.

It’s important to note that Janda’s terms of “tonic” and “phasic” do not refer to the specific muscle fiber physiology commonly known as Type I and Type II metabolisms; rather, these terms refer to muscles involved in flexor and extensor synergies, respectively. Therefore, Janda’s system is based more on neurological coordination of movement patterns than on isolated muscle function.

Janda recognized that each group of muscles has a predisposition to become either tight or weak, likely because of their central nervous system (CNS) regulation (Janda, 1987). In particular, postural muscles are prone to tightness, while phasic muscles are prone to weakness (See the table). While Janda observed that certain muscles are more prone to tightness or weakness, it’s important to remember that each muscle may or may not follow these patterns. In older adults, muscle imbalance is likely related to a combination of biomechanical and neurological factors; therefore, Table 1 includes muscles that may be either tight or weak in an older adult with muscle imbalance.

Table: Postural and Phasic Muscles (adapted from Janda, 1987)

* indicates the muscle may also be weak or tight

Consequences of Muscle Imbalances

Muscle imbalances may be both beneficial and harmful. For example, “functional” muscle imbalances are necessary for repetitive movements, including sports like swimming or tennis. Muscle imbalances may also assist in correcting structural faults such as leg length discrepancies. Many older adults have developed functional muscle imbalances to compensate for life-long structural faults; in these cases, attempts to re-balance muscles may actually cause more harm than good.

In contrast, “pathologic” muscle imbalances may lead to excess stress on structures, leading to joint degeneration. Long-standing imbalance of opposing muscle groups may place joints (such as the cervical spine, shoulder, and hip) in positions that abnormally stress the joint surfaces over time. The cumulative effects of small constant or repeated stresses over a long period of time can cause as much damage as sudden, severe trauma to tissues (Kendall et al., 2005).

Identifying Muscle Imbalances

In isolation, muscles may be either tight or weak, indicating a normal response to a more structural pathology; however, if muscle imbalance presents in the presence of pain in characteristic patterns of tightness and weakness, suspect a functional pathology. Professor Janda identified 2 common muscle imbalance syndromes associated with chronic musculoskeletal pain: Upper Crossed Syndrome (UCS) and Lower Crossed Syndrome (LCS) that can lead to chronic musculoskeletal pain (Figure 1). These patterns of muscle imbalance suggest a functional pathology of the central nervous system (Janda, 1987).

Upper Crossed Syndrome is characterized by tightness of the upper trapezius, levator scapulae, sternocleidomastoid, and pectoralis muscles, along with weakness of the deep neck flexors, lower and middle trapezius, and serratus anterior (See Figure 1). Lower Crossed Syndrome is characterized by tightness of the thoraco-lumbar erector spinae, rectus femoris, and iliopsoas, along with weakness of the abdominals (particularly transversus abdominus) and the gluteal muscles (gluteus maximus and medius).

To read the entire article, go to Active Aging Today. If you’re not a subscriber, subscribe now.