The importance of muscle dysfunction in various pain conditions is increasingly appreciated and the number of papers specifically showing muscle weakness as an element of pain syndromes is rapidly increasing. Patients with low back pain (LBP) have lower mean trunk strength than asymptomatic subjects.1-10 Lifting strength is also decreased in people with chronic LBP.11-13 Pain itself is possibly a strength-reducing factor, as is the duration of back pain.14 The muscle weakness etiology for low back pain is in line with the common impression that pain makes muscles difficult to use and less powerful.15
Because of this growing body of evidence, the lack of muscle strength has frequently been cited as the suspected etiology of LBP. These studies are one of the reasons Lamb and others argue that manual muscle testing (MMT) has content validity for the evaluation of LBP. Lamb states that MMT has content validity because the test construction is based on known physiologic, anatomic and kinesiologic principles.16 A growing number of papers have specifically described the validity of MMT in relationship to patients with LBP.17 Let's review what the research suggests regarding the potential correlation between "inhibited" or "weak" MMT findings and LBP.
Janda reports: "A great amount of our often surprising therapeutic success depends on the fact that we managed to facilitate the inhibited motor neurons quickly." In his opinion, "There is now enough evidence that impaired function of muscles occurs in close relationship with development of joint dysfunction, which is considered to be the most common cause of painful conditions such as back pain."18 He also found that in patients with a sacroiliac distortion, there was a concomitant and striking inhibition of the gluteus maximus or medius muscles during attempts at hip extension and abduction or during specific MMT.19 In fact, this weakness may even be pathognomic for a sacroiliac distortion.
Hodges has repeatedly shown that spinal manipulative therapy increases muscle strength in muscles that are weak, but not in normal muscles, and that this improvement in strength also improves the patient's clinical condition.20 Hodges, et al., have shown that there is a consistent delayed activation of the muscle that occurs in cases of LBP, such that the order in which the individual muscles are activated changes.21-23 This is a pattern of muscle activation called "synergist substitution," wherein the inhibited muscle recruits other muscles to assist it in its effort. During MMT, this can be identified as a slight shift of the patient's body in order to activate synergist (not prime mover) muscles.24
The LBP complications resulting from just one inhibited muscle in are significant. The gluteus maximus muscle is a major stabilizer of the pelvis and low back and is especially important for sacral, iliac and coccygeal stability. Smooth gait observed during running and walking is, in large measure, due to gluteus maximus function. A common postural fault that results from weak gluteus maximus muscles is accentuation of the A-P spinal curves, with an anteriorly rotated pelvis and hyperlordosis in the lumbar spine. However, the cause of this muscle's weakness may be from remote factors such as failure of the foot's positive support reaction.25-26
The positive support reaction is a fundamental reflex of posture. When pressure is applied to the plantar surface of the foot, the limb should extend strongly enough to support the body's weight, leaving you standing in a strong position. When the foot is weight-bearing, the force upon the interphalangeal joints and the stretching of the interosseous, adductor hallucis and other muscles stimulate the joint receptors and muscle spindles to provide facilitation of the extensor muscles, which is the essence of the positive support reaction.
The easiest method for evaluating the positive support reaction is with the patient prone, making it easy to evaluate extensor muscles such as the hamstrings, gluteus maximus, deep neck extensors and upper trapezius. The muscles should first be tested to determine that they are functioning normally. If not, make corrections with the usual applied kinesiology approaches, such as correcting vertebral subluxations. When the intrinsic muscles of the foot are stretched to simulate weight-bearing, the previously strong postural extensor muscles should remain strong or even test stronger because these muscles are normally facilitated by stimulation to the receptors of the positive support reaction.
Simulation of weight-bearing is done by flattening the longitudinal arches and spreading and flattening the metatarsal arch. Immediately after the challenge, one or more of the extensor muscles should be tested. A positive indication of foot involvement is weakening of the extensor muscle(s).
If the primary cause of the spinal joint dysfunction is left uncorrected, the secondary problem will continue to recur. For example, if the patient has a facet syndrome from an anterior pelvic tilt and lumbar hyperlordosis (both due to gluteus maximus weakness), the primary complaint may be LBP. If extended pronation and dysfunction of the positive support reaction are the cause of the weakness, or if the upper cervical fixation remains uncorrected, you will be unsuccessful in obtaining lasting improvement of the LBP.
Obviously, when the foot is found to be the cause of a remote problem such as LBP, the patient has to understand the mechanism taking place before they will accept the therapeutic approach necessary for the correction of the condition. Many patients are lost to appropriate chiropractic therapies simply because the doctor fails to adequately explain the connection between a remote primary problem and the patient's area of complaint.
If doctors cannot recognize the cause of health problems remote from the low back because they are unfamiliar with the "challenge" or "therapy localization" methods in AK, or if they do not test remote muscle impairments after treating the causative factors, they will not succeed in their treatments. In this common example, treatment may be directed to LBP over and over, yet the basic underlying cause of the problem is not found until the patient fortuitously develops a symptomatic condition in the foot or comes into contact with a doctor who is knowledgeable about the ramifications of foot dysfunction on extensor muscles in the low back and hips.
Many of the specific "name techniques" in chiropractic have presented landmark texts on spinal joint dysfunction.27 However, far too many inexplicably fail to describe or offer a method for measuring the potential impact of muscular attachments and inhibitions upon joint dysfunctions. These muscle-joint interactions have been extensively described and researched, and these muscle-joint realities should become a part of standard diagnostic testing in the chiropractic care of LBP.
Chiropractic clinical experience and research have demonstrated that dysfunction in a muscle may be caused by a failure of many different groups of muscles, structures, and mental and chemical disorders. The MMT response can provide important clues regarding the origin of that dysfunction. Applying the proper therapy results in improvement in the inhibited muscle, joint biomechanics, pain, movement and posture.
Functional muscle inhibition as detected with MMT is a distinctively chiropractic diagnosis. It is unique in contemporary alternative practice in that it is considered to be central to the practice of physical diagnosis, yet it is not organic pathology. It is functional impairment. Functional muscle inhibitions are present to a greater or lesser degree in most individuals with LBP. Therefore, MMT should be a part of our overall approach in the care of most patients who have muscle impairments in their presentation of LBP.
The evaluation of therapeutic results in LBP must be done both objectively and subjectively. Because the research cited here consistently points toward muscular dysfunction as an underlying cofactor (whether cause or effect), chiropractic clinical tests of muscle strength must be employed. These tests would ideally be something the patient can feel as well as one the doctor can measure. MMT is just such a tool and is recommended.
References
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