Why do most athletes include strength training as a critical part of their preparation for athletic performance? Strength training has numerous benefits. For athletes, these benefits may include the ability to generate more force during a movement, more power production [(force x distance)/time], and the ability to repeatedly develop and sustain near maximal or maximal forces. For example, the golfer who is required to maintain a semi-standing position while putting may require less muscular effort to do so; or the sprinter who requires strength to generate the necessary power for propulsion. Improved strength and co-ordination can also decrease the incidence of future injury and contribute to better structural integrity. Many Activities of Daily Living (ADL) involve combinations of muscular strength, muscular endurance, and flexibility, three components that together have been referred to as ''muscular fitness.'' Enhancements in strength have manifestations, some of which are perceived, some not. In addition, improved strength allows performance of tasks with less cardiovascular perturbation.

Several studies have examined the relationship between chiropractic adjustments and muscular strength (1). These studies have shown improvement of strength in athletes, young subjects, males and females, and those with or without symptoms. Clinically, strength can be defined as the ability of a muscle to produce force on an external object. There are three primary factors that influence the ability of a muscle to produce force: 1. neural factors, 2. muscular factors and 3. biomechanics (1). Other factors including hormones, cardiovascular function, the environment, psychological factors and others also can facilitate or inhibit strength, however these factors can be subsumed under one or more of the primary factors.

How do chiropractic adjustments improve muscular strength? To improve strength, one must impact the three primary factors as mentioned above. Proposed mechanisms of adjustments (1) congruent with these principles include: removal of potential nerve root pressure reversing compression effects, removal of motion restrictions and restoration of normal joint biomechanics, and by restoration of appropriate patterns of sensory input. Muscle factors such as fiber type are dependent upon neural input while length-tension relationships are intimately connected to neural and biomechanical factors and hence could be altered by modifying either input.

Postural control and movement dynamics are at the core of athletic performance. Thus, to improve athletic performance, chiropractic needs to improve the individual’s biodynamics. Due to its role in development, dependence on multiple organ systems and its need for accurate regulation, human posture and the behaviors it facilitates could represent an ideal construct for use in the measurement of health and wellness (2). Everyday task performance demands strength, balance, task specific postures, co-ordination and flexibility. Athletic endeavors require these same essentials, but typically, to a much higher degree of speed and regulation (i.e. regulating gait parameters in a long jump). Several interacting subsystems of the body are involved with postural dynamics and locomotion, including skeletal, neuromuscular and sensory contributions (visual, vestibular, somatosensory). Co-ordinated control of the moving person is a complex behavior and involves organization of the multiple degrees of freedom of the system.

Posture has traditionally been characterized as a form of biomechanical linkage, often in terms of rotations/translations from a given symmetrical vertical "ideal" posture. The traditional approach therefore assumes a "static" representation of the body in space. Since posture is dynamic (even during quiet stance) it can be considered a behavior whose goal is to facilitate other overt behaviors (actions) that we perform (2). This dynamic assumption of posture is consistent with the fact that postures are rarely isolatable responses, and the responses themselves tend to be anticipatory, ongoing and/or corrective in nature (3). As chiropractors looking to facilitate sport/athletic performance, we must examine postural (behavioral) dynamics objectively (e.g. movement variability, accuracy, etc.) pre and post chiropractic care. Other objective measures will need to be sport specific such as distance jumped in a long jump, time required for a sprint, or weight lifted during a power lift.

Two studies in particular address the benefits of chiropractic care on athletic performance. An activity that requires great co-ordination, strength, balance and flexibility is dance. Waters and Boone (4) examined the effect of spinal misalignment on dance performance. It was found that spinal misalignment has a negative effect on overall dance performance relative to muscle balance. This study indicates that absence of vertebral subluxation, which could allow the body to express normal muscular balance is a positive contribution to dance. Another study by Schwartzbauer et al. (5) analyzed athletic performance in baseball players following upper cervical chiropractic care. Twenty-one male university baseball players without physical injury completed the study, nine in the chiropractic group and twelve in the control group. The control group did not receive chiropractic care. Subluxations were determined from radiographic analysis and the Palmer toggle-recoil adjustment in side posture with a drop head piece was employed. The results showed significant improvement (p < 0.05) at fourteen weeks of care in muscle strength (repetitive shoulder abduction), long jump distance, and capillary counts in the group receiving adjustments.

Humans operate in a manner that tends to minimize the amount of energy necessary to fulfill a given goal state. We call these movements economical. In other words, people "prefer" to function in a manner that allows for optimal performance with minimal effort. However, there are many examples of behaviors that are not economical. Consider a person asked to stand quietly looking at a picture on a nearby wall. This type of task should elicit little if any activity from the paraspinal musculature as measured by EMG. Thus, widespread activation of trunk and paraspinal muscles would be considered inappropriate and not economical (i.e. does not contribute to improved performance) for this task. Interestingly, research has shown that EMG activity is reduced in resting muscle following adjustments (6-7). Future investigations may wish to look at the potential association between metabolic economy and chiropractic adjustments.

As an example, a given marathon runner tries to get to the finish line first, in the least amount of time. The time it takes to run the race is a function of the average horizontal velocity over the specified distance. Kinematics, gait patterns and the kinetics of running are likely related to running economy. Runner A, who performs significantly more side-to-side or lateral work than Runner B, given all other factors equal, would exhibit less efficiency than Runner B. Perhaps the need to generate more lateral work is due to thoracic or extremity subluxation, restricting normal joint motion and producing compensatory actions. Or, perhaps neural interference from a subluxation alters normal perceptual and motor responses, resulting in the same uneconomical behavior. Chiropractic care may improve athletic performance in this situation by improving the runner’s biodynamics.

What is clear is that the available evidence, albeit limited, supports the notion that chiropractic can positively impact both strength and athletic performance. Many chiropractors that have worked with athletes have experienced these types of anecdotal improvements. Given that subluxation has potential negative consequences for both muscular strength and postural dynamics, athletes who exhibit subluxations, seem prone to performance decrements. These decrements or interferences in behavior may be improved through chiropractic care. One avenue to investigating the health/wellness benefits of the chiropractic adjustment is to examine its effects on behaviors typically associated with sports and athletics (such as strength, running, jumping). By means of systematically working with athletes in this fashion, the practicing chiropractor can contribute to our knowledge by publishing case studies or case series of the results of chiropractic adjustments on athletic performance.

References:

1. Smith DL, Cox RH. Muscular strength and chiropractic: theoretical mechanisms and health implications. J Vertebral Subluxation Res 1999-2000;3:1-13.
2. Smart LJ, Smith DL. Postural dynamics: clinical and empirical implications. J Manipulative Physiol Ther 2000. In press.
3. Reed ES. Changing theories of postural development. In: Woollacott MH, Shumway-Cook A, editors. Development of posture and gait across the lifespan. Columbia: University of South Carolina Press; 1989.p.3-24.
4. Waters KD, Boone WR. The relationship of spinal misalignment elements to muscle imbalance in dance performance. J Chiro Research Clinical Invest 1988;1(2):49-58.
5. Schwartzbauer J, Kolber J, et al. Athletic performance and physiological measures in baseball players following upper cervical chiropractic care: a pilot study. J Vertebral Subluxation Res 1997;1(4):33-9.                                                      
6. Shambaugh P. Changes in electrical activity in muscles resulting from chiropractic adjustment: a pilot study. J Manipulative Physiol Ther 1987;10(6):300-04.                                      
7. Grice AS. Muscle tonus change following manipulation. Journal of the Canadian Chiropractic Association 1974;12:29-31.

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