I can be a bit of a skeptical person.  This makes me difficult to live with but it also makes me a pretty good researcher. It is because of this trait that I shy away from emotion-driven claims on the power of chiropractic to raise the dead and heal all wounds.  That being said, a recent review of the literature has opened my eyes and has me a little emotional myself.  The research states that my two favorite things- chiropractic and strength gains make each other better.  I’m stoked.

I performed a very simple review of literature where I searched the terms “Chiropractic, Strength” in a Pubmed search engine. This is what I found:

I found seven studies that compared full force isometric contraction of muscles including the quadriceps, rotator cuff, cervical extensors and low back extensors before and after chiropractic adjustment of the spine. All seven found increased strength and/or decreased muscle inhibition post adjustment (1,4,6,7,9,10,11).  One study showed as much as 66.8% increase in muscle recruitment as tested with surface electromyography (4).  These results are compared to placebo groups and control groups.  Summary:  This is a big deal.  

Two of these studies were of particular interest from a sports medicine perspective as they studied the effect of chiropractic adjustment on the vastus medialis oblique of subjects with anterior knee pain (6,7). Both studies found decreased inhibition immediately post adjustment. This is very useful information to anyone trying to regain strength after a knee injury. As any Physical Therapist will tell you, post injury the primary focus of rehabilitation is to activate the VMO which easily inhibits and leads to lifelong strength and stability problems. Take home- if your knee is injured, get your spine adjusted.

The first question I am usually asked when I tell people about the previous studies is “how long after the adjustment do these effects last?”  I like this question.  It means to me that the patient is trying to plan how fast they’ll have to drive after leaving my office to take full advantage of this phenomena.   Two of the above studies attempted to answer this question by performing multiple strength assessments at predetermined intervals following the adjustment.  Wang and Meadows 2010 (9) found a measurable increase in the strength of the rotatror cuff muscles for up to ten minutes after the cervical spine was aligned and Grindstaff, Hertel 2009 (10) found acute measurable increases in quadriceps strength of 3% and muscle recruitment of 5% for twenty minutes post adjustment.  3% may not sound like much but if you have a 400 pound back squat, 3% means another twelve hard-earned pounds.

These serial assessments are a difficult way to determine how long the effects last because fatigue obscures maximum efforts. What is also missed by these studies are the accumulative effects of multiple adjustments over the span of a treatment period.  These only measure the acute changes after a single adjustment.

I did come across a long-term case study (2) that followed the treatment of an 18 year old patient who had a greatly reduced lumbar curve.  The clinician sent the patient to his high school strength coach to test his baseline bench press single repetition max which was 245 pounds.  Over the course of a 16 week treatment period which restored his lumbar curve to 31 degrees his bench press max increased by 60 pounds to 305 pounds and he was not bench pressing at all over that entire four month stretch.  You would expect his numbers to go down not up.  This study supports the premise that a normal lumbar lordosis provides inherent biomechanical stability and strength.

This case study shows the potential for long-term effects but the duration of the study allows for potential confounding factors that can be difficult to control:  was he eating and sleeping the same?  Was he under the same amounts of stress?  It is hard to say.  With a larger study with more subjects to compare individual results to you can account for these factors and that sort of a follow-up study is necessary to validate the findings of this case study.

Discussion:

These findings are exciting to most people and everyone who I’ve told about them wants to know:   “Why are people stronger after adjustment?”  They typically speculate that increased circulation is at work but the idea of increased circulation does not look likely.  That wouldn’t explain why adjusting the lower back benefits the strength of muscles up stream in the neck and shoulder girdle (2,3).

It is my opinion that there are two contributing factors to this phenomena:
1)  Decreased muscle inhibition, and 2) the effect of stability on strength.

1)  Muscle inhibition (MI)-
Would you rather lift a weight with 90 or 99% of your available muscle cells?  Decreased muscle inhibition as was demonstrated in the above studies (4,9) means that you have access to a greater percent of your total muscle mass.  Inhibited muscle is the percent of your muscle cells that are dormant even during maximum effort.  When you decrease MI you are firing on more cylinders.  This factor has far reaching potential for long-term strength gains far superior to the acute effects documented in these studies.  Accessing and training a greater percent of your muscle mass chronically would have a potentially compounded training effect.

“Why are muscles inhibited in the first place?”
Injury frequently results in muscle inhibition.  You could also argue that our modern lifestyle is one that actively fosters MI.  How many hours a day do you spend in arch supported shoes and low back supporting chairs?

2)  The effect of stability on strength:  If the hips are out of alignment and it is potentially dangerous to have the strength to pull 300 pounds over your head your body may have neurologic controls to ensure that you don’t have the muscular strength to over shoot the limitations of your stability.  Having a huge overhead press and no lumbar curve is like having a cannon on a canoe.

The old adage “the iron never lies” explains why post adjustment strength is an excellent outcome to measure objectively.  The previous studies isolate the effect of an adjustment on a single muscle or group of muscles involved in a motor task but the measurable impact of an adjustment is most likely body wide as is demonstrated by Giggey, Tepe (3) in a 2000 study that exhibited an 8.18 lbs. increase in cervical muscle extensors following a sacroiliac adjustment.  This is also evident from the 2009 study by Smith and Dianoff (8) that  demonstrated a 9.2% improvement in fine motor skills with a movement time test (rapidly point a pointer at a target on a computer screen) post adjustment compared to control groups.

To better understand the body wide effects of the chiropractic adjustment on the body, a more diverse set of outcome markers that include more motor units would be essential.  The above studies only use isometric/static contractions to measure strength changes and it is likely the case that the sort of neurological adaptation facilitated by adjustments is best displayed with explosive movements that involve motor skills and maximal muscle recruitment.  Olympic style functional movements such as snatch and clean and jerk would assess both motor skills and muscle strength.

All of your basic body functions may very well benefit from these same adjustments just as skeletal muscle does but it is not as easily studied or measured.  The general strength of your immune system for instance may be enhanced post-adjustment but until a “maximum effort contraction” of the immune system is devised we won’t be able to quantify that effect.   That will take some clever research design and I will report back when it happens.

1) Hillermann, Gomes, Journal Manipulative Physiological Therapeutics – 2006

2) Morningstar, Grand Blanc, Journal Chiropractic Medicine, Autumn 2003; 2(3):137-41

3)  Giggey K, Tepe R., Journal of Chiropractic Medicine 2009 June

4)  Keller TS, Colloca CJ, Journal of Manipulative Physiological Therapeutics. 2000, Nov-Dec

5) Jorgen Sandell, Palmgren, Journal of Chiropractic Medicine. 2008 June

6)  Suter, McMorland, Herzog, Journal of Manipulative Physiological Therapeutics. 1999. March-April

7)  Suter, McMorland, Herzog. Journal of Manipulative Physiological Therapeutics. 2000 Feb

8)  Smith, Dianoff, Smith.  Journal of Alternative and Complimentary Medicine 2009 March

9)  Wang SS., Meadows J.  Journal of Manipulative and Physiological Therapeutics.  2010 Feb

10) Grindstaff, Hertel. Manual Therapy 2009 Aug

11)  Suter, McMorland.  Clinical Biomechanics  2002 Aug