After a knee injury or knee surgery, many people find they can’t tighten their thigh muscle properly. They try to tighten it and very little happens. Part of the problem is the nervous system holding the muscle back.
This is called arthrogenic muscle inhibition. It helps explain why quadriceps strength can be slow to come back after knee surgery.
- Swelling and pain in a joint can switch off the muscles around it through reflexes in the spinal cord.
- As little as 20 to 30 ml of fluid in a healthy knee reduced reflex activity in the inner thigh muscle in experiments.
- After ACL surgery, the operated leg was on average 23% weaker at 6 months.
- Exercise straight after cooling the knee gave better strength results than either on its own.
- A related pattern of muscle wasting has been seen in a deep back muscle in acute back pain.
What arthrogenic muscle inhibition is
A review of the research describes it as a long-lasting inability to fully activate the quadriceps. It is caused by changes in the signals from sensory receptors in the joint, triggered by swelling, inflammation, joint laxity or damage to the nerves in the joint. Those signals feed into reflex pathways in the spinal cord that reduce the drive to the muscle [1].
In plain terms, the injured joint tells the nervous system to protect it, and the nervous system does that by turning down the muscle that moves it. When that lasts for weeks or months, it gets in the way of rebuilding strength.
Swelling alone can do it
Researchers have tested this by injecting saline into healthy knees. In a 1984 study, inhibition of the vastus medialis, the inner thigh muscle, began with between 20 and 30 ml of fluid, and its reflex response fell to about 56% of its starting level as more fluid was added. The other thigh muscles needed 50 to 60 ml. A local anaesthetic in the joint stopped the inhibition [2].
Later studies confirmed it. With 30 ml and 60 ml of saline in the knee, quadriceps activity dropped, and at 60 ml people landed differently from a single-leg drop [3]. Another study found that pain and swelling were about equally powerful at switching off the quadriceps [4].
Why it matters after ACL surgery
Quadriceps weakness is one of the most stubborn problems after anterior cruciate ligament (ACL) reconstruction. A review found that the operated leg was on average 23% weaker than the other leg at 6 months after surgery and 14% weaker at 12 months [5]. A meta-analysis of 28 studies found deficits in both muscle activation and strength that can persist for years after surgery [6].
Our ACL rehab page sets out the stages of rehab and the criteria for returning to sport.
What helps switch the muscle back on
Cooling the joint, then exercising
A scoping review of treatments for this problem found moderate-quality evidence for cryotherapy (cooling the joint) and exercise [7]. The order seems to matter. In a trial of 30 people at least 6 months after ACL reconstruction who still had quadriceps inhibition, those who exercised straight after cryotherapy gained more quadriceps strength than those who had cryotherapy alone or exercise alone [8].
Electrical stimulation
Electrical stimulation can help the muscle contract when the nervous system won’t fully do it. A meta-analysis found that neuromuscular electrical stimulation improved quadriceps strength in the first 4 to 12 weeks after ACL surgery, although study quality varied widely [9]. In a review of treatments aimed at reducing inhibition, TENS showed the largest effects, from a small number of studies [10]. We don’t use electrical stimulation at Aspire Physio Bangkok. Our rehab is built on progressive exercise.
Lighter loads with blood flow restriction
Blood flow restriction training uses a cuff on the upper thigh so that light weights produce a training effect closer to heavy ones. In a trial after ACL reconstruction, 8 weeks of leg press at 30% of maximum with blood flow restriction gave similar gains in strength and muscle size to training at 70% of maximum. The blood flow restriction group had less knee pain and swelling and better overall function [11].
Progressive strength training
Whatever helps the muscle switch on in the early weeks, the long-term answer is loading it progressively until the strength gap between legs is closed. That takes months, which is why rehab after knee surgery shouldn’t stop when the pain does.
It isn’t only the knee
Researchers have seen a related pattern in the lower back. In people with acute low back pain on one side, a deep back muscle called the multifidus was on average 31% smaller on the painful side, at a single level of the spine [12]. In a later trial in people with a first episode of back pain, the muscle did not recover by itself when the pain settled, but it did recover in people who did specific exercises [13]. At one year, 30% of the exercise group had another episode of back pain, against 84% of the group who didn’t do the exercises [14].
Later research has tempered that result. A Cochrane review found that this kind of motor control exercise is no more effective than other forms of exercise for long-standing back pain [15]. A meta-analysis on prevention found that exercise combined with education reduced the risk of a new episode of back pain [16]. The lesson is to keep exercising after back pain settles, rather than to rely on one special exercise. See our back pain page.
How we approach it
At Aspire Physio Bangkok, rehab after knee injury or surgery focuses on settling swelling and getting the quadriceps working again, then moves to progressive strength work on the gym floor in the same building. See post-surgery rehab, knee pain and sports injury rehab.
Book a free first consultation through our contact page or call 082-281-8009.
Common questions
Why can’t I tighten my thigh muscle after knee surgery?
Usually because of arthrogenic muscle inhibition. Swelling, pain and damage inside the knee change the signals the joint sends to the spinal cord, and the nervous system responds by holding back the quadriceps. The muscle is still there; it isn’t getting the full signal.
Can knee swelling cause muscle weakness?
Yes. In experiments where small amounts of saline were injected into healthy knees, quadriceps activity dropped. In one study, inhibition of the inner thigh muscle started at between 20 and 30 ml of fluid.
How long does quad weakness last after ACL reconstruction?
Longer than most people expect. Across studies, the operated leg was on average 23% weaker than the other leg at 6 months and 14% weaker at 12 months, and a meta-analysis found activation and strength deficits that persisted for years in some people.
Does electrical stimulation help after ACL surgery?
It can help in the early months. A meta-analysis found that neuromuscular electrical stimulation improved quadriceps strength 4 to 12 weeks after surgery, though the studies varied in quality. It is used alongside exercise.
References
- Rice DA, McNair PJ. Quadriceps arthrogenic muscle inhibition: neural mechanisms and treatment perspectives. Semin Arthritis Rheum. 2010;40(3):250-266. doi.org/10.1016/j.semarthrit.2009.10.001
- Spencer JD, Hayes KC, Alexander IJ. Knee joint effusion and quadriceps reflex inhibition in man. Arch Phys Med Rehabil. 1984;65(4):171-177. PMID 6712434. pubmed.ncbi.nlm.nih.gov/6712434/
- Palmieri-Smith RM, Kreinbrink J, Ashton-Miller JA, Wojtys EM. Quadriceps inhibition induced by an experimental knee joint effusion affects knee joint mechanics during a single-legged drop landing. Am J Sports Med. 2007;35(8):1269-1275. doi.org/10.1177/0363546506296417
- Palmieri-Smith RM, Villwock M, Downie B, Hecht G, Zernicke R. Pain and effusion and quadriceps activation and strength. J Athl Train. 2013;48(2):186-191. doi.org/10.4085/1062-6050-48.2.10
- Lepley LK. Deficits in quadriceps strength and patient-oriented outcomes at return to activity after ACL reconstruction: a review of the current literature. Sports Health. 2015;7(3):231-238. doi.org/10.1177/1941738115578112
- Lisee C, Lepley AS, Birchmeier T, O’Hagan K, Kuenze C. Quadriceps strength and volitional activation after anterior cruciate ligament reconstruction: a systematic review and meta-analysis. Sports Health. 2019;11(2):163-179. doi.org/10.1177/1941738118822739
- Sonnery-Cottet B, Saithna A, Quelard B, et al. Arthrogenic muscle inhibition after ACL reconstruction: a scoping review of the efficacy of interventions. Br J Sports Med. 2019;53(5):289-298. doi.org/10.1136/bjsports-2017-098401
- Hart JM, Kuenze CM, Diduch DR, Ingersoll CD. Quadriceps muscle function after rehabilitation with cryotherapy in patients with anterior cruciate ligament reconstruction. J Athl Train. 2014;49(6):733-739. doi.org/10.4085/1062-6050-49.3.39
- Hauger AV, Reiman MP, Bjordal JM, et al. Neuromuscular electrical stimulation is effective in strengthening the quadriceps muscle after anterior cruciate ligament surgery. Knee Surg Sports Traumatol Arthrosc. 2018;26(2):399-410. doi.org/10.1007/s00167-017-4669-5
- Harkey MS, Gribble PA, Pietrosimone BG. Disinhibitory interventions and voluntary quadriceps activation: a systematic review. J Athl Train. 2014;49(3):411-421. doi.org/10.4085/1062-6050-49.1.04
- Hughes L, Rosenblatt B, Haddad F, et al. Comparing the effectiveness of blood flow restriction and traditional heavy load resistance training in the post-surgery rehabilitation of anterior cruciate ligament reconstruction patients: a UK National Health Service randomised controlled trial. Sports Med. 2019;49(11):1787-1805. doi.org/10.1007/s40279-019-01137-2
- Hides JA, Stokes MJ, Saide M, Jull GA, Cooper DH. Evidence of lumbar multifidus muscle wasting ipsilateral to symptoms in patients with acute/subacute low back pain. Spine. 1994;19(2):165-172. doi.org/10.1097/00007632-199401001-00009
- Hides JA, Richardson CA, Jull GA. Multifidus muscle recovery is not automatic after resolution of acute, first-episode low back pain. Spine. 1996;21(23):2763-2769. doi.org/10.1097/00007632-199612010-00011
- Hides JA, Jull GA, Richardson CA. Long-term effects of specific stabilizing exercises for first-episode low back pain. Spine. 2001;26(11):E243-E248. doi.org/10.1097/00007632-200106010-00004
- Saragiotto BT, Maher CG, Yamato TP, et al. Motor control exercise for chronic non-specific low-back pain. Cochrane Database Syst Rev. 2016;(1):CD012004. doi.org/10.1002/14651858.CD012004
- Steffens D, Maher CG, Pereira LSM, et al. Prevention of low back pain: a systematic review and meta-analysis. JAMA Intern Med. 2016;176(2):199-208. doi.org/10.1001/jamainternmed.2015.7431
