After knee replacement surgery, many patients meet this situation. Lying flat on the bed and trying to lift the leg, the knee sags first and the heel stays on the bed whatever the effort. Yet when the therapist lightly supports the lower leg, the knee can straighten completely. Where is the problem? Is the muscle totally weak, or is something else going on?
The answer is often an "extension lag": the knee can be straightened fully passively but not actively through the last part. This happens because joint swelling "mutes" the quadriceps through a reflex arc, which is arthrogenic muscle inhibition (AMI), and does not mean the muscle is dead. Once the swelling is dealt with first, and activation is restarted through isometric contractions and electrical stimulation, the muscle can recover step by step. The sections below start from this phenomenon and go on to explain the nerve mechanism by which swelling makes the muscle unable to respond.
The difference between active and passive extension
Being clear about the difference between the two is the starting point for understanding the whole matter. An "extension lag" (also called quadriceps lag) means that when the therapist supports the lower leg or the patient is completely relaxed, the knee can straighten all the way. When the patient is asked to straighten the last ten or so degrees with his own effort, however, there is a "gap" and the last part cannot be completed. This is the opposite of "structural loss of extension", where the knee cannot be straightened even passively because of the joint, soft tissue or a mechanical block. In an extension lag, the range of movement in the joint is enough, and the problem is only that active activation of the quadriceps has not reached the level needed.
| Extension lag (an activation problem) | When the patient is relaxed or someone supports the lower leg, the knee can straighten fully, but with his own effort he falls short of the last part. In this case the quadriceps at the front of the thigh needs to be restarted. |
| Structural limitation (a joint problem) | If the knee cannot be straightened even with help from another person, the problem is in the joint, the soft tissue or a mechanical block. The focus then is on the joint itself, and practising muscle effort alone is not enough. |
| How to tell at the bedside | In a straight leg raise (SLR), as soon as the thigh lifts, the knee bends and sags first. This is the typical sign that the thigh muscle cannot make the effort to straighten the knee. |
One third of patients still have a lag of 15° or more at discharge
Extension lag is not a subjective impression that "the patient is lazy". It is a measurable and clinically meaningful marker. McGinn and colleagues (2018) followed 150 knee replacement patients and found that 51 of them (34%) had an active extension lag of 15° or more at discharge. The average lag angle was 17.1° in the lag group and only 5.7° in the control group, a clear difference. More worrying still, over an average follow-up of about two and a half years, the lag group had higher long-term pain scores. Although the main functional difference between the two groups was not large, the warning sign of a "gap in active extension" deserves to be followed up properly and should not be taken lightly.
How joint swelling inhibits the quadriceps through nerve reflexes
A study design that is almost a "pure experiment" explains the mechanism most clearly. Spencer and colleagues (1984) injected saline into the knees of healthy people (with no pain and no structural damage) to create an artificial effusion, and measured the H-reflex of the quadriceps (an indicator of reflex activation at spinal cord level). Once the effusion formed, the reflex amplitude fell straight away. Vastus medialis fell to 55.7% of the control value, rectus femoris to 69.1% and vastus lateralis to 65.3%, and vastus medialis began to be inhibited with only about 20 to 30 ml of effusion. The most important step was that, when the researchers injected a local anaesthetic (lidocaine) instead, the inhibition disappeared at once. This strongly shows that the inability to produce force is a reflex triggered by sensory signals coming from inside the joint, and it is neither mechanical pressure from the fluid nor damage to the muscle itself.
Put another way, swelling acts like pressing a "mute button". Sensory receptors in the joint send signals into the spinal cord and reflexly turn down the output of the motor neurons to the quadriceps. The review by Rice and McNair (2010) points out that this arthrogenic muscle inhibition (AMI) of the quadriceps is triggered by joint swelling, inflammation and changes in the firing of sensory receptors, and keeps suppressing the muscle through several spinal reflex pathways. It becomes the main limit on rehabilitation: as long as the inhibition has not been lifted, however hard the training, the result is halved.
Problems of muscle activation at brain and spinal cord level
One point needs to be made clear: AMI is not a simple one-way process in which "swelling always weakens the brain's signal". Rice and McNair (2010) stress in their review that arthrogenic inhibition is a phenomenon across levels. Besides the spinal reflex pathways above, the cortex (brain level) is also involved, and the changes are quite complex and cannot be put simply as "the signal is always weaker". The accurate understanding is therefore that failure of activation involves several levels in the spinal cord and cortex, and should not be reduced to "swelling uniformly weakens the brain signal".
This multi-level nature explains why restarting activation cannot rely on passive waiting. Petterson and colleagues (2011) observed in knee replacement patients that, at about four weeks after surgery, strength depends mainly on "whether the muscle can be activated", and only after about 12 weeks does the effect of muscle cross-sectional area (that is, size) gradually take the lead. In other words, the key to improving an extension lag is to "restart the signal". If one only waits for the muscle to grow by itself, the early rehabilitation window that should be used most is wasted. The difference in timing between quadriceps inhibition and wasting is explained in more detail in the article on thigh weakness after knee replacement.
Three steps to restart the muscle
Once the mechanism is understood, the direction of management is quite clear: first turn off the "mute button", then wake up muscle activation step by step. Each of the three steps below has its own job, and none is a single "magic tool".
The order for restarting an extension lag:
- Control swelling and pain first to lower inhibition—Use cold therapy, raise the leg and control pain as the doctor advises, to reduce joint effusion. Since the effusion itself is the source that triggers reflex inhibition (Spencer 1984), the effusion must be reduced first before the muscle can be made to respond again.
- Isometric contractions to rebuild activation—Lie flat on the bed with the knee straight and press the back of the knee firmly down, feeling the muscle at the front of the thigh tighten. Make sure the muscle can be made to respond before moving on to straight leg raises. If the knee bends as soon as it lifts during a straight leg raise (SLR), go back to this step and practise again.
- NMES to bypass the inhibition and activate directly—If the situation is suitable, some rehabilitation programmes use external electrical current from 48 hours after surgery to stimulate the muscle to contract directly. This bypasses the inhibited nerve signal, clearly reduces early loss of strength and improves the ability to extend actively (Stevens-Lapsley 2012).
Whether NMES is worth adding, when to start it and how large the benefit is are separate questions, covered in the article on whether NMES electrical stimulation after knee replacement is worth doing. For goals for knee bending as well as straightening, see straightening and bending angle goals after knee replacement.
The role of biofeedback in rehabilitation
Many patients ask about "electromyographic biofeedback (EMG biofeedback)", in which a device turns the muscle contraction signal into a picture or a sound so the patient can "see" whether he is making effort, which in theory helps to relearn how to activate the muscle. The idea is reasonable, but the research evidence is mixed, and this has to be stated plainly. A randomised controlled trial by Sklempe Kokic and colleagues (2022) (59 in the experimental group and 58 in the control group) found that both groups improved from day 1 to day 21, but at day 21 there was no significant difference between the groups on any measure, so the authors concluded that adding EMG biofeedback did not bring extra benefit.
Biofeedback suits use as a teaching aid and is not a necessity. In managing an extension lag, biofeedback mainly helps the patient to confirm whether the muscle has really been made to respond. It is not the core treatment that turns the result around. Studies disagree on its extra benefit (Sklempe Kokic 2022). It can be used to support practice when needed, but it need not be treated as an indispensable device.
When to ask a therapist for help
The most common situation at a home assessment is that the family wrongly thinks the older adult is "unwilling to walk and pretending to be weak", when in fact there is an extension lag. When someone supports the leg it lifts easily, but as soon as the patient is asked to lift it alone it sags at once. The therapist demonstrates on the spot the difference between passive and active extension, explains to the family that being unable to produce force is not laziness, and starts with isometric contractions together with swelling control, to avoid more resistance and fall risk from pushing the patient too hard. Another common picture is a patient who, afraid of joint stiffness, practises only bending and neglects activation in the straightening direction, and often complains that the knee "will not straighten". If the examination confirms an extension lag (a gap actively, full extension passively), home rehabilitation should first make sure that active contraction in the straightening direction is in place, and should not hurry to add resistance or bending angle.
If the straight leg raise still cannot be done, the extension lag does not go away, or the knee can straighten passively but sags as soon as the patient makes an effort, a therapist should be asked to assess early. The registered physiotherapists at Nova Health provide physiotherapy at home, can tell accurately whether the lag comes from insufficient activation or from a structural limit, plan the order of swelling control and isometric restarting according to the degree of effusion, and guide the family in the right way to help. For a home programme that moves from isometric contractions to resistance-band work step by step, see the four home stages of quadriceps strengthening.
Local resources. To ask about general public hospital services and referral to public orthopaedic and physiotherapy services, call the Hospital Authority hotline 2300 6555 on weekdays from 08:45 to 17:45. For community support, the Hong Kong Society for Rehabilitation provides community physiotherapy and referral for exercise guidance after surgery, telephone 2549 7744. Living spaces in Hong Kong are generally small, and the patient can sit at the edge of the bed with the leg raised to apply cold to reduce swelling. Straight leg raises (SLR) and isometric quadriceps contractions can be practised just by lying flat on the bed without any equipment, which suits a tenement flat or a very small unit.