The muscle that guards the knee.

Walk down a flight of stairs and pay attention to the front of your thigh. With every step, the quadriceps lengthens under load and lowers your whole body weight onto the knee below, gently, instead of letting it drop. Most people never think about this. It happens thousands of times a day, and when it works, it is invisible.

Researchers describe the job in plainer mechanical terms. When a muscle lengthens while it works, it absorbs energy, and the quadriceps does this at the knee during ordinary walking and on the way down stairs.¹ In effect, it is one of the knee's shock absorbers. Which makes what happens to it with age worth understanding.

What the years take

Muscle mass declines with age, but more slowly than is often claimed. A quantitative review of the published studies found a median loss of about 0.47% a year in men and 0.37% in women, or roughly 4.7 and 3.7% of peak muscle per decade. The authors also traced the widely repeated figure of a 1 to 2% loss every year after 50, and found that the study it is usually credited to does not report it.²

Strength is a different story. The same review reports that at around 75, men lose strength at 3 to 4% a year and women at 2.5 to 3%. Across studies, strength goes two to five times faster than muscle.² Scientists now separate the two: sarcopenia for the loss of muscle, and dynapenia for the loss of strength and the wider "functional compromise of the entire neuromuscular apparatus" behind it.²

The Health, Aging and Body Composition study measured this directly in 1,880 older adults over three years. Leg strength fell about three times faster than leg lean mass, and men lost almost twice as much strength as women. Gaining lean mass did not stop the decline in strength.³ The authors read this as a fall in muscle quality: the muscle was there, but it produced less force.

Strength drains faster than muscle

Approximate loss over five years among 1,678 older adults (%)

Men: knee extensor strength16%

Men: thigh muscle mass5%

Women: knee extensor strength13%

Women: thigh muscle mass3%

Source: Mitchell et al., Frontiers in Physiology (2012), summarising longitudinal data

That distinction matters for daily life. In the review's words, "loss of strength is a more consistent risk for disability and death than is loss of muscle mass."²

A weak link at the knee

In 1997, researchers measured leg strength in 462 volunteers aged 65 and over. Relative to body weight, knee extensor strength was about 20% lower in people with osteoarthritis on X-ray. The weakness was in the quadriceps, not the hamstrings, and it appeared even in women whose knees showed osteoarthritis but did not hurt, and whose leg muscle mass was normal.⁴

That last finding was the surprising one. If weakness were only the result of a painful knee being rested, it should not appear in knees that do not hurt. The authors suggested quadriceps weakness might itself be a risk factor for pain, disability and joint damage.⁴ A review two years later made the same case, arguing that muscle problems "are not simply a consequence of joint damage" and that well conditioned muscles may delay or prevent osteoarthritis.⁵

The evidence has grown since. A 2022 systematic review pooled 11 studies with 46,819 participants. People with weaker knee extensors had higher odds of going on to develop painful knee osteoarthritis: an odds ratio of 1.85 in women and 1.43 in men. For osteoarthritis seen on X-ray, the odds ratios were 1.43 in women and 1.39 in men. The studies were observational, so they show a link rather than proof of cause, and the authors rated the quality of the evidence as low.⁶

The circle

Once a knee hurts, the circle tightens. Pain makes walking and stairs less appealing, and researchers note that under-use of the muscles around the knee because of that pain may itself add to the weakness.¹

The joint can also switch the muscle off directly. A change in the signals from the knee's own sensory receptors can produce a "long-lasting inability to fully activate the quadriceps muscle" known as arthrogenic muscle inhibition. It is seen across many knee conditions and remains, in the words of one review, "a significant barrier to effective rehabilitation" in people with arthritis.⁷

And weakness feeds back into pain. The Multicenter Osteoarthritis Study followed 2,404 adults aged 50 to 79 for five years. Women in the weakest third for quadriceps strength had a 28% higher risk of worsening knee pain or knee replacement than women in the strongest third. In men, no such link was found.⁸ Weaker muscle, more pain, less use, weaker muscle: it is a loop that can feed itself. Getting up from a chair is often where it first shows, which we wrote about in another post.

What training can do

The encouraging part is that the ageing quadriceps still responds. In a small study published in 1990, ten frail nursing home residents with an average age of 90 did eight weeks of high intensity strength training. Among the nine who finished, strength rose by an average of 174%, thigh muscle area by 9%, and tandem walking speed by 48%.⁹

Larger evidence points the same way. A Cochrane review of 121 randomised trials with 6,700 older participants, most training two or three times a week, found a large effect on strength and a modest gain in walking speed of 0.08 metres per second. In six trials with 503 people with osteoarthritis, participants reported less pain. The reviewers also noted that side effects were poorly recorded, and that joint pain and muscle soreness were reported in many of the studies that looked for them.¹⁰ Guidelines for knee osteoarthritis put exercise at the centre of care, as we described in our post on the years before surgery.

The quadriceps guards the knee quietly, one step at a time, and loses its strength faster than its size. That is part of why Veema focuses on the movements where this muscle works hardest: standing up, walking and the stairs at home. Whether an exoskeleton can support those movements for people with weak or painful knees is a question only studies with older adults can answer.

These figures describe the need. They are not results or claims about Veema.

Sources

1. Hernandez et al., Frontiers in Medicine (2015): Used for the quadriceps' eccentric role in energy absorption at the knee during gait and descending stairs, and the point that under-use due to pain may contribute to weakness.
2. Mitchell et al., Frontiers in Physiology (2012): Median muscle mass loss 0.47%/year (men) and 0.37%/year (women), 4.7% and 3.7% per decade; 1 to 2%/year after 50 figure misattributed; strength loss at 75 of 3 to 4%/year (men) and 2.5 to 3%/year (women), strength lost 2 to 5 times faster than mass across studies; 5-year data in 1,678 people (men 16% strength vs 5% mass, women 13% vs 3%); dynapenia definition and quote on disability and death.
3. Goodpaster et al., Journals of Gerontology Series A (2006): 1,880 older adults over three years; leg strength declined about three times faster than leg lean mass; men lost almost twice as much strength as women; gaining lean mass did not prevent strength loss.
4. Slemenda et al., Annals of Internal Medicine (1997): 462 volunteers aged 65 and over; extensor strength to body weight about 20% lower with radiographic OA; quadriceps not hamstring weakness; weakness in women with OA without knee pain and normal lean mass; weakness proposed as a primary risk factor.
5. Hurley, Rheumatic Disease Clinics of North America (1999): Quote that muscle dysfunctions 'are not simply a consequence of joint damage' and that well-conditioned muscles may delay or prevent OA.
6. Øiestad et al., British Journal of Sports Medicine (2022): 11 studies, 46,819 participants; symptomatic knee OA OR 1.85 (women) and 1.43 (men); radiographic OR 1.43 (women) and 1.39 (men); low quality evidence.
7. Rice and McNair, Seminars in Arthritis and Rheumatism (2010): Definition of arthrogenic muscle inhibition as a long-lasting inability to fully activate the quadriceps, caused by changed discharge of articular sensory receptors, ubiquitous across knee pathologies, a significant barrier to rehabilitation in arthritis.
8. Glass et al., Osteoarthritis and Cartilage (2013): MOST cohort, 2,404 participants aged 50 to 79, 5 years; women in lowest strength tertile RR 1.28 for worsening knee pain or knee replacement; no association in men (RR 1.01).
9. Fiatarone et al., JAMA (1990): 10 frail institutionalised volunteers aged 90 ± 1 years, 8 weeks of high-intensity training; among 9 completers, strength +174%, midthigh muscle area +9.0%, tandem gait speed +48%.
10. Liu and Latham, Cochrane Database of Systematic Reviews (2009): 121 RCTs, 6,700 participants, mostly 2 to 3 sessions a week; large effect on strength (SMD 0.84); gait speed +0.08 m/s; OA pain reduced in 6 trials with 503 participants; adverse events poorly recorded.