Is StairMaster Low Impact or Hard on Your Knees?

A StairMaster is not truly “low impact” in the strict biomechanical sense, but calling it “hard on your knees” overstates the reality for most people. The forces your knees absorb during stair stepping are higher than flat walking yet meaningfully lower than running or jumping, and the machine’s controlled, repetitive motion avoids the sudden jolts that tend to cause joint problems. Where things get interesting is in the details: how much your kneecap specifically is loaded, why going down matters more than going up, and what the research says about stair climbing when your knees already hurt.

What “Low Impact” Actually Means on a StairMaster

The term “low impact” gets thrown around loosely in fitness marketing. Strictly defined, it means at least one foot stays on the ground (or a pedal) at all times, eliminating the airborne phase that makes running high-impact. By that definition, a StairMaster qualifies: your feet never leave the pedals, so there’s no landing shock. But the force your joints experience is a separate question from whether your feet leave a surface.

Research measuring ground reaction forces during stair climbing found that vertical loads can reach about 1.6 times body weight on a steep staircase, and both the variability and side-to-side asymmetry of those forces increased on steeper inclines.¹ For context, level walking produces roughly 1.0 to 1.2 times body weight, while running generates around 2.5 to 3 times body weight or more. A StairMaster sits in the middle of that spectrum. That makes it substantially easier on your joints than a treadmill set to a running pace, but it is not in the same category as cycling or swimming, where forces on the knee are minimal.

The reason this matters is that people often choose a StairMaster specifically because they think it protects their knees. It does protect them, compared to running. It just doesn’t eliminate knee loading altogether, and understanding where the load lands helps explain who does well on the machine and who runs into trouble.

Where the Load Actually Goes in Your Knee

The knee is not a single hinge. It contains two main contact zones: the tibiofemoral joint (where the thigh bone meets the shin bone) and the patellofemoral joint (where the kneecap glides along the front of the thigh bone). Stair climbing disproportionately loads the kneecap side.

A biomechanical study comparing patellofemoral mechanics during stair climbing versus walking found that at the early phase of knee bending, stair climbing produced higher peak pressure on the kneecap, pushed force distribution more to the outer side, and tilted the kneecap more laterally than walking did. At deeper knee bending, both average and peak kneecap pressures were higher during stair climbing.² The researchers suggested this increase in pressure and lateral loading offers a biomechanical explanation for why people with kneecap-related pain so often report that stairs bother them more than flat ground.

This does not mean stair climbing damages healthy kneecaps. It means the kneecap joint is the part of the knee working hardest during this movement. If your knee pain is specifically behind or around the kneecap, a StairMaster will test that area more than walking or cycling would. If your knee issue is elsewhere, the machine may feel perfectly comfortable.

Going Down Is Harder Than Going Up

This is one of the most underappreciated facts about stair-related knee stress: descending stairs is harder on the knee than climbing them. On an actual staircase, your quadriceps must work eccentrically (lengthening under load) to control your descent, and the demand is enormous. Research on older adults found that stair descent required knee extensor effort equal to about 120% of their maximal isometric strength, while stair ascent required roughly 103%.³ In other words, going downstairs can demand more from your quads than they can maximally produce in a static test.

Here is where the StairMaster has a genuine advantage over real staircases. On the machine, there is no descent phase. You are always stepping up. The pedals sink under your weight in a controlled motion, so your muscles never have to manage the high eccentric braking forces that make walking downstairs so demanding. This single design feature is a meaningful knee-sparing benefit that gets overlooked in most discussions about the machine.

The Closed-Chain Advantage Over Other Gym Exercises

StairMaster stepping is a “closed kinetic chain” movement, meaning your foot stays planted on a surface while your body moves over it. Squats and lunges are closed-chain too. The alternative, “open kinetic chain,” describes exercises where your foot moves freely through space, like a seated leg extension machine.

This distinction has real consequences for knee ligament stress. A study comparing tibiofemoral forces during open- versus closed-chain exercises found that the closed-chain exercise produced significantly less shear force across the knee in nearly all positions tested. The reduction came from a combination of a more vertically directed load and greater co-contraction of opposing muscle groups around the knee, which stabilizes the joint.⁴ A separate study confirmed this pattern, finding that during closed-chain exercises, the shear force direction stayed posterior (toward the back of the knee) throughout the range of motion, whereas open-chain knee extensions generated anterior shear force, the kind that stresses the ACL, in the final portion of the movement.⁵

For people worried about their knees, this means the StairMaster’s stepping pattern inherently produces less ligament-straining shear than popular open-chain leg machines. It’s one of the reasons physical therapists often favor step-based exercises in knee rehabilitation programs.

How StairMaster Compares to an Elliptical

The elliptical is the other machine people reach for when they want cardio that’s gentle on the joints. A study comparing biomechanics between an elliptical motion and a stepping motion on the same adaptive trainer found meaningful differences. The stepping motion produced about 42% higher knee extensor moments than the elliptical motion, meaning the quads worked harder during stepping. But the elliptical motion generated roughly 268% greater anterior joint reaction force at the knee.⁶

That anterior force on the elliptical is the kind that pulls forward on the shin bone, which can be relevant for people with ACL concerns. The stepping motion loads the quadriceps more heavily but in a direction that compresses the joint rather than shearing it. Neither machine is categorically “better for knees.” The right choice depends on which part of the knee you are trying to protect. If your concern is the kneecap and the quad tendon, the elliptical may feel easier. If your concern is ligament stability or anterior knee laxity, the stepping motion’s force profile is more favorable.

What Happens When You Already Have Knee Osteoarthritis

People with knee osteoarthritis often assume stair climbing is off-limits. The evidence is more encouraging than that assumption suggests.

Research on people with patellofemoral osteoarthritis found that they naturally adapted their stair technique: compared to healthy controls, they climbed and descended with lower knee extension moments, lower quadriceps forces, and lower kneecap joint reaction forces, accompanied by increased forward pelvic tilt.⁷ In other words, the body automatically compensates to reduce the load on damaged cartilage. This is protective in the short term, though it shifts work to the hip and may create its own issues over time if the imbalance goes unaddressed.

More directly, a 12-week study of patients with knee osteoarthritis found that stair-climbing training led to significant improvements in both quadriceps and hamstring strength, postural stability, and self-reported pain and function scores. Pain ratings dropped by about half, and composite function scores improved substantially.⁸ The improvements occurred both in a group that also received electrical stimulation and in a group that did stair-climbing training alone, though the combination group saw greater gains. The takeaway: controlled stair climbing does not appear to worsen knee OA and may actively improve symptoms when introduced appropriately.

Patellofemoral Pain Syndrome and the StairMaster

Patellofemoral pain syndrome (PFPS), the most common cause of pain at the front of the knee in active people, has a more complicated relationship with stair climbing. People with PFPS already have altered movement patterns on stairs. Research found that individuals with PFPS showed shorter activation of key stabilizing muscles around the kneecap and delayed firing of the gluteus medius, alongside higher knee abduction moments during both stair ascent and descent.⁹ Separately, a study of stair stepping mechanics found that people with PFPS exhibited greater knee flexion during stair climbing at comfortable speeds, along with greater hip adduction and internal rotation during descent.¹⁰

These movement differences mean the knee tracks differently, which can amplify kneecap pressure in people whose tracking is already problematic. If you have PFPS, this does not necessarily mean you should avoid a StairMaster entirely, but it does mean the machine will expose the weakness. Strengthening the hip stabilizers, particularly the gluteus medius, and focusing on keeping the knee aligned over the foot during stepping are more productive strategies than simply avoiding the activity.

Does Body Weight Change the Equation?

It’s reasonable to assume that heavier individuals experience dramatically more knee stress on a StairMaster. The reality is more nuanced. A study comparing stair biomechanics in people with obesity versus those at a healthy weight found that when knee moments were normalized to body mass, there were no significant differences between the groups. In absolute terms, people with obesity did produce larger peak knee extension moments during both stair ascent and descent, but the knee abduction moments, the forces that push the knee inward and are associated with medial compartment wear, were not significantly different between groups even in absolute terms.¹¹

This suggests that the knee handles the proportional increase in load reasonably well during stair climbing, at least from a moment standpoint. Heavier individuals do absorb more total force, which matters for cartilage health over time, but the biomechanical pattern itself doesn’t break down just because body weight is higher. The practical implication: if you’re carrying extra weight, the StairMaster isn’t uniquely risky for your knees compared to other weight-bearing activity, though starting at a lower intensity and building gradually remains good advice.

The Dose-Response Puzzle for Cartilage

Perhaps the most surprising finding in the stair-climbing literature comes from a dose-response analysis using data from a large osteoarthritis cohort. The study found that moderate (“middle”) volumes of stair climbing were associated with roughly triple the odds of patellofemoral cartilage progression compared to low volumes, with an adjusted odds ratio of about 3.2. High-volume stair climbing also showed elevated odds ratios, but those results did not reach statistical significance. When the kneecap and the groove it rides in were analyzed separately, the groove cartilage showed the strongest association with middle-volume climbing.¹²

This is a single observational study, so causation is uncertain, and the finding that high-volume climbers did not show statistically significant damage is hard to explain with a simple “more stairs equals more damage” narrative. The authors speculated that people who climb a lot of stairs may have better-adapted musculature that protects the joint, while those in the middle range may load the cartilage enough to cause wear without having built the protective strength. This hypothesis remains unproven, but the study is a useful reminder that the relationship between stair volume and cartilage health is not straightforwardly linear. If you are using a StairMaster daily at moderate intensity, paying attention to progressive strengthening, not just cardio endurance, likely matters for long-term joint health.

After ACL Surgery

The StairMaster has been specifically studied as a rehabilitation tool following ACL reconstruction, with reassuring results. In-vivo measurement of ACL strain during StairMaster use at two different speeds (80 and 112 steps per minute) found that the average peak strain values were low, around 2.7%, and did not differ between the two cadences. However, the researchers noted that strain values varied considerably from person to person, with some patients producing high strain and others very little.¹³

A clinical trial comparing StairMaster use to stationary cycling during post-operative ACL rehabilitation found no harmful effect of stair climbing on knee strength or limb measurements, confirming that a StairMaster can serve as a safe alternative to a bike in rehab programs.¹⁴ The individual variability in ACL strain is worth noting, though: if you are early in ACL recovery, your surgeon or physical therapist should clear you for StairMaster use based on your specific graft and progress, rather than on general guidelines alone.

Muscle Activation and What It Means for Knee Protection

One reason the StairMaster is generally knee-friendly is that it activates the muscles that stabilize the knee. A systematic review of exercises that promote co-activation of the hamstrings and quadriceps found that the StairMaster, along with squat variations and lateral step-ups, produced higher quadriceps activation than hamstring activation.¹⁵ Quad-dominant activation is expected during a stepping motion, and it is not inherently a problem. But if you have a history of ACL issues or anterior knee pain, supplementing your StairMaster sessions with hamstring-focused work (like Romanian deadlifts or Nordic curls) can help balance the forces around the knee and improve overall joint stability.

Similarly, the hip muscles matter more than most StairMaster users realize. As the patellofemoral pain research showed, weakness or delayed activation of the gluteus medius allows the knee to drift inward during stepping, increasing kneecap stress. Cuing yourself to push through the whole foot and keep your knee tracking over your second toe can reduce this drift. Separate hip strengthening exercises like side-lying leg raises or banded clamshells complement StairMaster use by addressing the weak link that often turns a knee-neutral exercise into a knee-aggravating one.

Practical Adjustments for Sensitive Knees

A few modifiable factors influence how much your knees feel a StairMaster session. Step height is the most obvious one. Research on people with patellofemoral pain found that despite greater knee bending at higher step heights, the timing and magnitude of quadriceps activation did not change significantly across different step heights.¹⁶ That’s somewhat reassuring: a taller step bends the knee more but doesn’t appear to disrupt muscle firing patterns. Still, deeper knee flexion does increase patellofemoral contact pressure, so starting with a shallower step and progressing gradually is a reasonable approach if your kneecap is sensitive.

Speed is another variable. Faster stepping rates increase cardiovascular demand without necessarily increasing peak knee forces proportionally, since the motion stays within the same range. However, fatigue is the hidden risk factor. As your quads tire during a long session, your form deteriorates: you may lean more heavily on the handrails, let your knees drift inward, or allow your trunk to flex forward excessively. Each of these compensations shifts load in ways that can irritate the knee. Shorter, focused sessions with good form are likely better for knee health than marathon sessions where fatigue erodes your mechanics in the final 10 minutes.

Handrail use is a related consideration. Gripping the rails tightly offloads your legs, which reduces the training stimulus but also reduces knee loading. A light touch for balance is fine; white-knuckling the rails defeats the purpose of the exercise and can also encourage a hunched posture that changes hip and knee alignment. If you need to death-grip the rails to keep up, the machine is set too fast.

When a StairMaster Probably Isn’t the Right Machine

For most people with healthy or mildly symptomatic knees, the StairMaster occupies a sensible middle ground between high-impact running and low-load cycling. But there are situations where a different choice makes more sense. Acute knee swelling or inflammation, a recent meniscus tear that hasn’t been evaluated, or a locked knee are all reasons to stay off the machine until cleared by a clinician. People with significant patellofemoral cartilage damage who experience pain during the first few steps of stair climbing, not just after prolonged use, may find that a recumbent bike or pool walking provides the cardio stimulus without the kneecap pressure. And anyone in the early weeks following knee surgery should follow their rehabilitation protocol’s timeline rather than self-selecting exercises based on how the machine feels.

For people who tolerate the machine well but want to protect their knees long-term, the research collectively points in one direction: build the muscles around the joint, not just the cardiovascular fitness. The StairMaster trains both simultaneously, which is part of its appeal. But adding dedicated hip and hamstring strengthening, maintaining a reasonable training volume rather than assuming more is always better, and paying attention to form as fatigue sets in are the practical steps that keep the machine working for your knees rather than against them.