Introduction

When people experiencing pain are diagnosed with knee osteoarthritis (OA), they’ll often get a biomedical explanation that their cartilage is damaged. Sometimes messages of “wear and tear” or “bone on bone” are used which may create or even increase fear of further loading the knee. People’s participation in daily activities and recreational sports may be lowered as they fear causing further damage and subsequently worsening pain. However, we know that exercise is safe en particularly recommended in these people. But to guide them in their exercise participation, we must understand what happens in the knee joint during these everyday activities. This study examined knee joint loading in osteoarthritis in people with knee varus alignment during the execution of three commonly used exercises.

Methods

This study had a cross-sectional design where the authors included participants with knee osteoarthritis and varus alignment. Knee muscle forces were calculated during a lunge, a squat, and a single-leg heel raise and compared to the forces produced upon walking.

The included participants were above 50 years of age and had knee pain with walking of at least 4/10 NRS. They experienced pain on most days of the previous month and this had been ongoing for more than 3 months. On the Kellgren and Lawrence scale, they had a grade 2 or more, representing mild joint space narrowing and the presence of osteophytes. Their knee had a varus alignment which was defined as “an anatomic axis angle of <183° for males and <181° for females, measured on weight-bearing anterior–posterior x-rays”.

Barefooted they completed 5 trials for each weight-bearing exercise. These included single-leg heel raise, double-leg squat, and forward lunge. Every exercise was divided into 3 phases for the analysis.

• ascent/descent from the starting pose to the end of the self-selected range (phase 1);
• a 3-s isometric hold, verbally counted by the researcher (phase 2);
• ascent/descent back to the initial starting pose (phase 3).

Hereafter, 5 barefoot walking trials were obtained across a 10m path. The trial was executed at the participant’s preferred speed. The exercises were compared against the walking trial to analyze the forces on the knee joint. To summarize the loads on the knee joint during these exercises, the medial tibiofemoral contact force was calculated.

The medial tibiofemoral joint contact force analysis was done by integrating data obtained from the EMG, skin markers, and force plate with the data from MRI images of the knee. The knee muscle forces were used as inputs into a planar knee mechanism to estimate MTCF normalized for the participant’s body weight. The peak force for each muscle group and peak normalized MTCF were extracted. The mean muscle and external load contributions were obtained and expressed as a relative percentage of the total contact loading experienced by the medial tibiofemoral compartment.

Results

Twenty-eight participants were included. They were overweight on average with their mean BMI being 29.6 kg/m2. The mean age of participants was 63 years. The median duration of their symptoms was 44 months and the median levels of pain were 6/10. Most of them had grade 2 or 3 OA.

The analysis of knee joint loading in osteoarthritis revealed that during the squat movement, the peak knee extensor and flexor forces were higher than during walking. Squatting led to a lower peak hip abductor force. No difference was found between the peak forces produced with single-leg plantar flexion upon squatting or walking.

The lunge revealed higher peak knee flexor and extensor forces than produced during walking. No differences were found between hip abduction and plantar flexor forces produced during walking and performing the lunge.

When doing the single-leg heel raise, this produced a lower peak hip abductor force compared to walking. No difference in forces between walking and this exercise was found for the peak knee flexor and extensor force, nor for the peak plantar flexor force.

A squat produced lower peak knee contact force compared to walking. Single-leg heel raises produced lower peak knee contact forces compared to walking and doing a lunge led to no difference in peak knee contact force compared to walking.

Questions and thoughts

Participants were recruited from a larger clinical trial investigating the effect of wearing a valgus knee brace. Yet the current sample did not wear a knee brace before or during the study period.

The use of surface EMG has its limitations, for example, cross-talk from adjacent muscles cannot be avoided.

The exercises were performed using the participants’ preferred strategy. This is good as there is no good or bad performance for an everyday task, yet this may have created variability in the outcomes. Every participant may have his movement strategy and may have done the exercise differently. For example, a shift in body weight was observed in the contralateral leg. The participants avoided loading the knee fully, it appeared. Movement speed may also have influenced the outcomes.

Talk nerdy to me

A cross-sectional study design was chosen. This means that the current study only told us something about the measurements at one particular moment in time. It cannot say anything about changes over time (for example strength gains). The knee joint loading in osteoarthritis was studied at one particular moment in time.

The order of the exercises was not randomized and this could have led to fatigue which may have been influencing the last trials of exercises. The walking trial was always held at last.

It was tried to ensure a decent trial was recorded. When the participant lost his balance, this trial was excluded from the results. This way, only those trials with a good execution were analyzed. This could have created more homogeneity in the results.

Some approaches to standardize the movements were implied. For the squat exercise, a 15° wedge was used so the effect of possible restrictions in hip and/or ankle range of motion was minimized. For the lunge, the distance between the feet was normalized to 70% of the leg length, which was measured from the lateral malleolus to the greater trochanter. Ground reaction forces omitted through the chair the participants were allowed to take some support for balance during the single-leg heel raise were examined to see whether the participants relied too much on their hands for balance. If this was the case, this trial was excluded.

Take home messages

If walking is considered a safe exercise alternative for people with knee osteoarthritis, then doing lunges, squats, and single-leg heel raises are possibly even safer. They produced higher or equal knee muscle forces but lower or similar medial tibiofemoral knee joint contact forces. As such you should not fear those exercises to use in rehab. You can promote the safety of exercising. As these exercises produce higher muscle forces, they would likely help to increase muscle strength around the knee joint. However, the latter was not tested as this was a cross-sectional study, capturing the data at one point in time.

Reference

Starkey SC, Diamond LE, Hinman RS, Saxby DJ, Knox G, Hall M. Muscle Forces during Weight-Bearing Exercises in Medial Knee Osteoarthritis and Varus Malalignment: A Cross-Sectional Study. Med Sci Sports Exerc. 2022 Sep 1;54(9):1448-1458. doi: 10.1249/MSS.0000000000002943. Epub 2022 May 12. PMID: 35551169.