Australian journal of physiotherapy

The effects of strengthening exercises on male and female adults with osteoarthritis of the knee


Osteoarthritis (OA) is a degenerative disease affecting the joints in the body, whereby the surface of the joint is damaged (Arthritis Research Council 2005). It is commonly defined by radiological and/or clinical features. The National Collaborating Centre for Chronic Conditions (2008), report that OA is the cause of joint pain in 8.5 million people in the United Kingdom (UK). It is characterised by synovitis and damage to the cartilaginous surfaces, and is often accompanied by functional limitations and reduced quality of life (Doi et al. 2008). It is a common cause of disability amongst over fifty-year olds than any other disease (Fransen et al. 2008).

The knee is a common site for OA, with obesity being one of the main risk factors in terms of its development and progression (Hammerman 1995). Its prevalence increases with age, and is higher in women than in men (Symmons et al. 2006). Around 20% of adults aged between forty-five and sixty-four years suffer OA knee pain (National Collaborating Centre for Chronic Condition 2008). This figure is likely to rise as the proportion of the population increases. Individuals with knee OA complain of aching pain, stiffness and joint immobility (NICE 2008). It therefore has a considerable impact upon healthcare, with two million people a year visiting their General Practitioner (Arthritis Research Council 2005). It is necessary to carry out this review on the effects of strength training on knee OA as it is a continuing public health problem. This is due to increased life expectancy, which leads to a rise in risk factors, such as obesity and poor physical activity.

At present, there is no specific cure for OA, but muscle weakness (particularly the quadriceps), pain and poor physical function that correspond to OA has the potential to be influenced by strengthening exercises (Deyle et al. 2000; Quilty et al. 2003; Topp et al. 2002). The NICE guidelines for OA (2008) therefore recommend that exercises, including strengthening and aerobic fitness, should be a core treatment for adults suffering from OA of the knee. However, there are no current guidelines describing the appropriate exercise dosage.

A systematic review published in 2008, looked at strength training for treatment of OA of the knee (Lange et al. 2008). In 50%-70% of studies included in this review, muscle strength, pain and physical activity improved with strength training. They found that quadricep weakness can be modified by strength training. However, they concluded that more research should investigate the specific dose, and the incidence and progression of strength training on knee OA. This identifies a gap in the literature. A Cochrane Review in 2008 looked at exercise for OA of the knee and found that exercise has short term benefits in reducing pain and improving physical function (Fransen et al. 2008). These reviews were used to identify relevant information and support this literature review.

The aim of this review is to accumulate, and critically appraise the relevant literature on the effects of strength training on male and female adults with OA of the knee.


Six electronic databases were searched: MEDLINE (1950 to October week 4 2009), PEDro, CINAHL (1981 to present), Web of Science, PubMed, and The Cochrane Database of Systematic Reviews. The search terms used were "strengthening", "osteoarthritis", and "knee". These terms were combined using "AND". All database searches were limited to 2007 to present, and published in the English language. Studies from 2007 and onwards were required as a recent systematic review on strengthening exercises and OA was completed in 2007. However, four studies were chosen prior to 2007 due to limited research. Nine articles from the databases were appropriate for this review, with the remaining four found by performing a manual search of authors and reference lists of included studies. Inclusion criteria for the chosen studies were male and female adults, strengthening exercises, and osteoarthritis of the knee. A copy of the search screen can be seen in appendix 2.

All twelve studies were critically appraised using the Critical Appraisal Skills Programme (CASP) (2006) for judging the quality of them (see appendix 3).

Literature Review:


Ages of participants in all studies (Huang et al. 2003; Jan et al. 2008; Lin et al. 2009; Bennell et al. 2005; Jenkinson et al. 2009; Shakoor et al. 2008; Iwamoto et al. 2007; Chaipinyo et al. 2009; Gur et al. 2002; Lund et al. 2008; King et al.2008; Mikesky et al. 2006) ranged from forty-one years (Gur et al. 2002) and onwards. This is relevant as the age range reflects those with knee OA that are seen in clinical practice. The majority of studies (Huang et al. 2003; Jan et al. 2008; Lin et al. 2009; Bennell et al. 2005; Jenkinson et al. 2009; Shakoor et al. 2008; Iwamoto et al. 2007; Chaipinyo et al. 2009) had more female participants than male. This gives us confidence to make comparisons between studies and practice, as the prevalence of OA is more common amongst females. This however does cause some concern when trying to generalise the results to the male population. Jenkinson et al. (2009) looked at the overweight/obese population with knee OA, which gives a good reflection of the population who suffer from OA.

Definitions of OA:

The definition of knee OA in most studies (Huang et al. 2003; King et al. 2008; Jan et al. 2008; Lin et al. 2009; Shakoor et al. 2008; Iwamoto et al. 2007; Mikesky et al. 2006; Gur et al. 2002; Jenkinson et al. 2009) was according to the Kellgren and Lawrence plain radiograph classification. According to Schiphof et al. (2008) descriptions of the Kellgren and Lawrence classification differs greatly amongst studies, and this was found to be the case in the studies reviewed. Therefore there is a need to form a valid and feasible classification system. All twelve studies recruited participants with any stages of knee OA, from acute to chronic. This can have a considerable impact on the effects of strengthening exercise, as someone with mild OA may respond better to exercise compared to someone with severe OA.

Sample size:

The sample sizes varied greatly amongst studies, from fourteen participants (King et al. 2008) to three hundred and eighty nine participants (Jenkinson et al. 2009). Considering that OA is the most common musculoskeletal disorder in the world (Lange et al. 2008), one would expect that a small sample size would fail to detect the clinically important impact of strength training on OA of the knee. More importantly, of the twelve studies, four (Huang et al. 2003; King et al. 2008; Iwamoto et al. 2007; Gur et al. 2002) failed to carry out a power analysis, which is problematic as the reader is unable to establish whether non-significant results are a consequence of a scarce sample size, or whether significant results correspond to a large sample size (Harris and Taylor 2007). All twelve studies contained a p-value which illustrates the level of statistical significance. More importantly five studies (Huang et al. 2003; Jan et al. 2008; Shakoor et al. 2008; Iwamoto et al. 2007; Gur et al. 2002) failed to state their confidence intervals, disillusioning one about the range within which the size of effect may lie.

Outcome measures:

The Western Ontario and McMaster Universities Arthritis Index (WOMAC) and a dynamometer were two commonly used outcome measures. The WOMAC score has shown to be a responsive, valid and reliable instrument for assessing the severity of knee OA (Roos et al. 1999; Salaffi et al. 2003). The dynamometer has also shown to be reliable in minimising measurement errors on strength values (Gragnon et al. 2005). This assures us that we are able to draw comparisons and apply the results to clinical practice. It must be noted that additional outcome measures looking at psychosocial issues, such as the effects on strength training on pain, were only looked at amongst five studies (Bennell et al. 2005; King et al. 2008; Huang et al. 2003; Jenkinson et al. 2009; Mikesky et al. 2006). This has important considerations for the impact of OA on the individual, as it has the ability to have an effect on motivation to exercise.

WOMAC was used in six studies (Jan et al. 2008; Lin et al. 2009; Bennell et al. 2005; Jenkinson et al. 2009; Shakoor et al. 2008; Mikesky et al. 2006). Mikesky et al. (2006) found that strength training did not have a significant effect on changes in WOMAC knee pain, and dynamometer results revealed that adults with knee OA lost more strength than those without OA (p= 0.041). The lack of change in knee pain could be due to half of the participants not possessing knee pain at baseline. Nonetheless, the remaining five studies reported a significant improvement in pain and function in WOMAC scores, of which three of them (Jan et al 2008; Lin et al. 2009; Jenkinson et al. 2009) reported treatment effect sizes, which suggests a study of high quality and statistical significance. All but one of the studies (Jenkinson et al. 2009) used a dynamometer to measure strength at baseline and at post-intervention, which enables us to observe the effects of strengthening exercises. Whilst Bennell et al. (2005) failed to mention the effects of the dynamometer on pain and function in knee OA, Iwamoto et al. (2007) found that a combination of strengthening exercises of the knee with a Medx knee machine improved the knee flexor and extensor muscles at 3-6 months (P<0.0001), and were maintained up to three years. It must however be noted that participants in this study had mild-to-moderate OA of the knee, and pain and physical function were not assessed as participants were receiving other modalities, for example, intra-articular injections.


All twelve studies looked at the effects of strength training on OA knee pain and function. The types of exercises performed were multimodal and ranged between studies. Machine-based resistance exercises were used in four studies (Huang et al. 2003; Iwamoto et al. 2007; Jan et al. 2008; King et al. 2008). The remaining eight studies used specific lower limb exercises, particularly to retrain the quadriceps muscles. The duration of the intervention in the majority of the studies (Jan et al. 2008; Shakoor et al. 2008; Huang et al. 2003; Lund et al. 2008; Lin et al. 2009; Gur et al. 2002) were eight weeks long, and five studies (Iwamoto et al. 2007; Bennell et al. 2005; Jenkinson et al. 2009; King et al 2008; Mikesky et al. 2006) were from twelve weeks and onwards. Trial duration has an important clinical effect on the improvements in pain and physical functioning and the progression of knee OA. Nonetheless, no studies have currently suggested an adequate amount of strengthening time needed and maintained to see a significant impact in both the short and long-term.

Frequency, intensity and intervention duration gives us an idea of the exercise dosage that needs to be prescribed to our patients, to see a significant improvement in strength. Improvements in pain and function rely heavily on the intervention dose. Variations occurred throughout all twelve studies. Exercising three times a week was commonly performed (Mikesky et al. 2006; Bennell et al. 2005; Lin et al. 2009; Jan et al. 2008; King et al. 2008; Huang et al. 2003; Gur et al. 2002) with positive results. This gives us confidence that we can prescribe strengthening exercises for three times a week for our patients. The duration of each session was only reported amongst five studies (Mikesky et al 2006; Bennell et al. 2005; Jan et al. 2008; King et al. 2008; Lund et al. 2008), the average being about thirty minutes long. Intensity was only reported in two studies (Mikesky et al. 2006; Iwamoto 2007). Only half of the studies (Mikesky et al. 2006; Bennell et a. 2005; Lin et al. 2009; Jan et al. 2008; Shakoor et al. 2008; Jenkinson et al. 2009) provided progression to exercises, which is essential to see an improvement in strength. Although exercise dosage was only vaguely reported across all twelve studies, they did show that strengthening exercises improved strength of male and female adults with OA of the knee. King et al. (2008) recommended that high-intensity resistance training, three times a week, for forty-five minutes can produce an increase in knee extensor and flexor strength on patients with advanced knee OA. Whilst this is an excellent recommendation, this was a pilot study with a small number of participants, who were younger and predominantly male, with advanced knee OA, limiting its ability to be applied to the knee OA cohort.

Eight of the studies (Jenkinson et al. 2009; Lund et al. 2008; Shakoor et al. 2008; Chaipinyo et al. 2009; Bennell et al. 2005; Huang et al. 2003; Iwamoto et al. 2007; Mikesky et al. 2006) asked patients to perform exercises at home. This has a clinical significance as it encourages individuals suffering from OA of the knee to take responsibility for their management of their condition. This is currently a key strategy that the National Health Service is encouraging. Although a home exercise program is beneficial, it does allow us to question the issue surrounding exercise compliance. Generally, exercise compliance is poor, and realistically researchers tend not to monitor home exercise participation everyday. Of the eight studies performing a home exercise program, only two studies (Shakoor et al. 2008; Jenkinson et al. 2009) called their adults to reinforce the exercise program and to provide support and encouragement. Two studies (Bennell et al. 2005; Chaipinyo et al 2009) asked participants to keep a log book to monitor compliance, whilst another study (Mikesky et al. 2006) gave out a home exercise program and a videotape to guide participants through their exercise sessions. From reviewing the literature it seems that participants need self-motivation and supervision during their exercise program to promote compliance. This will in turn provide clinical benefits, i.e. improved quadricep strength, a reduction in OA knee pain and improved function.

Of the twelve studies, four did not have a control group (King et al. 2008; Shakoor et al. 2008; Iwamoto et al. 2007; Chaipinyo et al. 2009) making it difficult to determine the effectiveness and relate the results of improved muscle strength. A control intervention in an OA cohort does however cause ethical concerns. It is not always practical to ask patients who are suffering with OA to take part in an ineffective program (Fransen et al. 2008).

Blinding of participants, clinicians and statisticians is also a consideration in exercise interventions. Jenkinson et al. (2009) did not blind participants nor clinicians as they conducted an open trial. Chaipinyo et al. (2009) felt that it was ethically required to explain the nature of the intervention to their participants, whilst Lund et al. (2008) felt it was necessary to blind staff and study personnel but failed to mention whether participants were blinded to the intervention. Mikesky et al. (2006) felt that an unblinded assessment of the isotonic strength group was biased in favour of showing an improvement in strength in the strength intervention group. Lack of blinding, particularly of participants, has the potential to introduce bias, which can affect the results of strength training on OA of the knee. In order to avoid selection bias, nine studies randomised participants to the appropriate intervention group. Shakoor et al. (2008); Iwamoto et al. (2007); King et al. 2008 failed to randomise their patients, and this could be due to them not being randomised controlled trials. Only five studies (Iwamoto et al. 2007; Chaipinyo et al. 2009; Mikesky et al. 2006; Lin et al. 2009; Bennell et al. 2005) described an intention-to-treat analysis whereby patients who dropped-out were still accounted for in their results. This gives us confidence that these studies provide us with a better understanding about the effects of strength training in OA of the knee.

The main limitations of this current review are that only one of the studies selected (Jenkinson et al. 2009) was performed in the UK. The remaining eleven studies were performed outside the UK, therefore are not the best representative sample of our NHS patients suffering from OA . People in Japan and Thailand may have very different cultural and health beliefs than those in the UK. The positive effects of strength training on OA of the knee seen in these studies, affect the generalisation to the UK population. Another limitation of this review is that of the twelve studies selected, four were not randomised controlled trials making it difficult to draw comparisons between studies.


This review has critically evaluated the relevant literature on the effects of strengthening exercises on male and female adults with OA of the knee. It can therefore be concluded that strength training has a positive impact on pain and physical functioning. However, adherence to exercise is essential in order for these benefits to become clear, particularly for patients with long-standing chronic knee OA.

The research protocol, which will be completed in April 2010, will look into the impact of strength training in patients with OA of the knee on quality of life.

Recommendations for future:

As the prevalence of knee OA is growing, future research needs to address exercise dosage, for short-term and long-term benefits. Further research into what type of strength training, i.e. isometric, isotonic, isokinetic, is beneficial in terms of establishing clinical significance, as different people will respond to different types of exercise. In sight of everything, it would be important to analyse how quality of life improves with strength training as OA has a big impact on an individual's well-being. It would also be of interest to examine disease progression of OA in relation to strength training dosage. Lastly, any study wishing to look at strength training should include a control group so that comparisons can be made and the validity of the chosen intervention can be observed.

Implications for practice:

This review gives us confidence strengthening exercises are effective for our patients suffering from OA of the knee, so long as it is done regularly and with guidance. At the moment, it is unclear what type of strength training is most effective in improving pain and physical function, nonetheless any type of strengthening exercise will not have a detrimental effect.

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