Isokinetic evaluation of the wrist flexors and extensors in women with postmenopausal osteoporosis

1. Introduction

Osteoporosis is a skeletal disease characterized by low bone mass and microarchitectural disruption of bone tissue [1]. Fractures are a major problem in patients with osteoporosis, are a major cause of morbidity, and can even result in death. Peak bone mass is affected by hereditary, endocrinological factors, and life style choices, including smoking and diet [2]. Depletion of the estrogen level in postmenopausal women results in a decrease in bone mass and an increase in the risk of fractures, as compared to similarly aged men [2, 3]. In particular, hip fractures in postmenopausal women are associated with an elevated mortality rate. Vertebral fractures are usually asymptomatic, but can cause kyphosis, and obstructive and restrictive lung disease in symptomatic cases. Distal radius fractures are the third most common fracture type and has a tendency to occur in those aged over 65 years [4]. Distal radius fractures often result in disability, and only 50% of such patients have a good functional outcome 6 months post fracture [5].

In osteoporotic postmenopausal patients there is positive correlation between muscle strength and bone loss. Sarcopenia shares some pathophysiological mechanisms with osteoporosis [6, 7]. The risk of fracture is greatly increased by muscle weakness, and muscle strength is positively correlated with bone mineral density [3, 8]. Objective measurement values are given by isokinetic devices in utilization of muscle strength in several body regions [9, 10]. It was reported that the isokinetic strength of hip abductor muscles was associated with femoral bone mineral density in postmenopausal women [11]. The same study also observed that handgrip strength was lower in osteoporotic postmenopausal women than in non-osteoporotic postmenopausal women. It was also reported that forearm isokinetic muscle strength was related to bone mineral density in male osteoporotic patients [12]. However, to the best our knowledge isokinetic wrist muscle strength has not been studied in osteoporotic postmenopausal women. Accordingly, the present study aimed to compare wrist flexor and extensor muscle strength, based on isokinetic measurement, in osteoporotic postmenopausal and non-osteoporotic postmenopausal women.

2. Methods

The patient group included 20 osteoporotic postmenopausal women (mean age: 66.4 ± 7.5 years) with a dual energy X-ray absorptiometry (DXA) T-score ⩽ - 2.5 and the control group included 15 non-osteoporotic postmenopausal women (mean age: 55.4 ± 5.6 years) with a normal DXA T-score ( ⩾ 1). All of the participants were evaluated for osteoporosis and underwent DXA measurement for the first time, and all had a negative history of antiresorptive therapy for osteoporosis. Patients who had an osteopenic DXA T-score and distal radius fracture during the previous year were excluded, as were those with a neurologic disorder or any illness (such as myopathy) that can cause muscle weakness. The study protocol was approved by the Ankara Physical and Rehabilitation Medicine Research and Training Hospital Ethics Committee.

Demographic and clinical characteristics of the participants, including age, weight, height, BMI, and DXA T-score, were recorded. Bone mineral density and T-scores were measured using a LUNAR DPX IQ DXA device. Measurements were performed with the participants in the supine position with internal rotation of the femur. Vertebral (L1-L4), femoral neck, and femoral total measurements were obtained.

Wrist flexion and extension concentric-concentric muscle strength of the dominant limb at 60 and 180 ∘ /s were tested using a Biodex System 3 (Biodex Medical Systems, Inc., New York, USA). The participants were positioned for measurement according to the isokinetic system manufacturer’s instructions. Briefly, participants were seated on the test chair with the backrest set to 90 ∘ . The arm to be tested was secured to the chair’s forearm rest. The wrist joint was positioned in neutral, flexion, and extension. The forearm stabilizer pad and strap secured the distal aspect of the forearm of the test hand. Right and left seatbelts secured the torso. Wrist flexion and extension range of motion were set within each participant’s available range. All the participants were permitted ⩾ 3 experimental trials to become familiar with the process for each angular velocity, and then they performed 3 maximal repetitions at 60 ∘ /s and 10 maximal repetitions at 180 ∘ /s. Consistent verbal encouragement for maximal effort was given to each participant throughout the testing procedure. Participants rested for about 10 s between each of 2 sets of movement. All isokinetic testing was conducted between 13:30 and 16:00. None of the participants reported any discomfort while being tested.

3. Results

The participants’ demographic and clinical characteristics are given in Table 1. Age and BMI differed significantly between the 2 groups ( 0.05). As outlined in Table 2, the PMs of the wrist flexors at both velocities were significantly lower in the patient group than in the control group.

Moreover, agonist/antagonist ratio parameters at both angular velocities were lower in the patient group than in the control group, which indicated wrist flexor weakness, but the differences were not significant. Extensor muscle strength did not differ significantly between the 2 groups. Finally, there were no significant correlations between wrist flexor and extensor PM values and DXA vertebral (L1-L4), femoral neck, and femoral total measurements ( $P>$ 0.05).

The main findings of this study show that wrist flexor muscle group strength in osteoporotic postmenopausal women is lower than in its non-osteoporo- tic counterpart. Although sarcopenia and dynapenia are normal variations of old age, some postmenopausal women may develop decreased muscle strength, independent of the existence of osteoporosis [13, 14, 15]; however, it was also reported that muscle strength decreases as bone mineral density decreases [3, 16, 17]. Zhou et al. [3] reported that a decrease in bone mineral density (especially total bone mineral density and the L2-L4 level) is strongly correlated with hip and trunk isokinetic strength. Similarly, Blain et al. [18] observed that reduced knee strength is correlated positively with decreased femoral bone mineral density. Similarly, Bayramoğlu et al observed that hip abductor strength is weakly but positively correlated with femoral bone mineral density and that osteoporotic women had significantly lower grip strength than non-osteoporotic women. Additionally, Taaffe et al. [19] noted that dynamic leg muscle strength was a strong predictor of bone mineral density in elderly females. The present findings similarly show that wrist flexor muscle strength in patients was lower than in controls.

Low wrist muscle strength might be a risk factor for distal radius fractures [5]. Earlier, Ozdurak et al. [12] reported decreased forearm muscle strength (at 60 180 ∘ /s ) in male patients with osteoporosis, and that there is a significant positive correlation between forearm muscle strength and forearm bone mineral density. Likewise, in the present study wrist flexor muscle strength was lower in the patients than in the controls. Wrist flexor muscle weakness might lead to distal radial fractures, as wrist flexors can play a protective role during falls. Pre-activation of wrist muscles during a fall positively affects joint stability and decreases the risk of injury and fracture [20]. Based on the present findings, we think that wrist flexors weakness plays an additional role in protecting against distal radial fractures, as the protective effect of wrist flexors during falls might reduce the force of the fall upon the radius. Exercises for strengthening the wrist flexors might be a useful method to help prevent distal radius fractures in osteoporotic postmenopausal women as described previously [21]. The present study is noteworthy, as to the best of our knowledge it is the first to evaluate wrist flexor and extensor muscle strength in osteoporotic postmenopausal women.

The present study has some limitations. Mean age and mean BMI differed between the patient and control groups; however, we think that the difference in age between the groups did not significantly affect wrist isokinetic strength, as weakness was only noted in the patients’ wrist flexors, not in their extensors. Had the observed loss of muscle strength been related to age it would have been uniform and also affected the wrist extensors. Furthermore, it is known that upper extremity isokinetic strength is not significantly affected by age, whereas lower extremity isokinetic strength is [22]. The effect of BMI on upper limb isokinetic muscle strength is not clearly known. It was reported that physical performance decreases with increasing age as BMI increases [23]. Although BMI was positively correlated with grip strength in older men, this correlation was not observed in older women. We suggest that the effect of the difference in BMI between the patient and control groups on the present study’s findings is limited. Nonetheless, additional comparative research on osteoporotic and non-osteoporotic groups of similar age and BMI will be helpful for further studies. Another limitation of the present study is the small patient group; however, we think that this limitation was adequately compensated for by use of appropriate statistical methods.

5. Conclusions

In conclusion, the present findings show that wrist flexors are weaker in osteoporotic postmenopausal women than in non-osteoporotic postmenopausal women. We speculate that such muscle weakness may play a role in distal radial fractures. Thus exercises for strengthening the flexors may have a preventive value but prior to that, the association between wrist flexor muscle strength and distal radius fracture should be further elucidated via additional well-designed randomized controlled trials.