Main Findings
The main objective of this study was to determine the association between knee extension rate of torque development and physical function among individuals with knee OA. Knee extension rate of torque development was not associated with scores from the self-reported measures of function but was significantly associated with scores from the clinical tests of function. Individuals who displayed greater rates of torque development were able to complete the 20-meter walk test faster than individuals with lower rates of torque development. While knee extension rate of torque development during all three time intervals measured was significantly associated with 20-meter walk times, only rate of torque development measured during the first 100 ms of muscle contraction was significantly associated with scores from the stair climb test. Knee extension rate of torque development displayed a moderate negative correlation with both the 20-meter walk and stair climb test. Rate of torque development was not associated with scores from the 30- second chair stand test.
Importance of Association Between RTD and Walking Speed
Previous research58,59 suggests that walking speed is an important predictor
of various measures of health in an aging population. A decline in walking speed among older adults is associated with increased disability58, and older adults with
slower walking speeds face an increased mortality risk.59 Walking speed is a
significant concern for individuals with knee OA, as individuals with knee OA walk slower than their healthy counterparts.60 Individuals with knee OA are also at
greater risk of experiencing a rapid decline in walking speed.61
In this study, we observed a significant association between greater rates of torque development and faster walking speeds among individuals with knee OA. Additionally, rate of torque development was more closely associated with walking
speeds than peak torque or voluntary activation. Neither peak torque (rho=-.430, p=0.058) nor central activation ratio (rho=-.125, p=0.6) were significantly
associated with 20-meter walk times. Since only rate of torque development was significantly associated with 20-meter walk times, rate of torque development may be a more sensitive indicator of slow walking pace than quadriceps strength or activation. While previous research has indicated that quadriceps strength11,23 or
activation22 play a role in the decreased physical function of individuals with knee
OA, this study indicates that knee extension rate of torque development is also significantly related to physical function, especially fast-paced walking speed. Based on these findings, low knee extension rates of torque development may be a
potential mechanism to explain slow walking speeds among individuals with knee OA.
Non-Significant Associations
Knee extension rate of torque development was not significantly associated with scores from the 30-second chair stand test, WOMAC function assessment, and SF-12. Previous research27 linking knee extension RTD with physical function
among individuals receiving knee arthroplasty also found significant associations between RTD and speed in walking and stair climbing tests, but this study did not use a chair-stand test as an outcome measure. Our findings suggest that knee extension rate of torque development may be more associated with performance in walking and stair climb tests than a chair-stand test among individuals with knee OA. While a 30-second chair stand test has been previously used as a measure of muscle endurance62, rate of torque development is generally linked with muscle
power and performance in short-duration powerful activities25. This may explain
why knee extension rate of torque development was not significantly associated with scores from the 30-second chair stand in this study.
A previous study28 found that individuals with hip OA with higher quadriceps
rate of torque development scored higher on self-reported measures of physical function. This significant association between knee extension RTD and self- reported function was not found in our study using individuals with knee OA,
perhaps because the sample used was not large enough. Among this sample of twenty individuals with knee OA, greater rates of torque development were correlated with better self-reported function, but the associations were not statistically significant. If an association does exist between rate of torque
development and self-reported physical function, then a larger sample size would have more power in detecting significance than our sample of twenty participants.
Interventions to Increase Rate of Torque Development
If knee extension rate of torque development is significantly associated with faster walking speeds among individuals with knee OA, it may be beneficial to employ interventions aimed at increasing RTD in this population. In a younger population, explosive strength and plyometric training has been shown to increase knee extension rate of torque development63, likely due to neural changes64. An
explosive strength training intervention was also successful at increasing plantar flexion rate of torque in an elderly population54. Future research should investigate
whether a strength training intervention could successfully increase knee extension rate of torque development among individuals with knee OA, with the goal that greater rates of torque development will lead to increased walking speeds and improved physical function. However, this may be difficult to implement in this population due to functional and pain-related limitations among individuals with knee OA. Other interventions, such as whole-body vibration52 and TENS therapy65,
are being investigated for their neuromuscular effects on individuals with knee OA. It is worthwhile to investigate whether these interventions have any impact on rate of torque development in this population.
Limitations and Areas for Further Investigation
This study had two significant limiting factors. Only twenty participants were included. While enough to show significant associations between RTD and measures of function, the number of participants was not enough to generalize these findings to the greater population of all individuals with knee OA. All
quadriceps activation deficits, so these results cannot be generalized to individuals who do not fit this study’s inclusion criteria. Further research should continue investigating the association between knee extension rate of torque development and physical function in a larger, more diverse sample that better represents all individuals with knee OA. The associations that we found that were not significant may be underpowered to detect significance. A larger sample size may provide greater power to detect a significant association between rate of torque
development and measures of physical function in this population, especially self- reported physical function.
Additionally, this study only tested participants at one point in time.
Therefore, we were only able to determine a correlational association between the outcome measures. In order to investigate a causal relationship between knee extension rate of torque development and physical function, a longitudinal study is needed to measure the change in outcome measures over time. Future research should investigate whether changes in knee extension rate of torque development associate with increases in walking speed over time, and whether an intervention that increases rate of torque development could be effective at increasing the walking speed in this population. Future research into this association could affect how clinicians aim to improve physical function in patients with knee OA.