Older adults often experience muscle dysfunction and mobility challenges that place them at an increased risk for falling.1–5 Furthermore, the ability of older adults to perform mobility tasks such as walking, stair negotiation, or rising from a chair may require increased effort compared with young adults and could result in an accident.6 Specifically, a fall can occur when negotiating stairs, with more incidents occurring during stair descent.7 In addition to maximal force production, submaximal muscle function has also been suggested as a potential contributor to fall risk.8 It is not clear, however, if the ability to control submaximal force is directly linked to performance on mobility tasks in older adults who have fallen.
The force produced during a muscle contraction fluctuates around an average value.3,9–12 The amplitude of force fluctuations can provide information about the ability of the nervous system to regulate force output. The force fluctuations during submaximal muscle contractions, or variability in motor output, are increased for older adults compared with young adults and are often characterized as muscle force steadiness (MFS).9,13–16 Furthermore, increased force variability may adversely impact movement precision depending on the type of muscle contraction or age of the individual.3,9,17–20
In addition to MFS, muscle force accuracy (MFA), which measures the ability of an individual to achieve a target force (TF) for a period of time, has also been utilized to assess the control of submaximal force output.15 Although data on MFA are limited and less frequently reported compared with MFS, MFA has been found to be impaired in older adults during relatively slow contractions and rapid, discrete contractions.15,20–22 It is not known whether MFA is associated with decreased mobility in older adults who have fallen. It may seem intuitive that increased variability in motor output, characterized as MFS and MFA, may be associated with decreased mobility in older adults who have fallen. However, to our knowledge, no evidence has been reported in this older adult population.
The purpose of this single group correlational study was to determine the association between concentric (CON) and eccentric (ECC) submaximal motor control of the knee extensors and the performance of mobility tasks in older adults who had fallen. We postulated a link between mobility and MFS and MFA during CON and ECC knee extensor contractions.
Twenty older adults volunteered to participate in a 1-day experimental session lasting approximately 2.5 hours. The participants (see Table 1) included adults aged 65 years or older who experienced at least 1 fall in the past year, were able to ambulate within the community with or without an assistive device, and were managing 2 or more comorbid medical conditions. A fall was defined as an event resulting in an unexpected contact or resting position with the ground or floor for medically unexplained reasons. Exclusion criteria included progressive neurologic disorders, cancer treatment, chronic heart failure, or unstable medical conditions. Participants were recruited from the Salt Lake City community. Participants provided signed confirmation of their informed consent. The institutional review board at the University of Utah approved the procedures used in this study.
Knee Extension Maximal Muscle Torque Measurements
Participants performed maximal voluntary isometric contractions (MVIC) of the knee extensor muscles at a joint angle of 45° of knee flexion on a KinCom dynamometer (KinCom 500H, Chattecx Corp; Harrison, TN). The center of the knee joint was aligned with the axis of the dynamometer, the mid-thigh was secured with a strap, and the distal leg was secured to the lever arm with a velcro strap. Before testing, participants performed 3 warm-up submaximal contractions, 1 at 50% and 2 at 75% of their perceived maximal effort. Following a rest period of 1 to 3 minutes, participants performed 3 maximal isometric contractions by pushing as forcefully as possible for 3 seconds while receiving strong verbal encouragement. One minute of rest was given between each trial. The maximal peak torque of the 3 trials, with a difference no greater than 5% between trials, was used as the MVIC. A fourth trial was performed if the aforementioned criteria were not met. Following MVIC testing and before measuring MFS, participants were given 10 minutes of rest.
Muscle Force Steadiness and Muscle Force Accuracy
Muscle force steadiness was measured by having the participants perform submaximal CON and ECC contractions of the knee extensors while attempting to match a TF line of 50% of their MVIC. Using a KinCom dynamometer (KinCom 500H, Chattecx Corp; Harrison, TN) set at a speed of 15 degree per second, data were collected between 75 and 15 degrees of knee extension. The monitor was positioned approximately 1 m in front of the participant. Participants were instructed to perform a smooth, steady contraction while attempting to match the TF line. Participants initially performed 2 familiarization trials followed by a series of test trial sequences (TTSs), with each series containing 3 trials per TTS for a total of 30 trials. Each trial lasted 8 seconds in duration, the first 4 seconds consisting of a CON contraction immediately followed by a 4-second ECC contraction of the knee extensor muscles. Participants were given a 1-minute rest period between each test trial and a 3- to 5-minute rest period between each series of 3 trials. The first 6 TTSs (a total of 18 trials) were not included in the data analysis as they were considered practice trials to ensure that participants understood the task and to avoid the potential contribution of early motor learning effects reported by others.15,23 Muscle force steadiness and MFA measures were collected on the weaker limb, as determined by the MVIC force. Muscle force steadiness was characterized by the coefficient of variation (CV = standard deviation (SD)/mean × 100), an accepted method for quantifying normalized force variability.3,24 Muscle force accuracy was defined as the difference between the average force produced and the submaximal TF (Figure 1).
Standardized tests of mobility were used including the 6-minute walk test (6MWT), the Timed Up and Go test (TUG), and the stair test (stair ascent = StA and stair descent = StD). These tests have been reported to be reliable (intraclass correlation coefficient [ICC] range: 0.90-0.96) in older adults.5,25–28
Six-Minute Walk Test
The 6MWT measures the total distance individuals can walk in 6 minutes and assesses their walking ability, a functional component of activities of daily living.29 Participants were instructed to walk as far as they possibly could in 6 minutes around a rectangular course 89.45 m in length, and the total distance walked was recorded for data analysis. The 6MWT has been shown to have a high test retest reliability.27
Timed Up and Go Test
The TUG test has been used to measure mobility and has been shown to be related to increased fall risks in older adults.5,30 Participants started in a seated position with their backs against the backrest of a chair (43 cm seat height). They were instructed to rise from the chair as quickly and safely as they could, walk a distance of 3 m, turn around a cone, and walk back to the chair and to sit back down. Timing commenced once movement occurred and ceased when participants returned to the starting position with their backs against the backrest of the chair.30 Participants were given 1 practice trial, followed by 3 test trials, with at least a 1-minute rest period between trials. The mean of 3 trials was used for data analysis.
The StA and StD tests were administered by having participants positioned at the base of a well-lit 10-step (17.15 cm steps) staircase with handrails. Under close supervision, participants were instructed to safely ascend and descend the stairs as quickly as possible. Timing commenced once movement occurred and stopped when both feet reached the top or bottom landing.31 Participants were given 1 to 3 minutes rest between trials. The mean of 3 trials was used for data analysis. The validity of these test measures has been reported previously.28
The torque signal from the dynamometer was analog-to-digital converted at 1000 samples/s (National Instruments Corporation, Austin, TX) and low-pass filtered (20th-order Butterworth) at 100 Hz. The middle 2 seconds from each 4-second force trace was used to measure MFS to avoid the ramping up and down of force production. After linear detrending of the middle 2 seconds, mean torque, SD of torque, and CV of torque were calculated using Matlab software (Mathworks, Inc; Natlick, MA; Matlab 2009).32 The MFS test trials where the mean torque produced was within ±20% of the TF were included as part of the data analysis. Muscle force accuracy was expressed as a percentage of the TF (%TF). The mean force produced was divided by the TF and multiplied by 100 ([mean force * target force−1]*100). A value of 100%TF signified a mean force equal to the TF. Values for MFA%TF greater than or less than 100% indicated overshoot or undershoot of the mean force relative to the TF.
To determine whether an association was present between MFS, MFA, and mobility tasks, the Pearson product moment correlation coefficients were utilized to evaluate the bivariate relationships. The statistical significance level was set at P < .05. Data analysis was performed using SPSS version 20 (IBM SPSS Inc, Armonk, NY).
All 20 participants completed the study. Demographic characteristics expressed as group means and SD for age, height, weight, body mass index, MVIC, the number of falls, and the number of comorbidities were included in Table 1. There was a greater proportion of females (n = 15) than males (n = 5). Participants appeared to be typical of older community dwelling adults for age, sex, height, and weight as observed by others.33–35 Group means and 95% confidence interval of the mobility test measures for the 6MWT, TUG, StA, and StD were included in Table 2.
Muscle Force Steadiness and Mobility Tests
No significant correlations were detected for CON MFS and mobility tasks for the 6MWT (r = −0.22; P = .93), TUG (r = −0.13; P = .96), StA (r = −0.14; P = .55), and StD (r = 0.02; P = .93). Likewise, ECC MFS was not significantly correlated with mobility tasks consisting of the 6MWT (r = −0.39; P = .09), TUG (r = 0.42; P = .07), StA (r = 0.27; P = .24), and StD (r = 0.30; P = .20).
Muscle Force Accuracy and Mobility Tests
Although CON MFA was not correlated with mobility tasks for the 6MWT (r = −0.10; P = .67), TUG (r = 0.23; P = .33), StA (r = 0.17; P = .47), and StD (r = 0.38; P = .10), there were moderate to good correlations for ECC MFA and the 6MWT (r = −0.48; P < .05), TUG (r = 0.68; P < .01), StA (r = 0.60; P < .01), and StD (r = 0.75; P < .01) (Figure 2).
The purpose of this study was to determine if the steadiness and accuracy of submaximal knee extensor force was correlated with performance on mobility tasks in older adults who had fallen. Interestingly, only force accuracy during ECC contractions was correlated with performance on mobility tasks. Force accuracy during ECC explained between approximately 23% and 56% of the variance in the performance of the mobility tasks. However, no significant associations were found between force steadiness measures during either CON or ECC contractions and mobility performance. In addition, no significant correlations were detected between force accuracy during CON contractions and mobility performance. These findings are novel and unique in that—(1) this is the first report to document knee extensor force accuracy as a feature of submaximal muscle function linked to mobility tasks; (2) this is the first study to show that this relationship is specific to submaximal ECC contractions; and (3) these relationships may point to an important feature of muscle impairment in older adults who have fallen.
Previously reported evidence regarding MFA and mobility tasks is very limited. Only Seynnes et al36 reported that submaximal isometric MFS was associated with chair rise time, yet no associations were detected between MFA and mobility tasks. And though Hortobagyi et al15 did not correlate motor control measures (MFS and MFA) with functional tasks, they reported that older adults demonstrated overshooting of the TF by 2 times the amount during CON and 3 times the amount during ECC contractions of the knee extensor muscles compared with young adults. Moreover, Carville et al8 did not correlate steadiness measures with mobility, but rather reported that at-risk older adults who had fallen exhibited greater variability in motor output during ECC but not CON contractions of the knee extensors than older adults who had not fallen. Collectively, the reported evidence of associations between knee extensor steadiness and accuracy (MFS and MFA) and mobility tasks is very limited and varies between contraction types.
Interestingly, others have compared steadiness and accuracy in older adults with performance tasks including general limb movement trajectories or motor tasks that require movement precision as performed by muscles of the hand.16,17 The general reported findings were that older adults demonstrated greater variability when performing ECC submaximal muscle contractions, particularly at low TF levels,9,15,19,37 than young adults.3,18,20,24,38 Furthermore, it is the relationships between task performance, movement precision, and force steadiness that may present additional challenges for older adults who have fallen.11,36,39
The plausible mechanisms that may explain associations between ECC force accuracy and mobility include (1) unique characteristics or variations in the descending command during ECC compared with CON or isometric contractions; and (2) ability to modulate presynaptic inhibition of Ia afferents during ECC contractions.
Reviews by Enoka,40 Duchateau and Enoka,41 and Duchateau and Baudry42 have suggested how the motor output or descending command may vary when performing ECC compared with CON contractions. In general, when a CON contraction is performed, the torque produced by the muscle force produced is slightly greater than the torque produced by the applied load in order for the muscle to produce a shortening contraction. However, during an ECC contraction, the applied load torque is greater than the muscle force torque and the muscle performs a lengthening contraction.40,41 The muscle activation levels during submaximal ECC contractions may differ from that of CON contractions, and the specific modulation of the activation signal to the targeted muscle may vary.9,43 For example, during an ECC contraction, reported evidence suggests that in addition to derecruitment of active motor units, the discharge rate also tends to decline.44 Accordingly, the overall activation level of a muscle is less during ECC contraction compared with CON.40 Furthermore, changes in the neuromuscular system due to aging include reductions in the number of motor neurons and muscle fibers, motor unit remodeling via collateral sprouting from adjacent motor neurons, and increased size of existing motor units.45,46 Collectively, these age-related changes to the motor unit can affect the activation and discharge rate of the motor units, thereby contributing to the variability in the force output.3,9,14
An additional mechanism that may play a contributory role to the increased variability in motor output is presynaptic inhibition of Ia afferents. Input from Ia afferents to the active motor neuron pool can be inhibited due to the depolarization of primary afferent fibers by interneurons and consequently affect the activation of the motor units to the targeted muscle.47,48 Furthermore, depending on the task that is to be performed, whether position control or force control, older adults demonstrate a decreased ability to regulate presynaptic inhibition compared with young adults. For example, Baudry et al47 reported that the ability of older adults to regulate presynaptic Ia afferent inhibition was reduced when individuals were tasked to control hand position while supporting a mass (position control) compared with exerting the same force against a rigid constraint (force control).
The clinical relevance of the reported findings in this study are such that MFA may be an additional factor to consider when exploring force control in older adults, particularly in those who have fallen, as this may impede or reduce their ability when performing mobility tasks. Since resistance exercise training can improve MFS in older adults,10,13,15,17,49 further investigation is warranted on the effects of such training on MFA measures, particularly during ECC contractions.
This study included a relatively small number of individuals and multiple bivariate correlations. The results should be interpreted in light of the concept that statistical associations between ECC MFA and mobility tasks do not equate to causation. For example, discretion should be used with interpreting these particular outcomes to design interventions for improving force control in older adults who have fallen. However, these findings are novel and are at least suggestive that inaccurate submaximal force output during lengthening contractions may contribute to impaired mobility in older adults at risk for falling.
Overall, there were moderate to good correlations between the ability to produce accurate force during ECC contractions and performance on mobility tasks in older adults who fall. These findings are novel and suggest that a decreased ability to regulate ECC forces may contribute to impaired mobility in older adults who are at risk for falling or have fallen. Further investigations should determine whether training-related improvements in force accuracy during lengthening contractions result in improved mobility in older adults at risk for falls.
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