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. 2017 Jan 11;117(4):1483–1488. doi: 10.1152/jn.00640.2016

Motor plan differs for young and older adults during similar movements

Agostina Casamento-Moran 1, Yen-Ting Chen 1, Neha Lodha 1, Basma Yacoubi 1, Evangelos A Christou 1,2,✉
PMCID: PMC5376608  PMID: 28077666

We aimed to determine whether the altered activation of muscles in older adults compared with young adults during fast goal-directed movements is related to an altered motor plan. For matched movements, there were differences in the coordination of antagonistic muscles but no differences in the individual activation of muscles. We provide novel evidence that the differential activation of muscles in older adults is related to an altered motor plan.

Keywords: aging, electromyography, goal-directed movements, motor plan, neural control of movement

Abstract

Older adults exhibit altered activation of the agonist and antagonist muscles during goal-directed movements compared with young adults. However, it remains unclear whether the differential activation of the antagonistic muscles in older adults results from an impaired motor plan or an altered ability of the muscle to contract. The purpose of this study, therefore, was to determine whether the motor plan differs for young and older adults. Ten young (26.1 ± 4.3 yr, 4 women) and 16 older adults (71.9 ± 6.9 yr, 9 women) participated in the study. Participants performed 100 trials of fast goal directed movements with ankle dorsiflexion while we recorded the electromyographic activity of the primary agonist (tibialis anterior; TA) and antagonist (soleus; SOL) muscles. From those 100 trials we selected 5 trials in each of 3 movement end-point categories (fast, accurate, and slow). We investigated age-associated differences in the motor plan by quantifying the individual activity and coordination of the agonist and antagonist muscles. During similar movement end points, older adults exhibited similar activation of the agonist (TA) and antagonist (SOL) muscles compared with young adults. In addition, the coordination of the agonist and antagonist muscles (TA and SOL) was different between the two age groups. Specifically, older adults exhibited lower TA-SOL overlap (F1,23 = 41.2, P < 0.001) and greater TA-SOL peak EMG delay (F1,25 = 35.5, P < 0.001). This finding suggests that although subjects in both age groups displayed similar movement end points, they exhibited a different motor plan, as demonstrated by altered coordination between the agonist and antagonist muscles.

NEW & NOTEWORTHY We aimed to determine whether the altered activation of muscles in older adults compared with young adults during fast goal-directed movements is related to an altered motor plan. For matched movements, there were differences in the coordination of antagonistic muscles but no differences in the individual activation of muscles. We provide novel evidence that the differential activation of muscles in older adults is related to an altered motor plan.

most movements of daily living are goal-directed by nature. During goal-directed movements of the upper and lower limbs, the overall performance and the activation of the agonist and antagonist muscles differ for young and older adults (Chen et al. 2014; Kwon et al. 2014). An important but unresolved question is whether the differential activation of the antagonistic muscles in older adults during goal-directed movements results from an altered motor plan or changes in the ability of the muscles to execute the same motor plan (Frontera et al. 2000; Janssen et al. 2014; Lexell et al. 1988).

In the present study, we investigated possible age-associated differences in the motor plan by comparing the individual activation and the coordination of the agonist and antagonist muscles during similar goal-directed movements. To match the performance of goal-directed movements, we selected movements with similar end points for young and older adults and examined the activation and the coordination (temporal coactivation) of the agonist and antagonist muscles. Differences in individual muscle activity could reflect an altered motor plan or an altered muscle activity. Nonetheless, coordination differences with similar individual muscle activity would suggest an altered motor plan. The purpose of this study was to determine whether the motor plan differs for young and older adults during fast goal-directed movements. We hypothesized that older adults would exhibit altered coordination of the agonist and antagonist muscles and thus an altered motor plan.

MATERIALS AND METHODS

Participants

Ten young (26.1 ± 4.3 yr, 4 women) and 16 older adults (71.9 ± 6.9 yr, 9 women) participated in the study. All participants reported being healthy without any known neurological impairments and were right-footed (Elias and Bryden 1998). The Institutional Review Board at the University of Florida approved the procedures of this study. All participants signed a written informed consent before participating in the study.

Experimental Approach

Participants completed one testing session in which they performed 100 trials of an ankle dorsiflexion goal-directed task. At the beginning of the session, we explained the experimental procedures and the goal-directed task (ankle dorsiflexion movements) to the participants. Each participant performed the following procedures within the session: 1) maximal voluntary contraction (MVC) with ankle dorsiflexion and plantarflexion; 2) 3–5 goal-directed movement practice trials at a different target from the actual target; 3) 100 goal-directed movement trials with ankle dorsiflexion; and 4) repetition of the MVC task.

Experimental Setup

Participants sat with their left hip joint flexed to ~90° with 10° abduction, the knee flexed to ~90°, and the ankle plantarflexed to ~15°. The left foot rested on a customized foot device with an adjustable footplate and was strapped over the metatarsals to ensure a secure position and simultaneous movement between the device and the foot (Fig. 1A). The axis of rotation of the customized foot device was positioned in line with the axis of rotation of the left ankle to allow only dorsiflexion and plantarflexion of the ankle.

Fig. 1.

Fig. 1.

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Experimental set up and motor output selection. A: schematic drawing of the experimental setup and arrangement of the left foot. The left foot was placed and rested on a customized foot device with an adjustable foot plate and secured by a strap over the metatarsals. Participants performed fast back and forth goal-directed movements with ankle dorsiflexion. B: 5 selected movement end points for each category of motor output (fast, accurate, and slow) for 1 young and 1 older adult participant. Note the accuracy for each motor output category was the same for young and older adults. The 5 trials were uniformly selected across the 100 practice trials for both young and older adults.

Limb displacement.

The displacement of the ankle (dorsiflexion) was measured using a low-friction potentiometer (SP22G-5K; Mouser Electronics, Mansfield, TX) located directly lateral to the fibular malleolus. The ankle position signals were sampled at 1,000 Hz with a NI-DAQ card (model USB6210; National Instruments, Austin, TX) and stored on a personal computer.

EMG measurements.

Muscle activation was recorded with a Trigno wireless electromyography (EMG) system (Delsys, Boston, MA) and Bagnoli EMG system (Delsys) from the tibialis anterior (TA) and soleus (SOL) muscles. The recording electrodes were placed on the skin and in line with the muscle fibers. Specifically, the electrode for the TA was placed at one-third on the line between the proximal border of the fibula and the tip of the medial malleolus. For the SOL, the electrode was placed halfway between the end of the head of the gastrocnemius and the origin of the Achilles tendon. The EMG signals were bandpass filtered from 20 to 450 Hz, amplified 1,000 times, sampled at 1,000 Hz with a DAQ card (model USB6210; National Instruments), and stored on a personal computer.

MVC task.

We identified the MVC for ankle dorsiflexion and ankle plantarflexion. Participants increased force to their maximum in 3 s and maintained the maximal force for ~3 s. They exerted 3–5 MVCs or until two MVC trials were within 5% of each other. One-minute rests were provided between consecutive trials. To normalize the EMG during the goal-directed movements, we recorded the peak EMG (average of 0.5 s around the peak EMG of the trial) during MVC.

Goal-directed movements.

Participants performed unloaded goal-directed movements that involved accurately matching a target by performing a single ankle dorsiflexion, as described in Chen et al. 2014. In brief, the goal was to match the peak displacement of the movement (accomplished with a forward and backward movement) with the target. The target consisted of a peak foot displacement of 9° in 180 ms. We did not provide specific instructions about how to execute the movements, allowing participants to choose the motor strategy that worked best for them.

The task consisted of the following three phases: 1) get ready, 2) move, and 3) feedback. The get ready phase began with the presentation of a red target on the monitor for 2 s. The red target cued participants to be ready for the move phase. The move phase began when the red target switched to green. The change in target color cued participants to perform the goal-directed movement. The green target stayed on the monitor for 3 s, and participants initiated the movement at their convenience (no reaction time). The recording of the task began when the participant initiated the movement. The feedback phase started at the end of the move phase and lasted for 5 s. Participants received visual feedback of their performance (movement trajectory) relative to the target (9°, 180 ms). The visual gain was kept constant with a visual angle of 1° (Vaillancourt et al. 2006).

Data Analysis

Data were analyzed offline using custom-written programs in MATLAB (R2013a; The MathWorks, Natick, MA).

Movement end point.

We quantified the position and time coordinates to calculate the movement end point. Position was quantified as the absolute vertical deviation from zero to peak displacement, whereas time was quantified as the absolute horizontal deviation from zero to peak displacement. The hypotenuse between the position and time coordinates represents the shortest distance from zero to peak performance and thus movement end point (Fig. 1B). To be able to quantify the hypotenuse, we normalized the position and time coordinates to have the same units (%). We normalized position error to the targeted peak displacement (9°; Eq. 1) and time error to the targeted time to peak displacement (180 ms; Eq. 2) to have both variables with the same units (%) and quantify overall error (Eq. 3).

Position (%) = peak displacement (degrees)targeted peak displacement (degrees)× 100 (1)
Time (%) = time to peak displacements (ms)targeted time to peak displacements (ms) × 100 (2)
Movement End Point (%)=(position)2+(time)2 (3)

We specifically focus on movement end point because it is the main variable controlled in the task and because it summarizes the performance in terms of space and time.

Motor output selection.

From the 100 trials performed by each participant we arbitrarily selected 5 trials that belonged to 3 movement end-point categories (Fig. 1B): 1) fast: trials had faster timing (70–140 ms) and accurate position (7°–11°); 2) accurate: trials had accurate timing (160–200 ms) and accurate position (7°–11°); and 3) slow: trials had slower timing (220–300 ms) and accurate position (7°–11°). If for any given participant there were more than 5 possible trials in each category, we further selected the trials as the ones that had the position closest to 9° and the timing closest to 70, 180, and 300 ms for the fast, accurate, and slow categories, respectively. All the selected trials were evenly distributed across the 100 trials in similar proportions for young and older adults in each motor output category.

Neuromuscular activity.

The interference EMGs were rectified and smoothed with a fourth-order Butterworth digital filter (filtfilt) with a cutoff frequency at 6 Hz (Poston et al. 2008) (Fig. 2). This filter was used to identify the amplitude, onset, and offset of the EMG burst of the primary agonist (TA) and the primary antagonist muscle (SOL) of the ankle.

Fig. 2.

Fig. 2.

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Agonist-antagonist EMG activity. We quantified the EMG bursts from the agonist and antagonist muscles by detrending, rectifying, and 6-Hz low-pass filtering the interference EMGs. We then determined the start, end, and peak of each burst to calculate individual muscle activity and antagonistic muscle coordination. TA-SOL delay was defined as the time difference between the times of the peak EMG burst amplitude of the 2 muscles. TA-SOL overlap was defined as the percentage overlap between the activity of the 2 muscles (A/B × 100).

Muscle activity.

We examined muscle activity by quantifying the EMG burst in the following ways. The EMG burst duration was the time between EMG onset (15% of the peak EMG) and EMG offset (15% of the peak EMG). The EMG burst amplitude was the peak EMG activity within the EMG burst duration normalized to the EMG recorded during the MVC. The EMG burst time to peak was the time between time of EMG onset (15% of the peak EMG) and the time of peak EMG activity.

Muscle coordination.

We examined the coordination of the agonist and antagonist muscles in two ways. The agonist-antagonist (TA-SOL) overlap (Fig. 2) was defined as the percentage of overlap between the two muscles’ activity A and B, as indicated in Eq. 4:

TA-SOL Overlap (%) = (A)Offset TA EMG burst − Onset SOL EMG burst(ms)(B)Offset SOL EMG burst−Onset TA EMG burst(ms) × 100. (4)

The agonist-antagonist (TA-SOL) peak EMG delay (Fig. 2) was defined as the time difference between the times of the peak EMG burst amplitude of the two muscles, as indicated in Eq. 5.

TA-SOL Delay(ms) = TA Peak EMG(ms) − SOL Peak EMG(ms). (5)

Statistical Analysis

We performed a mixed ANOVA [2 age groups × 3 motor output categories (fast, accurate, and slow)] with repeated measures on motor output categories to compare the movement end point and muscle activity of young and older adults. Significant main effects and interactions were followed with Bonferroni post hoc comparisons.

Analyses were performed with the IBM SPSS Statistics 21.0 statistical package (IBM, Armonk, NY). The α level for all statistical tests was 0.05. Data are reported as means ± SD in the text and figures.

RESULTS

Demographics

Young and older adults had similar height (1.73 vs. 1.70 m, young vs. older; P > 0.05), weight (72.45 vs. 78.42 kg; P > 0.50), and body mass index (23.94 vs. 26.87; P > 0.55).

Movement End Point and Aging

Young and older adults exhibited similar performance in each of the three motor output categories. There was a nonsignificant main effect of age (F1,24 = 0.8, P = 0.38) and a nonsignificant age × motor output category interaction for movement end point (F2,48 = 1.5, P = 0.23; Fig. 3, top). As expected, there was a significant main effect of movement end point among the three motor output categories (F2,48 = 476.7, P < 0.001).

Fig. 3.

Fig. 3.

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Movement end point and Individual muscle activation for young and older adults. Young and older adults exhibited similar movement end points in each of the selected motor output category. Independent of movement end point, older adults exhibited similar agonist and antagonist activation. This is demonstrated with no differences in the amplitude, duration, and time to peak of both the agonist (TA) and antagonist (SOL) muscles.

Neuromuscular Activation

Independently of the motor output category, older adults exhibited similar activation of the agonist (TA) and antagonist (SOL) muscles compared with young adults (Fig. 3, middle and bottom), as measured with EMG burst duration, amplitude, and time to peak. The main effect of age for the amplitude (TA: F1,24 = 0.03, P = 0.86; SOL: F1,24 = 0.96, P = 0.34), duration (TA: F1,24 = 0.9, P = 0.35; SOL: F1,24 = 0.5, P = 0.51), and time to peak (TA: F1,24 = 0.2, P = 0.69; SOL: F1,24 = 0.02, P = 0.90) for both muscles (Fig. 3, middle and bottom) was not significant. Additionally, the age × motor output interaction for amplitude (TA: F2,48 = 1.6, P = 0.2; SOL: F2,48 = 1.8, P = 0.18), duration (TA: F2,48 = 0.07, P = 0.9; SOL: F2,48 = 0.9, P = 0.4), and time to peak (TA: F2,48 = 1.3, P = 0.3; SOL: F2,48 = 0.7, P = 0.5) for both muscles was not significant.

In contrast, older adults exhibited different coordination of the agonist (TA) and antagonist (SOL) muscles. Specifically, older adults exhibited lower TA-SOL overlap (F1,23 = 41.2, P < 0.001; Fig. 4A) and greater TA-SOL Peak EMG delay than young adults for every motor output category (F1,25 = 35.5, P < 0.001; Fig. 4B). The age × motor output interaction was not significant for both of the muscle coordination variables (TA-SOL overlap: F2,44 = 1.1, P = 0.3; TA-SOL Peak EMG delay: F2,48 = 0.4, P = 0.6).

Fig. 4.

Fig. 4.

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Agonist and antagonist muscle coordination of young and older adults. Independently of movement end point, older adults exhibited different agonist and antagonist muscle coordination. This is demonstrated with decreased agonist-antagonist TA-SOL overlap (top) and increased TA-SOL delay (bottom) during fast (right), accurate (middle), and slow (left) goal-directed movements. *P < 0.05, significant differences between young and older adults.

DISCUSSION

The purpose of this study was to determine whether motor planning of goal-directed movements differs for young and older adults. We selected trials with similar movement end points and quantified the individual muscle activity and the coordination of the agonist and antagonist muscles. We found differences in the coordination but no differences in the individual activation of the agonist and antagonist muscles. Therefore, these findings provide novel evidence that during fast goal-directed movements, the differential muscle coordination in older adults is likely induced by an altered motor plan instead of an altered individual muscle activity.

Aging and Agonist-Antagonist Muscle Coordination

To our knowledge this is the first study to demonstrate that independently of movement end point, the agonist-antagonist coordination differs for young and older adults. Below we provide supporting evidence that the age-associated differences in the coordination of the antagonistic muscles reflect an altered motor plan.

Differences in the coordination of the agonist and antagonist muscles between young and older adults can be due to different movement kinematics, an altered muscle activation, or differences in the motor plan. Our experimental design ruled out the possibility that different movement kinematics (i.e., range of motion and timing of the movement end point) caused the observed coordination differences between the two age groups. Similarly, we have ruled out the possibility that the observed coordination differences were caused by differential activation of the individual muscles. We found that the TA and SOL muscles exhibited similar activation for young and older adults. Therefore, the coordination differences in the antagonistic muscles for young and older adults are likely related to an altered motor plan.

Fast goal-directed movements, such as those performed in our study (140–240 ms), are primarily controlled by preplanned descending cortical commands that are not influenced by online sensory or visual feedback (Desmurget and Grafton 2000; Gordon and Ghez 1987; Seidler et al. 2004; Shadmehr et al. 2010). In our task, participants can only perform preplanned trial-to-trial adjustments that do not involve performing online error corrections or responding to perturbations. Consequently, for these fast movements, participants organize and execute the motor plan without producing any adjustments while performing the task (feedforward portion of the movement) (Wolpert and Miall 1996). Therefore, the observed difference in muscle coordination between the two age groups are not likely due to feedback. This supports previous finding that in fast movement conditions, young and older adults use different strategies to plan and control their movements (Boisgontier and Nougier 2013).

The difference in the selected motor plan of young and older adults can be due to age-related changes of the central or peripheral motor system. With aging, the activity (Talelli et al. 2008), connectivity (Bo et al. 2014; Fling et al. 2011), and morphology of the cortical centers (Sowell et al. 2003) change, and the number and quality of the projections from cortical centers to the muscle decrease. For example, there is a decline in the number of spinal motor neurons (Tomlinson and Irving 1977) and motor units (Campbell et al. 1973) in older adults. Therefore, the difference in the selected motor plan in older adults could be an adaptation to age-related changes in the central and peripheral motor system.

Limitations

The findings of this study are limited to the muscles examined and the sensitivity of the surface EMG. We examined the tibialis anterior (TA) as the primary agonist and the soleus (SOL) as the primary antagonist muscles during ankle dorsiflexion. However, it is likely that other muscles, specifically the gastrocnemius and peroneus, contribute differently to the movement (Smeets 1994). Future studies should examine in more detail how additional agonist and antagonist muscles are activated differently in young and older adults. Cross talk is a potential risk when working with surface EMG (Farina et al. 2004). Although the analyzed signals could be contaminated, no major signs of cross talk were detected. Moreover, the same positioning of electrodes and experimental setup was used in young and older adults, making our findings independent of cross talk.

In addition, the findings are limited to inferences from the coordination of the agonist and antagonist muscles. To further examine the idea that older adults implement a differential motor plan, future studies should also examine how supplementary motor areas in the brain are activated for young and older adults.

Conclusion

In summary, our findings provide evidence that the motor plan of goal-directed movements differs for young and older adults. This is demonstrated by differences in the coordination of the agonist and antagonist muscles and similarities in the activation of individual muscles during fast goal-directed movements. In addition, this study is important at a theoretical level; it suggests that similar movement outputs do not necessarily indicate a similar motor plan.

GRANTS

This work was supported by National Institute on Aging Grant R01 AG031769.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

A.C.M. and Y.-T.C. performed experiments; A.C.M. and Y.-T.C. analyzed data; A.C.M., B.Y., and E.A.C. interpreted results of experiments; A.C.M. prepared figures; A.C.M. drafted manuscript; A.C.M., N.L., B.Y., and E.A.C. edited and revised manuscript; E.A.C. approved final version of manuscript.

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