Assessing articulatory speed performance as a potential factor of slowed speech in older adults.

JSLHR

Research Article

Assessing Articulatory Speed Performance as a Potential Factor of Slowed Speech in Older Adults Antje S. Mefferda and Erin E. Cordera

Purpose: To improve our understanding about the underlying factors of aging-related speaking rate decline, the authors sought to determine if lip and jaw speeds are physiologically constrained in older adults. Method: Thirty-six females—10 young adults (ages 22–27 years), 9 middle-aged adults (ages 45–55 years), 10 young-old adults (65–74 years), and 7 very old adults (ages 87–95 years)— completed metronome-paced syllable repetitions while moving the lower lip or jaw to a fixed target with each repetition. Metronome paces incrementally increased from 1.4 Hz to 6.7 Hz. Lip and jaw movements were tracked using a 3-dimensional motion capture system. Results: Older adults’ maximum percent increase in lip and jaw peak speed was comparable to or tended to be even

greater than that of middle-aged and young adults. By contrast, lip and jaw stiffness, indexed by peak speed–displacement ratios, tended to decrease with age during fast and very fast repetition rates and were associated with mildly prolonged movement durations. Conclusions: The findings suggest that lip and jaw speeds are not constrained in older adults. The trend of reduced stiffness during fast rates, however, suggests that fine-force regulation becomes difficult for older adults. Thus, older adults may implement reduced habitual speaking rates as a behavioral strategy to compensate for diminished articulatory control.

A

explain at least in part the slowing of speaking rate, relatively little is known about how aging affects the speech motor system and its potential contribution to a slowed rate of speech. One of few well-documented aging-related changes in the speech motor system is the decline in orofacial muscle strength (e.g., Clark & Solomon, 2012; Robbins, Levine, Wood, Roecker, & Luschei, 1995; Wohlert & Smith, 1998; Youmans, Youmans, & Stierwalt, 2009). The hypothesis that reduced strength may result in insufficient muscular power to propel the articulator in a timely manner into proper position led to studies of the potential associations between orofacial strength and speaking rate; yet no significant relations were found in typical speakers or in speakers with motor speech impairments (Langmore & Lehman, 1994; Neel & Palmer, 2012; Solomon, Robin, & Luschei, 2000). Because speech is produced in a low-force but high-speed fashion (Barlow & Burton, 1990), the speed at which articulators can be moved may have a greater impact on speaking rate than orofacial strength alone. This notion is supported by findings of moderate correlations between articulatory speed and speaking rate in persons with amyotrophic lateral sclerosis (ALS), a progressive neuromuscular disease (Yunusova et al., 2012). Further, Langmore and Lehman

robust finding in the aging literature is the slowing of speaking rate with advanced age. Slowed speech has been observed across a wide range of speech tasks: conversational speech, paragraph reading, sentence productions, and syllable repetitions (e.g., Amerman & Parnell, 1992; Duchin & Mysak, 1987; Goozée, Stephenson, Murdoch, Darnell, & LaPointe, 2005; Jacewicz, Fox, & O’Neill, 2009; Ramig, 1983; Ryan & Burk, 1974; Smith, Wasowicz, & Preston, 1987; Wohlert & Smith, 1998).The underlying factors contributing to the speaking rate decline with advanced age are, however, only poorly understood. In general, cognitive-linguistic and speech physiologic constraints are known to affect speaking rate (Nip & Green, 2013). Although cognitive-linguistic performance decline has been well documented in older adults (e.g., Bryan, Luszcz, & Crawford, 1997; Burke & MacKay, 1997; Elgamal, Roy, & Sharratt, 2011; Glosser & Deser, 1992; Hultsch, Hertzog, Small, & Dixon, 1999; Lamar, Resnick, & Zonderman, 2003) and may

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Wichita State University, KS

Correspondence to Antje S. Mefferd: [email protected] Editor: Jody Kreiman Associate Editor: Robert Fox Received August 19, 2012 Revision received May 8, 2013 Accepted July 1, 2013 DOI: 10.1044/2014_JSLHR-S-12–0261

Key Words: aging, speech motor control, physiology

Disclosure: The authors have declared that no competing interests existed at the time of publication.

Journal of Speech, Language, and Hearing Research • Vol. 57 • 347–360 • April 2014 • A American Speech-Language-Hearing Association

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(1994) reported that the maximum rates of repeated contraction of orofacial muscles were strong predictors of maximum syllable repetition rates in persons with ALS. Although in older adults articulatory speed performance is relatively unexplored, aging-related motor slowness (bradykinesia) has been well documented in limb studies. For example, older adults walk or reach with slower movement speeds than their younger peers (e.g., Bellgrove, Phillips, Bradshaw, & Gallucci, 1998; Haaland, Harrington, & Grice, 1993; Himann, Cunningham, Rechnitzer, & Patterson, 1988; Ketcham, Seidler, Van Gemmert, & Stelmach, 2002; Welford, 1977, 1984; Yan, Thomas, & Stelmach, 1998). The underlying causes of aging-related motor slowness in the limbs, however, remain highly debated. Motoneuron degeneration and subsequent remodeling of motor units as well as muscular atrophy (sarcopenia) are considered contributing factors (Korhonen et al., 2006). Although these aging processes have also been documented for orofacial muscles, important differences between skeletal and orofacial muscles also have been noted. For example, a relative decrease of fast-twitch type II fibers is commonly reported for aging skeletal muscles, but it is not evident in cranial muscle groups such as jaw closers (Monemi, Eriksson, Eriksson, & Thornell, 1998; Monemi Eriksson, Kadi, Butler-Browne, & Thornell, 1999; Monemi, Liu, Thornell, & Eriksson, 2000). Instead, a relative increase in fast-twitch type II fibers and hybrid (IM/IIC) fibers has been observed with advanced age (Monemi et al., 1999). Reasons for such differences between aging skeletal and cranial muscles are currently unclear; however, they suggest that findings of aging studies on limb motor performance may not be generalizable to speech motor performance in older adults. Despite histological differences between skeletal and orofacial muscles, general overlap has been found in the motor control mechanisms that underlie limb and speech movements (Ostry, Keller, & Parush, 1983). For example, manipulating movement durations, or rate, will elicit changes in speed and displacement. However, the nature of the task appears to dictate whether speed, displacement, or both are varied to achieve durational changes. Specifically, it has been shown that, during discrete limb movements, individuals prefer to modify speed rather than displacement in order to alter duration, even when speed and displacement are free to vary (Feldman, 1980; Hoffman & Strick, 1986). In contrast, when asked to increase the repetition rate of cyclic movements (e.g., finger tapping), individuals reduce displacements and only minimally alter speed (e.g., Konczak, Ackermann, Hertrich, Spieker, & Dichgans, 1997), a movement strategy thought to economize effort (Nelson, 1983). When altering speaking rate, speakers implement a variety of movement strategies to achieve faster speech. For example, some speakers reduce their articulatory displacement while holding speed constant or they reduce speed along with displacement (Kent & Moll, 1972; Kuehn & Moll, 1976; Westbury & Dembowski, 1993). Others increase speed while maintaining movement displacement (Abbs, 1973; Kuehn & Moll, 1976) or increase speed while reducing displacement (Gay, 1981). It is currently unclear which factors

determine the specific movement strategy for each speaker and whether these strategies are implemented consistently or vary depending on the context in which rate modulations are to be achieved. Regardless of the wide variety of movement strategies implemented by speakers to achieve changes in speaking rate, the ratio of peak speed and displacement, a kinematic index of stiffness, has been found to remain constant for a given movement duration (Munhall, Ostry, & Parush, 1985). Further, changes in the ratio of peak speed and displacement are known to have predictable effects on durational changes (e.g., McClean, 2000; Munhall et al., 1985). Specifically, a greater ratio, or an increase in stiffness, is associated with shorter movement durations, whereas a smaller ratio, or a decrease in stiffness, is associated with longer movement durations. Due to such strong interrelations between speed, displacement, and duration, the motor speech system has been likened to a linear mass-spring system (Kelso, Saltzman, & Tuller, 1986; Kelso, Vatikiotis-Bateson, Saltzman, & Kay, 1985; Munhall et al.,1985; Vatikiotis-Bateson & Kelso, 1993). In the context of a mass-spring model, movement duration, speed, and displacement are thought to be centrally controlled, mainly by regulation of stiffness and endpoint equilibrium. Based on observations that displacement and peak speed are strongly correlated (Feldman, 1980; Ostry et al., 1983; Munhall et al.,1985), many aging studies on limb motor performance varied the displacement (i.e., distance to target) to assess aging effects on speed performance (e.g., Cooke, Brown, & Cunningham, 1989; Ketcham et al., 2002; Lee, Fradet, Ketcham, & Dounskaia, 2007; Walker, Philbin, & Fisk, 1997). Commonly, an increased distance to the target during reach-to-point or reach-to-grasp tasks resulted in a proportional increase in peak speeds as long as movement duration was held constant by the participant. Similarly, given a specific distance, peak speeds were assessed by asking participants to increase their movement rate. In these studies, aging-related motor slowness was commonly evidenced by the combination of reduced peak speeds and prolonged movement durations (e.g., Ketcham et al., 2002; Lee et al., 2007). However, specific durational targets, for example metronome paces, were often not provided. Thus, older adults could have moved at a slower rate than young adults because they may have been cautious and preferred a slower rate. For this reason, a frequent alternative interpretation of motor slowness has been that older adults trade speed for accuracy, particularly during tasks that have an increased noise-to-force ratio, such as those requiring a relatively large target distance or relatively high speed (Fitts, 1954; Walker et al., 1997; Welford, 1981). Given the currently limited understanding about the underlying factors of speaking rate decline in older adults, the purpose of this study was to determine how aging affects orofacial speed performance. Figure 1 presents the experimental framework of this study and illustrates expected motor behaviors in response to the experimental conditions. Specifically, the goal of this experimental setup was to evoke speaking rate changes by asking participants to manipulate

348 Journal of Speech, Language, and Hearing Research • Vol. 57 • 347–360 • April 2014 Downloaded From: http://jslhr.pubs.asha.org/pdfaccess.ashx?url=/data/journals/jslhr/930116/ by a Univ Of Newcastle User on 06/25/2017 Terms of Use: http://pubs.asha.org/ss/rights_and_permissions.aspx

Figure 1. Experimental framework to test articulatory speed using a metronome-paced fixed-target paradigm.

their speed rather than displacement, a movement strategy that is naturally implemented by some speakers (Kuehn & Moll, 1976). A reaching-to-point task, which is commonly used in studies on upper limb movements, provided a suitable experimental paradigm to assess speed performance while controlling for movement distance. Such a fixed-target paradigm was recently adapted to assess speed performance in the orofacial system, specifically to evaluate lip and jaw speed performance in persons with ALS (Mefferd, Green, & Pattee, 2012). Speakers were instructed to increase the repetition rate of jaw and lip movements according to incremental increases in metronome paces while tapping the fixed target with their articulator with each beat. Hence, the fixed target predetermined the movement distance, and the metronome pace predetermined the movement duration. As can be seen in Figure 1, movement duration inversely co-varies with peak speed performance when displacement remains unchanged. The metronome-paced fixed-target task offers, therefore, the opportunity to assess individuals’ ability to increase peak speeds as well as the extent to which the ratio of peak speed and displacement (i.e., stiffness) can be increased in response to rate changes. Given the previous findings of aging studies on limbs, the ability to increase speed was expected to decline with age. A constrained ability to increase speed in older adults would, consequently, result in lower peak speed–displacement ratios when compared with those of middle-aged and young adults, particularly during very fast metronome paces. Finally, because of inverse relations between peak speed– displacement ratios and duration, older adults were expected to display longer movement durations than middle-aged and young adults, particularly during the fast repetition rates.

Method Participants This study was reviewed and approved by the university’s ethics committee. All participants gave written consent. A total of 36 volunteers participated and were grouped into the following age groups: 10 young adults who were

college students (Mage = 23.8, range = 22–27), nine middleaged adults who lived in the nearby community (Mage = 50.5 years, range = 45–55), 10 young-old adults who lived in the nearby community (Mage = 68.5 years, range = 65–74), and seven very-old adults who lived in an independent living facility (Mage = 90.3 years, range = 87–95 years). Because this study included a relatively small number of participants per group, only data of the female participants were selected and analyzed to reduce heterogeneity within each age group. Exclusion criteria were a history of neurological disease, orofacial tissue repair (e.g., cleft lip), speech or language impairment, and prescription medication other than those needed to control blood pressure. To ensure that hearing was within functional limits, all participants passed a hearing screening at 0.5 kHz, 1 kHz, 2 kHz, and 4 kHz at 35 dB in at least one ear or reported a check-up of hearing aids within 1 year of the time of data collection. Finally, all participants had adequate dentition or well-fitting dentures to perform the experimental tasks.

Experimental Setup The methodology of this study closely paralleled that of a previous study by Mefferd et al. (2012). Each participant was fitted with the headgear device that had a strike target mounted on a lever arm, which was placed underneath the jaw or lower lip depending on the structure to be examined (Figure 2). The distance of the strike target relative to jaw or lower lip was determined by each participant’s articulatory displacement during repetitions of the syllable /fa/ at the slowest metronome pace (1.4 Hz or syllables per second). This distance to the strike target was not changed during the experiment. The strike distance was determined for each speaker individually because there was no a priori reason to assume that mean strike distances would differ significantly among age groups. Rather, we expected the strike distances to be randomly distributed around a mean distance across all participants. Syllable repetition rate was provided by an auditory metronome (the “weird metronome” available at www. weirdmetronome.com/) via loudspeakers set to a comfortable

Mefferd & Corder: Aging Effects on Lip and Jaw Peak Speeds


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Figure 2. Experimental setup of fixed-target device and placement of reflective markers. From “A Novel Fixed-Target Task to Determine Articulatory Speed Constraints in Persons With Amyotrophic Lateral Sclerosis” by A. S. Mefferd, J. R. Green, & G. Pattee, 2012, Journal of Communication Disorders, 45, p. 38. ã 2012 by Elsevier. Adapted with permission.

loudness level. Participants were asked to complete the syllable repetitions at seven metronome paces: 1.4 Hz (slow), 2.5 Hz, 3.3 Hz (moderate), 4.2 Hz, 5.0 Hz ( fast), 5.8 Hz, and 6.7 Hz (very fast). We included these paces because most of them represented the range of durational change typically observed when speakers manipulate speaking rates (e.g., Adams, Weismer, & Kent, 1993). Further, the fastest metronome pace was set slightly above the reported maximum syllable repetition rates for young adults to prevent ceiling effects in speed performance (Kent, Kent, & Rosenbek, 1987).

Experimental Tasks First, all participants were asked to repeat the syllable /fa/ and strike the target with their jaw with each metronome beat. The syllable /ba/, which is commonly used to assess lip and jaw movements, was not well suited for the purpose of this study because preliminary data showed that participants move the upper lip towards the lower lip to achieve lip closure at fast metronome paces. This articulatory strategy would shorten the movement distance of the lower lip, which was unsuitable for the purpose of this study. Thus, the syllable /fa/ was used because the upper central incisors provided a fairly fixed boundary during oral closure. It should be noted that the tissue of the lower lip may be deformed when contacting the teeth, hence slightly affecting the displacement; however, this is unavoidable due to the anatomic properties of the lower lip. Participants produced syllable repetitions at each metronome pace for approximately 15 seconds, which equaled about 20 repetitions at the slowest metronome pace.

Participants were asked to complete the syllable repetitions until the experimenter stopped the metronome. Further, participants were instructed to take a breath when needed and then restart syllable productions when ready to do so. Typically, participants stopped one to two times during the recording to take a breath once syllable repetition rate exceeded the moderate pace. During data collection, one experimenter monitored the marker motion tracking on the computer screen, while another experimenter directly observed the task performance and encouraged the participant to reach the strike target with each metronome beat, particularly during the fast paces. All participants were provided breaks of approximately 30–60 seconds before proceeding to the next metronome pace. After participants completed this task by moving their jaw, they were asked to complete the same task with their lower lip. It is important to note that participants in the very old age group did not complete this task because it was not possible to place the strike target underneath the lower lip without the target rubbing against the skin due to the lip shape (thin lower lip) in these older women. Therefore, only young adults, middle-aged adults, and young-old adults performed the metronome-paced fixed-target task with the lower lip. To prevent the jaw from moving together with the lower lip, a bite block was inserted between the upper and lower molars on the right side of the mouth. The bite block was custom made for each participant prior to data collection. A small piece of dental putty (Condensation Putty, manufactured by Henry Schein) was placed between the molars on the right side of the mouth, and participants were asked to close their jaw until they bit down onto a 10-mmthick piece of wood positioned between the upper and lower frontal incisors. After approximately 30 seconds, the piece of putty was removed from the oral cavity and placed on a clean paper towel to harden. Although participants were not explicitly instructed to change the production of the syllable /fa/ for the lower lip task, the vowel in the syllable /fa/ changed from an open vowel produced during the jaw task to a centralized vowel (/fə/) in the lower lip task for all participants—a natural consequence of the constrained jaw position.

Data Acquisition Jaw and lower lip movements were captured using a three-dimensional (3D) optical motion capture system (Motion Analysis, Ltd.) consisting of four infrared Eagle cameras with a resolution of 1.3 million pixels at 1280 × 1024 full resolution. The movements were sampled at a frequency of 120 Hz. Four small (4-mm) reflective markers were placed on the participant’s lips, and three reflective markers were placed on the jaw. Eight reflective markers were placed on the tip of the nose, nose bridge, and forehead as reference markers (see Figure 2). Only the right jaw marker, the center lower lip marker, and the right bottom head reference marker were analyzed for the purpose of this study. The right jaw marker was used instead of the center jaw marker to reduce measurement errors because previous analyses have


shown that markers located off midline of the face, away from the fleshy part of the chin, are less affected by skin and tissue movements compared with markers placed at the chin’s center (Green, Wilson, Wang, & Moore, 2007). A miniature microphone was mounted on the head strap of the fixed-target device to record the audio signal as a reference simultaneous with the movement data.

Figure 3. The top two panels show an example of the 3D Euclidean distance signal between the right jaw marker and right bottom head marker as a function of time and the derived speed signal during the slow metronome pace. Movements were produced by a participant from the very old age group. The shaded areas indicate the closing movements during each syllable repetition. The bottom two panels show the same signals during the very fast metronome pace. Arrows indicate selection of closing movements based on extent of maximum displacement. The speed signal was not viewed by the experimenter during any part of the data analysis.

Data Analysis The 3D Euclidean distance signals between the right jaw marker or lower lip marker and right bottom head reference marker were used for further data analysis. These movement signals were smoothed with an 18-Hz low pass filter using a custom-written MATLAB algorithm. The top panel of Figure 3 displays an example of the 3D Euclidean distance signal based on the right jaw and right bottom head marker recorded during the slowest metronome pace produced by a very old participant. The second panel shows the algorithmically derived movement speed signal. The next two panels display the same information as above for the very fast metronome pace. The onset and offset locations for the closing stroke of the syllable repetitions are indicated in Figure 3 and were based on the maximum and minimum positions of the jaw or lower lip excursions, respectively. The jaw and lip speed signals, however, were not displayed during data analysis. Positional extrema (peaks and troughs) were derived algorithmically based on the zero-crossings of velocity and marked in the displacement signal. In case the syllable production contained small submovements with several positional peaks during the maximum opening (predominantly occurring during the slowest metronome pace; see also Adams et al., 1993), the peak located closest to the onset of the closing movement was selected as the onset of the closing stroke. Similarly, the first trough during the positional minimum of the overall closing of the jaw or lip was selected as the offset. This approach was chosen to exclude the steady states of the syllable production in durational measures. Although multiple peaks in speed could be observed during slow movements, only the largest peak speed was used to report peak speed in this study. The custom-written MATLAB code provided an output of peak speed, displacement, and duration of the selected movement segment. Because it became evident during the course of the experiment that all participants had difficulty with consistently reaching the target as the metronome pace increased, particularly during the lip task, 10 repetitions with the largest displacements were selected for data analysis at each metronome pace. This approach was thought to capture each participant’s best effort to reach the target. The ratio of peak speed and maximum displacement was calculated based on the 10 peak speed and maximum displacement measures of each participant at each metronome pace.

Statistical Design The statistical analyses of the jaw and lower lip movement data were completed separately. Only the data from four metronome paces (slow, moderate, fast, and very fast)



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were included to limit the number of post hoc analyses. For the jaw task, separate 4 × 4 factorial analyses of variance (ANOVAs) were completed for duration, displacement, and peak speed. Further, the 10 calculated ratios of peak speed and maximum displacement for each participant at each metronome pace were averaged and submitted to a 4 × 4 factorial design. If Mauchly’s test of sphericity was significant, then Greenhouse–Geisser adjustments were implemented. If significant interactions were found, significant main effects were not further investigated; however, interactions were explored by completing simple comparisons of cell means using t tests with Bonferroni corrections. Statistical analysis of the lower lip data was completed in a nonparametric fashion because tracking errors of the lower lip marker resulted in missing data points of the repeated measure within each group, which resulted in small sample sizes: n = 7 in young and middle-aged adults; n = 4 in young-old adults. Friedman’s tests of variance by rank were used to test within-group effects of all kinematic measures. Post hoc tests were completed using the Wilcoxon signed-rank approach with a critical alpha level set to a = .05. To determine between-group differences of kinematic measures, Kruskal–Wallis tests were conducted for each variable at each metronome pace. Post hoc tests were completed using the Mann–Whitney U approach using a critical alpha level of a = .05.

Measurement Reliability Because the 10 largest displacements were not determined algorithmically, interrater reliability was estimated by remeasuring data of four randomly selected participants (one of each age group; however, the participant of the very old age group was not included in the reliability test for the lower lip task because this task could not be completed by this age group). The measured and remeasured durations, peak speeds, and maximum displacements of the jaw and lower lip task were averaged for each participant across 10 repetitions, and the means of all four metronome paces were submitted to a Pearson’s correlation analysis (n = 16 for jaw, n = 11 for lower lip measures; the randomly selected participant of the young-old group was missing data for the moderate metronome condition of the lower lip task due to tracking errors). Results showed a mean Pearson correlation coefficient of r = .97 across all conducted correlational analyses. The lowest coefficient value was observed in the lower lip peak speed measure, [r(10) = .93, p < .001]. The highest coefficient values were observed for jaw durations as well as jaw peak speed measures, [r(15) = .99, p < .001]. Based on these results, interrater reliability was determined adequate for this study.

Results Jaw Movement Durations The first section of Table 1 shows group means and standard errors of jaw movement durations for each age group at each metronome pace. Statistical analysis yielded

a significant main effect of pace on movement duration, F(1.85, 59.46) = 477.37, p < .001, hp2 = .937, effect size large. No significant Pace × Group interactions or between-group differences were found. Post hoc analysis of the main effect revealed that all simple comparisons were statistically significant at or below p = .02. In Table 1 brackets further indicate the significant cell mean comparisons within each age group.

Maximum Jaw Displacements Table 1 displays group means and standard errors of maximum jaw displacements for each age group at each metronome pace. Results yielded significant main effects of pace, F(2.12, 67.75) = 7.992, p = .001, h p2 = .200, effect size small, and age, F(3, 32) = 3.270, p = .034, h p2 = .235, effect size small, on jaw displacements as well as a significant Pace × Age interaction, F(6.35, 67.75) = 3.141, p = .007, h p2 = .229, effect size small. Post hoc analyses showed that jaw displacement significantly decreased as a function of metronome pace in young adults, F(1.59, 14.3) = 16.689, p < .001, h p2 = .650, effect size moderate. Significant paired t tests within the young adult group are shown in Table 1. Between-group comparisons of jaw displacements at each metronome pace yielded significant findings for the fast, F(3, 35) = 3.089, p = .04, hp2 = .225, effect size small, and the very fast metronome pace, F(3, 35) = 7.053, p = .001, h p2 = .398, effect size moderate. Significant independent t-test findings are indicated in Table 1.

Jaw Peak Speeds The bottom section of Table 1 shows the group means and standard errors of jaw peak speeds for each age group at each metronome pace. Significant main effects for pace, F(1.91, 61.02) = 68.694, p < .001, hp2 = .682, effect size large, and age, F(3, 32) = 2.993, p = .045, hp2 =.219, effect size small, on jaw peak speed were observed. A significant Pace × Age group interaction was also found, F(5.70, 60.79) = 3.228, p = .009, hp2 = .232, effect size small. Post hoc analyses showed that jaw peak speed significantly increased as a function of metronome pace in young adults, F(3, 27) = 12.096, p < .001, hp2 = .573, effect size moderate; middle-aged adults, F(1.67, 13.35) = 26.55, p < .001, hp2 = .768, effect size large; young-old adults, F(1.50, 13.52) = 15.48, p < .001, hp2 = .632, effect size large; and very old adults, F(3, 18) = 19.164, p < .001, hp2 = .762, effect size large. Significant paired t-test results for all cell mean comparisons are shown in Table 1. One-way ANOVAs were conducted for each metronome pace with age group as the between-group variable. Significant between-group differences were found for the very fast metronome pace, F(3, 35) = 4.89, p < .007, hp2 = .314, effect size small. Significant independent t-test results for all cell mean comparisons are shown in Table 1. Because the analysis of jaw displacements revealed significant between-group effects, and displacement and speed are strongly correlated, it was necessary to convert jaw peak speeds into percentages and use each participant’s peak


Table 1. Group means and standard errors of jaw kinematic measures. Metronome pace Variable

Group

Slow M (SE)

Jaw movement duration (ms)

Young

340 (18)

181 (5)

118 (4)

108 (11)

Middle-aged

403 (18)

196 (18)

118 (5)

94 (3)

Young-old

362 (16)

188 (7)

123 (11)

115 (15)

Very old

354 (8)

169 (7)

130 (8)

111 (9)

Young

6.2 (0.6)

6.5 (0.6)

5.2 (0.4)

3.9 (0.5)

Middle-aged

7.6 (0.7)

7.8 (0.8)

6.9 (0.7)

6.4 (0.8)

Young-old

7.7 (0.9)

8.0 (0.9)

7.2 (0.9)

7.0 (0.8)

Very old

7.3 (0.4)

8.0 (0.5)

8.5 (1.0)

8.5 (0.7)

Young

45.6 (3.7)

73.6 (5.2)

82.4 (5.6)

72.7 (8.29)

Middle-aged

51.5 (5.8)

90.4 (12.4)

111.9 (15.0)

129.9 (14.9)

Young-old

54.0 (4.8)

98.3 (14.1)

118.3 (16.0)

131.4 (19.8)

Very old

55.8 (4.3)

95.5 (10.0)

127.4 (15.34)

152.9 (17.3)

Jaw maximum displacement (mm)

Jaw Peak Speed (mm/s)

Moderate M (SE)

Fast M (SE)

Very fast M (SE)

Note. Young: n = 10; middle-aged: n = 9; young-old: n = 10; very old: n = 7. Brackets indicate significant simple comparison (Bonferronicorrected p values). For within-group comparisons, only significant comparisons between adjacent metronome paces are indicated.

speed during the slowest metronome pace as their own baseline (100%). Then, the maximum percentage increase— regardless of metronome pace—was selected for each participant. This approach was taken because a pace effect on speed was not as much of interest at this point as the betweengroup difference in achieved increase in peak speed. Thus, maximum percentages were submitted to a one-way ANOVA with age as the between-group variable. The top panel of Figure 4 displays the group means and standard errors of the maximum percent increase in jaw peak speed across all metronome paces. No significant differences between age groups were found; however, very old adults tended to have greater percent increases in jaw peak speed than did young adults, F(3, 36) = 2.415, p = 0.085, hp2 = .185, effect size small.

Jaw Peak Speed–Displacement Ratios The top panel of Figure 5 displays the means and standard errors of the maximum jaw displacements (x-axis) and the means and standard errors of the jaw peak speeds (y-axis) for each metronome pace. The ratios of the jaw peak speeds and displacement were calculated and the resulting means and standard errors for each group at each metronome pace are shown in the bottom panel of Figure 5. The statistical analysis revealed a significant main effect for pace, F(1.87, 59.76) = 211.524, p = .001, hp2 = .869, effect size large. Post hoc comparisons between metronome paces were all

significant. No significant age effects or Pace × Age interactions were found. As can be seen in Figure 5, although age effects were not significant, very old adults tended to have lower ratios than did all other age groups during the fast and very fast metronome pace.

Lower Lip Movement Durations The first section of Table 2 displays group means and standard errors of lip movement durations for each age group as a function of metronome pace. Friedman’s test revealed significant effects of metronome pace on lip movement durations for young adults, c2(3) = 16.2, p = .001, Kendall’s W = .77, effect size large; for middle-aged adults, c2(3) = 19.97, p < .001, Kendall’s W = .95, effect size large; and young-old adults, c2(3) = 11.1, p = .011, Kendall’s W = .93, effect size large. As indicated in Table 2, post hoc analyses confirmed that movement durations were significantly shortened as a function of metronome pace, except for the comparison between the fast and very fast metronome paces in young adults and young-old adults. Between-group comparisons were nonsignificant.

Lower Lip Displacements The middle section of Table 2 displays group means and standard errors of maximum lower lip displacements for



353

Figure 4. Group means and standard errors of the maximum percent increases in jaw peak speed (top panel) and lip peak speed (bottom panel). No significant between-group differences were found for lip or jaw percent increase in peak speed.

each age group at each metronome pace. A significant effect for metronome pace on lower lip displacements was found in young adults, c2(3) = 16.2, p = .001, Kendall’s W = .77, effect size large; in middle-aged adults, c2(3) = 19.97, p = .001, Kendall’s W = .95, effect size large; and young-old adults, c2(3) = 10.8, p = .013, Kendall’s W = .90, effect size large. Significant findings of post hoc comparisons are displayed in Table 2. Post hoc comparisons yielded a significant decline in lower lip displacement between the moderate and fast as well as between the fast and very fast metronome paces in all three age groups. No significant between-group differences in lower lip excursions were found.

Figure 5. Top panel: Mean jaw peak speeds versus displacements for each age group. The regression lines were based on the means of each participant within each metronome pace. Bottom panel: Group means of the jaw peak speed–displacement ratios as a function of metronome pace. All comparisons of ratios between metronome paces were significant (p < .001). No significant between-group differences were found. Error bars represent standard errors.

Lower Lip Peak Speed The bottom section of Table 2 shows the group means and standard errors of lower lip peak speeds for each age group as a function of metronome pace. Within-group analyses yielded significant effects of metronome pace on lower lip peak speeds for young adults, c2(3) = 14.83, p = .002, Kendall’s W = .71, effect size large. Post hoc comparisons of adjacent metronome paces showed that the significant main effect was driven by an increase in lower lip peak speed


Table 2. Group means and standard errors of lower lip kinematic measures. Metronome pace Variable

Group

Slow M (SE)

Moderate M (SE)

Fast M (SE)

Very fast M (SE)

Lip movement duration (ms)

Young

355 (21)

158 (3)

115 (7)

120 (10)

Middle-aged

363 (21)

166 (13)

121 (9)

96 (5)

Young-old

333 (22)

158 (11)

112 (5)

104 (8)

Young

9.3 (0.8)

8.9.1 (0.8)

6.4 (0.6)

4.9 (0.6)

Middle-aged

10.7 (0.9)

9.7 (0.6)

7.6 (0.6)

5.5 (0.7)

Young-old

12.2 (1.8)

11.9 (0.5)

8.2 (0.4)

7.3 (0.6)

Young

74.4 (11.6)

123.7 (12.0)

105.4 (6.0)

82.8 (8.1)

Middle-aged

79.7 (9.4)

123.8 (7.0)

129.8 (10.5)

113.8 (11.5)

Young-old

90.5 (19.2)

161.5 (10.0)

140.5 (5.8)

133.2 (3.6)

Lip maximum displacement (mm)

Lip Peak Speed (mm/s)

Note. Young: n = 7; middle-aged, n = 7; young-old, n = 4.

between the slow and moderate pace (Table 2). Betweengroup comparisons did not reveal significant differences in lower lip peak speeds. Although no statistical differences in lower lip displacement were found, older adults tended to move their lower lip further than middle-aged and young adults during the slowest metronome pace. Therefore, the same approach of data analysis used for jaw peak speeds was implemented for lip peak speeds. The bottom panel of Figure 4 displays the group means and standard errors of the maximum percent increase in lower lip peak speed across all metronome paces. A Kruskal–Wallis test with age as the between-group variable revealed no significant differences between age groups. Similar percentages of increase in lower lip peak speeds were observed across all age groups.

Lip Peak Speed–Displacement Ratios The top panel of Figure 6 displays the means and standard errors of the maximum lower lip displacements (x-axis) and the lower lip peak speeds (y-axis). The resulting means and standard errors of the peak speed–displacement ratios are shown in the bottom panel of Figure 6. Friedman’s tests revealed significant main effect of pace on these ratios in the young adults, c2(3) = 16.2, p = .001, Kendall’s W = .77, effect size large; in middle-aged adults, c2(3) = 21.0, p < .001, Kendall’s W = 1.0, effect size large; as well as in young-old adults, c2(3) = 11.1, p < .011, Kendall’s W = .925, effect size large. Post hoc comparisons between metronome paces were significant, except for the comparison between the fast and the very fast metronome pace in the young adults and young-old adults. Between-group analysis revealed no statistically significant findings. However, for the very fast metronome pace the main effect of age on peak speed–

displacement ratios approached significance, c2(2) = 5.536, p = .063. Post hoc comparisons revealed that younger adults had significantly lower ratios than middle-aged adults, U = 6, p = .018. Further, young-old adults tended to have lower peak speed–displacement ratios than middle-aged adults.

Discussion In this study, we sought to determine if lip and jaw peak speed performance was constrained in older adults. A fixed-target task was used to assess the ability to increase articulatory speed in response to increasing metronome pace and to discourage participants from displacement reductions, which would require only minimal modulation of movement speed to increase rate. Based on previous research that reported reduced limb movement speed with advanced age, we expected the ability to increase lower lip and jaw speed to be constrained in older adults. Further, we expected peak speed–displacement ratios to increase as a function of metronome paces for all participants; however, the ratios of older adults were expected to increase less than those of young adults.

General Findings In contrast to the stated research hypotheses, our findings showed that the ability to increase jaw peak speeds did not decline with advanced age. However, young adults had smaller jaw displacements than did all other age groups, and their jaw displacements decreased as metronome paces increased. Jaw peak speed–displacement ratios generally increased with pace; yet, during fast and very fast repetition rates, ratios tended to be lower for older adults than for



355

Figure 6. Top panel: Mean lip peak speeds versus displacements in each age group. The regression lines were calculated based on the means of each participant within each metronome pace. Bottom panel: Group means of the lip peak speed–displacement ratios as a function of metronome pace. All comparisons of ratios between metronome paces were significant (p < .001). Between-group findings are marked by brackets (p = .018). Error bars represent standard errors.

increases in metronome pace. Although lower lip displacements were not significantly different among age groups, older adults tended to produce larger lip movements than middle-aged and young adults across all metronome paces. As expected, the lip peak speed–displacement ratios generally increased as a function of metronome pace for all participants; however, ratios of middle-aged adults were significantly greater than those of young adults and also tended to be greater than those of young-old adults. This finding was not expected for young adults but was anticipated for older adults. Lower ratios of young and older adults were associated with mildly prolonged lip movement durations during fast repetition rates.

Current Findings in the Context of Aging Studies on Limb Motor Performance

young and middle-aged adults. As expected, lower ratios were associated with longer movement durations. For lip kinematic measures, our hypothesis of a speed decline with advanced age was also not supported; no differences were observed between age groups when comparing maximum increases in lower lip peak speed; however, all participants failed to maintain their lip displacement with

The lack of decline in lip and jaw peak speeds with advanced age did not concur with previous findings in the limb literature reporting reduced speeds in older adults (e.g., Bellgrove et al., 1998; Haaland et al., 1993; Himann et al., 1988; Hoffman & Strick, 1986; Ketcham et al., 2002; Welford, 1977, 1984; Yan et al., 1998). The discrepancies between limb and orofacial speed performance may be due to aging-specific differences in the skeletal and cranial system (e.g., differences in the relative changes of type I and type II fibers with advanced age; potential differences in the extent of sarcopenia). However, methodological differences to evaluate speed should also be considered as a potential source for discrepant findings between previous studies and the current one. In many limb studies, for example, durational demands in reach-to-point tasks were coarsely defined as “move as fast as possible” rather than precisely modeled by metronome paces. Consequently, older adults may have moved their arm or leg at a lower speed than young adults simply because they practiced caution and were concerned with pointing accuracy. In fact, Walker et al. (1997) as well as Cooke et al. (1989) demonstrated that when accuracy demands were minimal, or participants were specifically told that inaccuracy would not be penalized, older adults produced similar peak speeds during reach-to-point tasks as their younger peers. Walker and colleagues concluded that previous aging studies may have underestimated the impact of older adults’ strategy to preserve accuracy and overestimated physiologic constraints affecting limb peak speed. In this current study, participants were instructed to strike the target with each metronome beat; however, it was not further specified whether the contact with the strike target had to be a light touch or a forceful push. In that sense, the fixed-target task also required minimal accuracy, which may explain why our findings were similar to those of limb studies that also involved minimal accuracy (e.g., Cooke et al., 1989; Walker et al., 1997).

Durational and Spatial Task Performance During fast and very fast metronome paces, only young-old and very old participants maintained their jaw displacements—however, at the cost of longer movement


durations—compared with younger age groups. To also achieve adequate durations during these paces, older adults should have increased jaw peak speeds to a greater extent than they did. It is possible that older adults reached their speed performance limit during the very fast pace and consequently failed to sufficiently shorten movement durations. During the fast pace, however, their jaw peak speeds also did not adequately scale relative to their displacements, and yet speeds—as well as the ratio of peak speed–displacement (i.e., stiffness)— could be further increased during the following metronome pace condition. Thus, during the fast pace condition, longer movement durations were neither the result of a jaw speed constraint nor the result of a limited ability to increase stiffness. Findings suggest that older adults had difficulty regulating stiffness and generating adequate force to sufficiently shorten durations during the fast metronome pace. Speed, however, did not appear to be physiologically constrained. During the very fast metronome pace of the lip task, young adults displayed the longest durations of all age groups. These relatively long durations were associated with relatively low peak speed–displacement ratios (i.e., stiffness) when compared with other age groups. Further, young adults did not achieve any change in stiffness or duration from the fast and the very fast pace despite attempts, as noted by displacement and speed reductions. Taken together, these kinematic observations suggest that young adults reached their performance limit during the fast metronome pace in the lip task. Young-old adults, who also demonstrated an inability to increase lip speed past the moderate pace, did increase stiffness and shorten durations during the very fast pace. Only middle-aged adults demonstrated an ability to slightly increase lip speed during the fast pace. Given these between-group differences in performance, it appears that middle-aged adults were the top performers of this task. Figure 7 shows the relation between changes in peak speed–displacement ratios and durations. It can be seen that

although peak speed–displacement ratios and durations differ among age groups during both the lip and jaw task, they vary in a predictable fashion regardless of age ( jaw: y = 188x2 – 135.46x + 30.57, R2 = .90; lip: y = 177.16x2 – 128.37x + 30.03, R2 = .93). Further, aging effects on motor performance as well as the function of ratios and duration appear to be similar for the lower lip and the jaw. In light of anatomical/histological and biomechanical differences between these two articulators, these similarities are quite remarkable.

Problems with Performing the Fixed-Target Task Difficulties with maintaining lower lip displacement across metronome paces were also observed in our previous study (Mefferd et al., 2012). We speculate that, with increased effort to move fast, the lower lip is pulled in a retracted, less flexible position. This may have contributed to the observed difficulty with keeping lip displacements consistent across metronome paces. Because movement repetitions of the largest displacements (i.e., the best efforts to complete the fixed-target task) were selected for all participants, maximum lip speed performance could be derived despite the challenges to complete this task. The reason for all young adults failing to maintain a consistent jaw displacement during the jaw fixed-target task is currently not clear. During the data collection, participants of all ages commented on their difficulty with reaching the target; all participants were verbally encouraged in their efforts to move to the target with each beat. In contrast to young adults, very old adults displayed a minimal increase in displacement despite the fixed target. This could be due to increased force generation associated with faster speeds and insufficient stiffness regulation to decelerate their movements quickly enough, resulting in skin tissue compression when the strike target was hit more forcefully.

Figure 7. Group means of the peak speed–displacement ratio as a function of movement duration during the jaw task (left panel) and lip task (right panel).



357

Limitations A potential limitation of this study is that the target distance was determined by each individual’s displacements during the slow metronome pace. The rationale was to use a fixed target to discourage displacement reductions and instead elicit increases in speed in response to rate manipulations. Assuming a normal distribution around a mean displacement, a self-selected displacement was thought to facilitate individual success in the completion of the fixedtarget task. A predetermined distance may have forced participants to overreach or not move far enough compared with their natural speech excursions. Whereas jaw displacements and jaw peak speeds of middle-aged, young-old, and very old adults did distribute around the same mean during the slow metronome pace, young adults started with smaller jaw displacements and lower speeds than all other age groups. The reason for the difference in jaw displacements is currently unclear and was not expected prior to the experiment. Differences in displacements during faster metronome paces could not be avoided, however, because the two task demands (matching durations and keeping target distance) were interrelated with speed. Thus, if sufficient speed were not generated, either the durational or spatial demand or both (duration and distance to target) would not be able to be fulfilled. During the slow metronome pace of the lip task, displacements tended to differ among all three age groups. Therefore, we compared relative increases in peak speeds among age groups to control for this offset in displacement and speed during the slowest metronome pace. Another potential limitation of this study is that participants were grouped merely by their chronological age regardless of their biological age. Several studies have shown that biological age is a more accurate predictor of performance decline, including speaking rate, than chronological age (Iinuma et al., 2012; Mueller & Xue, 1996; Ramig, 1983). Future studies should address overall physical condition of participants when assessing orofacial speed performance.

Conclusion Taken together, our findings suggest that the ability to increase lip and jaw speed does not decline with age. However, the findings suggest that older adults may have more difficulty with stiffness regulation and adequate force production when producing fast syllable repetition rates. Thus, slowed speech may be primarily a compensatory movement strategy to maintain speech accuracy in the presence of diminished articulatory control.

Acknowledgments This work was supported in part by the American SpeechLanguage-Hearing Association (ASHA) Advancing Academic Research Careers (AARC) mentoring grant, provided to the first author. Parts of this study were conducted as a master’s thesis by the second author. Preliminary results were presented in November

2011 at the Annual Convention of the American Speech-LanguageHearing Association in San Diego, CA, and in February 2012 at the Motor Speech Conference in Santa Rosa, CA. We would like to thank all participants for their willingness to complete this study and the Kansas Masonic Home for their support. We also thank Sean Hess, Carol Hassebroek, Cliff Bragg, and Ali Sanderson for their valuable contributions and their assistance with data collection and analyses as well as the two reviewers for their helpful comments and suggestions.

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