Comparative Primate Electromyographic Data

We compared treeshrew EMG data to primate EMG data collected from greater galagos (Otolemur crassicaudatus and O. garnetti), ring-tailed lemurs (Lemur catta), owl monkeys (Aotus trivirgatus), callitrichids (Callithrix jacchus and Sagunius fuscicollis), macaques (Macaca fascicularis and M. fuscata) and baboons (Papio anubis) (Hylander et al., 2000, 2002, 2005; Vinyard et al., 2001). All primate data were collected and analyzed in a similar fashion to the treeshrew EMG data following the methods outlined in Hylander et al. (2000). We calculated grand means for primate W/B ratios and peak firing times from these sources following the same methods described above for treeshrews. All comparisons between primates and treeshrews are based on these grand means.

5 The 95% confidence intervals based on this bootstrapping approach gave very similar results to the 95% confidence intervals for the grand means calculated using a typical parametric approach. The coincidence of these values supports the use of the bootstrapped confidence intervals (Manly, 1997).

RESULTS

Treeshrew Electromyography

Masseter W/B ratios

The grand means for the superficial and deep masseter W/B ratios are similar in treeshrews (2.7 and 2.8, respectively) (Table 2). On average, treeshrews recruit almost three times as much relative working-side masseter activity when compared to their balancing-side masseter. The confidence intervals (CIs) for the grand means for both W/B ratios range from a little over 2 to about 3.5 (Table 2). Individual treeshrews clearly vary in their average superficial and deep masseter W/B ratios. Specifically, treeshrew 1 tends to have much higher levels of relative balancing-side superficial and deep masseter recruitment (i.e., W/B ratios of 1.5 and 1.7, respectively) than treeshrew 2 who recruits the working-side muscles up to five times more than the balancing-side muscles (i.e., W/B ratios of 5.0 and 4.6, respectively) (Table 2).

Temporalis W/B ratios

The grand means of the W/B ratios for the anterior temporalis and posterior temporalis are also quite similar to each other (2.1 and 2.0, respectively).

Table 2. Treeshrew jaw muscle average W/B ratios

Subject

N

Superficial masseter

Deep masseter

Anterior temporalis

Posterior temporalis

Mean

Log10

Mean

Log10

Mean

Log10

Mean

Log10

SD

SD

SD

SD

Treeshrew 1

Subject mean

202

1.5

-

1.7

-

1.1

-

1.2

-

Treeshrew 2

Subject mean

257

5.0

0.04

4.6

0.18

3.0

0.20

2.5

0.02

Treeshrew 3

Subject mean

295

2.0

0.08

1.9

0.08

1.4

0.11

1.6

0.03

Treeshrew 4

Subject mean

214

2.6

0.10

2.4

0.10

2.3

0.16

1.9

0.17

Treeshrew 5

Subject mean

324

2.4

0.21

3.2

0.10

2.3

0.27

2.4

0.07

Grand mean

2.7

0.20

2.8

0.19

2.1

0.22

2.0

0.13

95%CI for—

grand mean

2.13

3.37

2.25-

3.50

<1.62-

2.69

1.71

-2.32

N = number of chewing cycles; Log10 SD = standard deviation of Log10 transformed ratios.

95% CI = Confidence Interval for the Grand mean based on bootstrapping the 13 experimental means.

N = number of chewing cycles; Log10 SD = standard deviation of Log10 transformed ratios.

95% CI = Confidence Interval for the Grand mean based on bootstrapping the 13 experimental means.

Both are lower on average than the grand means for the two masseter W/B ratios (Table 2). The confidence interval (CI) for the grand mean of the anterior temporalis W/B ratio ranges from 1.6 to 2.7. The CI for the posterior temporalis is slightly narrower ranging from 1.7 to 2.3. Treeshrews 1 and 2 also set the range of variation in averages among treeshrews (Table 2). Similar to the pattern seen with the masseters, treeshrew 1 temporalis W/B ratios indicate a near equivalence in amounts of relative working-versus balancing-side muscle recruitment (W/B ratios of 1.1 and 1.2, respectively).

Alternatively, treeshrew 2 tends to have the highest levels of relative working-side recruitment for the temporalis (W/B ratios of 3.0 and 2.5, respectively) (Table 2).

Jaw-muscle firing patterns

The treeshrew working-side deep masseter (WDM), balancing-side superficial masseter (BSM) and balancing-side deep masseter (BDM) each peak on average before the peak activity of the working-side superficial masseter (WSM) (Table 3). The BSM and BDM both peak about 10 ms prior to the WSM. The WDM peaks on average only 3 ms prior to the WSM peak. The confidence interval (CI) for each of these muscles support this observation in that the CIs for the BSM and BDM only shows slight overlap with the WDM confidence interval and the CI of the WDM does not include zero. Individual treeshrew averages generally uphold this timing pattern of peak EMG activity where the BSM peaking first followed by the BDM, then the WDM and finally the WSM.

Peak firing in the working-side anterior temporalis (WAT) and posterior temporalis (WPT) muscles occurs on average at about the same time (Table 3). Both peak before the WSM. The grand mean and confidence intervals for the WAT and WPT are essentially the same. The grand mean for the time of peak activity in the balancing-side anterior temporalis (BAT) precedes the average peak of the balancing-side posterior temporalis (BPT) by about two milliseconds. Even though this two millisecond difference is small, the CIs for the two grand means show little overlap suggesting that there may be a general tendency for the BAT to peak just prior to the BPT (Table 3).The balancing-side anterior temporalis and BPT both reach peak activity after the WSM on average. Furthermore, comparison of individual treeshrew means tends to support this pattern of the BAT peaking after the WSM, but prior to the BPT.

The two working-side temporalis muscles are the first muscles on average to show peak activity during a chewing cycle (Table 3). Alternatively, the BAT

Table 3. Treeshrew timing differences in ms between peak jaw-muscle activity relative to the working-side superficial masseter

Subject

Table 3. Treeshrew timing differences in ms between peak jaw-muscle activity relative to the working-side superficial masseter

Subject

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Treeshrew 1

Subject mean

202

-1

-

6

-

1

-

11

-

-2

-

17

-

-4

-

Treeshrew 2

Subject mean

257

1

2

7

6

6

7

9

3

-5

3

11

5

-6

1

Treeshrew 3

Subject mean

295

6

5

6

4

8

5

13

4

-1

2

11

1

-3

3

Treeshrew 4

Subject mean

214

2

7

13

4

9

7

13

3

-3

1

11

2

-3

2

Treeshrew 5

Subject mean

324

5

4

14

3

12

3

13

2

0

3

15

3

-5

1

Grand mean

3

5

10

5

8

6

12

3

-2

3

12

3

-4

2

95% CI for the

Grand mean

0.9-

-5.5

7.3-

-12.5

5.4-

-11.1

10.5-

-13.7

-1.0-(-

-3.5)

10.5-

-13.7

-3.3 -

(-5.3)

Positive values indicate that peak EMG activity of the muscle precedes peak EMG activity of the working-side superficial masseter. Negative values indicate the reverse condition. N = number of chewing cycles; SD = standard deviation.

95% CI = Confidence Interval for the Grand mean based on bootstrapping the 13 experimental means.

Positive values indicate that peak EMG activity of the muscle precedes peak EMG activity of the working-side superficial masseter. Negative values indicate the reverse condition. N = number of chewing cycles; SD = standard deviation.

95% CI = Confidence Interval for the Grand mean based on bootstrapping the 13 experimental means.

and BPT are the last muscles to exhibit peak activity during a chewing cycle. Thus of all of the muscles analyzed, the temporalis muscles show the greatest amount of offset in timing of peak activity between the working- and balancing-side muscles during chewing in treeshrews.

Figure 4 shows the progression of average peak firing times among the treeshrew jaw-closing muscles. The WAT, WPT, BSM, and to some extent the BDM, form a group of jaw-closing muscles that tend to fire first during a chewing cycle (Triplet I). The BAT, BPT and WSM, and arguably the WDM, form a second group of jaw-closing muscles that show peak activity later in the power stroke (Triplet II).

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