Shaking Lines

In manga and anime, lines are drawn surrounding an object when it's supposed to be wobbling, trembling, or shaking, or, alternatively, lines drawn around a character when they're quivering or twitching because they're afraid, startled, shocked or excited.

In Japanese, they're called yure-sen 揺れ線, "wavering lines," or furue-sen 震え線, "trembling lines."

Anime: Kimetsu no Yaiba 鬼滅の刃 (Episode 11)

• Usage
• Styles
  
  • Smooth Curves
  • Stiff Straight Lines
  • Segmented Lines
  • Shaky Lines
  • Flickering Lines
  • Spiky Lines

Usage

The shaking lines are used in manga and anime for two distinct reasons:

1. To depict movement.
2. To depict a reaction;

In the first case, we're talking about motion lines, which are lines used in comics and manga panels to give movement to the static drawings.

In anime, this sort of line is usually unnecessary, because you can just animate an object shaking instead of drawing lines around it.

However, sometimes the same sort of line is used on a character when they're trembling or shaking in reaction to a situation.

In this case, the lines are drawn EVEN IN ANIME, because those lines are a sort of manga symbol for fear, shock, excitement, and so on.

• Sweat drops, vertical lines drawn on a character's forehead, specially blue lines, and drawing their face turning blue, are other common symbols used when a character is scared.

Styles

Depending on the sort of movement depicted, the style of the contour changes.

Smooth Curves

Smooth lines, curves around an object means their movement is equally smooth.

Manga: Initial D, 頭文字Ｄ (Chapter 5, リベンジ宣言！！ 吠える１３Ｂターボ)

• In the first pair of panels, a hand turns left and right to let drip the water from the cup.
• In the second pair, a hand manipulates an alarm clock.
• In the third panel, a character's head wobbles as he washes his face.

These lines can also be used when an object is growing larger or shrinking over time.

Stiff Straight Lines

When hard objects that don't bend jiggle around, the contour is rendered with straight lines—no curves.

Manga: Ashita no Joe あしたのジョー (Volume 1, Page 110)

• The lamp post, made out of metal, stiffly trembles as the girl climbs it.

Similarly, when a character stiffens up in reaction to something, they're contoured with straight lines.

Manga: Karakai Jouzu no Takagi-san からかい上手の高木さん (Chapter 1, 消しゴム)

• Context: a boy stiffens up after a girl turns her head to look at him.
• doki'
  ドキッ
  *thump*

Segmented Lines

Lines that are extremely segmented signify an interval between movements, like trembling or twitching.

Manga: Gabriel DropOut, ガヴリールドロップアウト (Chapter 3)

• Context: Satanichia twitches with excitement.
• zoku'
  ゾクッ
  *tremble*

Longer lines with longer gaps give the impression of greater movement with greater pause, like twitching, conversely, a lot of short lines give the impression of repeatedly moving very slightly, trembling.

Anime: Asobi Asobase あそびあそばせ (Episode 4)

• Context: Olivia オリヴィア trembles with horror.

Shaky Lines

Contours drawn shaky, approaching and distancing itself from the object over and over, represent shaking.

Anime: Kimetsu no Yaiba 鬼滅の刃 (Episode 11)

• Context: Agatsuma Zen'itsu 我妻善逸 trembles with fear.

Flickering Lines

In anime, shaking lines are sometimes animated flickering around the character as they tremble.

Character: Yoshida Yuuko 吉田優子, Shamiko シャミ子
Anime: Machikado Mazoku まちカドまぞく (Episode 10)

Spiky Lines

Extremely shaky contours have noticeable spikes at certain points. These exaggerated contours represent some forceful, quick movement. Twitching, convulsing, and so on.

Typically, they're drawn on joints, specially on the shoulders, when a character is startled or feels a chill down their spine.

Anime: Dumbbell Nan Kilo Moteru? ダンベル何キロ持てる？ (Episode 1)