diff --git a/src/engine/character/mod.rs b/src/engine/character/mod.rs
index 3de743c..5494844 100644
--- a/src/engine/character/mod.rs
+++ b/src/engine/character/mod.rs
@@ -20,6 +20,7 @@ use engine::physics;
 use engine::math::{
     duration_as_f32,
     duration_as_frame,
+    clamp,
 };
 
 /// The different sides a character can face.
@@ -185,16 +186,24 @@ impl Updatable for Character {
         let duration = duration_as_f32(duration);
 
         // Compute acceleration
-        let accel = physics::G;
+        let accel = physics::G + force * 64.0 * 64.0;
 
         // Compute speed
         self.speed.x *= physics::GROUND_FRICTION.x;
-        self.speed += accel * duration + force * 64.0;
+        self.speed += accel * duration;
 
         if self.speed.y > physics::MAXIMUM_VERTICAL_SPEED {
             self.speed.y = physics::MAXIMUM_VERTICAL_SPEED;
         }
 
+        let limit = if Key::Space.is_pressed() {
+            physics::MAXIMUM_RUNNING_SPEED
+        } else {
+            physics::MAXIMUM_WALKING_SPEED
+        };
+
+        self.speed.x = clamp(self.speed.x, -limit, limit);
+
         // Compute position
         self.position += self.speed * duration;
 
diff --git a/src/engine/map/mod.rs b/src/engine/map/mod.rs
index f93274f..52844e2 100644
--- a/src/engine/map/mod.rs
+++ b/src/engine/map/mod.rs
@@ -6,7 +6,10 @@ use sfml::graphics::{
 
 use engine::texture::Texture;
 use engine::renderer::Drawable;
-use engine::math::Matrix;
+use engine::math::{
+    Matrix,
+    clamp,
+};
 
 /// This enum represents if the collision happens on the X axis or the Y axis.
 #[derive(Copy, Clone)]
@@ -506,13 +509,4 @@ pub fn overlap(box1: FloatRect, box2: FloatRect) -> bool {
     box2.top  + box2.height > box1.top
 }
 
-/// Clamp a number between two boundaries.
-pub fn clamp(number: f32, min: f32, max: f32) -> f32 {
-    if number < min {
-        min
-    } else if number > max {
-        max
-    } else {
-        number
-    }
-}
+
diff --git a/src/engine/math/mod.rs b/src/engine/math/mod.rs
index 29b2909..1729dd6 100644
--- a/src/engine/math/mod.rs
+++ b/src/engine/math/mod.rs
@@ -5,6 +5,17 @@ use std::ops::{
     IndexMut
 };
 
+/// Clamp a number between two boundaries.
+pub fn clamp(number: f32, min: f32, max: f32) -> f32 {
+    if number < min {
+        min
+    } else if number > max {
+        max
+    } else {
+        number
+    }
+}
+
 /// Converts a duration into an animation frame number.
 pub fn duration_as_frame(duration: &Duration, total: usize) -> i32 {
     let secs = duration_as_f32(duration);
diff --git a/src/engine/physics/mod.rs b/src/engine/physics/mod.rs
index b100677..c182744 100644
--- a/src/engine/physics/mod.rs
+++ b/src/engine/physics/mod.rs
@@ -11,3 +11,9 @@ pub const JUMP_SPEED: Vector2<f32> = Vector2 { x: 0.0, y: -12.5 * 32.0 };
 
 /// The maximum vertical speed.
 pub const MAXIMUM_VERTICAL_SPEED: f32 = 10.0 * 32.0;
+
+/// The maximul walking speed.
+pub const MAXIMUM_WALKING_SPEED: f32 = 2.5 * 32.0;
+
+/// The maximum running speed.
+pub const MAXIMUM_RUNNING_SPEED: f32 = 5.0 * 32.0;