===================================================================
--- original.js
+++ change.js
@@ -9,8 +9,9 @@
 /****************************************************************************************** */ 
 var Athlete = Container.expand(function (line) {
 	var self = Container.call(this);
 	self.speedX = -1 * globalSpeedPerLine[line];
+	self.nextHurdleSpeedX = 0;
 	self.jumpSpeed = -17;
 	self.lastJumpTime = 0;
 	self.interJumpDelayMs = 500;
 	self.gravity = 0.5;
@@ -126,18 +127,24 @@
 				if (self.isRunning) {
 					self.runAnim();
 				}
 			} else {
+				var deltaSpeed = globalSpeedPerLine[self.lineIndex] - globalSpeedPerLine[0];
 				if (self.lineIndex == 3) {
-					log("Current pultiplier=" + Math.abs(globalSpeedPerLine[self.lineIndex] / 10));
+					log("Delta Speed" + deltaSpeed + " /Multiplier=" + Math.abs(globalSpeedPerLine[self.lineIndex] / 10));
 				}
-				self.speedY += self.gravity * Math.abs(globalSpeedPerLine[self.lineIndex] / 10); //self.lineIndex
-				self.y += self.speedY * Math.abs(globalSpeedPerLine[self.lineIndex] / 10); //self.lineIndex
+				//self.speedY += self.gravity * Math.abs(globalSpeedPerLine[self.lineIndex] / 10); //self.lineIndex
+				//self.y += self.speedY * Math.abs(globalSpeedPerLine[self.lineIndex] / 10); //self.lineIndex
+				self.speedY += self.gravity * Math.abs((globalSpeedPerLine[self.lineIndex] - deltaSpeed) / 10); //self.lineIndex
+				self.y += self.speedY * Math.abs((globalSpeedPerLine[self.lineIndex] - deltaSpeed) / 10); //self.lineIndex
 				//log("self.speedY=" + self.speedY, " / self.y=" + self.y);
 				if (self.y >= linesGroundLevels[self.lineIndex]) {
 					self.y = linesGroundLevels[self.lineIndex];
 					self.isOnGround = true;
 					self.speedY = 0;
+					// Define speed for next hurdle
+					var difficultyLevel = 1; // TODO : Move to global
+					self.nextHurdleSpeedX = -6 + difficultyLevel * 3 * Math.random();
 				} else {
 					self.jumpAnim();
 				}
 			}
@@ -152,42 +159,41 @@
 		self.updatePosture();
 		/* ********************************************************** ATHLETE AI **************************************************** */
 		if (self.isAi) {
 			var nextX = self.x;
-			if (!self.isFalling) {
-				if (athlete.isFalling) {
-					if (self.lineIndex == 3) {
-						log("Player falls add=" + self.speedX);
-					}
-					//nextX += self.speedX;
-					nextX -= globalSpeedPerLine[self.lineIndex] * 1.5;
-				} else {
-					//nextX += globalSpeedPerLine[self.lineIndex] * 0.5 * Math.random();
-				}
-				if (!athlete.isFalling && !athlete.isOnGround) {
-					// Little advance when player plays jumping
-					if (self.lineIndex == 3) {
-						log("Player jumping add=" + self.speedX * 0.1);
-					}
-					//nextX += self.speedX * 0.1;
-				}
+			self.previousX = self.x;
+			// Do nothing during fall or jump
+			if (self.isFalling || !self.isOnGround) {
+				return;
 			}
-			// Auto jump hurdles in obstacles[self.lineIndex]
-			if (self.isOnGround) {
-				if (globalSlowDownAfterJump) {
+			// Advance right if athlete is falling
+			/*
+			if (athlete.isFalling) {
+					var advanceRate = -1.25 * globalSpeedPerLine[self.lineIndex]; // speedX;
 					if (self.lineIndex == 3) {
-						log("globalSlowDownAfterJump SET=-10");
+							log("Player falls add=" + advanceRate);
 					}
-					globalSpeedPerLine[self.lineIndex] = -10;
+					nextX += advanceRate;
+			}*/
+			var deltaSpeed = globalSpeedPerLine[self.lineIndex] - globalSpeedPerLine[0];
+			if (self.lineIndex == 3 && deltaSpeed) {
+				log("Delta speed=" + deltaSpeed);
+			}
+			nextX += -1 * deltaSpeed;
+			// Auto jump first hurdle of the line
+			if (obstacles[self.lineIndex].length) {
+				var distance = obstacles[self.lineIndex][0].x - nextX;
+				if (self.lineIndex == 3) {
+					log("distance=" + distance);
 				}
-				if (obstacles[self.lineIndex].length) {
-					var distance = obstacles[self.lineIndex][0].x - nextX;
-					if (self.lineIndex == 3) {
-						log("distance=" + distance);
-					}
-					if (distance > 0 && distance <= 250 + self.speedX) {
-						self.jump();
-					}
+				// calculate optimal distance to jump depending on current hurdle speed (globalSpeedPerLine[self.lineIndex]) 
+				// and jump speed (self.jumpSpeed) : trunk must be higher than hurdle when reaching it                
+				var optimalDistance = 0; // To be defined
+				if (distance > 0 && distance <= 250) {
+					self.jump();
+				} else if (distance > 1000 && !deltaSpeed) {
+					// When hurdle is far, apply inter-hurdle randomness
+					nextX += self.nextHurdleSpeedX;
 				}
 			}
 			self.x = nextX;
 			self.previousX = self.x;
@@ -403,12 +409,12 @@
 
 /**** 
 * Game Code
 ****/ 
-// Enumeration for game states
 /****************************************************************************************** */ 
 /************************************** GLOBAL VARIABLES ********************************** */
 /****************************************************************************************** */ 
+// Enumeration for game states
 var GAME_STATE = {
 	INIT: 'INIT',
 	MENU: 'MENU',
 	HELP: 'HELP',
@@ -597,13 +603,9 @@
 	track4.y = 2732 - 1024;
 	// Initialize ground levels & speeds
 	for (var line = 0; line < numberOfLines; line++) {
 		linesGroundLevels[line] = groundLevel - 200 - 220 * line;
-		if (typeof globalBaseSpeed !== 'undefined') {
-			globalSpeedPerLine[line] = globalBaseSpeed;
-		} else {
-			console.error('globalBaseSpeed is not defined');
-		}
+		globalSpeedPerLine[line] = globalBaseSpeed;
 	}
 	// Add Button in the center of the screen
 	startButton = LK.getAsset('button', {
 		anchorX: 0.5,