1.3 Biomechanics & Human Movement Science

Biomechanics

Biomechanics is the science concerned with the internal and external forces acting on the human body and the effects produced by these forces. For a personal trainer, understanding biomechanics is the key to optimizing exercise technique, maximizing muscular tension, and preventing catastrophic injuries. It bridges the gap between anatomy and physics, explaining why certain postures are stronger and how minor adjustments can drastically change an exercise's difficulty.

Levers in the Human Body

A lever is a rigid bar that rotates around a fixed point called a fulcrum. In the human body, bones act as the rigid bars, joints serve as the fulcrums, muscles provide the effort (force), and the weight of the body part plus any external weight acts as the load (resistance). There are three classes of levers, determined by the relative arrangement of the fulcrum, effort, and load.

1. First-Class Levers (Fulcrum in the Middle): The fulcrum is positioned between the effort and the load, like a seesaw.

   • Example in the body: The atlanto-occipital joint in the neck. The joint is the fulcrum, the weight of the head is the load, and the neck muscles pulling down on the back of the skull provide the effort to keep the head upright.
   • Mechanical Advantage: Can favor either force or speed depending on the exact placement of the fulcrum.

2. Second-Class Levers (Load in the Middle): The load is positioned between the fulcrum and the effort, like a wheelbarrow.

   • Example in the body: Plantarflexion at the ankle during a calf raise. The ball of the foot is the fulcrum, the body weight is the load, and the calf muscles provide the effort pulling upward on the heel.
   • Mechanical Advantage: Favors force production. A small effort can move a large load, but at the cost of speed and range of motion.

3. Third-Class Levers (Effort in the Middle): The effort is applied between the fulcrum and the load, like using a pair of tweezers or swinging a baseball bat.

   • Example in the body: The elbow joint during a biceps curl. The elbow is the fulcrum, the weight in the hand is the load, and the biceps tendon attaches to the forearm between the elbow and the hand, providing the effort.
   • Mechanical Advantage: Favors speed and range of motion over sheer force. This is the most common lever system in the human body, allowing us to move our limbs quickly and through wide arcs, even though it requires the muscles to generate more force than the actual weight of the load.

Force and Torque

In biomechanics, Force is a push or pull that alters the state of motion of a body. When training, the primary forces we manage are gravity, friction, and muscular contraction.

Torque is the rotational equivalent of force. It is a measure of how much a force acting on an object causes that object to rotate around an axis.

Torque = Force × Moment Arm

The moment arm is the perpendicular distance from the axis of rotation (the joint) to the line of action of the force. In practical terms, the longer the moment arm, the greater the torque, and the harder the exercise feels. For example, performing a lateral raise with completely straight arms creates a long moment arm, making the dumbbell feel very heavy. Bending the elbows brings the weight closer to the shoulder joint, shortening the moment arm, reducing the torque, and making the exercise feel easier, even though the physical weight hasn't changed.

Muscle Actions

Muscles don't just 'flex'; they interact with resistance in three distinct ways, collectively known as muscle actions.

The Importance of the Eccentric Phase

Many novices focus entirely on the concentric (lifting) phase of an exercise and let gravity handle the eccentric (lowering) phase. However, biomechanically, muscles can generate significantly more tension eccentrically than concentrically. Furthermore, eccentric training is highly associated with muscle damage, which is a primary driver for muscle hypertrophy (growth). A skilled personal trainer ensures clients control the eccentric phase, often prescribing tempos like a 3-second lowering phase to maximize time under tension and stimulate optimal adaptations.

The Length-Tension Relationship

The length-tension relationship states that there is an optimal resting length for a muscle where it can produce the maximum amount of force. At this optimal length, the actin and myosin filaments have the maximum degree of overlap, allowing for the highest number of cross-bridges to form.

If a muscle is stretched too far, the filaments barely overlap, and few cross-bridges can form, resulting in weak force production. Conversely, if a muscle is already fully shortened, the filaments are completely overlapping and crumpled, leaving no room for further contraction, again resulting in weak force production. This is why you are strongest in the middle range of motion during most exercises. Understanding this concept allows trainers to position clients properly to maximize strength output or to intentionally train muscles at disadvantaged lengths to promote specific structural adaptations.