The Sum of All Forces


Summation of Forces is Important in Performance Training 

Biomechanics is the study of how the human body interacts with the physics world to impact human performance. Having a good understanding of this interaction is a key component of having a sound coaching program that is ultimately beneficial to the athlete’s development and performance.

There are a few concepts that are important in understanding biomechanics and performance – force, mass, acceleration. Remember that formula – F=m x a? This is the fundamental equation when trying to understand how these three concepts interplay and impact performance. Force is a push or pull that results in a movement or action and it is affected by mass and acceleration. The greater the mass, the more force is needed to move an object. And mass is inversely proportional to acceleration. That is, as mass increases, acceleration decreases and vice versa. To further understand this relationship, we have to understand what mass is as mass and weight are often used interchangeably but they are not the same thing. Mass is the amount of matter that an object has. In the physics world, mass is described as the amount of force needed to cause an object to accelerate. Weight, on the other hand, is a measure of the force of gravity on the object. When you weight yourself, your weight is actually the measurement of force of gravity pulling down multiplied by your mass.

How, then does all of this apply and affect performance? Mass is finite in the sense that an object has a large mass or small mass or anything in-between and it’s a simple straightforward quantity. Your body has the same mass running down the track or on the field or in space. However, when looking at force and acceleration, it gets a little more complicated since they both have magnitude and direction. And force can be a single force or multiple forces. When sprinting down the track for example, different body parts produce different forces to produce the desired effect – moving the body in a given direction as fast as possible.

The multiple forces acting on the body and the combination of all forces acting on the body together is the theory of summation of forces. In the human body, movement is achieved by a complex and highly coordinated mechanical interaction between bones, muscles, ligaments and joints within the musculoskeletal system under the control of the nervous system. Muscles generate tensile forces and apply torque at joints in order to provide static and dynamic stability of the body under gravitational forces and other loads while regularly performing precise limb control. In other words, force is created by these joints so that specific movement occurs. For example, when sprinting down the track, hips, knees, ankles, elbows, shoulders create the forces that propel the body forward with counter rotation forces created by the arms so that the sprinter doesn’t fall over. But it’s not as simple as isolating any one movement such as hip positioning, lifting your knees, pulling your elbows back, etc… because everything has to be timed, coordinated and moving in the direction of the force. Also, let’s not forget about mass. Remember the inverse relationship between mass and acceleration – as mass goes up, acceleration goes down. So when you increase the mass – of your leg, arm, overall body – you need greater force to move the body. Where does the force come from? That’s were relative strength comes into play. Being able to appropriately apply your strength to produce power creates that connection you need to propel the body forward. If you or your athlete isn’t strong enough, then you/they are not going to be able to lift any body part in a way that would result in positive acceleration. So when a coach screams ‘knees up’, ‘toes up’ and any other popular cue…understand that those cues are only a small part of a larger equation. Cues without a periodized program is a short sighted approach when developing the athlete – this is treating the symptom not the cause. Center of mass also has an essential role in performance. When sprinting, shifting your center of mass by leaning forward causes gravity to pull you forward thus helping you accelerate and move faster down the track. Not having enough of a lean forward will shift your center of mass causing the effects of gravity to pull you straight down so you become inefficient. So if you are straight up or leaning back, guess what? This is like pumping the brakes and coming to a slow stop.

We now know that the body uses forces that act together to cause the body to move. Another critical component of this theory is the sequencing of forces. We go back to that famous cue of lift your knees or that catch phrase, your glutes aren’t firing. Muscles are always firing, but that’s a topic we already covered and you can read about it here. As we mentioned, muscles don’t work in isolation and movement actually happens in specific sequence. The strongest and lowest body parts around the center of mass (e.g. trunk and thighs) move first, followed by the weaker, lighter, and faster extremities. When a basketball player is taking a shot, the movement begins with the legs pushing into the ground, the force is then returned back up to the legs (equal and opposite reaction), then the force is at the shoulders travels to the forearms and finally to the fingertips as the player releases the ball. This is precise sequencing is known as sequential acceleration and results in successive force summation. To obtain a maximum force, summation needs sequential stabilization of body parts, with some body parts having to be fixed at stable points while other parts produce the effective forces. For example, the force of a punch in boxing or throwing is greater when initiated from the lower body and hips, rather than from the shoulders. Any injury or muscle weakness that causes the body to circumvent the sequence of the musculoskeletal system will change the mechanical interaction and can cause bad form or injury.

Coaching isn’t just coaching cues. Coaching is constantly learning about and using fact based, peer reviewed science and research that has been replicated to understand how each exercise movement impacts the human body and using that knowledge to determine how to produce the desired effects, whether it’s to be faster, swim better, swing harder, be more coordinated. Having a sound understanding of biomechanics, loading of each element, how forces interact with the body is important to really have a successful training program that allows the athlete to maximize their potential.