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Functional Training, Training Response & the Optimal Training Dose

April 06, 2023

Krisna Bhargava PhD

functional training and training response

Why tackle muscle hypertrophy?

Better understanding muscle hypertrophy helps you to identify how many sets and reps are required for you to meet your fitness goals. Our mission at ARENA is to empower you on your journey towards a stronger, more vibrant life, regardless of who you are or your background. We believe that strength truly is the fountain of youth, and we want to help you tap into it.

Our way of solving how to best address muscle hypertrophy opportunities is based on the energetics of loaded training, and is pioneered here by ARENA.

We explain that the concept of "Optimal Training Dose", the central tenant behind ARENA technology, enables you to get from A to B–whether A means that you are old or young, injured or spry, amateur or athlete. 

In this article, we also delve into basic concepts in physics, such as force, momentum, energy, and intensity. We will also cover concepts in training style, stress, and load.

In doing so, we uncover the limitations of traditional training programs and develop a more sustainable and optimized model for progressive training that leverages ARENA's robotic strength training technology.

This appeals to people who are already ARENA users, who would like to speed up their fitness journey. It will appeal to those interested in the science and numbers of vitality:

  • Coaches
  • Trainers
  • Former and current athletes
  • “Quantitative self” enthusiasts

Let's dig deeper.

Table of Contents

Who ARENA is for

ARENA is designed for functional loaded training, with features such as digitally-controlled resistance that can be adjusted instantly, compound motions, and guided classes.

While it may appear simple, ARENA is a powerful tool for anyone interested in improving their level of fitness and achieving muscle hypertrophy! However, it may not satisfy all of the needs of those who practice "barbell sports" such as powerlifting, Olympic lifting, or CrossFit. If the barbell is the focus of your fitness regimen, ARENA is probably best used for your rehabilitation and accessory work.

For most people, ARENA is a turnkey solution for progressive and functional strength training!

Strength training is the 'Fountain of Youth'

Building muscle mass not only helps us look and feel better, but it can also help us live longer. As we age, our muscles naturally begin to deteriorate, leading to a loss of strength and mobility!

However, by engaging in regular strength training exercises, we can slow down this process and maintain our muscle mass for longer. This is important because having stronger muscles means that we are less likely to suffer from falls or injuries as we age, and allows you to maximize muscle hypertrophy, if that is your goal!

Additionally, studies have shown that individuals with higher levels of muscle mass tend to have lower rates of chronic diseases such as diabetes and heart disease, which can ultimately lead to a longer and healthier life.

If you want to live a long and vibrant lifestyle, focus on getting stronger!

Strength training is an effective means for improving overall physical health and preventing injury. By building strength in the muscles, tendons and ligaments, individuals can reduce their risk of common injuries such as sprains, strains and tears.

Additionally, strength training can aid in post-injury recovery by providing a layer of muscle tissue that helps support the healing process!

Strength training offers more benefits than just physical improvement:

  • Increase self-confidence and improve body image.  
  • Regulate hormone levels and improve mood and energy levels.  
  • Improve immune strength and reduce the risk of certain cancers.  
  • Improve sleep quality.  
  • Reduce anxiety and depression and improve cognitive function. 

Basic concepts in physics

Mass and Weight

Mass is the amount of substance present in an object, measured in kilograms [kg] or pounds [lb] (more specifically, pounds-mass [lbm]).

Mass represents the sum total of all the atoms, molecules, and other particles that make up an object! In essence, it is a measure of how much "stuff" there is.

Objects with mass attract one-another through the force of gravity. We imagine that there is an invisible gravitational field around them, especially when an object like Earth has a very large amount of mass. 

Weight is the force exerted on an object due to gravity caused by Earth, measured in newtons [N] or pounds [lb] (more specifically, pounds-force [lbf]).

Weight is directly proportional to an object's mass, meaning that the more massive an object is, the greater its weight will be under the same gravitational force.

We use weight as a concept because we experience it in every day life. We have an intuitive sense of what it feels like to pick up a 10 lb dumbbell, a 40 lb toddler, or a 50 kg sack of rice. 

It is easy to confuse mass and weight. That is because most of the objects that we interact are significantly smaller than the Earth.

However, the weight of an object can vary depending on the strength of the gravitational field at that point. You may not notice a difference on Earth, since the Earth’s gravitational field is approximately the same everywhere on its surface.

Let’s say you bring that 40 lb toddler to the Moon. Since the Moon’s graviational field is weaker than the Earth’s, your 40lb toddler would weigh a lot less on the Moon (6.8lbs to be precise).  

Force and Momentum

Force is the influence or push/pull that one object exerts on another object. A force can cause an object to accelerate, change its direction, or change its shape.

Force is measured in units of newtons [N] or pounds [lb] and is equal to the mass of an object multiplied by its acceleration. There are many different types of forces, including gravitational force, electromagnetic force, and contact forces like friction and tension.

When you lift a 10 lb dumbbell, you exert a mechanical force that counters the force of gravity pulling down, also known as its weight.

💡  ARENA technology uses advanced robotics to generate a lot of force with very little internal mass! The ARENA platform has a mass of ~50 lb but can generate ~200 lb of force. We’ll talk about how this is possible when we discuss the concept of Energy.

Once a mass is set in motion, it continues moving until something stops or slows it down. This is due to the object's inertia, or its tendency to resist changes in motion.

Inertia can cause an object at rest to stay at rest, and an object in motion to stay in motion! 

Momentum, on the other hand, is a measure of the amount of motion a mass has, and is calculated by multiplying the mass of an object by its velocity. 

When you raise a barbell off the ground during a power clean or other explosive move, you are exerting a large amount of force quickly to get it moving and relying on momentum to keep it moving.

Conversely, when you lower the barbell, you are fighting its growing momentum due to gravity.

Understanding force and momentum is crucial to understanding how objects move and behave in the physical world!

Energy, Work, Intensity, and Power

Energy helps us understand how physical systems interact and is measured in joules [J] or calories [cal].

'Energy' refers to the ability of things in motion to interact, transfer momentum, and do work!

When we move an object a certain distance, we expend mechanical energy. When heat flows from a hot object to a cold one, thermal energy is transferred between them.

An object raised to a height has potential energy due to gravity, and when it falls, it gains kinetic energy due to its motion. In this way, energy can be converted from one form to another, allowing us to build incredibly complex machines like cars.

The chemical energy in gas is released in a car's engine through combustion. A cleverly designed powertrain converts the explosion into the rotational kinetic energy in the wheels needed for driving. Our body also uses food's chemical energy in complex reactions that create heat, work, and electricity - hence all the calorie labels you see on the back of the food you eat!

In the context of energy, work refers to the amount of energy transferred when a force is exerted on an object, causing it to move a certain distance. We calculate work by multiplying the force by the distance covered as a result of the interaction.

For example, if you push a sled with a force of 10 newtons for a distance of 2 meters, then the work done would be 20 joules (10 N x 2 m = 20 J). 

💡 When you work out on an ARENA Platform, you are not lifting a heavy mass like you would in a traditional gym. To create resistance, energy stored as chemical energy in a battery is converted into electromagnetic energy in the motor, and then converted to mechanical energy through the drivetrain and cable. Some of the work that you are doing is also converted back into stored energy in the battery. This is how the platform can produce 200lbs of force while weighing so much less and staying portable!  

From the perspective of physics, intensity refers to work done over time and is measured in watts [W] or calories per second [cal/s].

Intensity is a term that can have a lot of different meanings in the fitness world. It can refer to the level of effort required during a workout. More often, when we think of high intensity workouts, we think of heart-pumping exercises that make you sweat and release endorphins. When we think of low intensity workouts, we usually mean yoga or stretching.

You can increase or decrease intensity by playing with how much force, distance, or time you are expressing in a workout. For example, resting for longer periods between sets lowers the workout's intensity, while performing reps quickly or with high weight increases its immediate intensity. 

Similar to intensity, power, is the rate at which work is done over time. In the context of fitness and lifting weights, power can be understood as the speed at which you can perform a specific task that requires force, like lifting a barbell.

We calculate power by multiplying force by velocity, a physics term used to express speed. The more weight you can lift and the faster you can do it, the more power you possess!

While power in fitness is closely related to strength and speed, it also helps us understand how the two tradeoff by considering how much capacity an individual has to generate power.

When bearing high weight loads, we tend to move pretty slow. This is because all of our power generation is shifted towards providing the force needed to deal with that weight. When moving into positions with a light weight load, we can move much faster, using our power to achieve high speeds.

Basic concepts in muscle training

“You are fit if you can adapt to the demands of your environment with ease and imagination…” ― Christopher McDougall, Natural Born Heroes: Mastering the Lost Secrets of Strength and Endurance

Fitness is a state of well-being that encompasses strength, endurance, flexibility, and overall health. It can be achieved through:

  • Various forms of exercise
  • Avoiding drugs and alcohol
  • Managing stress well
  • Eating nutritious foods
  • ...and of course, sleeping well!

Athleticism is fitness taken to the purpose of competition and sport. Not all fit people are athletes, but all athletes are fit. While you may be too old or injured to be an athlete, you are never too old or injured to be fit in your own right.

Strength training and conditioning are two important components of fitness that play complementary roles. Strength training involves lifting weights to increase strength and/or increase muscle mass. Conditioning focuses on improving cardiovascular endurance through activities like running or cycling.

Both forms of exercise help to build a strong foundation for overall fitness, as they improve muscular strength, endurance, bone density, balance, and coordination. By incorporating both strength training and conditioning into your fitness routine, you can achieve optimal physical health and performance in daily life activities.

What is 'functional training', anyways?

Functional training is a type of exercise that aims to improve your ability perform daily activities and focuses on seven types of movements: pull, push, squat, lunge, hinge, rotation and gait.

Examples of functional training exercises include:

  • Lunges
  • Squats
  • Push-ups
  • Pull-ups

Each of these functional strength exercises can be performed using bodyweight or with resistance (such as barbells, dumbbells, kettlebells, medicine balls, etc.). These movements are fundamental patterns that you incorporate into your everyday life!

So any time you climb a set of stairs, bend over to lift up a box, or push open a heavy door, you are incorporating one or more of these basic functional training movements. Incorporating functional training into your training routine can help improve your overall physical fitness, including balance, coordination, flexibility, and endurance.

What is 'progressive strength' training?

The idea behind strength training is to pick up heavy weights and put them down again until you're tired. Then you eat, sleep, and repeat this routine a few times a week.

When performed correctly, strength training regimens provide the benefits of strength training while avoiding injury!

Progressive strength training involves gradually increasing the weight or number of repetitions over time in order to stimulate muscle growth and adaptation. Progressive strength training results in stronger muscles and bones and better circulation, and is the tried and trusted way to see real results.  

Training volume is a measure of the amount of effort that is put into strength training to cause adaptation. It's essentially a yardstick for tracking progression in strength training and conditioning. There are two types of progression: vertical and horizontal.

Vertical progression involves increasing the weight for a given exercise from session to session.

Horizontal progression involves keeping the weight the same but increasing the number of reps from session to session.

Volume helps you quantify your progress by calculating the total amount of weight lifted over all sets and reps. Trainers will manipulate weight and rep targets in order to increase volume over time and ensure their clients make progress. 

This is not an exact science. Other systems exist for progressive training and there are all sorts of arguments about which ones are better and what type of nuances really lead to optimized progress.

For example, trainers sometimes manipulate density or the number of reps performed in a period of time to optimize volume. Others look at volume load, or the average weight per rep seen over a training session.

Ultimately, these measurements seek to address deficiencies in the traditional system so that they can personalize recommendations scalable for many types of clients. Keeping track of and manipulating these data can also be overwhelming, which is why many trainers and athletes keep extensive logs or use apps to make it easier to monitor their progress over time.

relationship between kilojoules exerted and time

High Intensity Interval Training (HIIT) and Circuit Training (CT)

HIIT and CT are two popular training styles that offer a more efficient use of time by mixing the use of weights and calisthenics with conditioning. This gives greater cardiovascular benefits compared to traditional strength training!

The high energy pace and full body movement variations common to HIIT and CT workouts often mean that they can only be sustained for less than an hour.

However, they have been shown to increase post-exercise metabolism for as long as a day, which may accelerate fat loss and increase overall fitness.

Breaking training up into sets is necessary to prevent fatigue when lifting weights. However, if you rest too little between sets, your muscular, nervous, and respiratory systems won't recover, which can limit your ability to get enough volume.

Conversely again, resting too much can lead to longer workouts and take away the challenging pace needed to train the cardiovascular system.

HIIT involves keeping your heart rate up by keeping the number of reps high and the rest periods short enough to allow for recovery before the next set. While training in this style, the overall level of intensity is higher and often features exercises done to maximum effort.

CT alternates between exercises that use different muscle groups so you are constantly moving during a circuit with active recovery!

While these training styles offer benefits in conditioning, they can be difficult to incorporate into progressive training - the tried and true way to improve strength. This is due to practical limitations of grabbing different equipment from all over the gym floor while managing short rest times.

HIIT and CT formats are particularly popular in a group class setting that solve for these limitations by limiting you to a few choices for weights and resistance bands, but throw out measuring volume. 

hiit classes are popular amongst people of all ages

Deficiencies of traditional training regimens

We have already discussed one serious deficiency with traditional volume progression based training programs: they don’t account for the intensity of exercise. However, HIIT and CT workouts are fundamentally more effective at improving most people’s physical fitness.

ARENA addresses this deficiency by bringing volume progression to functional, high intensity workouts.  

This is accomplished in a few ways:

  • You can change weight instantly via the Go Arena mobile app: no more running around the gym floor in search of equipment.
  • Guided workouts like the Lift or Strength for Runners Program are pre-composed with the appropriate rest times and variable movements. 
  • Volume is measured in an accurate and precise manner, based on the energetics of loaded muscular effort.

These benefits are intuitively felt on the ARENA. What separates a good experience from an optimal one is in how deficiencies in progressive volume systems are overcome by trainers.

Let’s break down these deficiencies in detail:

Traditional strength machines have a limited range of motion

In traditional systems, a rep is a rep, regardless of how long your legs or arms are, or how heavy your limbs might be. A short, stout person will have an easier time with squatting than a tall, stalky one!

A person with a previous injury or arthritis in their knees will not be able to bend to the same depth as someone who does not have these impairments.

Traditional volume measurement does not address differences in range of motion. 

Traditional strength machines permit limited velocity and tempo

When you lift a free weight, you do not bear the entire load on your muscles throughout the motion. Momentum is on your side–you need only to give the weight a strong enough push to get an advantage!

This limitation makes it very difficult to count on each rep having the same force or power profile, especially when you have poor form and lift too fast or do not have a coach/trainer handy.

Essentially, you face a tradeoff between force and velocity through traditional strength machines. More sophisticated measurement is a better measurement of the strength work being done.

Traditional strength training leads to 'progression bias'

When you try something new, it’s hard at first. After many tries, it gets easier!

We call this “muscle memory”, or skills competence. As your body and mind get confident with certain exercises, they become easier.

Traditional volume progression relies on you doing the same exercises all the time, biasing your measurement of progression. You may misestimate the actual amount of progress you can achieve because you've done them so many times before, leading to an overvaluing of their effects.

skills competence displayed through Kevin Durant


What is an 'optimal training dose?'

ARENA addresses these deficiencies by using basic physical concepts to measure the energetics of strength training

It starts with how weight, or resistance, is generated. ARENA products do not use a traditional weight stack, elastic band, or a similar mechanical source of load.

Instead, a miniaturized regenerative drivetrain inspired by high-performance electric cars, creates resistance that the user lifts against. Rather than moving mass against gravity, you are fighting electromagnetic force generated by a motor and battery. This has enabled the clever engineers at ARENA to develop software that manipulates how you experience lifting weights to be more efficient.

More specifically, ARENA technology generates isoinertial force at most normal weight settings and speeds. 

load on the muscles at different velocities

Historically, we think of one’s ability to generate force and speed as a tradeoff. It’s hard to get a grip on how much power one can generate when momentum subtracts sustained load from the muscles as you lift a weight. ARENA flattens the curve: the drivetrain keeps force constant regardless of your velocity, allowing you to increase or decrease power dynamically.

In other words, there is little to no momentum when you lift. This means that the force you set on the unit is what you get, regardless of how fast or slow you lift, fully loading your muscles through the entire range of motion.

Many users report that even simple bicep curls on the ARENA Platform feel more difficult than with a dumbbell or barbell in an apples-to-apples comparison. That is because they literally are more difficult: the weight is the same, but how much advantage you can give your body by imparting momentum to the weight has decreased to next to nothing.

The result is you need to generate more power in your muscles to move the weight across a range of typical working velocities.

When a trainer or athlete counts their reps, they are approximating the amount of displacement they affect on a free weight. When they select a free weight to work with, they are approximating the load their muscles will bear.

When they calculate volume by multiplying reps by weight, they approximating the work or mechanical energy transfer from their body to the weight. With ARENA, there is no variation in the weight due to momentum and displacement is measured digitally, allowing the app to report and log your true work, not just an approximation. 

sample strength report from the ARENA home gym

Measuring true work is a bit like doing a blood panel that gives you quantitative biomarker data. True work enables ARENA to build a “reference” data set of normal ranges across all sorts of body types, ages, genders, locations, impairments, and other variations in people’s lives in the most inclusive manner possible.

It also means that every workout you do is a biomarker measurement of where you are at in your fitness, in real time. The combination of these reference data and workout-as-a-biomarker enables ARENA to customize an optimal training dose for each user, every week, automatically.

What was once restricted to athletes and celebrities with elite trainers, is now in the palm of your hands!

Training Dose (TD) and Training Dose Response (TDR)

Training dose (TD) and training dose response (TDR) are two important concepts in the field of exercise science.

TD refers to the cumulative amount of stress placed on an individual from single or multiple sessions over time.

TDR is related to the responses or consequences of a session.

TD can be thought of as being "external to the body" and includes loaded exercise, recovery work, endurance work, and sports. On the other hand, TDR is "internal to the body" and can vary based on factors outside of training such as sleep, nutrition, stress, psychological state, and injuries.

ARENA’s Optimal Training Dose recommendation engine is a growing, living algorithm designed to find the least amount of TD that will result in the greatest amount of positive TDR for every individual each week. This not only minimizes the amount of recovery needed but also avoids injury and burnout while focusing on real gains that reinforce consistency!

Feeling good after a workout means you are more likely to come back and not waste time during your day. Using true work as a biomarker, combined with automated guidance through classes and other measurements of your activity, overcomes the limitations in traditional progressive training like intensity, range of motion, velocity/tempo, and skills competence.

relationship between training dose and training dose response

ARENA builds a picture of your TD-TDR relationship over time. This allows you to build progress efficiently and incrementally, eventually leading to compound gains. 

TDR is much more difficult to measure your directly. A trainer might ask a client how they are feeling that day and find out that they only got a few hours of sleep. Then that client might totally kill it and hit a personal record (PR) during their training session.

Maybe sleep didn’t have a real influence on their readiness?

Maybe just asking the question relieved related stress?

It’s impossible for us to know for sure what caused the PR, but we can at least measure how often and under what conditions PRs tend to occur. In other words, the best way to know if you can PR is to see if you can do it.

With the rise of wearables and quantified nutrition devices like consumer-grade continuous glucose monitoring, the backdrop for predicting TDR becomes more clear relative to TD history. By setting a target weekly TD, and measuring how far off you are from hitting it, ARENA is able to create a picture of these conditions and optimize your targets over time.

training dose response measurement inputs

An incredible number of controllable and uncontrollable factors contribute to your TD-TDR relationship. We cannot measure all of them (yet!), but what we can do is measure how your body adapts to the increasing stress from one training session to the next, and extrapolate which factors are the most important for you.

The bottom line

ARENA’s approach to setting an Optimal Training Dose based on TD-TDR measurement and isoinertial training brings the power of elite coaching and specialized tools to your doorstep. Practically speaking, it means you will see your efforts measured exactly and you will progress in the most efficient way possible.

We believe that when you see real results, you will keep coming back for more.

This is what sets ARENA apart.

If you are not able to leverage ARENA to your advantage, join the Membership program and connect to a concierge trainer who can help you get the most out of the ARENA Platform and Go Arena mobile app.

If you already know what to do in a gym, just throw it in Freestyle mode and get moving. Even if you start with just ten minutes a day, you’ll save an hour of commute time and will have knocked out isoinertial reps that are multitudes more effective than swinging around a dumbbell.

Just get moving, do a little work, and let the Optimal Training Dose do the rest in planning for progress.

ARENA is a great workout for all types of people!


- Krisna Bhargava, PhD - Chief Executive Officer, ARENA Innovation Corp
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