The Components of Fitness


For personal trainers, understanding the wide variety of fitness components is crucial in designing effective, comprehensive training programs for clients. 

We can group these components into the following categories: muscular strength, flexibility, muscular endurance, speed, muscular power, and aerobic capacity.

This article will provide you with a bit of a summary for each of these fitness components, explaining what it is, how personal trainers can develop it in their clients, how they can measure it and also the expected time scale it may possibly take to start seeing results!

The Components of Fitness

Muscular Strength

Muscular strength refers to the maximum amount of force a muscle can exert. It’s what allows us to lift heavy objects or perform strength based activities. Strength is a term that is often used quite generically, expressed through various rep ranges and through both isolation and compound exercises.

How to Develop Strength

PT’s should Incorporate resistance training into their clients’ routines, focusing on lifting heavier weights with fewer repetitions. Compound movements like squats, deadlifts, and bench presses are generally considered the most effective. PT’s should particularly focus on ‘progressive overload’, a term used to describe incremental and often linear increases in weight over time; whether that be every session, week or month.

How to Measure Strength

One rep max (1RM) tests are often cited as the best tests for assessing muscular strength. However, for safety, you might opt to test strength over a 2, 3, 4, or 5 rep range, and use an equation to predict a 1RM. PTs should note that they should always particularly carefully spot their clients whilst carrying out any kind of strength test, as the potential for their client to fail a rep is very high.

Time to Develop Strength

Clients may start seeing improvements in as little as two to four weeks with consistent training, but significant gains often take months or even years. This is highly dependent though on time the client already has training under their belt, nutrition, training frequency, programming etc. 


Flexibility is the range of motion available at a joint or group of joints. It’s essential for performing daily activities and exercises, and reducing the risk of injuries.

How to Develop Flexibility

Implement dynamic stretches as part of your clients’ warm ups and static stretching during their cool downs. Encourage clients to potentially practice yoga or pilates for added benefits. They do not need to go to a yoga or pilates studio for this; there are plenty of video options online.

How to Measure Flexibility

The sit and reach test is a popular method for assessing lower back and hamstring flexibility. Other joint specific tests can also be used depending on your client’s goals. Over time, you can measure whether range of movement has improved. 

Time to Develop Flexibility

Flexibility improvements can be seen relatively quickly, potentially within a few weeks. However, like all fitness components, maintaining and improving flexibility requires ongoing effort. If you don’t use it, you lose it!

Muscular Endurance

Muscular endurance is the ability of a muscle, or group of muscles, to perform repeated contractions over time without fatigue.

How to Develop Muscular Endurance

Your clients should focus on lower weights and higher repetitions. By performing low joint impact and high volume workout such as going swimming, cycling, or circuit training, you can improve your muscular endurance significantly without hammering your joints too hard.

How to Measure Muscular Endurance

Tests vary by muscle group, but common methods include press up, sit up, or plank tests, measuring how many repetitions a client can perform or how long they can hold a position for. Improvements in time or reps over a period of linear programming signify progress in muscular endurance. 

Time to Develop Muscular Endurance

Clients may notice improvements in muscular endurance within a few weeks, with significant enhancements occurring after consistent training over months.


Speed is the measurement of distance covered in a period of time. This can be by means of physically moving the body through space (e.g. running), or speed in performing repetitions of an exercise.

How to Develop Speed

High-intensity interval training, sprint drills, and plyometrics can improve speed. Focus on explosive, dynamic exercises in training sessions.

How to Measure Speed

Timed sprints over a set distance are straightforward and effective for measuring speed. Agility drills with time measurements can also assess speed in sport specific movements such as changing direction for football, tennis, MMA, badminton and other similar sports.

Time to Develop Speed

Speed can start to improve within a few weeks, but achieving notable changes often requires months of dedicated training and technique refinement. Generally, the ROI for developing speed starts to slow significantly, the more experienced you get. International swimmers for example will train 20+ hours per week, only to improve their 100m time by 0.01s across a year. 

Muscular Power

Muscular power is the ability to exert a maximum amount of force in the shortest possible time. It’s what helps us to jump higher, throw farther, or sprint faster. It is essentially the interplay between strength and speed.

How to Develop Power

Plyometric exercises and Olympic lifts are excellent for enhancing muscular power. Training should focus on fast, explosive movements with adequate rest between sets. Lower rep ranges, fewer than 5 reps, are best for developing and maximum power output.

How to Measure Power

Vertical jump tests or medicine ball throw distances are common ways to assess muscular power. Power output can also be measured using specialised equipment like force plates for jumps or presses.

Time to Develop Power

Often the development of power will be a 2-4 week block within an athlete’s programme (following a strength gaining phase), which implies that power can be significantly improved very quickly. 

Aerobic Capacity

Aerobic capacity, or cardiovascular endurance, is the ability of the heart and lungs to deliver oxygen to the body’s tissues during sustained physical activity. Your aerobic capacity is essentially determined by how long you can perform cardiovascular/aerobic exercise for.

How to Develop Aerobic Capacity

Personal trainers should encourage clients to partake in steady state cardio, interval training, or training in which they can control pace such as swimming, cycling, or running. 

One of my personal favourites whilst swimming is to perform 20x100m efforts, attempting to maintain a consistent pace throughout, on a time of 1 minute 30. This means that I swim 100m, and however long it takes, the remainder of the 1 minute 30 is my rest period before I go again. This similar principle can be applied to watt bike training, track running or treadmill running. 

How to Measure Aerobic Capacity

The VO2 max test is the most accurate and popular measure of aerobic capacity. However as a personal trainer, you may not have access to this testing equipment for clients. You can therefore generally choose a distance and measure time to complete. You can also use how your client feels as a measurement of whether their aerobic capacity is improving!

Time to develop Aerobic Capacity

Improvements can be noticeable within 6-12 weeks of consistent cardiovascular training, with ongoing improvements over time.


As demonstrated in this article, creating the perfect programme for clients requires a highly personalised approach depending on their goals, with further personalisation depending on factors such as starting point, age, weight, lifestyle etc. If you’re looking to become a personal trainer with the highest degree of expertise, be sure to check out our Level 3 Personal Trainer Course, a CIMSPA approved course with the gold standard of teaching and testing.