Principles of Physical Activity for Healthy Adults

Original Editor - Ewa Jaraczewska based on the course by Tarina van der Stockt

Top Contributors - Ewa Jaraczewska, Jess Bell and Tarina van der Stockt  

Introduction

Regular physical activity is vital for health, helping to prevent chronic disease, improve metabolic function, and enhance overall quality of life. It also supports mental well-being, reduces stress and helps to maintain functional independence across the lifespan.

The World Health Organization (WHO) has published guidelines that set population-level physical activity targets. For adults aged 18–64, the WHO recommends:

• at least 150–300 minutes of moderate-intensity aerobic activity per week, or 75–150 minutes of vigorous-intensity activity, or an equivalent combination
• muscle-strengthening activities on two or more days per week, working all major muscle groups at a moderate or greater intensity

• reducing sedentary time, replacing it with activity of any intensity

Physiotherapists, who work with movement and exercise daily, are well placed to turn these guidelines into individualised, sustainable plans for their clients.

Pre-Exercise Screening

Before prescribing exercise, a clinician must confirm that their client is safe to engage in unsupervised or self-directed physical activity.

Pre-exercise screening should determine whether the client has any diagnosed cardiovascular, respiratory, or metabolic disease, or symptoms such as chest pain, dizziness, or breathlessness that seems disproportionate to effort. They should ask about medications that alter heart rate or blood pressure, such as beta-blockers or diuretics, and about recent surgery, injury, or musculoskeletal pain that might limit activity. They should also ask about family history of sudden cardiac events at a young age, and, where applicable, about pregnancy or postpartum status.

The Physical Activity Readiness Questionnaire (PAR-Q+) is a validated tool that can be used to carry out pre-exercise screening in a standardised way. When screening identifies relevant risk factors, it is necessary to obtain medical clearance before proceeding. It is, however, important to note that the PAR-Q+ may over-identify clients who require further medical evaluation, and for some, this could "be a disincentive to get physically active."

Clinical tip: Screening is not a one-off event. It should be revisited periodically, including whenever the client mentions a new diagnosis, medication, or change in symptoms.

Physical Activity Intensity Categories

Physical activity intensity categories

Exercise and non-exercise physical activity can be classified by the level of intensity. The intensity of strengthening exercises is commonly gauged using repetitions in reserve (RIR).

• Light intensity: Minimal physiological strain, allowing for effortless conversation. For strengthening, there are more than 7–8 reps in reserve. This level of activity is important for daily health and to counter sedentary behaviour (e.g. slow walking, light housework, stretching).
• Moderate intensity: Promotes health benefits like improved blood pressure and mood. Heart rate and breathing increase, but a conversation can still be held with some effort. For strengthening, there are around 4–6 reps in reserve (e.g. brisk walking, recreational swimming, cycling at a comfortable pace, doubles tennis, and working major muscle groups enough to tire them after 10–15 reps).
• Vigorous intensity: Intensity is high enough that sustained conversation becomes difficult. For strengthening, there are fewer than 2–3 reps in reserve. Because one minute of vigorous activity roughly equals two minutes of moderate activity, it can offer similar health benefits in half the time (e.g. running, singles tennis, group fitness class).
• Very vigorous intensity: Near-maximal effort, and no reps in reserve. Techniques such as High-Intensity Interval Training (HIIT) and Sprint Interval Training (SIT) can improve cardiorespiratory and athletic performance, but they do not suit everyone and careful screening is required (e.g. sprinting, HIIT, SIT).

Physical Activity Intensity Measurement Tools

MET Scale

Intensity can be estimated using the following:

1. METs (Metabolic Equivalent of Task),
2. Talk test
3. Rate of Perceived Exertion (RPE)
4. Heart-rate monitoring.

A MET expresses the energy cost of an activity relative to rest. This makes it useful for research and for comparing the intensity of different activities, but it is not a practical measure for clients to use during activity.

The Talk Test

For a client who is new to exercise and has no equipment, the talk test is a good starting point, as it is fast and intuitive. As a client develops a more reliable sense of perceived exertion, RPE becomes more useful (either the 6-20 Borg scale or the CR10 scale).

Intensity can also be gauged using heart rate. Different exercise intensities correlate to different percentages of maximum heart rate (%HRmax). Maximum heart rate is difficult to measure directly, so it is typically estimated using a prediction equation. The Fox formula (220 minus age) is widely used because it is simple, but it tends to overestimate %HRmax in younger people and underestimate it in older people. Other equations, such as Tanaka's formula (208 minus 0.7 times age), were developed to reduce this error, but all age-based estimates remain approximations.

Table 1 shows how each of these approaches maps to different intensities of exercise.

Table 1. Physical Activity Intensity Categories and Measuring Tools

Intensity	METs	Talk Test	RPE (Borg 6-20)	RPE (CR10)	Heart Rate Maximum (%HRmax)	Heart Rate Zone
Light	1.5-3.0	Full conversation, can sing	<12	<4	50-60%	1-2
Moderate	3-6	Full sentences, singing difficult	12-14	4-6	60-70%	3 to lower 4
Vigorous	>6	Short phrases only	15–17	7-8	70-85%	Upper 4 to 5
Very vigorous	Near maximal	Speaking is uncomfortable	≥18	9-10	>85%

Heart rate zones

Many clients use wearables (smartwatches or fitness trackers) to track heart rate and daily activity. These devices tend to display effort using a five-zone model based on percentage of maximum heart rate. This data is worth engaging with, but with two caveats: accuracy varies between devices, and zone boundaries differ between manufacturers depending on how each estimates maximum heart rate. Having clients cross-check their wearable data against the talk test or RPE during sessions helps develop their internal sense of perceived exertion alongside external data.

Training intensity can also be set from a graded exercise test. This is common in rehabilitation and clinical exercise settings, though most healthy adults will not have had one.

Clinical pearl: No single measure tells the whole story. A wearable might show a client is training in a low zone when they are clearly working hard. Reading the zone alongside RPE gives a more complete picture.

FITT-VP Framework

The FITT-VP (Frequency, Intensity, Time, Type, Volume, and Progression) framework is commonly used to structure exercise prescription. The components are best understood through an example. Consider a 45-year-old office worker. She is largely sedentary, with no red flags on screening. Her goal is to reduce afternoon fatigue and improve general fitness. She enjoys walking and dislikes gym environments. Table 2 shows how the FITT-VP framework can be used to prescribe exercise for this client.

Table 2. Exercise Prescription Using FITT-VP Framework

Component	Prescription
Frequency	5 days/week aerobic; 2 days/week strength
Intensity	Light to moderate (RPE 12–14; able to hold a conversation with some effort)
Time	30 minutes/session aerobic; 20–25 minutes strength
Type	Brisk walking; resistance band circuit
Volume	~150 min/week aerobic exercise, plus 2 strength sessions (40–50 min)
Progression	Increase to 40-minute aerobic sessions by week 4; reassess RPE monthly

The same weekly total of 150 minutes of aerobic exercise could be met through three 50-minute sessions or five 30-minute sessions. Volume describes the total, while frequency and time describe how that total is distributed across the week. This distribution should follow the client's schedule and preferences, not a fixed template.

Physical Activity Programme Progression

"Progressions and regressions in exercise are a strategic approach aimed at optimising physical performance and health, preventing injury and avoiding overtraining through continuous and planned adaptation of training stimuli."

Progression must be built into exercise prescription. Without progression, even a well-constructed programme will stall, and clients will plateau once they have adapted to a load.

The general principle is to start low and progress gradually. A common approach is to first establish consistency and good technique at a comfortable intensity before adding load. Duration and frequency are often increased next, and once weekly training volume approaches the WHO targets, intensity may be increased. The timing of progressions is individual: for example, one client might spend the first few weeks building consistency, before increasing duration over the following weeks, and then increasing intensity. Perceived exertion and functional response should be reassessed periodically. If a session that used to feel like an RPE of 13 now feels more like 10, it is time to progress.

When progressing strength training, it is often better to change one variable at a time, such as load, repetitions, or sets, rather than adjusting several at once. This lowers injury risk and makes it easier to identify which change was responsible if a problem arises.

Common Errors When Prescribing Physical Activity

A few common pitfalls are worth flagging.

Prescribing volume before addressing the client's barriers to exercise. Even the most well-calculated programme won't produce results if it does not fit the person's life. Barriers such as time, cost, access, or confidence significantly impact adherence, and must be considered.

Leaning too heavily on age-predicted maximum heart rate formulas. Formulas like 220-minus-age are rough estimates, and planning load entirely around them can give a misleading target.

Treating wearable data as ground truth. Optical wrist sensors lose accuracy during high-intensity work or activities that involve significant wrist movement.

Transitioning a deconditioned or previously sedentary client directly into vigorous or HIIT-style training without a sufficient base-building phase. This approach will overload an unprepared musculoskeletal system, increasing the risk of significant delayed onset muscle soreness, overuse injuries, and drop-out from burnout.

Neglecting rest. Training the same muscle group on consecutive days without forty-eight hours of recovery can blunt adaptation and raise injury risk.

Focusing too heavily on structured exercise minutes and forgetting about sedentary time. Meeting the WHO targets does not, on its own, cancel out the risk of prolonged sitting. Reducing sedentary time should therefore be an additional goal.

Physical Activity Red Flags

Clinicians can generally guide healthy adults through a progressive programme without specialist input. Some presentations, however, warrant referral to a physician, cardiologist, or another specialised service. These include:

• symptoms that emerge during activity, such as chest pain, unusual breathlessness, dizziness, or palpitations
• a marked mismatch between a client's reported effort and their observed physiological response, such as unusually high perceived exertion at what should be a low workload
• musculoskeletal pain that does not resolve with reasonable activity modification

A client with multiple cardiovascular risk factors who wants to begin vigorous or HIIT-style training should be assessed by a specialist before starting the activity, rather than after symptoms appear.

Physical Activity Prescription Documentation

Good documentation for physical activity prescription facilitates continuity of care and provides the rationale for a clinician's reasoning. Documentation does not need to be lengthy, but should include:

• screening outcome (PAR-Q+ or equivalent) and any flags

• baseline activity level and relevant history (including positive or negative exercise experiences)
• FITT-VP prescription as agreed with the client
• method chosen to monitor intensity, and why
• planned review date and progression criteria
• client-identified barriers and how they were addressed

Summary

The WHO guidelines set a population-level target for physical activity, but turning that target into effective practice means screening before prescribing, matching the intensity-monitoring tool to the individual, structuring the prescription with FITT-VP around the client's context and needs, progressing deliberately while reviewing regularly, and recognising when to refer.

Exercise prescription is not just about hitting targets. Lasting change depends on a client enjoying their programme and being able to fit it into their life, not on the plan being perfect on paper.

Resources

• Corrective Exercise Red Flags: When Recovery Needs Intervention
• World Health Organization 2020 guidelines on physical activity and sedentary behaviour
• The Power of Sprint Interval Training (SIT): A Beginner's Guide