Physiological Strain during Physical Work | Research Methods in Sports Medicine

Load, Strain, Adaptation
Types of Muscular Work
The Cardiovascular Response
The Ventilatory and Metabolic Response

Learning objectives:

Define exercise physiology as the basis of sports medical functional diagnostics
Distinguish positive-dynamic, negative-dynamic and static holding work, and their metabolic cost
Quantify the cardiovascular, ventilatory and metabolic response to submaximal and maximal work
Derive V̇O₂max from its three determinants and state why arm work carries a different risk profile than leg work

1 Load, Strain, Adaptation

Three terms must be kept apart. Load (Belastung) is what is imposed from outside — watts, kilometres, kilograms. Strain (Beanspruchung) is the internal, individual physiological answer to that load. Adaptation is the structural and functional change that follows repeated strain, or its loss under chronic inactivity. Acute strain and chronic adaptation act at every physiological level: cell, tissue, organ, organ system.

Exercise physiology describes life processes and body functions in connection with movement, physical activity and performance capacity. It covers both the acute response — a single continuous run — and the adaptation to chronic physical (in)activity, as observed in the training process. It is the foundation on which sports medical functional diagnostics rests: without a model of the expected response, a measured response cannot be called normal or abnormal.

2 Types of Muscular Work

Positive-dynamic work: work-performing contraction alternates with relaxation (walking uphill).
Negative-dynamic work: braked muscle lengthening alternates with unloaded contraction (walking downhill).
Static holding work: quiet standing, holding a position.

Two or all three types are usually combined. In dynamic work mechanical work is performed on the environment; in pure holding work it is not — force × distance = 0 — yet chemical energy is still consumed and fully converted into heat. The product of muscle force and holding time is the measure of that cost. This asymmetry is why isometric tasks feel disproportionately hard relative to the external work they show, and why an ergometer that only records external power systematically under-reads the strain of holding tasks.

3 The Cardiovascular Response

Hard work requires up to 500 times more O₂ delivery to the musculature than rest, while the increased H⁺, CO₂ and lactate load must be removed. Cardiac output rises from 5–6 l·min⁻¹ at rest in untrained people to a maximum of roughly 20 l·min⁻¹, through sympathetically driven increases in heart rate (up to ~2.5-fold) and stroke volume (up to ~1.2-fold). In light and moderate work heart rate reaches a new steady value without fatigue; very heavy work must be terminated because the heart cannot sustain the required output.

The redistribution matters as much as the increase: skin perfusion rises for heat dissipation, while renal and splanchnic perfusion fall below resting values under sympathetic tone. Systolic pressure rises while diastolic pressure stays approximately constant, so pulse pressure widens by some 20–50 mmHg with only a moderate rise in mean pressure. The smaller the active muscle mass, the steeper the pressure rise — arm work (hedge trimming) produces a higher rise than leg work (cycling). Clinically: arm work therefore endangers patients with coronary artery disease or cerebrovascular sclerosis more than leg work does.

Muscle perfusion during maximal work reaches up to 2.5 l·min⁻¹ per kilogram of active muscle — about 10 % of maximal cardiac output — so fewer than about 10 kg of muscle, roughly one third of total muscle mass, can be maximally active at once. The required vasodilation is achieved largely locally: rising PCO₂, falling PO₂, falling pH, and NO release. In pure holding work the constantly tensed muscle compresses its own vessels and partly prevents this increase, which is why it fatigues faster than rhythmic dynamic work.

4 The Ventilatory and Metabolic Response

Minute ventilation rises from a resting value of about 7.5 l·min⁻¹ to a maximum of 90–120 l·min⁻¹, through both breathing frequency (max. 40–60 min⁻¹) and tidal volume (max. ~2 l). Together with the increased cardiac output this allows V̇O₂ to rise from about 0.3 l·min⁻¹ at rest to about 3 l·min⁻¹ in untrained people. The ventilatory equivalent — the litres of air breathed per litre of O₂ taken up — is about 25 at rest and rises to 40–50 beyond the endurance limit. That rise is the ventilatory signature the threshold concepts of Lecture 8 and Lecture 10 are built on.

Below the maximal limit, V̇O₂ rises linearly with power at about 10 ml·W⁻¹ independent of training status. Maximal oxygen uptake follows from its three determinants:

V̇O₂max = HR_max × SV_max × avDO₂_max

Expressed relative to body mass, V̇O₂max is conventionally treated as the “ideal” measure of physical capacity — a claim Lecture 10 revisits critically: the threshold concept discriminates endurance performance better in populations whose maximal values have converged.

One-Minute-Paper Topics

A One-Minute-Paper (OMP) is a short, focused prompt that students answer in ~60 seconds at the end of a session to consolidate learning, surface misconceptions, and provide formative feedback. When answering, be concise, specific, and use terminology from today’s session.

Why does static holding work consume chemical energy although the external mechanical work is zero?
A cardiac rehabilitation patient asks whether he may resume gardening. Which part of §3 is decisive for your answer, and why?
Give the three determinants of V̇O₂max and name, for each, one training adaptation that changes it.
Estimate the ventilatory equivalent at rest and above the endurance limit, and say what the change reflects physiologically.

References