OPTIMAL LOADING IN REHABILITATION

Load is a biological signal

Rehabilitation is often discussed in terms of exercises, sets and repetitions, but the underlying principle is much broader: mechanical load creates a biological response.

Through mechanotransduction, cells convert mechanical strain into biochemical signalling. Depending on the tissue involved, this can stimulate collagen synthesis, bone formation, cartilage turnover or muscle protein synthesis. The goal is therefore not simply to “exercise” an injured structure, but to expose it to enough load to stimulate adaptation without exceeding its current capacity.

This idea can be understood through the envelope of function. Every tissue has a range of loading it can currently tolerate while maintaining homeostasis. Too little load provides insufficient stimulus. Too much, too frequently, or without adequate recovery can provoke pain, swelling or tissue breakdown. Rehabilitation progressively expands this envelope.

The important point is that the optimal stimulus is tissue-specific.

Bone: load hard, then recover

Bone responds particularly well to high-magnitude, high-rate and relatively novel loading. Fast ground contacts, multidirectional loading and impact provide a stronger osteogenic stimulus than prolonged low-intensity or static loading.

However, bone's response appears to saturate quickly. After a relatively small number of effective loading cycles, continuing to repeat the same stimulus adds fatigue without producing an equivalent additional adaptive signal. Recovery between loading bouts restores mechanosensitivity.

For rehabilitation, this supports a strategy of short, meaningful loading exposures rather than endless repetitions, followed by sufficient recovery.

Tendon: heavy and patient

Tendon requires a different strategy. Its main stimulus is high tensile loading with sufficient time under tension.

Heavy Slow Resistance programmes typically use controlled concentric and eccentric phases and progressively increase tendon capacity. During more reactive or painful stages, isometric loading may first be used to restore tolerance before progressing toward heavier resistance and eventually energy-storage and plyometric work.

Recovery also matters. Tendon collagen synthesis remains elevated for many hours following heavy loading, supporting the common practice of separating demanding tendon sessions rather than performing maximal loading every day.

The lesson is simple: tendon adaptation takes time. The aim is not merely to reduce symptoms, but to gradually restore the tissue's ability to tolerate high tensile and eventually rapid elastic loading.

Muscle: restore tension and capacity

Muscle adaptation is primarily driven by mechanical tension and progressive overload, but after injury the tissue may initially be unable to tolerate conventional heavy resistance.

Low-load strategies such as blood-flow restriction training (BFR) can therefore be useful during early rehabilitation or periods of significant atrophy. They allow a meaningful muscular stimulus to be generated using relatively low external loads. As capacity improves, rehabilitation should progressively return toward heavier resistance, eccentric loading, higher velocities and eventually power.

This creates a continuum rather than a sudden transition from rehabilitation to performance: low mechanical cost first, then progressive tension, velocity and sport-specific demand.

Cartilage and meniscus: move often, gently

Cartilage presents another loading problem entirely.

Because articular cartilage has no direct blood supply, movement helps create the fluid exchange needed to transport nutrients and remove waste products. Moderate, repetitive cyclic loading therefore plays an important role in maintaining cartilage health.

Both extremes can be problematic. Excessive impact or prolonged static compression may be poorly tolerated, but complete unloading is not ideal either. The preferred strategy is generally frequent, controlled, low-impact movement such as cycling, gait and appropriate closed-chain exercise.

For the meniscus, the same principle must be combined with the specific characteristics of the lesion or surgical repair. Early rehabilitation may therefore emphasise frequent, gentle movement while delaying higher magnitude and impact loading until the tissue is ready.

Does the time of day matter?

One of the more interesting areas explored in the guideline is chronobiology.

Musculoskeletal tissues have their own circadian rhythms. Muscle force production, tendon collagen metabolism, bone turnover and cartilage activity all vary across approximately 24 hours. This raises the possibility that the same loading stimulus may produce slightly different responses depending on when it is performed.

For example, peak muscle strength and power commonly occur later in the afternoon as core temperature rises, while tendon collagen synthesis may theoretically benefit from loading after the morning cortisol peak.

However, this part of the evidence requires caution. Human trials showing that training at a specific hour produces superior long-term tissue adaptation are still limited.

For now, timing should be considered an optimisation tool rather than a fundamental rehabilitation rule. The correct exercise, dose and progression remain considerably more important than whether it is performed at 10:00 or 16:00.

Consistency may be the more useful chronobiological principle: regularly loading tissues within a relatively predictable daily pattern may help reinforce their normal biological rhythms.

Progress by criteria, not simply by time

Optimal loading also means knowing when the patient has earned the next level of exposure.

Return to activity should therefore be viewed as a continuum: return to run, return to play, return to sport and ultimately return to performance. Progression should consider pain and swelling, range of motion, strength symmetry, power, loading capacity, movement quality and psychological readiness rather than relying solely on the number of weeks since injury.

The broader clinical principle is that rehabilitation should continuously answer three questions:

Which tissue are we trying to adapt? How much load can it currently tolerate? And what dose, rate and recovery will move that capacity forward without exceeding it?

The best rehabilitation programme is therefore not the one containing the most exercises. It is the one that delivers the right mechanical signal, to the right tissue, at the right stage, with enough recovery for adaptation to occur.