Kelsey Matichuk, RMT

Here's a snapshot of the biological insult from international travel. It takes your body over two weeks to fully recover.

It's a big price tag.

One international trip per quarter is a reasonable balance.

Time to recover:
> sleep duration: 2 days
> grip strength: 5 days
> mood: 1 wk
> cortisol: 9 days
> sleep quality: 2 wks
> blood glucose: 2 wks

🧬 Your weekly science updates. Read more 👇🏻

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This Week in Science (25 May - 31 May 2026)

Just published 🔥

𝗖𝗿𝗼𝘀𝘀-𝗲𝗱𝘂𝗰𝗮𝘁𝗶𝗼𝗻 𝗼𝗳 𝘂𝗻𝗶𝗹𝗮𝘁𝗲𝗿𝗮𝗹 𝗿𝗲𝘀𝗶𝘀𝘁𝗮𝗻𝗰𝗲 𝘁𝗿𝗮𝗶𝗻𝗶𝗻𝗴 𝗮𝘀 𝗮 𝘀𝘁𝗿𝗮𝘁𝗲𝗴𝘆 𝘁𝗼 𝗺𝗶𝘁𝗶𝗴𝗮𝘁𝗲 𝗶𝗺𝗺𝗼𝗯𝗶𝗹𝗶𝘀𝗮𝘁𝗶𝗼𝗻-𝗶𝗻𝗱𝘂𝗰𝗲𝗱 𝗻𝗲𝘂𝗿𝗼𝗺𝘂𝘀𝗰𝘂𝗹𝗮𝗿 𝗱𝗲𝗰𝗹𝗶𝗻𝗲: 𝗔 𝘀𝘆𝘀𝘁𝗲𝗺𝗮𝘁𝗶𝗰 𝗿𝗲𝘃𝗶𝗲𝘄 𝗮𝗻𝗱 𝗺𝗲𝘁𝗮-𝗮𝗻𝗮𝗹𝘆𝘀𝗶𝘀

▶️ Immobilisation has been used in the treatment of upper and lower extremity injuries since ancient times in order to reduce pain, allow tissue healing, and prevent further damage (). However, modern forms of immobilisation such as casting, bracing, or surgical fixation substantially reduce mechanical loading and therefore induce rapid declines in muscle strength and muscle mass (https://pubmed.ncbi.nlm.nih.gov/19727027/, https://pubmed.ncbi.nlm.nih.gov/38895777/).

⬇️ These neuromuscular losses occur particularly quickly during the first days of immobilisation before beginning to plateau after approximately two weeks (https://pubmed.ncbi.nlm.nih.gov/36883219/). Daily strength losses of approximately 2.0% in the knee extensors and 1.2% in the elbow flexors have been reported (https://pubmed.ncbi.nlm.nih.gov/30900205/).

💡 Importantly, these early declines in strength are thought to be driven predominantly by neural mechanisms — including reductions in neural drive, motor unit excitability, and firing frequency — rather than by muscle atrophy itself (https://pubmed.ncbi.nlm.nih.gov/41106072/, https://pubmed.ncbi.nlm.nih.gov/33981206/, https://pubmed.ncbi.nlm.nih.gov/36088611/).

⬆️ Since persistent strength deficits and asymmetries are associated with an increased risk of reinjury and delayed return to sport or work (https://pubmed.ncbi.nlm.nih.gov/36965459/, https://pubmed.ncbi.nlm.nih.gov/35141554/), cross-education of the contralateral non-immobilised limb has emerged as a promising strategy to attenuate neuromuscular decline during periods of immobilisation itself.

📘 Carroll et al. (https://pubmed.ncbi.nlm.nih.gov/17043329/) showed that these contralateral strength gains reach approximately 52% of those observed in the trained limb, highlighting the importance of maximising trained limb strength to optimise the cross-education effect in the untrained contralateral limb.

💪 Effective cross-education interventions require near-maximal training intensities to maximise strength gains in the untrained limb (https://pubmed.ncbi.nlm.nih.gov/28936703/, https://pubmed.ncbi.nlm.nih.gov/37156010/, https://pubmed.ncbi.nlm.nih.gov/33984253/).

🧠 The magnitude of strength transfer is greatest when training emphasises eccentric muscle actions, which induce greater reductions in intracortical inhibition and greater increases in corticospinal excitability compared to other contraction types (https://pubmed.ncbi.nlm.nih.gov/26037804/, https://pubmed.ncbi.nlm.nih.gov/34488881/).

📘 In a brand-new study, Rodríguez-Coloma and colleagues (https://pubmed.ncbi.nlm.nih.gov/42141765/) quantified the effects of cross-education on muscle strength and size during unilateral limb immobilisation in healthy individuals. Subgroup analyses examined the moderating influence of training modality (i.e., eccentric, concentric, isometric, or combined), immobilisation model (i.e., proximal vs distal limb musculature), and muscle group specificity for preserving neuromuscular function.

👫 The review included eight experimental studies with a total of 189 healthy participants. In all studies, one upper limb was immobilised for 3 to 4 weeks while the opposite limb performed resistance training.

𝗠𝗮𝗶𝗻 𝗙𝗶𝗻𝗱𝗶𝗻𝗴𝘀

📊 Cross- education attenuated strength loss compared with immobilisation alone (g = 0.53, p < 0.001), with effect magnitude moderated by immobilisation location and training modality.

💪Proximal immobilisation (i.e. shoulder joint) yielded greater attenuation (overall: g = 0.62; eccentric: g = 0.82; concentric- eccentric: g = 0.68) than distal immobilisation (i.e. wrist joint, overall: g = 0.42; eccentric: g = 0.34; isometric: 43 g = 0.63).

📊 Regarding muscle size, cross education produced a small preservation effect in the immobilised limb (g = 0.19, p = 0.01). This effect was only evident in proximal immobilisation (g = 0.40) compared to distal (g = 0.06). Quantitatively, proximal effects were about 1.5 times greater than distal effects for strength, but markedly greater—about 6.7 times—for muscle size.

🧠 Possible neuroanatomical Explanation: Proximal muscles receive enhanced descending drive from the reticulospinal tract (mediating diffuse bilateral adaptations), whereas distal fine motor movements of the hand/forearm rely heavily on the corticospinal tract. This localized control restricts the cross-activation effect, rendering cross-education far less effective at protecting distal wrist musculature from structural atrophy.

📊 Strong positive associations were observed between adaptations in the trained and immobilised limbs for strength (r = 0.79) and muscle size (r = 0.81). These findings indicate that cross-education attenuates losses in muscle strength and size during immobilisation. However, these results should be interpreted with caution due to the varying risk of bias among the included studies.

💡 Several distinct neurophysiological and morphological mechanisms might explain theses results:

👉 Strength Sparing is Primarily Neural: Early strength losses during disuse are predominantly driven by neural decrements, such as reduced neural drive and altered motor unit firing rates, rather than immediate muscle atrophy. Cross-education acts as a direct countermeasure by maintaining cortical excitability and neural drive to the immobilised limb.

👉 Size Sparing is Limited by Lack of Load: Muscle hypertrophy and mass preservation are fundamentally dependent on direct external mechanical loading. Because the immobilised limb experiences no actual load, the cross-education effect on muscle size is predictably small.

👉 Potential Protein Synthesis Modulation: The small amount of muscle size preservation that does occur might be driven by sustained neural activation, which may weakly modulate local signaling pathways to abated decreases in muscle protein synthesis, effectively slowing down tissue atrophy.

𝗪𝗵𝗮𝘁 𝗮𝗿𝗲 𝘁𝗵𝗲 𝗽𝗼𝘀𝘀𝗶𝗯𝗹𝗲 𝗺𝗲𝗰𝗵𝗮𝗻𝗶𝘀𝗺𝘀?

The broader neural cross-transfer of strength is justified by two primary candidate cortical mechanisms:

🧠 The Cross-Activation Hypothesis (https://pubmed.ncbi.nlm.nih.gov/23908616/): Unilateral motor training produces spillover activation facilitating neural adaptations in both hemispheres, driving concurrent neural adaptations in both the trained and untrained descending pathways.

🧠 The Bilateral Access Hypothesis: The neuroplastic adaptations remain localized within the hemisphere controlling the trained limb, but the untrained hemisphere can directly "access" these motor schemes via transcallosal pathways (the corpus callosum) when trying to recruit the immobilised limb (https://pubmed.ncbi.nlm.nih.gov/23908616/).

📋 Exercise Prescription Guidelines

1️⃣ Implement cross-education immediately during the acute immobilisation phase to blunt the steep initial drops in voluntary neural drive.

2️⃣ Prioritize Eccentric Load: Incorporate high-intensity eccentric actions (e.g., eccentric preacher curls) where feasible to optimize neuroplasticity and maximize transfer magnitude.

3️⃣ Demand Near-Maximal Intensity: Exercise prescription should utilize near-maximal intensities, as cross-education effects are highly dose-dependent.

4️⃣ Target Homologous Pairs: Ensure targeting of the exact homologous contralateral muscle groups to exploit muscle-specificity advantages.