Intensified training with insufficient recovery can impair muscle function at joint and single fibre levels

Take home message

• Periods of intense training with insufficient recovery resulted in impairments in muscle function at both the joint and single fibre levels.
• At the single fibre level of muscle, calcium (Ca²⁺) sensitivity was impaired, resulting in prolonged low-frequency force depression at the joint level, making submaximal efforts harder than usual.
• A period of intensified training causes a reduction in muscle contractile function, but this can be overcome with adequate recovery to increase sport performance through subsequent adaptation.

Background

• Athletes commonly increase training volume to cause muscle adaptations that result in an improvement in sport performance.
• Periods of increased training volume with low levels of recovery, known as intensified training with insufficient recovery, can be detrimental to sport performance and cause the athlete to feel more tired.
• How intensified training with insufficient recovery affects muscle function at the joint and single fibre levels, and the mechanisms involved, are unclear.
• The Ca²⁺ sensitivity of muscle can indicate how easily the muscle can contract when Ca²⁺ flows into it; a muscle with decreased sensitivity requires more Ca²⁺ to elicit a contraction.

How the study was done

• Fourteen endurance-trained recreational athletes (4 female and 10 male) completed a 4-week intensified training protocol.
• Participants were asked to complete 70% of their normal training volume for the first week, then for the next 3 weeks up to 150% of their normal training volume including 3 additional high-intensity training sessions per week.
• Joint-level neuromuscular testing of the knee extensors (isometric Maximum Voluntary Contractions, electrically evoked twitch contractions, and the development of a force-frequency curve) was completed before and during the intensified training period.
• Biopsies were taken from the vastus lateralis before and after the intensified training period to measure single-fibre force production and mechanical properties in solutions of differing concentrations of Ca²⁺.
• Single fibres were then classified based on fibre phenotype (“slow” or “fast”) to highlight differences in the mechanical properties between fibre types after a period of intensified training.

What the researchers found

• Maximal force output remained unchanged at both the joint and single fibre levels following a period of intensified training.
• Submaximal force production was reduced at the joint level, as shown by the development of prolonged low-frequency force depression, highlighting that more force is required to produce submaximal movements or to complete training sessions.
• The mechanism behind the changes in submaximal force production at the joint level can be attributed in part to reductions in Ca²⁺ sensitivity that occurred at the single fibre level, which resulted in the reduction of contractile function at submaximal levels.

Conclusion

• This article highlights that increasing training volume without increasing recovery time can lead to muscle impairments, making training sessions feel harder due to changes at the single fibre level and potentially decreasing sport performance if continued long-term.
• Further work should be done to determine the mechanism responsible for changes observed in the Ca²⁺ sensitivity of the single muscle fibres after a period of intensified training with insufficient recovery.