Burr, J. | Search bioRxiv

bioRxiv2026

Leg blood flow and muscle oxygenation are reduced when exercising with loads >=40% 1RM compared to loads <=30% 1RM due to increases in contraction induced intramuscular pressure

The critical occluding tension (COT) is the load at which contraction-induced increases in intramuscular pressure (IMP) collapse intramuscular vasculature and impair oxygen delivery. For knee extension resistance exercise, the COT occurs at ~30-40% 1RM, yet few studies have measured IMP alongside convective oxygen delivery. This study examined the interaction between IMP, leg blood flow, and muscle oxygenation during resistance exercise. Twelve participants completed knee extension bouts across a range of loads while vastus lateralis IMP, femoral blood flow (Doppler), blood pressure, muscle oxygenation (NIRS), and electromyography (EMG) were recorded. Mean IMP increased alongside relative load (20% 1RM=19.1(9.4) mmHg; 30% 1RM=22.4(10.4) mmHg; 40% 1RM=27.0(8.7) mmHg; 50% 1RM=31.5(11.5) mmHg; 60% 1RM=41.14(13.8) mmHg; p<0.05). Leg blood flow scaled to rest was similar between 20% and 30% 1RM (p=0.13), 40%, 50%, and 60% 1RM (p>0.05), but different between the 20-30% 1RM and 40-60% 1RM groupings (p<0.05) (20% 1RM=2.6(1.4) times rest; 30% 1RM=1.8(0.5) fold of rest; 40% 1RM=1.4(0.6) fold of rest; 50% 1RM=1.1(0.4) fold of rest; 60% 1RM=1.1(0.5) fold of rest). Muscle oxygenation was lower at 40%, 50%, and 60% 1RM compared to 20% and 30% 1RM (p<0.05). Thus, at [≤]30% 1RM exercising blood flow increased well above rest, indicating preserved microvascular perfusion, whereas at [≥]40% 1RM the rise in IMP was sufficient to impair perfusion and limit blood flow increases. Exercise at or above the COT restricts convective oxygen delivery due to contraction-induced increases in IMP.

physiology↗

bioRxiv2023

Effects of intensified training with insufficient recovery on joint level and single muscle fibre mechanical function: The role of myofibrillar Ca2+ sensitivity

Intense exercise training with insufficient recovery is associated with reductions in neuromuscular performance. However, it is unclear how single muscle fibre mechanical function and myofibrillar Ca2+ sensitivity contribute to these impairments. We investigated the effects of overload training on joint-level neuromuscular performance and cellular-level mechanical function. Fourteen athletes (4 female, 10 male) underwent a 3-week intensified training protocol consisting of [~]140% of their regular training hours with three additional high-intensity training sessions per week. Neuromuscular performance of the knee extensors was assessed via maximum voluntary contraction (MVC) force, electrically evoked twitch contractions, and a force-frequency relationship. Muscle biopsies were taken from the vastus lateralis to assess single fibre mechanical function. Neither MVC force nor twitch parameters were altered following intensified training (all p>0.05), but a rightward shift in the force-frequency curve was observed with a 6-27% reduction in force at low-frequencies (5-20Hz, all p<0.05). In single fibres, maximal force output was not reduced following intensified training, but there was a rightward shift in the force-pCa curve driven by a 6% reduction in Ca2+ sensitivity as indicated by a lower pCa50 value (i.e., higher [Ca2+]) across fibre types (Pre=6.477{+/-}0.157, Post=6.088{+/-}0.480, p<0.05). These data indicate intensified training leads to impaired Ca2+ sensitivity at the single fibre level, which in part explains impaired neuromuscular function at the joint level during lower frequencies of activation. This is an important consideration for athletes, as performance is often assessed at maximal levels of activation, and these underlying impairments in force generation may be less obvious. New & NoteworthyIntense exercise training with insufficient recovery leads to impaired muscle contractile performance. These impairments often manifest at lower frequencies of muscle stimulation, termed prolonged low-frequency force depression. Impaired myofibrillar calcium sensitivity has been suggested as a potential mechanism of prolonged low-frequency force depression. Our work shows that impaired calcium sensitivity of single muscle fibres coincided with joint level prolonged low-frequency force depression after intense exercise training with insufficient recovery.

physiology↗