The extracellular K+ concentration ([K+]o) increases during physical exercise. We here studied whether moderately elevated [K+]o may increase force and power output during contractions at in vivo-like subtetanic frequencies and whether such potentiation was associated with increased cytosolic free Ca2+ concentration ([Ca2+]i) during contractions. Isolated whole soleus and extensor digitorum longus (EDL) rat muscles were incubated at different levels of [K+]o, and isometric and dynamic contractility were tested at various stimulation frequencies. Furthermore, [Ca2+]i at rest and during contraction was measured along with isometric force in single mouse flexor digitorum brevis (FDB) fibers exposed to elevated [K+]o. Elevating [K+]o from 4 mM up to 8 mM (soleus) and 11 mM (EDL) increased isometric force at subtetanic frequencies, 2–15 Hz in soleus and up to 50 Hz in EDL, while inhibition was seen at tetanic frequency in both muscle types. Elevating [K+]o also increased peak power of dynamic subtetanic contractions, with potentiation being more pronounced in EDL than in soleus muscles. The force-potentiating effect of elevated [K+]o was transient in FDB single fibers, reaching peak after ~4 and 2.5 min in 9 and 11 mM [K+]o, respectively. At the time of peak potentiation, force and [Ca2+]i during 15-Hz contractions were significantly increased, whereas force was slightly decreased and [Ca2+]i unchanged during 50-Hz contractions. Moderate elevation of [K+]o can transiently potentiate force and power during contractions at subtetanic frequencies, which can be explained by a higher [Ca2+]i during contractions.

The development of maximal velocity and power in muscle depends on the ability to transmit action potentials (AP) at very high frequencies up to about 400 Hz. However, for every AP there is a small loss of K+ to the interstitium, which during intense exercise, may build up to a point where excitability is reduced, thus limiting the intensity of further exercise. It is still unknown how the muscle responds to high-frequency stimulation when exposed to high [K+]. Contractile parameters of the muscles (force [F], velocity [V], power [P], rate of force development [RFD], and work) were examined during dynamic contractions, performed in vitro using rat soleus muscles incubated in buffers containing 4 or 8 mmol/L K+ and stimulated with constant trains of tetanic or supratetanic frequency or with trains initiated by a high-frequency doublet, followed by tetanic or subtetanic trains. At 4 mmol/L K+, an increase in frequency increased Pmax when using constant train stimulation. When stimulating with trains containing high-frequency doublets an increase in 120-msec work was seen, however, no increase in Pmax was observed. At 8 mmol/L K+, no differences were seen for either Pmax or 120-msec work when increasing frequency or introducing doublets. In all experiments where the frequency was increased or doublets applied, an increase in RFD was seen in both normal and high [K+]. The results indicate that stimulation with supratetanic frequencies can improve dynamic muscle contractility, but improvements are attenuated when muscles are exposed to high extracellular [K+].

Accelerometers are widely used to measure physical activity, but limitations in the ability to differentiate between running intensities have been reported. This problem may relate to accelerometer placement. In this study, we compare the validity of accelerometers placed on the hip and the thigh for the measurement of walking and running speed under laboratory and field conditions. Young healthy men and women wore ActiGraph GT3X+ accelerometers on the hip and on the thigh while performing walking and running activities in laboratory (n = 31) and field conditions (n = 17). Vector magnitude counts per minute (VM cpm) were correlated with speed of locomotion and, during laboratory trials, with oxygen consumption (VO2, ml·min−1·kg−1). Both hip- and thigh-placed VM cpm showed strong correlations with walking speed ranging from 3 to 7 km·h−1 (r = 0.93 and r = 0.95, respectively) and VO2 (r = 0.85 and r = 0.91, respectively). Compared with the hip-placed VM cpm, thigh-placed VM cpm showed significantly stronger correlations with running speed ranging from 7 to 20 km·h−1 (r = 0.29 and r = 0.89, respectively) and the corresponding VO2 (r = 0.25 and r = 0.87, respectively). Regardless of accelerometer placement, VM cpm were similar between laboratory and field tests performed at comparable walking and running speeds. These results show that accelerometers placed on the thigh, but not on the hip, provide proportional output across a wide range of walking and running speeds. Thus, thigh-placed accelerometers are able to differentiate between running intensities in both laboratory and field conditions.

Purpose: Moderate elevations of [K+]o occur during exercise and have been shown to potentiate force during contractions elicited with subtetanic frequencies. Here, we investigated whether lactic acid (reduced chloride conductance), β2-adrenoceptor activation, and increased temperature would influence the potentiating effect of potassium in slow- and fast-twitch muscle. Methods: Isometric contractions were elicited by electrical stimulation at various frequencies in isolated rat soleus and extensor digitorum longus (EDL) muscles incubated at normal (4 mM) or elevated K+, in combination with either salbutamol (5 μM), lactic acid (18.1 mM), 9-AC (25 μM) or increased temperature (30 to 35°C). Results: Elevating [K+] from 4 mM to 7 mM (soleus) and 10 mM (EDL) potentiated isometric twitch and subtetanic force while slightly reducing tetanic. In EDL, salbutamol further augmented twitch force (+27±3 %, P<0.001) and subtetanic force (+22±4 %, P<0.001). In contrast, salbutamol reduced subtetanic force (-28±6 %, P<0.001) in soleus muscles. Lactic acid and 9-AC had no significant effects on isometric force of muscles already exposed to moderate elevations of [K+]o. The potentiating effect of elevated [K+]o was still well maintained at 35°C. Conclusion: Addition of salbutamol exerts a further force-potentiating effect in fast-twitch but not in slow-twitch muscles already potentiated by moderately elevated [K+]o, whilst neither lactic acid, 9-AC nor increased temperature exerts any further augmentation. However, the potentiating effect of elevated [K+]o was still maintained in the presence of these, thus emphasizing the positive influence of moderately elevated [K+]o for contractile performance during exercise.