The monthly Logistics Managers’ Index (LMI), which tracks U.S. transport, warehousing and inventory activity, posted the second-highest reading ever last month, as the nation’s supply chains continued to strain under the trinity of surging demand, tight supply and logistics networks not configured for the unprecedented volatility wrought by the COVID-19 pandemic.

The LMI’s headline number, published on Tuesday, came in at a 75 reading, second only to the 78.7 reading in March/April 2018. (The index was published in two-month intervals back then.) The overall index rate has clocked in above the 70-point mark for a record five straight months. In addition, the three-month moving average of 73.6 in the calendar year’s second quarter is a record, indicating that in the quarter just ended the index experienced the fastest growth rate in its nearly five-year history.

Available transport capacity — one of eight categories that make up the LMI — fell to 34.5 in June, the 13th consecutive month of contraction and at what the group that publishes the index calls “extreme rates” of shrinkage. The FreightWaves Tender Rejection index, which was cited in the report, has hovered around 25% since last fall. That means one out of every four available loads is not being picked up for shipping.

The 2018 reading was fueled by the highest reading for transport prices in the index’s history, a legacy of the federal government’s electronic logging device mandate that took effect on Dec. 18, 2017. The mandate removed a great deal of capacity from the market as drivers chose not to comply with it or took vehicles off the road to get into compliance.

Last month’s score, by contrast, was mostly the result of soaring warehouse and inventory costs, though transport costs remain high. Perhaps the most striking statistic in the June index was the 22.4-point gap between spiking inventory costs at 89.4 and expanding inventory levels at 67.

According to the authors, rapidly increasing inventory levels have been overwhelmed by the surge in warehouse rents and leases, as well as by the blast-off in retail sales as the economy reopens and Americans, many flush with cash from various sources of capital, continue to spend furiously on goods. Retail sales in April were 48.1% higher than in April 2020, a comparison skewed by the wholesale lockdown of the American economy during the spring of 2020.

The authors said the unprecedented fluctuations in inventory levels since the pandemic took hold have made it “significantly more expensive for supply chains to hold and move” each unit of inventory. There are no signs of near-term reversal; of the eight metrics that compose the index, only one, transportation prices, backed off from May 2021 levels. The authors said there will be no change in the status quo until supply chains that were built to function in fairly predictable and static business cycles adjust to the realities of a post-pandemic economy, namely e-commerce volumes that have grown faster than anyone expected.

The index is published by five U.S. universities in conjunction with the trade group Council of Supply Chain Management Professionals (CSCMP).