DCT

1:26-cv-04386

Ritfit LLC v. Zhong Yin

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I. Executive Summary and Procedural Information

  • Parties & Counsel:
  • Case Identification: 1:26-cv-04386, N.D. Ill., 04/17/2026
  • Venue Allegations: Venue is alleged to be proper as Defendants are foreign residents who may be sued in any judicial district. Alternatively, venue is asserted based on Defendants' enforcement activities targeting Plaintiffs' product sales to consumers in Illinois, which constitute a substantial part of the events giving rise to the action.
  • Core Dispute: Plaintiffs seek a declaratory judgment that their leg exercise machine does not infringe the Defendants' patent and that the patent is invalid due to anticipation and obviousness over prior art.
  • Technical Context: The technology at issue involves mechanical designs for leg extension and curl exercise machines, a common type of equipment in the home and commercial fitness market.
  • Key Procedural History: The action was precipitated by Defendants' patent infringement complaint filed through Amazon.com's Patent Evaluation Express program, which resulted in the delisting of Plaintiffs' product. The complaint details the prosecution history of the patent-in-suit, noting that the patent only issued after overcoming rejections based on prior art, including one rejection based on the Defendants' own pre-filing product disclosures. Plaintiffs allege that critical prior art was not considered by the USPTO examiner.

Case Timeline

Date Event
2021-08-23 Proformance Plus Reference becomes publicly available via YouTube video
2022-07-29 Chinese Utility Model Patent No. CN217067550U is published
2024-02-22 Defendants' LIONSCOOL product becomes publicly available on Amazon.com
2024-06-13 Defendants' LIONSCOOL product becomes publicly available on their website
2024-09-20 Plaintiffs' RITFIT Product is publicly featured on their official website
2024-09-23 RITFIT Reference becomes publicly available via YouTube video
2024-09-30 Plaintiffs' RITFIT Product is publicly listed for sale on Amazon.com
2024-10-02 Altas Reference becomes publicly available via YouTube video
2024-10-07 Plaintiffs record their first customer sales order for the RITFIT Product on Amazon.com
2024-11-21 U.S. Patent No. 12,427,372 B2 Priority and Filing Date
2025-01-23 USPTO issues Non-Final Office Action rejecting claims of the '372 Patent application
2025-04-28 USPTO issues a second rejection of claims of the '372 Patent application
2025-09-30 U.S. Patent No. 12,427,372 B2 Issues
2026-02-16 Defendants file infringement complaint against Plaintiffs' product with Amazon
2026-03-10 Amazon.com delists the RITFIT Product from its marketplace
2026-03-11 Plaintiffs submit an appeal to Amazon.com
2026-03-13 Amazon.com denies Plaintiffs' appeal
2026-04-17 Complaint for Declaratory Judgment is filed

II. Technology and Patent(s)-in-Suit Analysis

  • Patent Identification: U.S. Patent No. 12,427,372, "Leg Training Device," issued September 30, 2025 (the "'372 Patent").

The Invention Explained

  • Problem Addressed: The patent's background section describes a problem with conventional leg exercise machines where the arc-shaped motion of the swing arm results in an uneven weight distribution during the exercise ʼ372 Patent, col. 1:21-34 The resistance is described as being too light at the beginning of the motion and increasing suddenly at the end, which can diminish the effectiveness of the workout and increase the risk of injury ʼ372 Patent, col. 1:21-34
  • The Patented Solution: The invention claims to solve this problem by using a rotatably disposed counterweight bearer connected to the leg exercise component via a load transfer mechanism, such as a rope and pulley system ʼ372 Patent, abstract ʼ372 Patent, col. 1:56-62 This configuration is designed to transfer the resistance load from the counterweight to the user's legs more constantly throughout the entire range of motion, providing a balanced and smooth lifting experience ʼ372 Patent, col. 4:9-16
  • Technical Importance: The described technical approach aims to improve the safety and efficacy of leg exercise equipment by providing a more consistent resistance curve compared to prior designs.

Key Claims at a Glance

  • The complaint identifies independent claims 1 and 10 as being at issue Compl. ¶¶36-37
  • Independent Claim 1 recites a leg training device with, among other elements:
    • a support frame, seat, leg exercise component, and a rotatably disposed counterweight bearer
    • a load transfer component connecting the leg exercise component and the counterweight bearer
    • a leg exercise component that comprises a roller pad, a link mechanism, and an adjusting mechanism
    • wherein the link mechanism comprises a first rod, a second rod, and a bolt connected in a specific configuration
    • wherein the adjusting mechanism comprises an adjusting disk and a rotating shaft
  • Independent Claim 10 recites a leg training device with, among other elements:
    • a load transfer component comprising a rope and a guide wheel
    • a support frame with a fixed seat for supporting the counterweight bearer at rest
    • a configuration where two parts of the rope "constitute a 1:1 transmission" and pull the counterweight bearer away from the fixed seat to turn it over
  • The complaint notes that dependent claims 2 through 9 and 11 are also at issue (Compl. ¶¶38; Compl. ¶56).

III. The Accused Instrumentality

Product Identification

  • The accused instrumentality is the "RITFIT Product," identified as RITFIT's leg extension and curl machine, ASIN B0DJ8Z95WX Compl. ¶2

Functionality and Market Context

  • The complaint describes the RITFIT Product as a "plate-loaded, cable-operated leg extension and curl machine" for both home and commercial use Compl. ¶14 The complaint alleges that the product's delisting from Amazon.com following Defendants' infringement notice has caused "immediate and substantial harm" and "irreparable competitive and financial harm" to RITFIT's business, suggesting its commercial significance Compl. ¶3 Compl. ¶25 For the purposes of its non-infringement argument, the complaint alleges the RITFIT Product employs a "materially different load transfer and resistance-adjustment structure" than that claimed in the '372 Patent Compl. ¶54 and uses a "3:2 mechanical advantage ratio," not the 1:1 ratio recited in claim 10 Compl. ¶55

IV. Analysis of Infringement Allegations

This is a declaratory judgment action in which the Plaintiff, RitFit, alleges non-infringement. The following tables summarize RitFit's allegations regarding how its product fails to meet specific claim limitations.

'372 Patent Infringement Allegations

Claim Element (from Independent Claim 1) Alleged Non-Infringing Functionality Complaint Citation Patent Citation
the leg exercise component comprises... a link mechanism, and an adjusting mechanism... wherein the link mechanism comprises a first rod, a second rod and a bolt... wherein the adjusting mechanism comprises an adjusting disk and a rotating shaft The RITFIT Product is alleged to employ a "materially different load transfer and resistance-adjustment structure" that does not literally meet, and is not equivalent to, the specific structural combination defined in Claim 1. ¶54 col. 8:26-42

'372 Patent Infringement Allegations

Claim Element (from Independent Claim 10) Alleged Non-Infringing Functionality Complaint Citation Patent Citation
the two parts of the rope constitute a 1:1 transmission The RITFIT Product's rope-and-pulley system is alleged to use a "3:2 mechanical advantage ratio," which is not the 1:1 ratio required by the claim. ¶55 col. 10:13-14
  • Identified Points of Contention:
    • Scope Questions: The case may raise the question of how broadly to interpret the structural elements of Claim 1, such as the "link mechanism" and "adjusting mechanism." The complaint alleges the RITFIT Product is "materially different," which suggests a dispute over whether the accused product's components fall within the scope of the claim language Compl. ¶54
    • Technical Questions: A key factual question is whether the RITFIT Product's transmission system operates with a "1:1 transmission" ratio as required by Claim 10. The complaint's specific allegation of a "3:2 mechanical advantage ratio" creates a direct, testable conflict on this technical point Compl. ¶55 The complaint notes that a screenshot, attached as Exhibit B, confirms the USPTO examiner cited RITFIT's own product website as non-patent literature during prosecution, suggesting the product's features were publicly known Compl. ¶17

V. Key Claim Terms for Construction

  • The Term: "a link mechanism comprising a first rod, a second rod and a bolt" and "an adjusting mechanism comprising an adjusting disk and a rotating shaft"

    • Context and Importance: These terms define the core mechanical structure that distinguishes Claim 1. RitFit alleges its product is "materially different" Compl. ¶54, making the precise scope of this language central to the non-infringement analysis. Practitioners may focus on this term because the complaint alleges that the applicants amended the claims during prosecution to add this specific configuration to overcome prior art, potentially limiting its scope through prosecution history estoppel Compl. ¶57
    • Intrinsic Evidence for Interpretation:
      • Evidence for a Broader Interpretation: A party seeking a broader interpretation might argue that the terms should encompass any functionally similar linkage that connects the user interface (roller pad) to the resistance mechanism (adjusting disk).
      • Evidence for a Narrower Interpretation: A party seeking a narrower interpretation would point to the specific arrangement of the "first rod," "second rod," and "bolt" as detailed in the patent's written description and figures (e.g., '372 Patent, FIG. 6). They would also cite prosecution history arguments where the applicant allegedly distinguished this specific structure from the prior art to gain allowance of the claim Compl. ¶57

  • The Term: "1:1 transmission"

    • Context and Importance: This term is a specific functional limitation in Claim 10. RitFit's non-infringement argument for this claim hinges on its allegation that its product uses a "3:2 mechanical advantage ratio," not a 1:1 ratio Compl. ¶55
    • Intrinsic Evidence for Interpretation:
      • Evidence for a Broader Interpretation: A party could argue the term allows for minor deviations from a mathematically perfect 1:1 ratio while still falling within the spirit of the invention, or that the ratio should be measured in a particular way.
      • Evidence for a Narrower Interpretation: A party arguing for a narrow, literal meaning would point to the patent's explicit language that the rope parts "constitute a 1:1 transmission" '372 Patent, col. 10:13-14 They may also link this ratio to the patent's stated goal of creating a "constant" weight transfer, arguing that only a literal 1:1 ratio achieves this stated objective '372 Patent, col. 13:62-67

VI. Other Allegations

  • Exceptional Case Allegations: The complaint seeks a finding that the case is "exceptional" under 35 U.S.C. § 285, which would entitle Plaintiffs to an award of attorneys' fees Compl., prayer D The basis for this allegation is that Defendants allegedly knew or should have known that the technology claimed in the '372 Patent was already publicly disclosed-including by Defendants' own LIONSCOOL product-months before the patent application was filed Compl. ¶35 The complaint alleges that asserting the patent despite this knowledge and the existence of other substantial prior art renders the case exceptional Compl. ¶35
  • Prosecution History Estoppel: The complaint alleges that Defendants made "narrowing amendments" to Claims 1 and 10 during prosecution to overcome a § 103 obviousness rejection Compl. ¶57 Specifically, it alleges the link mechanism configuration was added to Claim 1 and the fixed seat feature was added to Claim 10 to distinguish the invention from the Johns and Holman prior art references. The complaint argues these amendments create prosecution history estoppel, barring Defendants from asserting infringement under the doctrine of equivalents for any claim scope surrendered Compl. ¶57

VII. Analyst's Conclusion: Key Questions for the Case

  • A central issue will be one of patent validity: was the invention claimed in the '372 Patent obvious at the time of filing? The complaint builds a detailed case for obviousness based on numerous prior art references, including some cited by the USPTO and others allegedly not considered, raising the question of whether the specific mechanical combination in the final claims represents a patentable advance or a predictable design choice Compl. ¶¶39-47
  • A second key question will be one of claim construction and factual infringement: does the RITFIT Product's alleged "3:2 mechanical advantage ratio" fall outside the scope of Claim 10's "1:1 transmission" requirement? And does its allegedly "materially different" linkage mechanism fall outside the proper construction of Claim 1's structural limitations?
  • Finally, a critical legal question will be the effect of the prosecution history: did Defendants' amendments to the claims to overcome prior art rejections create prosecution history estoppel? If so, this could bar them from recapturing surrendered territory through the doctrine of equivalents, severely limiting their infringement case, as argued by the Plaintiff Compl. ¶57
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